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The gear within the photo above includes, from left to right: meccano hand-cranked gramophone with acoustic recorder (out of body) to chop 78 rpm disc information on wax or celluloid; 'Crosley' 1926-vintage two valve regenerative receiver (above) with 1925-vintage crystal radio below it. Stromberg-Carlson portable battery-valve broadcast band receiver c.1948 used as a BFO for my principal transistor receiver. Small multimeter in entrance of S-C radio. National Panasonic 4-band 11-transistor (all germanium!) radio set receiving 160 metres (VK3AML on that occasion) with my Emmco headphones linked to it. The curly wire above the transistor radio was our communal private telephone line to my associates David and Bruce Bowden, who lived around the nook in Pleasant Road. HMV portable wind-up gramophone kind C101, circa 1923, with carbon microphone to transmit 78 rpm discs down the telephone line. Various broadcast-band and shortwave DX QSL cards and a single-sided disc on the noticeboard above (sung by a Madame Alma Gluck, from reminiscence!). In my left hand, I held the oldest disc report then in my assortment, London, circa 1903, of a brass band playing the 'Tancredi' overture. On the suitable, spare parts for our non-public neighbourhood line, and an outdated electrodynamic speaker. This was the one photo of my 'radio shack' taken within the 1960s. It was the 'sleep-out' behind 6 Torring Road, East Hawthorn, five miles East of Melbourne's GPO (Australia). The house was demolished in 1997. Three residence items now stand there.

Author's observe, 28 April 2005: As this web page offers with my own modulated mild experiments I hope readers will tolerate these reminiscences earlier than I reproduce my 1979 article on the topic from 'Amateur Radio' magazine:

WHY MODULATE Light?

Today's younger Australians can barely imagine the frustration of teenagers within the so-referred to as 'radical' 1960s the place electronic communication was involved. By fashionable requirements, radio was locked in legislative totalitarianism, dominated by the governmental communication monopoly of the Australian PMG's Department, and run on inflexible submit-colonial British public service strains. Something as innocuous as an intercom line run alongside your back fence to adjacent friends was illegal - 'a monetary menace to the monopoly of the general public telecommunications network' within the eyes of the PMG. I found this once i arrange a neighbourhood cellphone community to other native youngsters' locations early in 1967! Music transmission via newbie radio had been banned in Australia since 1939, in order that the fabric legally conveyed on any 'wireless' system was limited, significantly for music-mad teenagers. CB radios, mostly of the single channel hand-held selection, were imported in limited quantities and offered freely - however paradoxically they could not be legally licensed or used. You needed to be fifteen years old to take a seat for the amateur radio license, then involving a trifecta of exams in full theory, rules, and Morse at 12 words per minute. For the speculation, discursive essay answers have been required, theprimefans.com and there was no level of idea examination beneath the one most standard. From a younger particular person's perspective, you had to drag your self over a area of broken glass to realize a legal entry point. Even then, you had to wait in your sixteenth birthday to function ham radio legally in Australia.

If, like me, you had been thirteen years previous in 1967 and had digital experimenter friends close by, your aspirations to communicate faced years of seemingly endless frustration. There were good causes to investigate 'different' communication technologies - including modulated light.

Together with that we had the Vietnam War; the imminent menace of the obligatory army draft; a conservative government in energy since our beginning; and British-styled faculty uniforms unsuited to our scorching summers. Australian teenagers of the 1960s had many valid causes for discontent. The protest movement had its roots in many features of Australian society, not just the political issues for which any street march was a convenient excuse...

During the next decade Australians noticed the advent of full citizenship for Aborigines (1967); the Vietnam Draft Resister's Union, and its pirate radio station '3DR' (1971); a radical Labor authorities coming to power (1972); the primary Sunbury rock festival, Australia's 'Woodstock' (1972); novice amateur radio licensing (circa 1973); community broadcasting (1974); ethnic, multicultural broadcasting (1975); and the native legalisation of CB radio (1977). Finally, within the 1980s, non-public carriers in the telecommunication marketplace had been allowed to compete with the PMG and its descendant organisation, Telstra. Communication monopolies were damaged and the authorized restraints on private electronic communication eased markedly. With the arrival of the Internet, how may or not it's otherwise immediately?

I would argue that Australia was quite in contrast to America within the 1960s, in that conservatism reigned by many of the 1960s here, notably in our communication laws. Australia's 'liberation' - if you may call it that - largely came after 1970...

The arrival OF Educational Solid-STATE KITSETS

So, how did your average child with more than average curiosity get into electronics, and more notably into optical communication, forty years ago?

Within the mid-1960s, when the price of germanium transistors had fallen to a reasonable level, digital building kits were made available to stimulate the minds of future radio hams and experimenters. These kits were not like the one-mission meeting outfits then accessible from Heathkit and Lafayette. Their accent was on schooling, offering a set of electronic components to be arranged in different configurations on an insulating 'breadboard' with spring clips and connecting wires. With the help of a guidebook explaining the operation of every association, greater than twenty totally different circuits might be constructed, including Morse code oscillators, easy radio receivers, excessive-gain audio amplifiers and low-energy radio transmitters. At this time, we also had the bonus of a local Melbourne magazine for young experimenters, 'Transistor Kits' published by Colin Witchell - more not too long ago of 'Talking Electronics' magazine fame - from a tiny shop in Church Street, Brighton. For individuals who were interested, the mysteries of electronics held the key to a brand new world of technical prospects - and many of Colin's projects involved mild detectors in some creative manner...

An previous friend from my primary school days, Rowland Legg, acquired a Philips twenty-in-one package for the Christmas of 1965, and I was given a similar Japanese 'Eleco' equipment on the identical day. My parents had apparently famous my early aptitude for building crystal radio receivers, so that they'd decided to 'push the envelope'. The natural outcome was that over the next five years Rowland and i spent many afternoons together, unravelling the mysteries of digital amplification. Whatever time wasn't spent indoors with the kits or on other digital initiatives, we spent up on our rooves erecting prolonged wire radio antennas, or trying to get them up on progressively increased trees or supporting masts.

During these antenna-elevating sessions, Rowland Legg and I found that we might see one another's houses from vantage points on our rooves. In an try to arrange a signalling system, we spent many evenings up ladders with kerosene lanterns, using a black card to cut off the light and send messages to one another in Morse code. The usual Morse alphabet appeared moderately advanced, so Row devised certainly one of his own, following a logical mathematical development:

This Also proved to be too complicated to memorise and, with gentle flashes, too tough to relate to any typed checklist! We looked for a extra facile technique of communication, with enough security not to attract attention from the licensing authorities.

OUR FIRST OPTICAL COMMS - JUNE 1968

In 1967 I grew to become aware of the potential for transmitting speech over modulated mild beams after i bought a duplicate of an historical, leather-bound guide 'Science For All' (1884), containing William Ackroyd's account of Bell and Tainter's then-new 'photophone'. Another early influence was a ebook printed in 1921, 'The Boy Electrician', which gave constructional details of selenium light delicate cells, Tesla coils, audio transmission by multi-flip induction loops (which I constructed) and even the small print of a small X-ray machine (which, thank God, I did not assemble).

Like many teenagers of the late 1960s, I experimented with modulated light communication utilizing amplifier-pushed torch globes or neon lamps for transmitting and CdS photoconductive cells or OCP71 germanium phototransistors for receiving. A page from my bench notebook dated 25 May 1968 - a number of weeks after my 14th birthday - reveals my earliest plans for an optical system:

On the next weekend, 1st June 1968, a high school buddy named Howard McCallum and i arrange the planned modulated mild system with an incandescent torch bulb for transmitting and a CdS LDR in series with a 9 Volt battery and headphones for receiving, collimating a light beam between the two with magnifying lenses. The outcomes over a distance of four metres have been loud, but very distorted with frequency doubling results - a result of our initial attempts to transmit with out DC bias on the filament lamp!

Soon afterwards, a Mr A G Murrell of Penola, South Australia, printed the main points of his simple 'photophone' in the 'A Reader Built It' web page of March 1969's 'Electronics Australia' (pps. 91-93). The challenge's simplicity made it an attractive proposition for young folks, and many Australian experimenters of my era will remember it:

The geographical setting for our personal reconstruction of Murrell's system was less than salubrious. In reality, it was one of the eccentric radio 'shacks' that I've ever encountered. At the underside of my friend Rowland Legg's back yard, a large wood packing crate originally used for transport a Volkswagen automotive to Australia was set up by Row's father, Ern Legg, as a tiny electronics room - 'the tin shed' as we referred to as it. To maintain out the drafts, its partitions were papered with out-dated advertising posters for Melbourne's weekly scandal-rag, 'The truth', obtained from our kindly native newsagent across the nook in Tooronga Road. It was the sort of newspaper that no person would admit to buying, though the newsagent assured us of its wonderful native circulation. From every angle in Rowland's radio shed, headlines in an infinite typeface assaulted the eye: "SHOCK BIKIE Film" - "BLACK PANTIES Murder" - "CATHOLIC FATHER Wants Sex Surgery"! The piece de resistance amongst these posters was tactfully hidden behind a cupboard door, its wording being something like - "UNWED Mother TELLS Court: 'HE SHAGGED ME; THEN WE HAD INTERCOURSE' !!" - and I'm still questioning concerning the implications of that!

In these inglorious surroundings, and in the backroom of my home proven in the picture at the highest of this web web page, we spent many weekends constructing electronics projects of each conceivable type. Together, we soon had a version of Murrell's photophone transmitter constructed into the optics of a small ex-WW2 Aldis signalling lamp with a concave parabolic mirror of about 7.5 cm diameter. The Aldis housing was eventually mounted (with yards of PVC tape!) on a hot water overflow pipe protruding from the tiled roof at 1 Cole Street, Rowland's home.

However, my own preliminary optical comms exams with the Murrell equipment have been accomplished round March 1970 from the the yard radio shack then occupied by Hughie Paton, VK3ZEP, at 49 Havelock Road, throughout that house's back backyard, down its again driveway and across Torring Road to the glassed-in entrance veranda of our family home (to the left of the entrance door within the photo under) - a distance of about 50 metres. The system offered a hyperlink in one route solely. These tests have been logged on open reel audio tape, in order that I may test my audio high quality without the necessity for an assistant. My mother often came onto our veranda to listen to the outcome and talk her reaction by waving by the home windows. The tape still indicates the fairly good audio high quality that one might get with a restricted depth of modulation. It additionally demonstrates the very high hiss stage of the OCP71, the precise gadget used on that day being seen within the picture on the fitting.

I was out within the open air on the back of the Havelock Road property, across Torring Road from my house, shown above, with the Aldis lamp's glass parabolic reflector optics on a telescope tripod. This had a three volt 300 mA torch globe at its focus. I later discovered that the high frequency response of the system was inversely proportional to the thermal inertia of the filament. The perfect excessive frequency response was obtained with lamps of lower rated most present, a skinny filament with thick lead-out wires to dissipate the heat rapidly.

The transmitter gear, consisting of a microphone pre-amp, 3 watt transistorised audio power amp with transformer output, and two size D torch cell battery supplies (one for the amp and one as DC bias for the lamp) all sat in a carton between the legs of the tripod. At the receiving end, a 7.5 cm diameter magnifying lens focussed the picture of the Aldis lamp onto an OCP71. I can not remember whether the OCP71 was straight related to the microphone enter of the Sanyo 5" open reel tape recorder (photovoltaic configuration), or whether it was working into the road enter by way of the standard two-transistor pre-amp (photoconductive association). I appear to keep in mind that both were tried at different occasions, with a minimal distinction of outcomes. An occasional temporary buzz could be heard when widespread Australian blowflies occurred to fly through the beam, modulating the light with their wing beats. In the latter a part of the extract, a mild shower of rain occurred, the droplets falling by way of the beam producing a form of smooth 'plip-plap' sound. The effect of waving one's hand by the beam was also demonstrated. Whenever the beam was interrupted the hiss degree rose substantially - an impact for which, even now, I haven't any clarification, besides that it clearly had something to do with the impedance of the phototransistor supply rising as the light enter fell.

The audio tape of the check was made on a warm, pretty overcast Saturday afternoon. In the background, Ghera Harris (1896 - 1991) and her architect daughter Berenice Harris (1925 - 2002), who owned the Havelock Road property, can be heard washing dishes after lunch and planting hop bushes within the garden. The first World War veteran aero-engine fitter Reg Harris (1894 - 1979), Ghera's husband, was apparently planning a brand new ingredient for some home brew! The son of the household, Brian Harris (1936 - 1992), was briefly VK3ZFH in the late 1950s. Brian showed the creator how you can tune up a transmitter and skim an oscilloscope on the age of 4, in 1958. This set Chris on the lifelong 'downward' path into technical interests... Certainly one of Brian's last jobs involved the design of a few of the communications gear for the Hubble area telescope. Suburban life could be attention-grabbing with neighbours just like the Harris household at forty nine Havelock Road!

On this ten-minute mp3 extract from the original 45-minute tape, the creator at the age of 51 in 2005 introduces his squeaky-voiced 16-yr-old self in 1970. Computer nerds didn't exist then - WE were RADIO nerds! Anyone involved can obtain the audio log file of this March 1970 check as an mp3 file here:

(Currently damaged - coming quickly) March1970.mp3

One curious aspect of the germanium phototransistor OCP71 was its excessive infra-purple sensitivity. It was attainable to transmit audio, as one can hear on the tape above, with the torch bulb operating at a voltage so low that it had no perceptible visible output. The germanium detector could resolve the modulated HEAT from the lamp filament. Its work function as a photodetector was very low, which meant that it was inherently topic to large amounts of thermal noise, a lot more than silicon and very a lot more than photomultipliers:

On 1 September 1970 we extended the vary of the system to transmit audio from Rowland's electronics room ('the tin shed') at 1 Cole Street to my dwelling at 6 Torring Road, East Hawthorn, where I stood atop a ladder next to our yard bungalow with an an optical unit made from tin cans, like Murrell's within the photograph above. The audio link coated about 600 metres, and i may clearly see the sunshine focussed on the OCP71 in the receiver tube. The truth is, I may goal the receiver by having the focussed spot disappear behind the square delicate material inside the phototransistor. The hiss level from the OCP71 germanium phototransistor was gross, the 7.5 cm diameter of our optics was insufficient for the vary and the trebles had been restricted by the thermal inertia of the torch bulb, however this was our first actual 'light beam DX'. At the age of 16, the excitement of listening to these outcomes brought on my hands to shake, so that my optical receiver's intention was erratic, however I managed to log the entire contact on an audio tape in two 'bursts', with a pause to phone Rowland to inform him that each one was being received, midway. Initially, Rowland and his pal Neil Florence have been simply relaying the 7:30 pm information broadcast from the Melbourne broadcast station 3AK with its objects on the Vietnam War, and floods in New Zealand. As I slowly managed to align the receiver, the transmission steadily rose out of the noise - though not by a lot! Later, typical teenage music of the day can be heard - The Crystals singing 'After which He Kissed Me' (with Rowland attempting to interpolate a 1970-vintage form of 'karaoke') and the record of 'Lay Down' sung by Melanie Safka. When Rowland switched to his carbon microphone to announce 'Hello, Chris... that is being transmitted on the primary day of Spring, the 1st of September 1970 - and if you cannot hear this in any case this bother I'll have to kill you!', I might easily have fallen off the ladder in amazement. We'd finally devised a technique of circumventing the unlawful utilization of a radio transmitter - although to be sensible, with this appalling signal-to-noise ratio the success was solely marginal:

http://www.bluehaze.com.au/modlight/1Sept1970.mp3

I've hardly ever skilled more thrill from experimental work than I did on that night time 35 years in the past, and that i want hardly add that immediately afterwards, adrenalin-charged, I ran non-cease to Rowland's to report our success! The three of us posed for a photo round that time, with me holding a pair of 1920s-vintage headphones which we frequently used. The image could be titled 'hear evil, SEE evil, DO EVIL' - but largely, our backgrounds have been just too 'Eastern Suburbs' and discreetly shy for any of that!

OPTICAL COMMS IN SOUTH AUSTRALIA - 1968 TO 1972

Elsewhere, different Australian experimenters have been attempting the potential of atmospheric optical communication extra significantly. In Adelaide throughout 1968, my future collaborator Mike Groth (at present VK7MJ, then VK5ZMG) and a lab assistant good friend, Stewart Powell, built a pair of optical communication units within the suburb of Hammersmith using torch globes and OAP12 germanium photodiodes. They had a most range of about 1 km, and, as Mike places it, "awful fidelity". Mike's spare time for optical checks was restricted at the time by the calls for of doing the third 12 months of a BSc - with a new spouse.

However, in 1969 Mike Groth did his Honours in Adelaide, and one of many course initiatives in that year involved modulated mild. He defined the subsequent events to me in a letter dated 28 February 1988:

"The mission involved the analysis of modulated gentle as a technique of transmitting geomagnetic data over short distances to keep away from wire hyperlinks to the distant sensors, which tended to introduce hum loops if not rigorously balanced. This was a golden opportunity to learn the theory behind optical links and search again by the literature on the University Library. It turned apparent that it would be no drawback to transmit data over just a few hundred metres on a transparent night utilizing the brand new infra-purple diodes as sources, but the reliability of the hyperlink was unknown, especially as it was for use at a area station in the Adelaide hills, the place fogs and mists were widespread. I constructed an infra-purple hyperlink at 930 nm which measured the trail loss over the winter and spring of 1969, and my estimates of the path losses [in the 1987 'Amateur Radio' article 'Photophones Revisited'] were based mostly on this knowledge.

Optical communications fell into the background for the next 16 years, as I spent 1970 in New Guinea teaching and moved to New Zealand in 1971 to do postgraduate work at the University of Otago in Dunedin. I returned to Australia at the tip of 1979, but was relatively inactive in experimentation till 1985, after i determined to write down my experiences as a evaluation of the prospects of optical links. The hassle took almost 18 months..."

Mike's resultant article published in 1987 with revisions from 2005 may be found at:

Probably the most powerful amateur atmospheric optical communication tests in Australia around 1970 were executed by an Adelaide team and reported in an extraordinary Tasmanian magazine. The Hobart-based mostly 'Electronics Exchange Bulletin' was published round this time by the Tasmanian staff of Leo Gunther VK7RG and Rodney Reynolds VK7ZAR (now VK3AAR). Their extraordinary journal inspired articles by native experimenters on every conceivable subject associated to electronics and communications. Through the pages of 'EEB' between August 1968 and October 1972, two college students, Kingsley Burlinson VK6ZEA and Robert Averay VK5ZGE described their experiments in modulating fluorescent and mercury vapour fuel discharge lamps, attaining atmospheric ranges in excess of 3.5 miles (about 5 km), just outdoors Adelaide in valleys shielded from the city lights.

Though their gear was massively bulky by trendy standards, Burlinson and Averay pursued a novel line by driving their gas discharge lamps with audio-modulated 10 KHz pulse width modulation from 'class D' switching output transistors, thereby avoiding linearity and modulation effectivity problems. In this manner, the efficiency was much like that of the celebrated (at the time) class-D British Sinclair 'X-20' transistorised audio amplifier of the mid-60s. I used to be not conscious of the stories of these tests until the start of 1976, otherwise I may have skipped the following few steps in my very own tests...

OUR Switch TO Gas DISCHARGE LAMPS

A seek for higher modulated gentle sources than incandescents drew my attention to neon lamps. Gas ionisation is a a lot quicker process than the incandescent heating of a filament. In those days, neons had been cheaply accessible in any size from a pea lamp to a full size "beehive" bulb from one marvellous supply. Melbourne experimenters energetic within the 1960s will remember Waltham's Trading Company in Elizabeth Street, Melbourne. At the end of a narrow stairway leading down from the pavement was a moderately grubby Aladdin's cave of tables laden with cable, struggle surplus junk (each struggle except Vietnam), boxes of valves and khaki-painted items of indeterminate origin. Cash-strapped adolescents milled about with down-turned eyes, slowly sifting row after row of cartons and crates stuffed with technical solid-offs of every description. I acquired a number of boxes of neons there, together with varied other gas discharge units to try. A few years later, I discovered that some of these contained traces of radioactive material to help ionisation. Let the purchaser beware!

I initially wired a neon lamp into the anode circuit of a Philips battery triode from the 1920s, sort B406. The orangey-pink glow around the cathode on these neons is fairly intense, although it would not even strategy the intensity of modern LEDs or lasers. After organising the neon modulator I used to be rewarded with close to good audio from my OCP71 for the first time. Fortunately, a parental veto on building mains-fed power supplies ended on my fifteenth birthday (eight March 1969), so the required 300 volt rail was no problem.

SIDETRACK INTO 'MECHANICAL' Television - 1971

I used the neon modulator and OCP71 as the basis for a simple television system in 1970, by adding a few Nipkow scanning discs to the outfit. This diverted me from the modulated light communication checks for a couple of years as I delved into all of the pre-warfare Baird television texts to deliver the images to an acceptable commonplace. This introduced me into collaboration with the late Dan Van Elkan (b.1952 - d.1986, name signal VK3UI) and Tony Sanderson (b.1945, VK3AML), now the moderator of the 'bluehaze' web site. They were the 'ringleaders' of a larrikin group of amateurs operating residence constructed AM transmitters on the 160 metre band, then occupying 1800 KHz to 1860 KHz. These guys' newbie radio interests had been distinctive and individualistic. The vast majority of conservative hf (shortwave) operators had a slender and obsessive emphasis on 'communication quality' modulation, usually 300 Hz - 3 KHz, clipped, non-linear and cruddy. Dan and Tony were each hi-fi fans. Their transmitters had been comparatively broadbanded and immeasurably low in distortion, exceeding the audio specifications of many broadcasters. AKG or Western Electric microphones and broadcast-quality audio peak limiters with dual time constants have been concerned. The design and development of their modulation transformers and amplitude modulators was nothing short of an art. Their 160 metre receivers, also, employed biased, low distortion envelope detectors working by means of fastidiously designed audio amps into huge speaker systems with vented enclosures. The content and audio quality of their in-depth conversations on communications know-how made listening to their transmissions an absolute pleasure. Even their normal of audio compression served to convey the listener into the acoustic setting of their houses - an underestimated aspect of creating a practical auditory illusion. Their activity ruffled quite a number of amateur operators' feathers on the time... which only elevated my youthful admiration for both of them.

I truly met Dan (3UI) - dare I admit it - on the air on 31st December 1969, as the result of a brief dalliance I had with pirate radio transmissions on 1.Eight MHz. He lived in Hawthorn only a mile from my home, near the corner of Glenferrie and Riversdale Roads. Naturally he was amongst the primary to listen to my feeble and unstable transmissions, and he encouraged me to experiment additional to achieve the information to get the novice 'ticket'. We have been both given a 'cease it or else' ultimatum by an over-zealous radio inspector known to the locals as 'Uncle Ugh' (many will nonetheless know who I imply), however Dan and that i became nice mates because of this. Dan was about two years older than I, and was extremely influential on the quick future course of my life. Because the year 1970 progressed, I found myself spending more time with newbie radio pals and less with Rowland and the old fashioned mob. Rowland ultimately carved out a really profitable career within the Victorian Police Force (ironical, isn't it?), and i consider he still does, but his early interest in electronics declined...

In subsequent many years, the amateurs have legislated themselves into 'band plans' with 'accepted modes' and 'accepted bandwidths' for various frequency segments. In consequence - and I will specific an opinion here - they have systematically eliminated the legal foundation for the type of justifiable experimentation that 3AML and 3UI used to undertake. Many 'hams' are now operators of economic 'black box' transceivers which might elegantly provide single channel telephone quality (or worse) on every obtainable band for $4000+, however which often can't be correctly adapted to every other mode or type of experiment. From my perspective, the result's that novice radio has had progressively less attraction as a pastime, and I'm certain I'm not alone in expressing this opinion. For my money, when you legislate against experiment you kill the only attraction that amateur radio ever had. So lengthy as the emissions do not unfold past the amateur band edges, what is the problem? Anyone who pushes the hoary outdated argument that "bandspace is at a premium, so transmissions should be of the minimal doable bandwidth" must be deaf and blind to the steadily declining stage of beginner band usage over the past fifteen years. Who's to say what amateur radio should entail, so lengthy because it offers training and encourages experiment? Many people marvel why I've never bothered to pursue an amateur radio license. I hope that I've justified my position. Thank God for alternatives with extra freedom of content and bandwidth, like mild beam communication!

Dan (3UI) and i ultimately organized test transmissions of slim band television utilizing mechanical disc scanners over his 160 metre transmitter early in 1972. The next couple of years gave all of us a superb grounding in the principles of gentle detection and modulation, video amplification and optics.

Eventually, with D B Pitt and others in the United Kingdom, we formed the Narrow Band Tv Association, still in energetic operation and now represented on a website:

http://www.nbtv.org

One notably kind donation to this mechanical Tv scanner project was supplied by the late Kevin Duff, VK3CV (b.1927 - d.1996). Kev worked in telecine at Melbourne's authorities Tv station, ABV channel 2 in Elsternwick, working an archaic monochrome Marconi 35mm film scanner. This was originally mentioned to have been designed for the 405 line British service and used at BBC Tv's unique studio on the Alexandra Palace in London. By 1972 it was solely getting used for 30 minutes a day, at about 4:00 pm, to broadcast 35 mm film episodes of the youngsters's serial "The Cisco Kid" - the only regular program materials they had which still used that gauge of movie. The EMI 6097 photomultiplers in this Marconi telecine have been written off as quickly as they developed spots on their photocathodes - Kev known as them 'dynode spots' - but they were still quite serviceable for light detection. In 1972 Kevin saved a few of these from the dustbin for us.

Dan (3UI) and that i adapted the EMI 6097 photomultiplier to be used in our experimental camera and for the modulated gentle receivers. Its sensitivity was such an enormous quantum leap from the OCP71 that it opened a complete new world of technical prospects to us. Further photomultipliers have been acquired from the late A H 'Mac' McKibbin, VK3YEO, who used 931A's for slow scan tv scanners in those days.

MOD Light ON 160 METRES - FIRST CROSSBAND Tests 1974

Through the Autumn of 1975, I constructed a modulated gentle communication link that was used briefly between two members of the 160 metre AM group, Paul Higgins (then VK3BEK, now VK3EN) and Dave Stewart (VK3ASE). Both had radio shacks at first floor level, going through one another throughout suburban Glenhuntly with an uninterrupted line-of-sight, and separated by about 700 metres. The modulator from my Baird mechanical tv receiver by then used a 6L6 beam pentode in sequence with the neon, which was re-mounted at the main target of a rough 30 cm moulded glass parabolic reflector equipped by Tony (3AML). The reflector had initially been used in a traffic signal.

This optical transmitter was positioned on Paul's balcony in Glenhuntly Road, on the corner of Clarke Avenue. The receiver was placed within the attic window of Dave's QTH in Burrindi Road, Caulfield South. It used a 13 cm diameter magnifying lens focussing onto a 0.5mm focal airplane aperture, with a 931A photomultiplier catching the transmitted mild behind it.

Using Dave's 160 metre amateur transmitter as the return hyperlink - the ultimate in split frequency operation - Paul managed to hold on a crossband contact by the neon lamp. Sig/noise was poor owing to the very poor spectral match between the orange neon and the blue sensitive photomultiplier. The neon was pushed past its current ratings, in order that its bulb was quickly blackened by cathode sputtering, however at least it may very well be fully modulated. I was later in a position to measure the bandwidth and was astounded to find that the neon might be modulated to round 500 KHz, which was a terrific enchancment on the torch bulbs. Distortion was extreme. We were pushing the modulation fairly arduous, owing to the poor sig/noise ratio. A sample of the audio log of the contact is at present obtainable on Dave's web site. The outcomes have been sufficiently good for us to realise that we have been heading in the right direction for additional enchancment. The contact was logged on tape and an extract will be heard on VK3ASE's internet site:

[ Actually, Dave seems to have eliminated this one for now. (Tony, VK3AML) ]

The primary downside to be overcome was the low characteristic intensity of the neon discharge. We additionally needed a source with important blue output, to match the spectral response of the photomultipliers, which might only detect violet, blue or green gentle. Development was accelerated by the involvement of one other member of the 160 metre cross-band contact clique, John Eggington (then VK3ZGJ, now VK3EGG) whom I met at the top of 1975.

BREAKING THE ONE-MILE BARRIER - DEC. 1975.

At my residence, I was lucky in having an elevated place near the top of a hill in East Hawthorn, South of Camberwell Junction and fairly near the excessive copper dome of 'Our Lady Of Victories' catholic church. Standing on our roof, the view of downtown Melbourne to the West and of the suburbs round to the North was unobstructed, encompassing all of Hawthorn, Kew, Toorak, Kooyong in addition to components of Malvern, Richmond, Abbotsford and Northcote in an unbroken a hundred and twenty diploma arc. Just on my aspect of the Yarra, on the highest of a rise in clear view, was VK3ZGJ, along with his shack going through me at the third floor rear of an previous Victorian mansion at 29 Shakespeare Grove, West Hawthorn. The topography was preferrred for optical communication tests, exactly two miles (about 3.5 km) on an East-West path.

Within the early 1970s, before I met him, John (VK3ZGJ) constructed a sequence modulator for fluorescent lamps, consisting of a number of 807 output valves in parallel, with the fluoro within the anode return. He also constructed a portable light dependent resistor (LDR) receiver with a FET preamplifier. The LDR operated with bias into a load of 10 megohms or more, and with its low noise preamplifier it had significantly better sensitivity and spectral match to a fluorescent lamp's output than my old OCP71. You had to arrange for the image of the fluoro to focus precisely onto the hole between the conductive combs on the LDR's sensitive floor. This involved peering at the LDR via a 'spy-hole' in the optical mounting while deftly manipulating the receiver's alignment.

LDRs have a very slow response, rolling off at the very least 6dB per octave above 50 Hz. Treble increase could only partly appropriate the problem, as John discovered. My photomultipliers provided the reply to John's receiver problems, just as his fluoro transmitter modulator permitted advances over my feeble neon. We pooled assets over the next 5 months.

Late in December 1975, John and that i bought the communication system working between our houses, using a vertically mounted fluorescent lamp on the roof which "broadcast" light in all instructions. The modulator consisted of several (4, I believe) sort 807 beam pentodes feeding the forty watt fluoro in their anode circuit. A rail voltage of about 600 volts DC was utilized to the lamp. We did not use a lamp starter or a ballast choke to operate our fluoros. Instead, we had a novel starting association consisting of a band of aluminium foil wrapped across the glass near the cathode end of the tube, which was related to the secondary of an automotive spark coil. To begin the discharge, you'd apply the 600 volt rail, then energise the ring around the cathode by applying a battery briefly to the spark coil primary. The high-voltage spikes utilized to the glass near the cathode started a barely perceptible glow discharge inside that end of the tube, which might instantly spread the whole length of the tube beneath the influence of the 600 volts DC rail. Standing current was diverse by altering the worth of the cathode resistor on the sequence 807's. The filaments at both finish of the tube were never heated up with this association, which appeared to increase the working life of the tube.

Our first forty watt 'fluoro' take a look at transmission got here simply after the Christmas of 1975. It was in a single direction only, with John transmitting and me receiving. 3ZGJ managed to arrange some fairly elaborate music packages interrupted by announcements, one in all which I recorded on tape. Within the early 1970s a British group, calling themselves "Radio Love" (hey man, actual groovy) had proposed a system of native gentle beam broadcasting. This seems to have been an attempt to bypass the heavy hand of British officialdom, which at the moment had forced a number of independent broadcasters (eg 'Radio Caroline') to transmit from ships anchored in International waters off the English coast. Nothing was heard from the group after the publication of its preliminary plans, including the diagram beneath which neatly summarised their ideas:

John 3ZGJ, in imitation of this 'Radio Love', jokingly introduced his packages as being transmitted from "Radio Hush". The title was a bit less poofy, yet nonetheless retained the important atmosphere of the legal fringe-dweller! Later we duplicated the system to supply full duplex communication (simultaneous transmit and obtain) in each directions. A typical extract from these fluorescent gentle transmissions in the last week of December 1975 may be heard on this tape, recorded from the photomultiplier output at my finish of the link, two miles from the transmitter:

RadioHushDec75.mp3

We used no reflector or collimator with these fluorescent lamps. The photomultiplier receivers solely managed to achieve 15 dB sig/noise over the 3.5 km vary with this association. Fluorescent lamps had more than their fair proportion of issues as a modulated supply. Their phosphor coating had time lag, the persistence limiting the upper modulated frequency to about 5 KHz. The time lag was not constant with the wavelength of the emitted light. The pink phosphor elements had very long persistence, while the blue phosphor was much sooner. The system's upper audio frequency limit subsequently diverse with the spectral response of the detector, however with a blue-sensitive photomultiplier it was greater than adequate for audio.

The fluorescent lamp discharge would wander and 'snake' contained in the tube, particularly at swap-on, interfering with the modulation. For some purpose which we could never explain, the output on the cathode end of the fluoro, and only on the cathode end, was modulated by an erratic whine at about four hundred Hz, presumably attributable to the discharge hopping about from one part of the coiled cathode filament to another. This always set a definite restrict to the sig/noise achievable, even where there was plenty of light sign to demodulate.

The audio frequency response of the fluorescent tube was unexpectedly uneven, an impact undoubtedly attributable to acoustic resonances of the modulated mercury plasma column within its tubular enclosure. One could certainly hear a faint acoustic radiation from the tube while it was in operation, as these acoustic waves actually penetrated the glass partitions. Certain modulation frequencies, clearly associated to 'organ pipe' resonances within the fluoro tube, would cause the mercury discharge to extinguish, or to break up into a sequence of spaced glow discharges alongside the tube. I later discovered that this problem had been noted by N C Beese, who wrote a chapter on "Light Sources for Optical Communication" in the book 'Infrared Physics' (Pergamon Press Ltd., London, 1961, Vol. 1, pps 5 - 16). To quote from Beese (pps 13 - 14):

"Enclosed arc lamps operated on a.c. energy within the audio-frequency vary, or on d.c. and modulated by a.c. currents might trigger sound vibrations to be produced within the arc chamber. They're brought on by thermally induced variations in fuel stress that outcome from modifications in current density in the arc. At sure important frequencies, resonance of appreciable intensity is constructed up by reflection from the bulb partitions. The scale and form of the bulb, form of gasoline or vapour filling, temperature and operating conditions decide the frequency of the plasma oscillations which might be similar to standing sound waves in the discharge. Ordinarily this phenomenon is not observed as a result of lamps are operated on d.c. or low frequency a.c. with adequate ballast to make sure stable operation. In lengthy tubes the discharges assume a constricted, snakelike look at the vital frequencies and are attributable to sound energy reflected from the ends of the bulb. In a spherical bulb the sound waves spread to the bulb partitions and are then focussed again upon the arc to supply instability at the electrodes [...]"

Beese goes on to analyse a Xenon discharge lamp 1.5 cm in diameter and 13.Three cm long with 3 Amp d.c. current utilized with a 2 amp a.c. modulation applied. He famous:

"[...] the arc showed violent distortions at 2250 Hz but was quiescent at 2000 Hz and 2500 Hz. With 5 A d.c. and 3 A a.c. modulation at 2500 Hz, the discharge once more confirmed pronounced disturbances, however was stable at 2300 and 2700 Hz. The instability could start at both electrode, whereupon the discharge constricts into a thin luminous ribbon with sinusoidal shape, and the voltage will increase due to elevated arc size. [Instability at] harmonics of the basic frequency may even be noticed[...]

"[...] In a spherical bulb with electrodes on the centre [like a Xenon arc] acoustical resonance happens when the bulb diameter is equal to at least one-half [of the acoustic modulation] wavelength [...] An arc centred in a spherical bulb will literally 'blow itself out' by its own sound waves if any of the strong resonance frequencies are utilized to the lamp for an appreciable time [...]

[...] In a low pressure discharge lamp [eg fluorescent], assuming a median gasoline temperature of 250 degrees Centigrade, the velocity of sound in mercury vapour was 19,000 cm/sec, calculated by Laplace's formulation. At 600 Hz, the wavelength equals 31.7 cm [...] Maximum disturbance or turbulence at the electrodes occurs at a half wavelength from the nodes which are on the ends and centre of the lamp."

Owing to the low intensity and prolonged source area of the fluorescent lamp, the radiated flux might by no means be properly collimated. These limitations, and the frequency response irregularities clearly indicated that our mild transmitter needed a change of strategy.

At this level, Rodney Reynolds VK3AAR drew our attention to EEB's publication of the sooner work of Burlinson and Averay, whose modulator circuit and mercury arc source was a wholly novel method. Their optics, nevertheless, were quite crude, not practically directive sufficient for our work in suburban Melbourne, the place road lights and illuminated promoting signs proliferate.

FROM FLUORESCENT LAMP TO MERCURY ARC

We needed a better depth supply, no fluorescent coating, and an output wealthy in blue gentle to match the photomultipliers. From the electrical provider Arthur J Veall in Bridge Road, Richmond, John and i obtained some Philips excessive stress mercury arc lamps in January 1976, of the type used for manufacturing unit lighting. Most of these had inconvenient fluorescent coatings, apart from the smaller lamps below 100 watts output, which sadly had frosted glass envelopes. We bought a few HP80 mercury lamps rated at 80 watts. Driving these was an actual downside. The fluorescent lamps that we previously used have been low present, high voltage gadgets, modulated merely with customary output valves. By comparison, the excessive strain mercury arcs ran at medium voltages (30 to a hundred volts) but with high present (0.5 to three amp standing present), and they exhibited a extreme detrimental resistance characteristic - their voltage drop decreased sharply with growing current movement.

Other issues arose owing to the instability of the mercury arc. Its striking voltage diversified widely with ambient temperature, and its standing present different with time as heat induced the mercury to vaporise and the arc pressure elevated. At its peak, the quartz arc tube had to withstand inside pressures of round 30 atmospheres, so it was a device that you simply had to deal with with nice respect!

The slightest overmodulation peak would extinguish the discharge, and the whole system then needed to cool earlier than it could possibly be re-struck at a moderately low voltage. The lamp polarity also needed to be reversed at frequent intervals with a DPDT switch to avoid premature cathode failure by ionic bombardment. The lamps were designed for AC operation, the place the effects of cathode heating were shared 50 instances per second by each electrodes. Regardless of utilizing DC bias on the tubes, for which they weren't designed, we managed to get just a few hundred hours out of them, which was Ok when the lamps only value $7 apiece. Today they price about ten instances that.

The modulator had to steer the present passing to the arc. Quiescent class-A amplifier situations demanded that at the very least half of the provision rail should be dropped by the collection modulator, the remainder being dropped by the mercury arc. The unfavorable resistance of the arc was our stumbling block. As the present by way of the arc elevated its voltage drop decreased. This dragged the voltage utilized to the series control gadget up in the direction of the rail voltage as present reached a most. With 200 volts-on the rail and a peak current approaching 5 Amps, no transistor generally accessible in 1975 might handle the job. They could not take the high peak currents concurrently with the high emitter-collector voltage that this load with its damaging resistance would current to the output transistor in school A. We had some expensive pyrotechnic shows of 2N3055 and BUX80 transistor failure owing to this secondary breakdown level being exceeded. Like many foolhardly experimenters, we rarely used fast-blow fuses in our gear. Youthful arrogance, I guess!

The elegant resolution, recommended many years later by Rod Reynolds (VK3AAR), was to run the control transistor in parallel with the arc, with a single collection resistor as much as the supply rail from each. In that configuration, the transistor passed maximum voltage at minimal present, and vice versa, so that a much smaller transistor might be used than within the collection modulator configuration, the tradeoff being only a decreased general power efficiency. However, at the age of 21 in 1975 - and John was 20 - neither of us considered that...

Instead, we used a category-A vacuum tube amplifier of elephantine proportions in sequence with the arc. The facility provide was able to 200 Volts DC at three Amps, and the reservoir capacitors totalled 1500 µF at a 350 Volt ranking! For the modulator, 4 hefty 6080 or 6AS7 excessive present, low gm triode regulators were placed in parallel with low-worth balancing resistors in their cathode returns. The anodes had a nasty behavior of glowing a dull crimson when the arc was initiated, because it took a few minutes for the mercury discharge to run up to its normal 100 Volts drop. During that point, the major part of the provision rail was utilized to the valves, which had been sent beyond their dissipation limit. Under these situations, one could see alarming high resistance 'spots' scintillating with a bright and sparkling yellow gentle on the large oxidised cathode surfaces of the 6080s. I always anticipated a 'bang' but someway I was at all times lucky...

Each 6080 filament consumed 2.5 Amps at 6.3 Volts. The valve heaters alone consumed a total of 63 watts! It was all brute drive, ignorance, bulk power and heat! Fan cooling was obligatory. We used a home fan of 1928 vintage with a bum sleeve bearing, which rattled continuously during QSO's. Nevertheless the system labored remarkably properly, and because of Melbourne's many disposals stores, it could possibly be built for a few dollars. Not so now! Electronic disposals stores of that type are now mostly a thing of the previous.

This mercury arc transmitter was reasonably unstable in operation, liable to permitting the arc to drift right into a sluggish thermal runaway. To appropriate this tendency, one had to track the current of the arc by manipulating the grid bias of the output tubes - therefore the rationale for the metering of arc current and voltage within the modulator circuit, above. The temperature of the quartz bulb was the most important variable, as this and the arc's standing current were carefully associated. The bulb temperature managed the mercury vapour strain in the quartz phial, and due to this fact the electrical resistance of the arc. With inadequate average present movement, the heat of the arc would be too low to maintain the mercury vapour pressure, inflicting the voltage throughout the arc to slowly fall, in flip resulting in an additional fall of current. If the standing present was too excessive, the arc would change into so sizzling that it not only became harmful, but was unattainable to modulate totally. One could not depart the system unattended for more than about five minutes. Arc present would differ, particularly in the time simply after 'change on', because the log document below clearly signifies. Warm-up would take around 15 to half-hour before the system lastly settled into static present circumstances, with the heat pumped into the arc electrically equalling the heat misplaced to the encircling surroundings. The average standing present varied with the audio program content material and with the symmetry of the audio waveform utilized.

A typical example is supplied by the following log that I took of the arc transmitter's electrical standing, measured by the evening of Saturday 31st January 1976. Operation on that evening was greatest described as 'intermittent'! Eventually, in response to 3ZGJ's insistence that I add a adverse peak clipper to my modulator, I fitted a diode to the grids of the output tubes to do the job!:

6:05 pm

eighty V @ 0.Forty two Amp, 33.6 Watts

REMARKS: Test TRANSMISSION Only before nightfall. Arc has been on for forty five mins of heat-up, approx 60% peak modulation, with music from 78 rpm discs (avoiding copyright points!). Ambient temp. 350 C. VK3ZGJ not listening yet. Power and heater transformers are each sizzling to touch, but Ok.

PHOTOMULTIPLIER RECEIVERS - 1975/76

The receivers used on this system had been also designed around parts obtainable from disposals sources. Photomultipliers are superbly sensitive, and not nearly so fragile or difficult to arrange as some so-known as consultants recommend. They require a easy one thousand volt supply at about 5 or 10 mA - easily arranged with an old valve radio transformer working into a voltage doubler. The hand-held photomultiplier receiver was a moderately hazardous system once you had been manipulating it at night time from the top of an earthed metallic ladder! Particularly so when the 1000 volt supply had 8 µF of oil-crammed block reservoir capacitor behind it (ugh!) and was fed to the receiver by flat 240 volt twin lead flex. It grew to become much more dangerous when rain was falling! In my youth I had scant appreciation of my own mortality. Few teenagers ever do!

The old disposals photomultipliers typified by the 931A had virtually no response to red light, in order that they could not be used with LEDs or HeNe lasers. More moderen photomultipliers with gallium arsenide photocathodes have good pink sensitivity, however at prices exceeding $a thousand they're out of my league.

Overall outcomes with this cumbersome link system have been remarkably good. Considering the potential theoretical issues of thermal lag and ionisation time, the mercury lamp's limit of 50% modulation at 10 KHz was fairly usable, completely acceptable for direct amplitude modulation at audio frequencies. Piles of signal were accessible. The output was terrific. At night, with one's eyes darkish-tailored, you needed to protect your eyes with dark glasses or welding goggles. But knowledge and narrow band video modulation revealed wild section shifts above 5KHz. I believe that the mercury plasma, when thermally modulated at an audio price, was making an attempt to increase and shrink in opposition to its quartz tube housing with each modu1ation cycle, interacting with the arc cavity in a fancy collection of acoustic resonances. Indeed, one might actually hear the modulation coming from the arc, significantly at high audio frequencies, by placing your ear down near the quartz bulb. One undesirable side effect was that sure modulation frequencies near the arc cavity's acoustic resonance extinguished the discharge. Sibilant sounds have been especially risky. In reply to a query about the modulator I remember saying over the link "it's working perfectly!" At the opposite end, all John heard was "it is working perf - Click". The arc extinguished itself, proper on cue!

Over the four months as much as April 1976 the system was in operation as much as six nights per week between VK3ZGJ and my QTH at Camberwell Junction. It was unaffected by all but the heaviest fogs and rain showers. On very popular nights, low frequency noise and quick flutter fade were evident, but hardly obtrusive. The impact was because of scorching air cells rising from the warm ground into the cooler night time sky. This triggered a heat shimmer, with a consequent scintillation of the obtained light. We thought of the use of FM subcarrier modulation to clip off the scintillation on the obtained mild modulation, but the mercury arc lamp hadn't a sufficiently quick rise time to support this. Subsequently I found that using longer wavelengths - red or infra-crimson gentle - enormously reduce scintillation. FM approach could also be desirable for links of over 5 miles vary, but for shorter distances amplitude modulation's higher obtainable base bandwidth in all probability makes it the preferred mode.

The previous mercury arc/ photomultiplier system might have had appreciable long range potential but we never had the chance to check it. The heavy energy demands of the terminal tools demanded a mains supply at both ends of the hyperlink. We never successfully tried DX away from our respective houses and two miles (3.5 km) was the best distance examined. We may generally achieve 45 dB sig/noise over that distance.

Communication ceased throughout the day owing to the scattered blue gentle of the sky. The photomultipliers had been saturated by it. Interference filters could have been used to extract a dominant spectral emission line of the mercury discharge from the general background radiation, but I was unable to obtain one till the 1980s. They're costly and suitable just for parallel rays of light. There are higher methods of attaining good sig/noise during the day, utilizing a monochromatic source and a wavelength away from the peak output of the solar.

However, I would power up the photomultiplier with a relatively low voltage - about 500 volts - just before sunset, to catch John's first distinct words as the ambient mild light. On hot evenings, the signal was at all times lower than anticipated. I couldn't account for this, till one night I climbed the ladder to regulate the receiver and seen an odd odor. The receiver's optical cavity was full of smoke! I had forgotten that John was nearly directly West of me, and as the summer season solstice handed, the position at which the solar set on the horizon was transferring North on successive evenings. That night, it was setting straight behind John's home - so my thirteen cm receiver lens was focussing the solar's image on the back of the receiver housing, which was made of black-painted particle board. Awk! We had been so fortunate to not have set hearth to every little thing - however I stored the incident a secret from my folks. Things that my mother and father didn't know could not damage them! With some embarrassment, I fitted the optics with a removable lens cover.

One potential use of the mercury arc transmitter that LED's, Luxeons and most low power lasers probably might by no means rival lies in non-line-of-sight (NLOS) linking by reflection off clouds. Some years in the past I acquired a pair of 1 metre diameter searchlight mirrors to strive the thought. The mercury arc will be mounted in a single, photomultiplier in the opposite, each tracking the identical cloud. Street lights at present use the same kind of mercury discharge because the modulated source, so that a hundred Hz hum background plus harmonics would be unavoidable in an urban surroundings. I've yet to attempt the idea, as direct linking is undoubtedly more dependable and environment friendly, and has subsequently been my fundamental line of research.

OPTICAL COMMUNICATION'S 'Comic CUTS' - 1976

Almost as quickly because the optical link between John VK3ZGJ and myself was established, he relayed me to 144 MHz on the cross-band discussions radiated by VK3AML on 160 metres, a number of instances each week. For all practical purposes, I was on an intercom in 3ZGJ's shack, and there was no regulation against that. Legally and technically, I used to be a supervised (ie, pull the plug out if he misbehaves) visitor who just occurred to be present close to 3ZGJ's microphone!

Through the early part of 1976, there were many cross-band contacts on 160 metres with me "on lightbeam" which appeared so as to add technical novelty to the proceedings. The discussions have been completely open to any topic, offered that the due decorum of the medium was maintained. On some events, it wasn't! One evening we one way or the other drifted onto the topic of funerals, morticians, Egyptian mummification and the 'expensive departed'. This contact, with many others from that 'gentle beam link' interval, was logged on audio tape by Tony VK3AML and by other listeners, and extracts might be downloaded from Tony's 'bluehaze' web site ("Multimedia Page 3") as "Death By Light Beam" (mp3).

Over time, the content varied from creditable technical depth to immature ratbag ramblings. I by no means kept observe of all the log tapes, and i by no means know when bits of it should re-emerge on the weekly replays which have been maintained by VK3ASE - and others on Internet - haunting me ever since...

Most of our problems with the outdated gas discharge lamp system have been more comical than theoretical, and most were completely unforseen. Moths, as an illustration, have been irresistibly interested in the extremely-violet output of the mercury arc. Bogong moths, large Emperor Gum moths, Christmas beetles, flying ants - the full field and dice! They swarmed the transmitter arc on scorching nights, attenuating the beam flux and suiciding in opposition to the recent lamp. Every couple of hours I'd need to scrape their smouldering bodies out of the lamp reflector. Their fluttering wings modulated the transmitted beam, and the sunshine they mirrored formed a suggestions path into the adjoining, constantly running, duplex optical receiver. If I had my transmitter microphone operating close to the monitor loudspeaker, the presence of an Emperor Gum moth (which, I ought to clarify for non-locals, are the size of a small dinner plate) was signalled by ear-splitting feedback warbling at their wing-beat frequency of about 15 Hz. This was typically followed by our neighbours lobbing pebbles onto the tin roof of my radio shack to get me to 'flip my bloody noise down'! As we regularly operated between midnight and three a.m., I can hardly blame them. We referred to as the impact "moth-again" .

Another unexpected fault took ages to detect. After a seemingly harmless rainstorm, the mercury lamps would stop to operate. The lamp itself seemed perfectly Ok, even on close examination. We lastly found that water had drained down the bulb into to lamp's Edison screw socket. There, the DC bias arrange an electroplating action, dissolving the wires connecting the base with the bulb. The problem was quite simply solved by mounting the lamps upside-down.

Our line-of-sight path, skimming the tops of suburban Hawthorn's buildings, garden foliage and energy traces made steady contact between the terminal stations the topic of persevering with concern - and some unusual QRM. The state of affairs worsened in windy weather as there was a tall gum tree very close the the optical path, in a garden a few half mile distant from 3ZGJ. In a Northerly wind this large nuisance had branches that would sway back and forth into the beam path, slicing phrases and phrases out of our conversations. Eventually the rattling thing grew to become such a drag that we made half-serious plans about carrying balaclavas on an illicit midnight raid on its owner's garden with ropes and a pruning saw! Another suggestion involved borrowing a surgical cutting laser from Melbourne University's medical faculty, and trimming the foliage from a distance - the gap of 3ZGJ's shack veranda, actually. Just a few years afterward, I took a have a look at the same path with binoculars, and located that the nuisance tree, by then with several others, had grown to completely block the optical path. L.O.S. paths may be impermanent! The rising of tall trees in suburban streets and gardens is now rather more fashionable than it was thirty years ago. It could also be unfortunate for optical communication lovers, but it surely provides an aesthetically pleasing approach of supporting h.f. wire antennas. (I'm humming a chorus of Monty Python's "Always look on the shiny facet of life" whereas I'm typing this)!

The color of the mercury lamp also offered issues. It seemed the same as every other road mild in Melbourne. One evening, we drove up to one Tree Hill in the Dandenong Ranges outdoors Melbourne in an unroadworthy Austin with the portable LDR receiver, optimistically hoping to set a distance file. At the highest of the old lookout tower there (eliminated in 1981) we seemed down on the lights of Melbourne, which were as quite a few because the grains of sand on a seashore. With John's optical receiver we searched the varied lights visible in the final path of Hawthorn for about half an hour, vainly attempting to hear audio modulation among the grunts and farts of mains-fed mercury street lights. We by no means did find our audio-modulated light. It had probably extinguished itself on a modulation peak. The police, nonetheless, did discover us, and we had a tense time