From 2SZ to Z-4AA October 18/19th Part 2 – By John Heys G3BDQ

The amateur radio scene in 1924 centered around simple regenerative receivers, and self excited power oscillators which coupled into Marconi antenna systems. Most of the distance record breaking stations used input powers of more than 250 watts.

Cecil Goyder’s transmitter at Mill Hill was no exception and for his record breaking contact with New Zealand he employed a pair of Mullard 0/250 triodes in parallel working as a Hartley oscillator on a wavelength of about 90 metres. To get the high voltage power supply needed, a sychronised motor driven mechanical rectifier together with a valve diode was used. His smoothing circuit had just a 12H LF choke with a couple of capacitors. There was a 2 uF input capacitor and just 1 uF of final smoothing. In 1924 Goyder had no way to properly monitor his output signal for tone, stability or chirp and relied upon the RF current meter in the antenna lead to tune up for ‘maximum smoke’. A rather poor photo of the transmitter reveals that there was no meter to measure the current to the oscillator valves. He cleverly avoided the use of a wide spaced high voltage tuning capacitor by having a fixed capacitor which could be connected across various turns on the oscillator coil. This coil was made with quarter inch diameter copper tubing and had 18 turns. No chassis or breadboard was used to construct the transmitter; components being fixed to the bench or positioned up above on a small shelf.

the-transmitter-part2-2 the-transmitter-part2-1The synchronised rectifier was on a small stool standing close to the transformers of the high voltage power supply. To get the best output into his antenna Goyder had to select seven coil tapping points before starting operations, a procedure which must have taken up a considerable time. Keying the transmitter and getting a side tone to help his Morse sending would have been quite a problem. I feel that the only way to key his transmitter was to key, via a suitable relay, the AC mains input to the high voltage transformer. This relay could also have additional connects that could switch in a buzzer to operate when the key was pressed.

One can only imagine what the 2SZ signal sounded like, and Bell  (Z-4AA) wrote “UR QSA RAC sigs” (Raw AC tone) on his QSL card confirming the historic contact. The frequency drift during and between overs must have been considerable and his simple TRF receiver had to be switched off when the transmitter operated. This send/receive switching must have made quite lengthy intervals between overs. Goyder’s power output can only be guessed at, but was perhaps quite low and at best between 25 and 30% of the input power. His valves had low purveyance and despite using an anode voltage of 2,500 would at best only draw about 200 mA of anode current (500 watts) so I think the output power was little more than 120 – 170 watts and possibly less.

Just a few years after this notable contact, Goyder was using and advocating the use of quartz crystal frequency control and the use of CO-PA transmitter circuits. This revolution in transmitter stability allowed the use of narrow bandwidth receivers like the superhet. The normal 10 watts input power imposed upon most British operators became enough to then allow long distance working when conditions were favourable, something that was impossible when employing free running oscillator transmitters.

The antenna used at Mill Hill by 2SZ was a typical amateur Marconi, for resonant Hertzian radiators were not being used at that time. At the School a couple of 25ft masts were arranged with one at each end of the building which allowed an antenna height of 45 ft. British Licence conditions then (and even for a short time after WW2) only allowed the use of 100 ft long aerials. This included any top wire or wires and the down lead. At Mill Hill there was a top made up with five parallel wires fixed to 12 ft wooden spreaders and a 5 wire ‘sausage’ using circular spreaders which descended for 45 ft to the radio room. The earthing system included an 8 wire counterpoise which also had a cage lead in. The 2SZ transmitter was not directly connected to either earth or the counterpoise. This is not like present earthing practice. I think Goyder was afraid that his counterpoise arrangement would be jeopardized if a real earth was connected to his antenna circuit.

By John Heys G3BDQ – Vital Spark April 2013.

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