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Old 21st Sep 2020, 6:28 pm   #1
G6Tanuki's Avatar
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 8,934
Default A 'Modern Vintage" 3.5MHz ATU.

I'm gradually getting things together with my AEL3030 transceiver - sourcing some crystals, a 6-pin Jones socket for the power-lead, rewiring the mic socket to match the Pye standard, all with the assistance of a manual supplied by forum member G3KVM_Roger (thanks!).

To put it on-air I needed an ATU to match its 50-Ohm output to my random-length end-fed. Theory, confirmed with a borrowed VNA, said the antenna appeared capacitive, so should be matchable by a series-inductor. I hooked-up an old 'roller coaster' coil to the VNA and found that something like 25 turns was needed.

So, off to the 'vintage RF parts' junkbox - which yielded a nice ceramic 3 inch diameter Eddystone ribbed coil-former of a type I recall from 1930s RSGB Handbook ads - with an existing thick copper winding, along with a 6-position heavy-duty ceramic rotary-switch, made by General Electric, whose first career was probably aboard one of Uncle Sam's larger WWII aircraft. The switch would select taps on the coil for fine-tuning.

Putting it all into a plastic box (plastic so as not to detune the coil - I've seen loads of commercial ATUs where the metal case is indecently close to the coils) I connected it to the antenna, and supplied a whiff of RF through my trusty Bird, the idea being to probe the coil with a cric-clip-lead to find the resonance-points between 3.5 and 3.8MHz, then make more-permanent connections with leads soldered between the coil and the tap-select switch.

Ah... with the full coil in circuit it was resonant spot on 3.8MHz! My calculations were out - I needed more inductance to get down to 3.5MHz! But how.... I pondered shoving a few lengths of ferrite-rod into the centre of the coil but started to fret about hysteresis-heating, Curie-points etc. Then I had an idea - the antenna being a capacitive impedance, what about making it a bit more-capacitive? So I fished-out a Jackson "C803" wide-spaced variable capacitor and hung it between antenna and earth terminals - I was delighted to find that adding 45pF got the thing to resonate nicely at 3.55MHz - but, alas, the Jackson capacitor was too big to fit inside the box!

There _was_ room though for a smaller 50pF variable capacitor. One with much smaller plate-spacing, intended for receiver applications. I worried about flashover, but then remembered that in plenty of transmitter designs shown in ARRL/RSGB handbooks a 'receiver type' variable-capacitor was used at the output of pi-networks in transmitters running hundreds of Watts.

Try it and see... Yes, it tuned-up fine, so I tried it with 100 Watts of continuous carrier. No flashover! I'm happy!

Just to prove a point, I left the transmitter running [on an empty frequency] for five minutes of continuous carrier, then powered down and felt all over the ATU for any signs of heating. No problem...

If only I'd known at the start I wouldn't have drilled the box for the big switch, and would have used the space for the larger Jackson capacitor.

[Photo shows unit in part-built, test state]
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