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Old 9th Jan 2018, 11:11 am   #31
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
Default Re: The Best GDO Ever Made?

Just curious as to why - in the 'Magacycle Meter' - the anode load resistor is made up of three in series - R10 (6K8), R15 (6K8) and R14 (5K6), totalling 19K2. Why not just a 20K resistor (+4%), which would have been a standard value at the time, or 2 x 10K? Just seems a bit ungainly to use a string of 3 resistors in series for no apparent reason, either technical or practical unless I'm missing something. Even 18K or 22K would surely have been fine - the anode voltage wouldn't have been that critical and would have no bearing on the performance or accuracy.

Back in the 1980s when I was active in amateur radio, I built my own antennas - a G4MH type mini beam, multi-band trapped verticals and trapped dipoles, for which I made my own traps. In 1985 there was an excellent PW article to make an FET Gate Dip Oscillator which covered from 1.8 MHz to 150 MHz in six overlapping ranges using home-wound plug in coils. The article had full constructional details, and there was a follow up 4-page in Dec 1985 on how to use a GDO.

http://www.americanradiohistory.com/...PW-1985-10.pdf

http://www.americanradiohistory.com/...PW-1985-12.pdf

I've attached some pics of the one that I built, which I was able to calibrate using a home-brew frequency counter. It worked very well. The last pic is of a 7 MHz trap I made for a trapped dipole using fibreglass spreaders to space the coil, and double sided PCB laminate for the 22pF capacitor. When adjusted using the GDO, the traps were housed in plastic tubes and sealed. The traps and hard drawn copper wire was up in the air from 1986 - 2003 when I took down all my antennas and called it a day.

The first pic shows the GDO with the six coils and a 7 MHz test coil.
Second pic shows the VHF coil at 150 MHz displayed on my homebrew PW 'Robin' frequency counter.
(Designed by the late Mike Rowe (AKA 'Swordholder', the original designer of the Sussex Valve Tester).
Third pic is the RF 'sniffer' attached to the Robin, sniffing the VHF coil.
Fourth pic is a display of 15 MHz RF from the GDO.
Last pic is of a 7 MHz trap, yet to be housed.

(From the pic of the trap, it may seem that both ends of the coil are soldered to the same side of the PCB capacitor, creating a dead short across the coil. That isn't so - there is a groove on each side of the PCB towards each end, obscured by the wire. The grooves create and area of copper on each side of the PCB, which - along with the fibreglass laminate - form a capacitor which will carry the full legal limit of 400W SSB).

All good fun at the time.

Back to the Acorn valved 'Megacycle meter' there was an article in Short Wave Magazine back in March 1957, which can be found here:

http://www.americanradiohistory.com/...WM-1957-03.pdf

Nice things to own, as curios and relics of a bygone era, but I wonder if anyone uses GDOs these days, and if so, what for, in a 'plug 'n play' era when antennas come from shops?

Hope that's of interest.
Attached Thumbnails
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Name:	PW GDO at 150MHz.jpg
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