[ARC5] QST articles.

[w7qho]

OK, how to feed commonly used ham antennas from an ARC-5/SCR-274N "command" transmitter (or other WW2 rigs originally designed to feed short whip or wire antennas).  How many times has this subject been addressed since  the late 1940's?

Basic consideration here is that these sets were designed to operate into antennas that presented a reactive load to the transmitter, in the case of the short radiators under discussion here, a relatively large capacitive reactance in  series with a low resistance, the latter value somewhere in the range of 10 ohms or less depending on  the frequency, length of the radiator and other factors.  The series inductor included in the antenna circuits of these radios served to cancel out the capacitive reactance leaving the resistive component which was coupled into the transmitter's PA tank circuit by means of a variable link.  Since the resistive value of the load was small, the link didn't need to have many turns on it to present the proper load to the PA, far too few turns, in fact, to load the PA with a 50 or 75 ohm load connected to it.

What we are doing here with the various series cap, parallel cap schemes used over the years is implementing networks to transform the 50-75 ohm loads presented by commonly used antenna systems to the much lower impedances the old PA tank coil/variable link assemblies can be happy with.  In the case of command sets operating on 75M, a 75 - 100 pf series cap works fine in my experience (more below).  The network is adjusted (tuned), of course, by means of the variable inductor.  There is, of course an equivalent parallel network and someone in the recent round of e-mails mentioned a capacitor value of 1200pf but I have never tried this.

Other load impedances will require different network schemes, of course.  An end-fed half wave wire antenna presents a (mostly) resistive impedance of around 2500  ohms and can be directly fed by an " L" network formed by shunting the antenna terminal on a command set to ground with a 75 - 100 pf capacitor on 75M, (conveniently) the same value used in the series circuit described above.

The networks described above also provide a measure of harmonic suppression above that which would be experienced with a simple link to antenna connection.

--
Dennis D.  W7QHO
Glendale, CA
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