[ARC5] ARC-5 antenna match

[Christopher Bowne]

I have found the best impedance matching network (note that I am not calling it an "antenna tuner", I agree the only way you tune an antenna is with sidecutters) to provide the most power transfer between the low imedance output (nominal 12 ohms) of a command set transmitter and your typical 50 ohm unbalanced impedance of an inverted vee, 1/4 wave vertical, etc. is the parallel tuned circuit autotransformer circuit described by Walt Hutchens, KJ4KV very early on in his ER in Uniform series of articles on the SCR-274/ARC-5 command sets.  It consists of a parallel tuned circuit where the antenna feed line is tapped on the coil about 5 turns up from the grounded (cold) end, and the transmitter is fed through a 200 pf series cap to a tap about 3 turns up from the cold end of the coil.  You tune the parallel  tank to resonance, and then adjust the roller inductor in the transmitter for maximum output.  With this arrangement I get nominally 45 to 55 watts
 output from a totally stock dynamotor powered transmitter into a Bird 50 watt dumny load, and 50 ohm coax fed inverted vees/dipoles on CW and about 20 watts on screen modulated AM.  There is more than adequate coupling range in the transmitter adjustable coupling  link, it will peak at about 1/2 to 2/3 of full range instead of never gettng to full output even at max coupling. Using just a 200 pf cap in series with the antenna feed directly to a transmitter without the autotransformer, I usually can get only 30 watts output with the coupling advanced to max. The parallel tank circuit also provides attenuation of harmonic output to the antenna.  I have also noticed that the heavy loading of the transmitter afforded by the autotransformer seems to improve the modulation quality in AM using the BC-456 screen modulator, the screen modulation seems to like the heavy loading.  

My tuned circuit coil was originallly  a two piece pi-network coil in an Eico 720 transmitter.  I use the both sections of the coil on 80 meters and short out larger of the two coils on 40.  I use a 200pf variable cap, it need to have fairly wide plate spacing as the RF voltage developed across the circuit at resonance is pretty high, I have had flash-over problems using receiving type variable caps.