Audio feedthrough w/o safety choke

Hi!

While experimenting with external plate modulation of a few small, classic CW transmitters (Ameco AC-1, Heath DX-20), I noticed an interesting effect that will occur in any plate modulated transmitter with a C-L-C pi network at the output, and which does not include a "safety choke" across the r.f. output:

The time-varying audio voltage on the plate will feed through the plate DC-blocking capacitor, and through the tank circuit... to produce an audio frequency component at the r.f. output of the transmitter. With respect to the applied modulating voltage.. the loading capacitor, in parallel with the tuning capacitor, and in parallel with whatever load is across the antenna terminals forms a voltage divider with the DC blocking capacitor.

If the load across the antenna terminals is (for example) a 50 ohm resistive dummy load... then (for a 2500pF DC blocking capacitor) the audio voltage across the antenna terminals is about 50/64,000 of the modulating voltage for a modulating frequency of 1kHz (~ 62dB down), and about 50/12,800 of the modulating voltage for a modulating frequency of 5kHz (~48 dB down).

HOWEVER, if the antenna is a 50 ohm resistive load at radio frequencies, but essentially a capacitor at audio frequencies (e.g., a real dipole instead of a dummy load)... then the voltage divider would produce a much larger audio frequency voltage across the output port... larger than 50% of the applied modulating voltage... depending upon the details of the component values for the DC blocking capacitor and the tank circuit capacitors.

I did notice that I could see the audio being detected by my modulation monitor (when I was not transmitting). I could also see the modulation, when not transmitting, causing some spurious effects on the SWR indicator of my SWR/power meter.

Putting a small 1mH choke across the attenuated input of my home-made mod monitor pickup (as Steve, WA1QIX does in his mod monitor pickups) eliminated the spurious mod monitor effect. Putting a physically large safety choke directly across the output of my transmitter also solved the problem.

For a safety choke to be used with low power transmitters, I am using about 8 turns of insulated wire wrapped on one of the #43 ferrite cores I use to build Class E transmitter transformers. This works fine for my low power transmitters. For the safety choke on my 1500 watt linear amplifier, I use 8 turns of #14 insulated wire wrapped on a bundle of four (4) #43 cores. It gets warm... but not hot... on long, old buzzard transmissions.

Best regards
Stu