Total posts : 45366
Tapping into the AVC circuit of an AM radio could work to get a relative FS reading which could be used for peaking.
In older radios made with discreet parts the AVC voltage was available as a DC voltage across the volume control. This voltage is not a linear representation of FS because the loop to control the receiver gain is closed. In the distant past I tried this with the loop both open and closed. With open loop the receiver can saturate which will not even provide a relative reading. Closed loop the voltage is usable as long as there is adequate signal received to produce a reading. I concluded that this was not suitable for meaningful measurements beyond peaking.
Newer radios use an IC for most of the receiving functions. The radio I have, a $15 GE portable, uses an IC where the
AVC voltage is produced internal to the IC but is available on one of the pins. From the datasheet for the IC it can be seen that this voltage varies very little once a usable signal is received and would not be very useful for FS indications and this was observed by experiment. This is not to dissuade anyone from experimenting but rather is a tip for what to look for.
For tuning my antenna I use a method similar to that mentioned by RFB. I have a homebrew RF current transformer which I place in line with the antenna system before the coil and can read the voltage/current phase angle at the feedpoint. Resonance is indicated when the phase angle is 0 degrees. This does not give field strength but does make tuning the antenna system easy.
The method mentioned by Ken Norris using a diode detector and DVM near the antenna is workable for peaking the antenna and may be the most practical means for antenna tuning. I used this method before I built the transformer system.
Harmonic radiation from a base coil loaded antenna will be negligible due to the pass characteristics of the filter formed by the coil and the antenna. It is possible, but unlikely, that the coil/antenna could be tuned to a harmonic but if so there would be essentially no detectable signal at the fundamental.