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.
Neil
The Zenith Transoceanic has a potentiometer that clicks into AVG mode when fully counter-clockwise. This of course is normal reception operation.
When turned clockwise and un-clicked the pot has direct control over the RF signal being received, and can be dialed through "no signal", carefully adjusted for "maximum signal", and turned higher for "over-saturated signal."
If such a control could somehow be patched into a portable radio receiver then we could build a tunable field-strength meter.
I have an old Lafayette Guardian 6000 portable radio with a switchable AGC and meter. I just tune in a commercial AM station that I know should have a certain feild stregnth in my area, (one close in frequency to the Pt. 15 I want to measure)switch off the AGC, and turn the manual gain up until the meter reads about mid-range. Let's say the station I tuned in is known to have a 1 mv/v fs here. To measure a Pt. 15 AM's fs, I'll just walk towards (or away from) the antenna until the meter reads the same as it did on the commercial station. At that point, I know the Pt. 15 station has an approximate 1 mv/m contour. It's not super accurate, but it gives me an idea of fs...and it's CHEAP!
Thing I would be concerned about on those older vintage radios are component degradation and tolerance shift due to age of those components..including potentiometers.
If were talking 20 plus year old radios..bet your radiating radio waves those pots and meters and coils in there are NOT where they once where when the thing was new..hence the readings your getting are MOST likely off by a long shot.
Your merely seeing a meter move up and down and adjustable by a pot..but none of it tells you anything about what is really going on with your system.
AGC or not, the inter-stage coils and tuned circuits are going to be off unless calibrated to a known calibrated meter and parts replaced with in-spec components.
Regardless, none of the indications by a consumer radio is going to give accurate measurements, but they can at least give you ball park guesstimates..and with the older radios sporting S meters...well those guesstimates will be an even further extension of guesstimates due to age and component degradation.
Given the plethora of schematics out here on the web for S meters and field strength meters, there isn't any reason why those should not be considered as those can be built with new in-spec components and will give a far better and more accurate result than an old radio with a noisy aged S meter pot and de-sense'd front end due to shifted coil and tuned circuit tolerances!
I'm not sure I would trust ANY radio receiver meter even with a sensitivity adjustment pot. More circuits in the thing than what is required to do the job..meaning more parts in there in the way of getting a decent reading. Why overtake the plumbing to only end up with a stopped up drain?
Keep It Simple Sam and it just might surprise you!
And I can guarantee anyone that building one of the field strength meters from the thousands of schematics on the web, will be plenty "cheap" than the price of those old radios in re-sale value now!
RFB
I have an idea..one that would serve both antenna peaking and field strength indication.
Place tuned detector/LED indicators at 1 meter intervals all around the antenna structure so that they spread away in a circle pattern from the antenna in the center. All LED's within the so called "200" foot by the book range can be green, and a few after that can be yellow, and then a super duper bright fat RED LED on the outer rim ring.
Tune for maximum illumination of the sensor circle and most distant ring of sensor LED's while keeping the fat RED one from lighting up. Although at that distance, that fat RED one might be flickering on its own from random spectrum junk!
Such a setup would also make a neat night time display!
RFB
Some junior pilot might look down at night and think you are an airport. Here he comes for a landing so maybe we should run away.