For those of you that have a (grid) dip meter and a spectrum analyser (or oscilloscope) -- could I ask you to do an experiment? The experiment is to observe how much the frequency of the dip meter is pulled, when it is brought near a resonant circuit.
I did this and was highly surprised by the results. I was wondering if my results are typical or not; hence the request for additional data.
In my case, the (surprising) results were as follows.
Just by moving the dip meter coil near the resonant circuit being measured (the DUT or Device Under Test), the dip meter's oscillation frequency was pulled by several hundred kHz. I observed the dip meter's oscillation frequency via a small pickup loop (about 2 cm diameter) placed at a fixed location near the DUT. This pickup loop was then connected to my RTL-SDR, functioning as a crude spectrum analyser. This test was at about 14 MHz.
When the dip meter is coupled to a DUT, and when slowly adjusting the dip meter's oscillation frequency around the region of a dip, the dip meter's oscillation frequency can suddenly jump by several tens or hundreds of kHz! This causes the "snapping" behavior when slowly tuning lower in frequency across a deep dip, whereupon suddenly the deep dip disappears and the meter's needle snaps back up. At the instant of the needle's snapping back up, the oscillation frequency (as observed by the radiated signal picked up by the RTL-SDR) jumps down by several hundred kHz. This frequency jumping is probably caused by an over-coupled, double-humped response of the two resonant tanks (the dip meter tank and the DUT tank) forming a sort of "combined, bandpass resonator" that determines, in a complicated manner, the oscillator's frequency, and that pulls the dip oscillator's frequency away from its natural frequency of oscillation if it were not coupled to the DUT. As the dip meter's tank is adjusted, the properties of the "combined, bandpass resonator" probably change until the the point where the influence of the coupled DUT's tank becomes too weak and the dip meter's own single tank suddenly dominates the oscillator behavior, causing the meter's needle to snap back, and causing the frequency jump.
If the coupling between the dip meter's coil and the DUT is reduced to a very low level such that the frequency jumping no longer occurs, then unfortunately (with my simple dip meter) the dip is no longer visible on the meter.
Even with coupling reduced to produce the weakest possible dip, the adjustment of the dip oscillator's frequency still was not smooth and exhibited sudden jumps in frequency in the region of the dip.
In practice, for a given coupling between dip meter and DUT, I found that the frequency where the deepest dip occurred, even though it pulled the oscillator frequency away from its natural oscillation frequency, was the most accurate indicator of the DUT's frequency, verified by measuring the DUT's resonant frequency on my NanoVNA (with a single-turn link winding through the DUT's toroidal coil). In other words, the frequency pulling doesn't seem to matter (as long as you can measure the frequency with a spectrum analyser or frequency counter) -- the frequency of the deepest dip (even if that frequency has been pulled by over-coupling to the DUT) still seems to correctly represent the resonant frequency of the DUT.
It may be that the above odd behavior is due to the limitations of my dip meter circuit. Or, it may be that all dip meters -- even the best ones -- exhibit frequency pulling and frequency jumping when tuning the oscillator around the dip.
To summarise, the surprising thing to me was that my dip meter's oscillation frequency was far different that what I expected. It was heavily pulled by by coupling into the DUT, and tuning across the dip yielded unexpected jumps in the oscillation frequency.
So my questions to you, if you have a dip meter, and a spectrum analyser, are:
Frequency pulling: Use a small pickup loop to detect the radiated signal from the dip meter, and display it on the spectrum analyser (or oscilloscope). When a dip is found, take a note of the oscillation frequency on the spectrum analyser (or oscilloscope). Then, completely remove the DUT and leave the dip meter tuning untouched. Does the oscillation frequency of the dip meter (as seen on the spectrum analyser) change? By how much?
Frequency jumping: When tuning across a dip, continuously observe the oscillation frequency (as observed on the spectrum analyser or oscilloscope). Does the dip meter's oscillation frequency suddenly jump by several 10s or 100s of kHz when tuning slowly across the dip?