Detect resonance of loop antenna

[Ronny Brakhya]

Hi! I have a small loop antenna with a very narrow bandwidth. It's coupled to an rf module exciting it at 169MHz. I would like to check if the antenna is tuned to the tx frequency and my idea was to try to rectify and measure the voltage on the loop. As i understood it, if i connect it close to the neutral center point, the peak voltage should be lower and the impedance should be higher than if i would try to measure it close to the capacitor.
-Is this a viable approach?
-How can i rectify it without detuning the antenna (rectifying diode capacitance is in the same range as loading capacitor)
-is there a different way to chech for resonance? I have a NanonVNA, but i'm looking for a cheap way to measure tuning in-circuit.

 

[vfone]

I don't understand why you want to rectify and measure the TX voltage, if you have a NanoVNA.

Also I don't know what kind of matching network you are using, but the simplest way to match a VHF loop antenna is by using a gamma match and a series (tunable) capacitor.
In this way you check the impedance (and the resonance) of the loop antenna using the NanoVNA connected at the 50 ohm coaxial port.

https://www.nonstopsystems.com/radio/img-ant/antenna-mag-loop-coupling-gamma-dims.jpg

 

[Ronny Brakhya]

I don't understand why you want to rectify and measure the TX voltage, if you have a NanoVNA.

Also I don't know what kind of matching network you are using, but the simplest way to match a VHF loop antenna is by using a gamma match and a series (tunable) capacitor.
In this way you check the impedance (and the resonance) of the loop antenna using the NanoVNA connected at the 50 ohm coaxial port.

https://www.nonstopsystems.com/radio/img-ant/antenna-mag-loop-coupling-gamma-dims.jpg

Thank you for your reply and i'm sorry i didn't explain why i want measure it this way.
So, the antenna is tuned using the VNA to tune the capacitor. Matching to 50 Ohm is done by changing the coupling loop size and shape, i guess the effect is the same as by using a gamma match. The problem is that the narrow bandwidth of the antenna can make it detune easily by changing the capacitive parasitic loading of its surroundings, and temperature coeff. of the capacitor, moisture etc. I want to implement automatic tuning of the capacitor for a fixed frequency. I would therefor like to maximize the resonance by exciting the antenna at this fixed frequency and tune the capacitor to find capacitance the voltage is at its greatest.
Does that sound sane?

 

[FvM]

SWR bridge technique sounds more appropriate for tuning antenna matching in operation.

 

[biff44]

yes, some sort of vector measurement of the reflected signal off of the antenna will tell you if it is at resonance.
at "resonance" the reactive parts will cancel, and the refelection coefficient angle will be zero (after calibrating out some line lengths).

I suppose you could also do the same with a scalar measurement of the reflected sinal if it is ok to dither the frequency + and - to detect the resonance.