Wednesday 3 April 2024

Antenna Experiments - Reprise

This is a follow-up to my earlier earlier post: if you haven't already done so, I suggest reading that one first.

I caved and bought a NanoVNA. It is a useful piece of equipment for not very much money, if you have an interest in RF.

Anyway, hooking up one of those "DF Robot" 433MHz antennae to it produced some interesting (in that they differ sharply from the previous results) results:-

The maximum return-loss/lowest VSWR occurs at 436MHz. Between 433.5MHz and 438.5MHz, the VSWR < 1.5. This is much closer to the expected values and suggest that my previous attempts to measure antenna resonance using just a spectrum analyser are not to be relied-upon.

Turning to the antenna consisting of a bit of coax with 16.5cm of exposed inner conductor, recall that theory suggested that the resonant frequency of that should be 454.5MHz.  It turns out that reality concurs pretty closely:-



 That red marker is at 452MHz, which is pretty close. It also suggests that the velocity factor of the signal in the coax should not be considered when doing the calculation. It isn't completely obvious (to me, anyway) why not: my intuition suggests that it should. But nothing I have read supports this intuition and now the measurements also indicate that my intuition is wrong. If you know why, please do leave a comment letting me know.

Also, it looks like the "rubber ducky" antenna is a much better antenna than the piece of coax at all of the relevant frequencies.