Antenna Experiments

Let me state from the outset that my knowledge of RF and antenna design is very rudimentary. I basically consider it to be black magic.

With that out of the way, I read something a while back that made me wonder if it was possible to measure antenna resonance with a track-generator equipped spectrum analyser.  In particular, with two identical antennae - one connected to the track-generator output and one connected to the spectrum analyser input - would there be an obvious peak in response at the antenna's resonant frequency.  That is an experiment worth trying.  

Here is the initial setup:-

 It is sweeping from 0 - 1.5GHz and there is already a very obvious peak.  Looking closer:-

The antennae are intended for use with DFRobot APC220 radios which have a frequency-range of 418-455MHz.  So in the ballpark, even if the measured peak is kind of low.  There is a significant difference between performance at the two ends of that frequency range, though. Reducing the frequency span on the spectrum analyser from to 418-455MHz:-

There is nearly 15dB difference between 418MHz and 455MHz.  Still...I can't say for sure what the resonant frequency of the antenna is actually supposed to be: perhaps it is 407.5MHz and the result is completely valid.

For the second test, I replaced the both antennae with a simple monopole consisting of a piece of coax with 16.5cm of the shielding stripped back.  This should resonate at a wavelength of 4 × 16.5cm = 66cm.  Assuming the speed of light is  3 × 10⁸m/s, that should be equivalent to 454.5MHz.  Let's see:-

Far from conclusive. The peak is at 152.5MHz, way over on the left.  There is a second peak at about 415MHz.  Zooming in to the 418-455MHz range:-

There is no sign of a peak of any kind near the resonant frequency.  

My "assumption" above about the speed of light wasn't entirely tongue-in-cheek.  The headline figure of 3 × 10⁸m/s is the speed of light in a vacuum. Signals travel through coax cable significantly slower than this.  According to a datasheet for RG179 coax that I checked (I think that the kind of coax I have), the velocity factor is 0.7.  That would put the speed of the signal propagating through that coax at closer to 2.1 × 10⁸m/s and - if the wavelength-to-frequency computation should take this into account - would put the resonant frequency at 318MHz.  I have no idea if the velocity factor should be applied here or not: if you know, please leave a comment.  There is certainly no bump in the output of the spectrum analyser at or near 318MHz that would give a clue either way.

Finally, I replaced the two coax monopoles with two lengths of wire, balanced precariously in the SMA connectors:-

The pieces of wire are identical and are both 10.5cm long. Looking at them as 1/4 wavelength monopoles and applying the equation, their resonant frequency should be 714MHz.  It doesn't appear to be:

There is indeed a strong peak, but it is nowhere near where it is supposed to be.  Interestingly, if I apply the velocity factor of 0.7, things look a lot better: that would imply a resonant frequency of 500MHz.  But I have no reason to believe that the velocity factor of a scrap of wire is 0.7, even if I knew that it was supposed to be included in the calculation.  I'm just fudging numbers here.

I got a similar result with 16.7cm of the same wire:

A good strong peak at 345MHz where is nowhere near the predicted 449MHz, but in the right ballpark if I apply the magic velocity factor of 0.7.

If you read down this far hoping for a satisfying conclusion and some enlightenment then I can only apologise for the disappointment you must be feeling.  I promise you, I feel your pain.

Nothing in any of this has done anything to shake my conviction that RF and antenna design is a black art.