[TowerTalk] Measuring resonance of a yagi element

[jimlux]

On 4/21/20 6:02 PM, Tom Hellem wrote:
> I would like to measure the self-resonant frequency of the reflector on a
> home made 20 meter yagi. It seems obvious the best way would probably be to
> open it up at the center and make it into a split dipole element and hook
> up my analyzer. However I would like to avoid cutting it and dealing with
> splicing it back together again. Can anyone suggest another way to
> accomplish my objective? The reason for doing this is because it is
> apparent from modeling in EZNEC that the
> length of the reflector is critical to the performance parameters of the
> antenna, e.g. gain, F/B, bandwith, etc., and it looks to me like it doesn't
> take much of a change in the length of the element
> 


Take your test meter and hook a 1 (or more) turn coil about 1" in 
diameter on it, and then couple the coil to the element in question. 
(i.e. have the coil's axis of symmetry in the same direction as the Yagi 
boom and put it next to it).

You should see the resonance, when the element "sucks out" the power 
from the coil.

You could probably also do some sort of simple "spread the wires and 
attach on either side" sort of a delta match? - you're looking for a 
resonance peak or dip, not a match to 50 ohms. The trick is that you 
don't want the measurement system to perturb the thing you are measuring.

This is how folks did it with a Grid Dip Meter back in the day.

A fancier technique would be to use a low loss ferrite core and make a 
transformer to couple your instrument to the element. It could be 1:1 or 
10:1 or whatever is convenient.  And you don't even have to be in the 
precise middle. An off center fed dipole resonates at the same frequency 
as a center fed dipole, just with a different impedance at resonance. 
Of course, if you're measuring off center, you have more possibility 
that your measurement system (or you) will couple to the antenna.