[TowerTalk] Measuring resonance of a yagi element

[VE6WZ_Steve]

Tom,
Yes.  there is a MUCH simpler way to do this without climbing the tower.

I have tuned a number of hombrew Yagis and vertical parasitic arrays, and used to think I needed to isolate the driver, then measure its resonance.  That is not required.

Because of mutual coupling, the REAL R (NOT IMPEDANCE) of the driver (feedpoint) will show a pronounced dip at the resonant frequency of the parasitic element.  This dip in real R is independent of the tuning of the driver.  You even can sweep the Yagi from the ground and it should still show a pronounced dip.  (it would be preferred to sweep it at the driver, or feedline corrected to the driver). Make sure to sweep well above and below the central QRG.  (like 13 Mhz to 15 Mhz.)

Look at your antenna model to confirm this. In your antenna model, if you do a frequency sweep and look at the output, you will see the dip in the real R at the frequency that the reflector is tuned to.  On the tower, you want to see this same dip in R at the same frequency.  This effect is a bit more complicated on a multi-element Yagi, but on a 3 el Yagi, you will still see the dip in the R at both the Reflector tuned QRG and the director QRG.

The mistake some make with this analysis is they look at the IMPEDANCE on the analyser.  That is wrong.  We see the dip in the real resistance.  ie. the R in Z= R+jX. Most analyzers will plot R vs frequency.

73, de steve ve6wz


> On Apr 21, 2020, at 7:02 PM, Tom Hellem <tom.hellem at gmail.com> 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
> 
> Thanks in advance for any replies.
> 
> Tom K0SN
> Missoula, MT
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