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Re: [TowerTalk] Measuring resonance of a yagi element

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Measuring resonance of a yagi element
From: Robert Harmon <k6uj@pacbell.net>
Date: Tue, 21 Apr 2020 19:04:03 -0700
List-post: <mailto:towertalk@contesting.com>
I new there was a reason to keep my old Millen grid dip meter  !

Bob
K6UJ

> On Apr 21, 2020, at 6:52 PM, jimlux <jimlux@earthlink.net> wrote:
> 
> 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.
> 
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> 
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