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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