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[TowerTalk] Resonance of loaded Parasitic Elements

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Subject: [TowerTalk] Resonance of loaded Parasitic Elements
From: (Tom Rauch)
Date: Thu, 6 Sep 2001 11:35:08 -0400

> Hi All , I am in the process of building a loaded two element 40 beam.
> I have the driven element finished and its total length is 43` it is
> resonate @ 7.150. What would the most practical way to tune the
> reflector be ? Should I split & insulate the reflector , mount it on
> the boom and check the resonate frequency ?  Is  5 %  longer a good
> length ? I am not quite sure of the best way method to use .  Would
> like to get some input on this .

Hi Kevin,

If you model a single element in space, you will find it makes an 
almost ideal reflector at normal spacings when it is self-resonant.

You want to be sure to do that at the LOWEST frequency where 
the antenna will be used. That is with that element a SINGLE 
element, no other elements around.

You will find the driven element will be slightly shorter than free-
space resonance, about 5%, for minimum reactance.

I just had a friend tune a quad by installing only the reflector. He 
fed the reflector like a dipole, and set it for resonance on the lowest 
frequency he planned on using (using an exact 1/2 wl of coax to 
the meter). He then closed the loop, and added the driven element.

He didn't have to touch a thing, it came out just as predicted.
73, Tom W8JI 

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