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[TowerTalk] 40M Beam Interaction

To: <>
Subject: [TowerTalk] 40M Beam Interaction
From: (
Date: Wed, 2 Jun 1999 12:33:17 -0600

On Wed, 2 Jun 1999 14:01:10 EDT writes:
>In a message dated 99-06-02 12:49:35 EDT, writes:
>> I have heard that the "new" Cushcraft, 2 el 40M beam (XM240)  has
>>  substantial interaction with nearby 20 M, full size, elements.  I
>>  understand that the older 40-2CD doesn't exhibit this tendency!  
>>  have any practical experience with these antennas stacked 9 - 12 
>with a
>>  20 M beam?  
>       I have installed 402CD's as close as six feet from a tribander 
>some degradation in SWR but you could live with it. I have recently 
>my first XM240 and even eight feet away from a TH11, it was terrible 
>affected both antennas. I turned it 90 degrees from the TH11 and it 
>WORSE. I moved the antenna up the mast a couple of more feet (inline 
>again) and figured we could live with the higher SWR.
>      Speculation from talking to a couple of TowerTalkians led us to 
>that the now insulated reflector could be the problem since that's 
>about the 
>only thing that's changed as far as we knew. Anyone else have any 

It is possible to have the 40 meter feedline just the
right (or wrong) length to make the 40 meter driven
element resonant or near-resonant on 10, 15, or 20.

A quick way to test for this condition is to observe
the swr on the high band antenna experiencing the
problem, while alternately disconnecting, shorting and
unshorting the 40 meter feedline where it normally
terminates in the shack. If the swr changes, experiment
with adding random lengths to the 40 meter feedline until
the problem goes away.

I have a 40-2CD 6 feet above a KT-34XA, with no noticeable
problems, certainly no swr problem.

Dave Hachadorian, K6LL
Yuma, AZ

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