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[TowerTalk] 40M Beam Interaction

To: <>
Subject: [TowerTalk] 40M Beam Interaction
From: (Eric Gustafson)
Date: Fri, 11 Jun 1999 11:11:11 -0700

I posted this a few days back.  But I don't remember seeing it on
the reflector.  I didn't get a delivery falure message either...

Anyhow, here goes again.

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Return-Path: <n7cl>
To: <>
Date: Wed, 2 Jun 1999 15:19:52 -0700
From: Eric Gustafson <>
In-reply-to: <> (
Subject: Re: [TowerTalk] 40M Beam Interaction

Hi TTians,

I'm sorry to have to say this because nobody wants to hear it

While changes in SWR are an indication that there _is_ some
interaction going on, these changes are _not_ the important
consideration.  The important (IMHO) consideration is "What does
the interaction do to the pattern of both antennas?".  Ask
yourself "Why do I want to have a beam in the first place?".

By the time that interaction is severe enough to produce an
observable SWR change, the point at which the pattern(s) _may_
have been severely degraded has been long since passed.  On the
other hand, as long as the pattern(s) are holding up, and the SWR
is still below 2:1, do you really need to care that the SWR

In the past, the only good way to verify whether the interaction
was detrimental was to put the antennas up and then verify (at
least the gross aspects of) the pattern.  Nowadays, as long as
the antennas aren't linearly loaded or trapped, you might be able
to save a lot of climbing and fiddling around by using a good
modeling program and feeding it a reasonable semblance of the
installation.  In fact, if the situation is "modelable" you might
even consider making adjustments to the affected antenna
(usually the smaller one) to compensate for the proximity of the
other skyhook.  Then, you could stack them at nearly any
separation that you liked.

Anyhow, don't just rely on the SWR to determine if there has been
a degrading interaction.  At least check the F/B ratio to make
sure that the beams have retained their directivity (they still

73, Eric  N7CL

To: <>
>Date: Wed, 2 Jun 1999 14:01:10 EDT
>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!  Anyone 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
>with some degradation in SWR but you could live with it. I have
>recently installed my first XM240 and even eight feet away from a
>TH11, it was terrible and affected both antennas. I turned it 90
>degrees from the TH11 and it got WORSE. I moved the antenna up
>the mast a couple of more feet (inline booms again) and figured
>we could live with the higher SWR.
>Speculation from talking to a couple of TowerTalkians led us to
>believe 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 insight?
>Cheers,   Steve    K7LXC
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