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# Re: Determining the length of a driven element

• Subject: [amsat-bb] Re: Determining the length of a driven element
• From: "Art McBride" <kc6uqh@xxxxxxx>
• Date: Fri, 22 Aug 2008 22:12:35 -0700

```Phil,
SWR should be less than 2:1 Several computer modeling programs exist that
will give you and answer to your question. Length is the resonant part of
the SWR, resistance is the other part, either can cause a high VSWR and
usually both contribute.

Art,
KC6UQH
-----Original Message-----
From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On
Behalf Of Phil
Sent: Friday, August 22, 2008 8:18 PM
To: amsat-bb@amsat.org
Subject: [amsat-bb] Determining the length of a driven element

Hello,

During a casual conversation recently, the topic of aerial trimming came up.

Perhaps this may be a little off-topic here but since I don't know the
answer
I thought this would be a good place to ask anyway.

Say a dipole has an SWR of 5:1 at one end of a band and 4:1 at the other
end.
Further, let's say that the dipole will be used at the centre of the band
and
with a reasonable SWR. It has been determined that the dipole needs to be
shortened. The question is by how much?

The person asking the question is reluctant to pull the aerial down a dozen
times so that short lengths can be nibbled off each end and instead wants to

know if the amount that needs to be cut off can be determined from the
difference in the SWR at both ends of the band.

A logical question I suppose but is there a reasonable answer?

--
Regards,
Phil
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```

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