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[TowerTalk] SWR question

To: <towertalk@contesting.com>
Subject: [TowerTalk] SWR question
From: nielsen@oz.net (Bob Nielsen)
Date: Thu Feb 27 21:43:39 2003
On Thu, Feb 27, 2003 at 04:38:10PM -0700, k1my wrote:
> My recent post asking for recommendations between a log and a SteppIR that
> claims 1:1 SWR across the band has yielded some interesting responses but
> has also raised a question in my mind. "Why is SWR important?"
> 
> Take this case:
> 
> option A - I have an SWR measured at the output of the amp of 1 : 1 and I
> have an indicated output power of 1 KW.
> 
> option B - I have an SWR measured at the output of  the amp of 1.5 : 1 and I
> have an indicated output power of 1 KW.
> 
> At the DX location, will the option A signal be stronger or the same,
> everything else being equal.

For a typical HF installation, the difference will be insignificant. 
VSWR will increase the loss in the transmission line, but this won't be
an issue unless the VSWR is much higher than 1.5:1.  Chapter 24 of the
ARRL Antenna Book (19th edition) has a figure which shows that when the
loss of a matched line is 2 dB, the additional loss due to a 1.5:1 VSWR
is only 0.1 dB.  If the line were terminated in a 5:1 VSWR, the
additional loss would be ~1.5 dB.  Some folks might claim to be able to
detect a 1.5 dB difference when listening to a DX signal, but I sure
can't. With a lower matched-line loss, the additional loss due to VSWR
is even less.

By the way, I looked at the spec pages on the SteppIR website and did
not find any claims of 1:1 VSWR.

73, 
Bob Nielsen, N7XY
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