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Re: [TowerTalk] Double shield

To: "Jim Brown" <>,<>
Subject: Re: [TowerTalk] Double shield
From: "Michael Tope" <>
Date: Wed, 13 Oct 2004 02:56:17 -0700
List-post: <>
Another way to look at it is to think about what length of run
would represent 3dB difference between the good feedline
and the cheap feedline:

                    LMR-400 vs RG-8X
1.8 MHz            1300 ft           Delta $ 390
3.5 MHz              870 ft           Delta $  260
7.0 MHz              600 ft           Delta $  180
14.0 MHz            380 ft           Delta $  114
28.0 MHz            260 ft           Delta $  78

Cost delta assumes $0.30/ft difference between LMR-400
and RG-8X

No surprise that at HF you have to have a pretty long feedline
before you start to see a 3dB delta between cheap and
expensive feedlines. Of course if you a trying to squeeze
every last dB out of your system, then the expensive line
might be worth it. For instance, if you have a 200' run to your
40 meter beam, then that extra 1dB of TX power will only cost
you $60. That is pretty cheap as dB's go (probably cheaper
than squeezing out an extra 1dB from your power amp or
getting 1dB more forward antenna gain). As to whether 1dB
is worth fretting over, of course, is a whole nuther' debate.

73 de Mike,

> Like any other engineering spec, it all depends on your application and
> what the higher spec buys you IN YOUR APPLICATION. That's why there are a
hundred or
> so different coax cables in the Belden book.  Of course LMR400 is serious
overkill at 40
> meters -- I wouldn't use it there unless I happened to have 3,000 feet in
my garage that I
> bought for $100 a spool. A run of the mill RG-8 or RG-11 is fine for a
full KW, and any
> decent RG-59 or RG-8X is fine for a few hundred watts. In fact, the
smaller cables are
> better -- there will be less sag and the antenna will be higher!  And if
you think that
> difference is small, so is the difference in loss.
> Jim Brown  K9YC


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