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Re: [TowerTalk] upgrading from a G5RV

Subject: Re: [TowerTalk] upgrading from a G5RV
From: "Dan Zimmerman N3OX" <>
Date: Tue, 19 Aug 2008 23:36:34 -0400
List-post: <">>
> There's something else at work there.

Yep, mismatched line loss.

Coax can be surprisingly awful when badly misterminated.  I brought home a
fresh roll of RG-6 from Home Depot and stuck a connector and a 1.5:1 unun on
it just for kicks.

SWR was <2:1 down to mid-HF somewhere according to my MFJ-259B

When you severely misterminate a low impedance line, the I^2*R losses in the
conductors can get pretty bad.

Very high impedance line tends to have less current to start with, so
conductor loss is less of an issue even for matched loss, and since the line
is high impedance and the typical mismatch with a random long (>1/2 wl)
doublet is always greater than |Z| > 50 ohms, so the SWR on the line is

The mismatched line loss doesn't actually have to go up, either.  For very
short lengths of line, the mismatched loss into some loads can actually be
lower than the matched loss.  I think this only happens if you don't have a
long enough line to get to a current maximum.  It's not a very important
case, but I think it's an interesting tidbit.  Line loss is a strong
function of load, and sometimes it's not obvious what will happen.

VK1OD's transmission line calculator is my tool of choice for estimating
mismatch losses:


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