[KB6NU] Youre all wet!
 

KB6NU's Ham Radio Blog

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Youre all wet!

Posted: 20 Jun 2016 11:54 AM PDT
http://feedproxy.google.com/~r/kb6nu...m_medium=email

450-ohm ladder line, like the feedline I use here at KB6NU.

Last week, we had a couple of rainy days here at KB6NU, and what that means
is that my homebrew Cobra antenna needed to be retunedÂ*once the ladder line
got wet.Â*It is kind of annoying to have to do this, but since this is about
the only negative thing I have to say about this antenna, I think that I
can put up with it.

I did get to thinking about this, though, and decided to do a little
Googling as to why and how much the rain actually affects the
characteristics of the 450 Ω ladder line feedline. One of the first
references was to an an item in the Elecraft forum on Nabble. It, in turn,
linked to an article by Wes Stewart, N7WS, Balanced Transmission Lines in
Current Amateur Practice Taking a closer look at “ladder line� and its
application.

This article had just the information that I was looking for. Not only
doesÂ*Wes describe whats going on with todays ladder line, but also gives us
a little history of the use of open-wire feedline. Â*N7WS purchased and
tested four samples of ladderÂ*line that he purchased from The WireMan, as
well as a sample ofÂ*air- insulated line that he made from 16 AWG enameled
wire spaced 0.75-in.

Because the lowest frequency he could test at was 45 MHz, he tested the
samples at 50 MHz. Nevertheless, he had some very interesting results:

The characteristic impedance of the dry samples actually varied from a low
of 359 Ω to 405 Ω. The characteristic impedance of the open-wire feederÂ*was
399 Ω.
The loss in dB/100-ft. ranged from 0.33 dB to 0.41 dB. The loss of the
open-wire feeder was 0.30 dB.
When wet, the characteristic impedance of the ladder line samples decreased
by a little more than 4%, and ranged from 343 Ω to 387 Ω. The
characteristic impedance of the open-wire feeder was unchanged.
The loss in dB/100-ft. ranged from 4.8 dB to 6.1 dB. The loss of the
open-wire feeder was unchanged.


The first thing to note that the maximum loss is only 6 dB. Thats only 1
S-unit, and this loss only occurs when the feedlineÂ*is wet. Not only that,
because the measurements were made at 50 MHz, the loss at say 7 MHz will be
lower. The equation that N7WS gives to approximate this loss at other
frequencies is:



If you plug in 7 MHz into that equation, youll find the approximate loss to
be about 2.25 dB, which is less than half an S-unit. So, if someone tells
you that they dont like ladder line because it gets lossy when it gets wet,
as someone did to me a couple of weeks ago, you can pretty much tell him
that hes all wet!

Having said that, I may just try to make my own open-wire feeder at some
point. It would eliminate having to re-tune the antenna when it gets wet,
and it would be fun to do, I think.

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