----- Original Message -----
From: "Pat Barthelow" <aa6eg@hotmail.com>
To: <towertalk@contesting.com>
Sent: Tuesday, April 05, 2005 11:27 AM
Subject: [TowerTalk] Quick Check for Coax Loss
> Folks,
>
> If I wanted to check a long run of coax to a monobander (20 meter)
> beam, to
> see if there are any surprise unexpected losses, I might disconnect
> the
> coax at the beam, and measure the indicated SWR at the radio.
> Assuming the
> disconnected coax represented infinite SWR, one could just look from
> a
> chart, or from formulas to convert the indicated SWR at the Radio to
> coax
> loss... been there, done that....
> Since it is a bit difficult to disconnect the coax at the beam on
> the tower,
> could I do a reasonably accurate measurement by exciting the 20
> meter yagi
> with 10 meter RF, and assume the 20 meter driven element impedance
> at 10
> meters would be extremely high nee, >>10SWR, and measure the
> reflected watts
> at the radio, and compute the coax loss? I guess what I really
> would like
> to know, is what (how high) would the feed impedance be on a 20
> meter beam,
> if fed with 28 mhz RF, making it a nearly full wave, Hi Z load?
Here's a repost of my previous msg on the subject:
Here's an easy procedure to make a good approximation of line loss,
without disconnecting the coax up on the tower:
1. Connect an antenna analyzer to the feedline in the shack.
2. With the analyzer in swr mode, vary the frequency, observing that
the swr goes through maxima and minima. Pick one of the maxima, in the
vicinity of 25 - 30 MHz. Note the swr and frequency. At that point,
the antenna up on the tower presents such an extreme impedance, that
you are essentially measuring the swr on an unterminated line.
3. Plug the observed max swr into the following equation:
loss (dB) = 10 log [(swr+1)/(swr-1)]
For example, if you measure a max swr of 9, at 26 MHz:
loss = 10 log(10/8) = 0.97 dB at 26 MHz.
4. Consult a coax loss chart to see how 0.97 dB at 26 MHz compares to
the specifications for new cable of that type and length at 26 Mhz.
The main limitation on this process is the ability of a cheap analyzer
to accurately measure high values of swr. That limitation is mitigated
by staying at the high end of the frequency range, where the swr
maxima have lower values.
Dave Hachadorian, K6LL
Yuma, AZ
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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