[CQ-Contest] Checking coax

k6ll at juno.com k6ll at juno.com
Thu Jul 9 05:31:47 EDT 1998

On Wed, 8 Jul 1998 17:42:22 -0500 (CDT) K4RO Kirk Pickering
<k4ro at k4ro.net> writes:
>I just installed several new runs of coax and hardline, and would
>like to know what methods you use to check on your feedline's
>W3LPL talked about this at Dayton a few years ago.  
>>From my notes, it went something like this:
>1) Find a point where your SWR is very high (about 5:1)
>2) Record this frequency
>3) Check the SWR periodically at this frequency
>If the SWR improves, your coax is *NOT* getting better. :-)
>Do I have this right?  Are there other methods people use?
>I'd like to run any tests now, while the installation is still 
>new.  The thinking is that I'll have some baseline data to compare 
>against in the future, to determine when something needs to be
>replaced again.
>If I have access to a Fluke TDM test unit for 50 ohm coax, what 
>measurements would be the most useful to make and record?
>Thanks for any input.
>-Kirk  K4RO

I was also in the audience when Frank made that recommendation at
Dayton. You have his comments exactly correct.

Just before the convention, I had made the recommendation, on one
of the reflectors, to find a frequency where the swr went through
a maximum, with the antenna connected, and to record the swr
at that point. Since the impedance of the antenna at that frequency
closely approximated an open circuit, the loss at that frequency could
be closely approximated by the following equation:

loss (dB) = 10 log (swr+1)/(swr-1)

Take the following example:

You have 100 feet of RG-213 connected to a tribander. You sweep
the frequency range with an antenna analyzer, such as the Autek,
and find that the swr goes through a maximum of 9:1 at 16 Mhz.

loss = 10 log 10/8
loss = .969 dB at 16 Mhz

If you look up the loss in a piece of brand new RG-213 at that
frequency, using the antenna handbook or a transmission line
computer program (e.g. TL), you will find that it is about .85 dB.
You can look at the performance graphs in the antenna
handbook to get an idea how the loss will vary with frequency
and to determine if the loss is acceptable for the intended use. 

A similar recommendation had been made in a then-recent issue of 
QST, but a graphical solution rather than an equation was provided
to find the loss. If you didn't have the graph in front of you,
you were s.o.l.

I talked with Frank after his presentation, and the reason that he
stopped short of endorsing my method was that he did not trust
most swr bridges to accurately read high values of swr. He has
a point there.

I must say though, that whenever I have used my method to test
brand-new coax installations, using the Autek analyzer, the
swr/loss values have always been right on the money, agreeing
exactly with the specifications of the coax. The peace of
mind provided by this one measurement is enough to justify
the price of the analyzer, at least to me. Note that the
analyzer must be able to read high values of swr.  

Dave Hachadorian, K6LL
San Diego, CA
K6LL at juno.com

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