On Tue, 14 Oct 2003 21:46:31 -0400 "Van Fair" <vfair@innova.net> writes:
> A friend has 60 to 70 feet of hard line (type unknown) buried to his
> tower and then RG8 up the tower. On the transmitter end it has 10
> feet of RG8 before the hard-line.
>
> He is having a problem with high SWR on his beam. In checking it out
> I put a 52 ohm dummy load at the end of the hard line at the base of
> the tower and found a 1-1 SWR at 12.53, 20.71 and 16.63 MHZ. Over
> the rest of the freq the SWR was much higher including 2-1 at 14.2,
> 1.9-1 at 21.3 and 1.5 -1 at 28.6 MHZ. This seems abnormal and my
> experience says it should be close to 1-1 on all freq. but I have
> only used 52 ohm coax. The hard-line may be 72 - 75 ohms. Would this
> cause the above type of readings or does this indicate some other
> problem.? It seems to me that mixing hardline and coax of different
> impedance is not good but does it work?? Thanks Van
That 52 ohm dummy load will get transformed all over the place
by the 75 ohm line, depending on how long the line is. When you look
at it with a 50 ohm swr bridge in the shack, wild variations with
frequency are to be expected.
If you put the swr bridge out near the tower, with rg-8 running
from the swr bridge up to the beam, and hard line running from the
transmitter to the swr bridge input, you will get a good idea of how
the swr looks on the beam. If there's only 70 feet of hard line running
to
the base of the tower, the swr issues are not worth the fraction
of a dB loss (on transmit only!) that the hard line buys you. I'd
replace it with 50 ohm Bury-flex.
Dave Hachadorian, K6LL
Yuma, AZ
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