Dick,
I assume you will have a separate coax for each antenna. If so, you can
use a simple series matching transformer at both ends. Assuming .66
velocity factor it would be about 90 inches of 75 ohm coax in series
with 90 inches of 50 ohm coax for 40M, half that number for 20M, 1/3
that number for 15M and 1/4 that number for 10M. The 75 ohm side of the
transformer would connect to the 50 ohm load and the 50 ohm side of the
transformer would connect to the 75 ohm load. Alternatively you can
live with the losses having to do with a 1.5/1 mismatch and cut the coax
to an electrical multiple of a half wavelength so the impedance will be
repeated on the other end. You will still have the losses associated
with the SWR on the line if you do not convert the impedance on both
ends - which is not that big a deal if it is 3/4 inch hardline. If you
are using a coax switch and want to have a coax that will repeat the
impedance at both ends, you would use a common denominator that would
work on all bands, 80-10. I think that length works out to something
like 220 feet for CATV stuff. If it is 3/4 inch hardline the connectors
are very easy to make and install using a copper fitting that will
accept an SO-238 on one end and have an internal thread with an opening
that will self thread onto the 3/4 inch CATV outer aluminum jacket. Use
some NOALOX or equivalent when you thread it on using a 1 1/8" wrench.
CATV hardline is really good stuff especially when it is free.
73...Stan, K5GO
----- Original Message -----
From: "Dick Dievendorff" <dieven@comcast.net>
To: <towertalk@contesting.com>
Sent: Monday, August 30, 2010 11:18 AM
Subject: [TowerTalk] 75 ohm hard line
> I've been offered a quantity of CATV 75-ohm hardline.
>
>
>
> The radio room will be about 120 feet from the base of the tower, and
> the
> highest antenna on that tower will be at 77'. The antennas on this
> tower
> are 40, 20, and 15 meter wide-band yagis.
>
>
>
> I'm trying to figure out what I need to consider when contemplating
> antennas
> and radios designed for 50 ohms with 75 ohm feedline. I'm worried
> not so
> much with power loss caused by the impedance mismatch, but the effort
> required to show the rig a 50 ohm load on various frequencies.
>
>
>
> The antennas on the tower are 50 ohm OWA antennas that were selected
> to
> avoid the need for an antenna tuner, a complexity I'd prefer to avoid
> in a
> high power SO2R contesting environment.
>
>
>
> Another antenna a little further away is a HyTower, and I expect to
> use an
> ATU for that one. But I'd probably use it on 75 and 160 mostly.
>
>
>
> I think I'm probably better off with a couple of runs of LMR-600, but
> I
> thought I'd ask for suggestions before turning down the hardline
> offer.
>
>
>
> I plan a K9AY receive loop fed with 75-ohm line some distance (less
> than 500
> feet) away. Should I bother with hardline for something like this?
>
>
>
> Dick, K6KR
>
>
>
> _______________________________________________
>
>
>
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