To: <towertalk@contesting.com>
> Date: Wed, 10 Feb 1999 10:41:29 -0500
> From: DavidC <davidc@bit-net.com>
Hi David,
> Local contester KM1H runs 75ohm CATV to all of his HF, VHF and UHF antennas
> without UNUNs and says the impedence bump over several hundred foot runs
> are
> unnoticable, especially since the loss in such cable is so low.
If a 50 ohm load is terminating a 75 ohm line, SWR at the
source end (when referenced to 50 ohms) will vary from 1:1 to 2:1 as
line length is varied. That is a very noticeable change.
> He also says that he has discovered that many antennas are closer to
> 60 ohms than 50, so 50 ohm cable also sometimes introduces a mismatch.
That sounds profound, but how was that opinion reached? I'm not
aware of any "impedance survey" that has demonstrated the majority of
antennas in use in the the world to be 60 ohms!
My verticals are all around 35 ohms, my dipoles 50 to 70. My yagis
are about 50 nominal, with an almost equal spread lower than 50 and
higher than 50 ohms. When I average out all my antennas, the
impedance comes out 48.2 ohms. It certainly doesn't apply here!
I prefer for ALL my antennas to present a 50 ohm load to the rig.
That way I can tune on a dummy load, and change from antenna to
antenna without touching a thing. That requires adjusting the system
so 50 ohms appears at the rig, no matter what goes on elsewhere.
Chad covered the facts very well.
73, Tom W8JI
w8ji@contesting.com
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