Date: Mon, 19 Jul 2010 18:26:27 -0700
From: "Rick Karlquist" <richard@karlquist.com>
Subject: Re: [TowerTalk] 75ohm cable
Gary Schafer wrote:
> You can have up to a 2:1 mismatch (SWR) at the transmitter using 75 ohm
> line
> if the antenna is a flat 50 ohms. If your 75 ohm line happens to be a
> multiple of a quarter wave length that will transform the 50 ohm load to
> 100
> ohms seen at the transmitter end.
>
> 73
> Gary K4FMX
Actually, the transmitter mismatch will be 2.25:1 (112.5 ohms).
Additionally, if the "50 ohm" antenna is actually 33.3 ohms,
it would be a 1.5:1 VSWR with 50 ohm coax but 169 ohms
with 75 ohm coax. Your 50 ohm transmitter (or tuner) would
effectively be working into a 3.4:1 VSWR. This may or
may not be OK.
Rick N6RK
## agreed. And if the "50 ohm" ant is actually less than 33 ohms,
it gets even worse.
## Run YC-156 power into high swr's = asking for trbl..even with the mating
vac caps tuner.
Everything from arced connector's to melted hot spots on the coax. If the
broadband
Array solutions 50:75 xfmr's are used at each end, then you won't have any
problems. With 9 x HF bands, and say just one remote switch box used... on the
end of 75 ohm coax... your chances of the 75 ohm coax being a multiple of a
quarter
wave long are very good. I operate at the upper/lower band edges [and in
between],
so the chances of a multiple of a qtr wavelength is pretty good.
## In Rick's example above, the swr at the feed point is really 1.5:1 but
the TX 'see's
3.4:1 . Who needs that?
Later... Jim VE7RF
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|