[TowerTalk] Cable TV hardline question

[jerryc]

Maby I am just trying to over-simplify this but what would be wrong with
connecting a 50 ohm. load to the far end of your 75 ohm hard line, putting
a section of rg213 (or your favorite 50 ohm coax) on the shack end and
then  trimming it to resonance at your desired frequency.  Coaxial cable
matchers only work at the design frequency.  I you need multiple bands
then why not use a UNUN to transform the impedence.  While not as cheap
as coax, they seem to work well.
JerryC
KC8TES






----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "David Robbins K1TTT" <k1ttt at arrl.net>; "'Tim Holmes'"
<W8TAH at Zoominternet.net>; <towertalk at contesting.com>
Sent: Friday, October 29, 2004 12:21 PM
Subject: Re: [TowerTalk] Cable TV hardline question


> I'm not saying with luck you can't do use 75 ohm cable at
> UHF in 50 ohm systems, but personally I'd never do it
> without matching at each end.
>
> > I use it on 6m,2m and 70cm with a ts-2000 and a couple
> tm-261's for 2m
> > packet and none of them complain.
>
> The impedance range of a 75 ohm cable feeding a 50 ohm load
> is from 50 ohms to 112.5 ohms, or a 2.25:1 SWR. Losses help,
> eventually pulling it down to a 75 ohm impedance (1.5:1) if
> the line is lossy.
>
> A 2:1 mismatch really isn't good for radios that are fixed
> tuned, whether they appear to like it or not. FM or
> non-linear PAs are less of a problem, but linearity can fall
> apart quickly when a PA designed to operate into a 50 ohm
> load is misterminated.
>
>  Plug it in and try it, if it isn't
> > good enough match for your radio add a short length of 75
> ohm to the
> > transceiver end, this will transform the impedance some
> more, if you
> > calculate it so the overall length is a multiple of 1/2
> wavelength you
> > get back whatever impedance is on the other end more or
> less... but I
> > have never found that to be really necessary in real
> situations.
>
> At about six inches per 1/4 wl section on 70cm, you'd go
> through a full cycle from 50 to 112 ohms and back with a one
> foot change. When you have multiple 1/4 wl sections in
> series, the SWR bandwidth decreases. The impedance error
> with frequency change of each small section adds. That's why
> a MFJ 259 analyzer can measure cable lengths into the
> thousands of feet using 10MHz as the test frequency.
>
> I'd be careful using CATV cable in 50 ohms systems where the
> cable is multiple wavelengths long.
>
>
>
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