# Re: 75 ohm CATV Cable on 70 cm

• Subject: [amsat-bb] Re: 75 ohm CATV Cable on 70 cm
• From: "i8cvs" <domenico.i8cvs@xxxxxx>
• Date: Tue, 22 May 2007 20:14:45 +0200

```----- Original Message -----
From: "Fred A Parker" <fparker@paulbunyan.net>
To: "amsat-bb" <amsat-bb@amsat.org>
Sent: Tuesday, May 22, 2007 4:29 AM
Subject: [amsat-bb] 75 ohm CATV Cable on 70 cm

> If it gives me any mismatch I just trim the length.

Hi Fred, KF0AK

Supposing that the impedance of your load i.e. the antenna is 50+j0 ohm and
the characteristic impedance of your transmission line is 75 ohm the SWR in
it is 75/50 = 1.5
Because of the above SWR your 75 ohm transmission line make an impedance
transformation in any point of it as you can see looking at the 1.5 SWR
circle in to a Smith Chart.
The only effect to trim the lenght of your transmission line will be that to
transform the 50+j0 ohm impedance of your load to another R+/-jX impedance
that as seen by your TX i.e. the 50+j0 ohm generator is in condition to
transfer the maximum possible power in your mismatched transmission line
but in any case the SWR in to the 75 ohm line is still 1.5 in any point of
it.

> Remember if the feed line is cut to an electrical half length the input
> impedance equals the output impedance.

It is correct as shown by one complete turn over the SWR circle of the Smith
Chart and if the impedance of your load i.e. the antenna is 50+j0 ohm than
your TX i.e. the generator will again see an impedance of 50+j0 ohm but the
SWR in to the 75 ohm line is still 1.5

> Fred (KF0AK)

73" de

i8CVS Domenico

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