[amsat-bb] Re: 75 ohm CATV Cable on 70 cm

[John Magliacane <kd2bd@xxxxxxxxx>]

--- Fred A Parker <fparker@paulbunyan.net> wrote:

> Remember this line is not designed for the high voltages you get with high
swr.

Correct.

However, trimming the line length to reduce a 50-ohm VSWR measurement in the
shack does not modify the SWR on the transmission line.

To reduce the SWR on the 75-ohm line, one could adjust a yagi's gamma or delta
matching network to produce a 75-ohm feedpoint impedance.

Additionally, "twelfth-wave" matching transformers (series-section
transformers)

      http://ourworld.compuserve.com/homepages/demerson/twelfth.htm

made from coaxial cable can be used to easily convert a 50-ohm source or load
for use in a 75-ohm system.

So, you could adjust your antenna(s) for 75-ohms, and use the transformer in
the shack to produce a 50-ohm load for your transceiver, while maintaining a
very low VSWR on the transmission line.

You could also connect one of these transformers to the "load" (antenna) port
of your 50-ohm VSWR meter to enable its use in accurately adjusting your
antennas for a 75-ohm feed point impedance (1:1 VSWR).


73, de John, KD2BD


Visit John on the Web at:

	http://kd2bd.ham.org/
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