75 ohm feed lines
engle at wdl.loral.com
Wed Jul 28 00:10:02 EDT 1993
Regarding the use of 75 ohm 7/8" Hardline.
There have been some comments regarding the need for matching
at both ends of TV-type 75 ohm hardline to mate the 50 ohm
radio and antenna.
I believe that you probably don't need to worry about it
at the radio end, IF you are using an amplifier
with a Pi or Pi-L output matching network or an
automatic antenna tuner. The only catch is to get 75 ohm
connectors on the coax and the amplifier/tuner. Most of the
connectors we buy are 50 ohm, but there are 75 ohm connectors
available. Andrews, for instance, has 75 ohm N connectors for
some of their smooth shield lines.
For the more adventurous, antennas can also be modified to
present 75 ohms to the feed line. Gamma matches can be adjusted
and feed points moved to get the desired effect. In some cases
mutual coupling between elements is used to modify the driven
element element impedance (most notably in the DL6WU design rules
for VHF/UHF Yagis, which call for the first director to be close
to the folded dipole driven element to lower its characteristics
impedance to 50 ohms).
Where there is a will there is a way.
David Engle, KE6ZE - engle at wdl.loral.com - 408/473-4419 @ work
Facts, what facts? I don't got to show you no stinking facts.
These are opinions expressed here.
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