"Jeff Liebermann" wrote in message
...

... am now a practicing member of the Cult of
75 ohms. The difference is subtle. While worship of the 50 ohm idol
results in the maximum power transfer, the change to 75 ohms results
in minimum coax loss and somewhat easier antenna design. However, the
major benefit is it allows one to minimize the tithe paid to vendors
as CATV coax is cheap and readily available. I suggest you consider a
religious conversion.

Yes. Somebody in this group (you, maybe) previously promoted 75 ohm cable.
This past spring, I was modeling my 20m Field Day dipole, varying size and
height for best pattern shape and minimum VSWR. At optimum, it had almost
exactly 75 ohm feedpoint impedance. Having plenty of TV coax, I didn't
hesitate to go with it. The antenna worked wonderfully well at FD.
Transmitter seemed to like it just fine.

"Sal"

"Sal" wrote in :

Yes. Somebody in this group (you, maybe) previously promoted 75 ohm
cable. This past spring, I was modeling my 20m Field Day dipole,
varying size and height for best pattern shape and minimum VSWR. At
optimum, it had almost exactly 75 ohm feedpoint impedance. Having
plenty of TV coax, I didn't hesitate to go with it. The antenna
worked wonderfully well at FD. Transmitter seemed to like it just
fine.

Sal, you will have discovered that the feed point impedance of a half
wave dipole at resonance is dependent on height above ground, type of
soil etc.

This need not be a choice between 50 ohms and 75 ohms.

My 40m inverted V dipole uses 75 ohm feedline (RG6), and the impedance
looking into the feedline is close to 50+j0. Dipole height was
determined prior to design of the matching system (ie height was not
constrained by the matching solution, rather the height set a
requirement for the matching solution). The transmitter has its rated
load impedance, the feedline is efficient and inexpensive. There is a
W2DU style choke balun near the feedpoint, and measured common mode
current is quite low.

Of course, the feedline has standing waves, and the dipole is not
resonant, to be frowned upon by some (many?). The antenna system can be
readily modelled, and it has higher efficiency than a resonant half wave
with comparable coax (say RG8X), indistinguisable pattern, marginally
higher gain.

Owen




"Sal"

On Wed, 28 Sep 2011 23:26:24 -0700, "Sal" wrote:

"Jeff Liebermann" wrote in message
.. .

... am now a practicing member of the Cult of
75 ohms. The difference is subtle. While worship of the 50 ohm idol
results in the maximum power transfer, the change to 75 ohms results
in minimum coax loss and somewhat easier antenna design. However, the
major benefit is it allows one to minimize the tithe paid to vendors
as CATV coax is cheap and readily available. I suggest you consider a
religious conversion.

Yes. Somebody in this group (you, maybe) previously promoted 75 ohm cable.
This past spring, I was modeling my 20m Field Day dipole, varying size and
height for best pattern shape and minimum VSWR. At optimum, it had almost
exactly 75 ohm feedpoint impedance. Having plenty of TV coax, I didn't
hesitate to go with it. The antenna worked wonderfully well at FD.
Transmitter seemed to like it just fine.

Probably me. I tend to promote the use of 75 ohm coax and systems.
Most of the coax cables to my rooftop antenna farm are 75 ohms. The
justification was convenience and price. I obtained five 1000ft rolls
of RG6a/u mutations (some with messenger wire), and some used hard
line, and couldn't resist using it. The only problems seem to be
getting accurate readings on my test equipment and having enough
F-to-N or F-to-BNC adapters.
http://www.qsl.net/n9zia/wireless/75_ohm_hardline.html

Note that using 75 ohm coax on a VHF/UHF repeater is a really bad
idea. Duplexers, isolators, circulators, cavities, and such are very
sensitive to mismatch. For repeaters, use only 50 ohms.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558