"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"