On 1/17/2012 3:40 AM, Paul Christensen wrote:
> That ties in with why we have a 50-ohm coaxial line standard for most 2-way
> communications purposes and it probably dates back to the Quackenbush era.
I don't think so. 50 ohms was the standard for VHF/UHF communications
gear because it more closely matched the impedance of the most common
antennas -- a quarter wave vertical with radials, or a quarter wave on
the roof of a vehicle. It did NOT become common in ham radio until the
days of solid state rigs with fixed tuned output stages, and the
designers of those rigs chose 50 ohms.
When I started in ham radio as a kid in the 50s, we used mostly RG59,
because it was closer to the feed impedance of a half wave dipole in
free space. Over the years, ham literature like ON4UN's book, which
plots the feedpoint impedance of a half wave dipole at various heights,
has made us more aware that 50 ohms cable is a better match to a low
half wave dipole. I feed my 110 ft high dipoles with 75 ohm coax, and my
low ones (on Field Day, for example) with 50 ohm coax.
Another point. The BEST RG11-size cables have lower loss than the BEST
RG8/RG213-size cables by virtue of their impedance, but RG-11 cables
with smaller center conductors or with copper coated steel center
conductors have significantly more loss on the lower HF bands than those
with a big (#14) solid copper center.
There's a tutorial on coax and stubs on my website.
73, Jim K9YC
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