On 12/16/2013 9:54 AM, n8de@thepoint.net wrote:
I wonder why no one has mentioned the fact that the "50-ohm antenna"
is NOT 'etched in stone'.
ANY antenna can be made to have a 75-ohm feedpoint, thus allowing the
use of 75-ohm feedine.
When the transmitter/amp/tuner looks at the feedline, it will be
75-ohms, which MOST GOOD rigs/amps/tuners can easily match.
I DID say exactly that, about two dozen posts back. I'll repeat part of
it for those who slept through it. :)
My high 80/40 fan dipoles (at 120 ft) over lousy ground measure in the
range of 85 ohms at resonance, which is also about what NEC predicts. I
feed them with Beldon 8213, which is their best RG11.
When I was putting together the design for our California QSO Party
expedition's antenna farm to Tehama co, I modeled dipoles for 80 and 40
at a height of 40 ft over lousy ground. NEC predicted something close to
50 ohms for the 80M dipole and 75 ohms for the 40M dipole. I decided to
use 50 ohm coax on the 80M dipole and 75 ohm coax on the 40M dipole. I
own both an MFJ259B and a vintage AEA analyzer, which has an internal
strap for 50 or 75 ohms, and I have it set for 75 ohms. When we
installed the dipoles and I checked the match, the 80M antenna was
clearly a good match to the 50 ohm coax at the predicted resonant
frequency, and the 40M dipole was a near perfect match to the RG11.
BTW -- the RG11 we use at CQP is a couple of long lengths of a Belden
triax that I found at a hamfest. Loss is slightly greater than their
best copper solid dielectric RG8.
Another point which I'll also repeat. The match that determines losses
in line is match between the antenna and the transmission line, NOT the
match of the line to the transmitter. Again, the adjustable output
network of most tube amplifiers, or an a good antenna tuner, take care
of providing the output stage with a load it is happy with. AND -- the
apparent SWR measured at the transmitter end of the line will be reduced
by the loss in the line.
Should the orginal poster take that coax? Hell yes! In addition that
great triax just mentioned, I bought a NOS 250 ft spool of vintage
Belden 75 ohm kW twin lead at a Chicago area hamfest about 25 years ago,
knowing that I would someday find a good use for it, and I did, 15 years
later. Likewise, I grabbed a nice half spool of Belden miniature 75 ohm
coax, which has found use making audio cables. And when a neighbor died
leaving a long spool of 75 ohm CATV coax in his yard, I quickly bought
that from his widow. I'm using about 300 ft of it to feed monoband
antennas. The Belden 8213 came as an 850 ft remainder on a 1,000 ft
spool that I talked a surplus vendor out of for cheap, noting that hams
don't like 75 ohm coax, so he would have trouble selling it.
IMO, the most critical specifications for transmission line are 1)
suitability to the environment in which they will be used, 2) loss at
the frequencies of interest, and 3) their cost to me.
73, Jim K9YC
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