On Mon, 20 Apr 2009 13:31:46 -0700, Paul Playford wrote:
>Be careful with cable company hardline. It is probably 75 ohms impedance
> and when you connect it to 50 ohm coax (RG-214) it will become a 1/4
> wavelength impedance transformer
Hold on a minute. A dipole in free space (or a high dipole) is a far better
match to 75 ohm coax than 50 ohm coax. And it is the match between the coax
and the antenna that determines the REAL SWR and the associated losses. A 50
ohm SWR bridge in a 75 ohm line will give wrong answers about that. LOW
dipoles tend to be a closer match to 50 ohm coax.
> and mess your antenna system up.
WRONG. First, ANY length of line transforms the impedance of a load that is
not a perfect match to it. BUT -- the details of the transformation depend
on the length of the line and the complex impedance. A piece of line is a
1/4 wave transformer only if it is 1/4 wave long.
Second, the transformation provided by the mismatched line simply presents a
different load to the rig, and the difference between 50 ohms and 75 ohm can
EASILY be matched by any GOOD antenna tuner.
When I started in ham radio 55 years ago, hams used 75 ohm coax (and 72 ohm
twinlead) to feed dipoles. When newer engineers began designing solid state
power amps, they close 50 ohm impedances because those engineers had cut
their teeth on commerical VHF radio, where 50 ohm coax was a good match for
"ground plane" antennas, which were then the dominant antenna for 2-way
radio on those bands. That's why ham rigs today use 50 ohm outputs. But the
most important thing about choosing a feedline is that it match the ANTENNA,
not the transmitter, because that's what determines the losses in the
feedline. It's the cart before the horse.
Don't believe me? Study the ARRL Handbook, ARRL Antenna Book, or ON4UN's
book, all of which address these issues, with graphs of impedance vs height.
Now, various beam antennas have been designed to be fed by 50 ohm coax, and
that's the best choice for those antennas. BUT -- the WORST CASE additional
loss due to a 1.5:1 VSWR is 0.18 dB, no matter how long the line, and 1.5:1
is the SWR of 75 ohm coax with a 50 ohm load.
Bottom line -- for a LONG run, the much lower loss of hardline as compared
to smaller coax is FAR more important than the slight increase in loss due
to mismatch.
73,
Jim Brown K9YC
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