On 1/16/2012 9:32 PM, Hank Garretson wrote:
> Antennas like this do not have to be precisely tuned.
Yeah, sort of. But any antenna that looks like a very high impedance at
the feedpoint is likely to result in a difficult to match impedance at
the TX end of the line. SO -- a "good" length is one that is not an
EXACT half wave, or an even multiple of half waves, at any frequency
where you want to use it. It is that high impedance that makes it more
difficult to match. A little time spent with a spreadsheet working the
wavelength equation is time well spent.
It is also VERY desirable that the antenna be symmetrical, so that
common mode voltage and current on the line is minimized, which in turn
minimizes noise pickup on the antenna and RF in the shack. While common
mode current can be greatly reduced by a good choke, high common mode
voltage can fry even the best of chokes. THAT'S why unbalance is such a
bad thing.
73, Jim K9YC
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