At 05:59 PM 8/9/02 -0600, n4kg@juno.com wrote:
>For intermediate lengths, multiples of 1/2 WL work fairly well
>over a few % bandwidth, i.e. 1/2 WL on 40M (~56 ft) is also a
>multiple of 1/2 WL on 20, 15, and 10M. Another useful length
>is 1/2 WL on 80M CW (3.6 MHz which is ~112 ft) and is also
>a multiple of 1/2 WL on 40, (close on)30, 20, 17, 15, 12, and 10M.
>
>Long runs get a little tricky on 10M because the band is so wide
>and there are LOTS of 1/2 WL's in a long run. For example,
>a frequency change from 28.0 to 28.56 (2%) times 12 half wavelengths
>(17 ft) creates a 1/4 WL shift in a 204 ft long line SO a 50 ohm load
>will look like 50 ohms if the line is tuned for CW but will look like
>100 ohms at 28.56 MHz. For 10M one can live with the
>minor SWR variations, tune the 10M antennas for 75 ohms,
>or install matching networks at each end.
I am using a single half-wave of CATV at 1750 KHz (about 210 feet),
measured at 28000 KHz, and find that it gives me acceptable SWRs (2:1 or
less) at the shack end on all bands, including on 10M. I may be just
lucky, but this particular combo does work. There are definitely some
impedance transformations going on in the line, because the valued of SWR
and the lowest SWR frequency do vary from the values measured at the base
of the tower, but they are all workable.
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