At 04:47 PM 10/5/98 -0400, W8JI wrote:
>Keep in mind the bandwidth of the cable, where it acts like an exact 1/2 wl
>section, goes down as the length of the cable increases. Because of that,
>you want to "TUNE" the cable on 28 MHz.
>An electrical length in feet of 278 feet just about optimizes the cable.
>That would give you only two main (non-WARC) bands where you'd have more
>than 1.5 VSWR at band edges. Ten meters would be 28 - 28.6 MHz and 80
>meters would be 3.28 to 3.88 MHz for VSWR<1.5. That assumes a very low loss
>cable.
Doesn't this calculation assume 1:1 (50 ohm) SWR at the various antenna
feedpoints? The picture gets more complex with real-world antennas and
some mix of R and J. Also, for the casual reader, this "electrical length"
must be multiplied by the velocity factor to get the physical length of the
cable.
>
>If I were you, I'd cut the cable a safe amount longer than expected to get
>an electrical length of 278 feet, terminate it in 50 ohms, and measure SWR
>on an accurate meter. You want the lowest dip in SWR at 28.3 MHz, and every
>1.76875 MHz below that. Check it on several frequencies (like 14.15 MHz,
>7.075, and 3.5375) to be sure you cut it right!
>
>The MFJ-259B will allow you to trim it within a foot reading directly off
>the dial! Remember, always measure SWR performance at the highest frequency
>for the most accuracy.
Mechanically, it's a lot easier to use a pigtail of RG-11 to do the final
trim. I wound up with about 200 ft of 3/4" hardline plus 20 feet or so of
RG-11. I coiled up the RG-11, and see no reason you shouldn't be able to
coil up your excess hardline (at either end of the run) in the same way.
Incidentally, I was warned by some local experts to expect some fairly
severe SWR excursions on 10 meters, since I wasn't terminating the feedline
in pure 50 ohms. Perhaps because losses in the cable are highest at that
end of the frequency range, I haven't found that to be so, even though my
measured losses are pretty low in absolute terms. Whatever the reason, the
installation has been very satisfactory.
73, Pete Smith N4ZR
In wild, wonderful, only middlin' rare WEST Virginia
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