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Bob,
I agree with you 100% only if the intent is to tune the antenna and cable to
the radio. However, I believe that my suggestion to lengthen or shorten the
cable was if the SWR result yielded over a 2:1 mismatch with a commercially
made antenna that was engineered to be a better than 2:1 SWR. Even with a
perfectly tuned cable you'll have standing waves. And I've seen 50 ohm
cables which yielded worse than 2:1 simply because the cable length was just
the right length to yield bad SWR. The lengthening and shortening of the
cable is NOT critical. You don't end up with a "tuned" cable. I may
install a 20 ft. length of 50 ohm cable. You may install a 25 ft. length of
cable. If you have a wierd SWR, cutting off 2 ft of cable may completely
eliminate it. You only really yield a "tuned" cable when you are not using
an off the shelf antenna and you are not using 50 ohm cable. Every bird out
there has a different length of cable.....I'd bet large sums of money on
this...and they work fine.
My suggestion to test only the center frequency was only if one is sweeping
a known commercial antenna. The odds of an antenna having bad SWR readings
across the bandwidth right out of the box are pretty slim. However, for
those who are somewhat anal retentive about things, it would be an excellent
idea to flip through the channels of the comm band and get a characteristic
of what your antenna and cable combination yield.
If you don't yield a better than 2:1 SWR, then you have only four choices:
1. Replace the antenna
2. Replace the radio
3. Replace the cable
4. Shorten or lengthen the cable to bring the bad SWR into better than a 2:1
ratio.
I've been in the RF business in one way or another since 1980. I wasn't a
designer, but I am a technologist. I worked in a firm which developed high
power amplifier systems for DOD contracts that were used in electronic
warfare. All the different bands. Plus, I've been a ham since '84. That
isn't as many years as you, Bob. But, I feel I'm qualified to comment on
such matters. I'm still in the RF business today and when I check my
antenna I'll check the low channel, the high channel, and the center channel
for their SWR reading. I've got somewhat of an advantage in that I can
borrow very expensive equipment to sweep through both transmission and
receiving antennae. Most people will be lucky enough to find some help
finding that kind of test equipment.
Anyway....healthy discussion....thank you.
Curtis Krouse N753K
> " 2) You suggest shortening or lengthening the coax cable to adjust the
> SWR."
>
> This is a BAD idea. What you are doing with this procedure is "tuning"
the
> cable/antenna combination to a single frequency. You still have standing
> waves on the cable even at the tuned frequency which results in radiation
> from the cable which mostly horizontal. This is wasted RF power. The
goal
> must be to install an antenna which has less than 2/1 VSWR over the entire
> frequency band referenced to 50 ohms. Connect it to the comm set with a 50
> ohm cable and you will have less than 2/1 VSWR at the cable over the
entire
> frequency band. You do not want a tuned cable. You want a flat cable
> (electrically speaking).
> Bob Jude, N65BJ
> I am qualified to comment on the subject --- 40+ years in antenna and RF
> equipment design and test.
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