[TowerTalk] antenna decisions

[Dick Green]

>I used a noise bridge when I did my 40-meter 4-square, but never felt
>comfortable with the nulls I got.  When I put my 80-meter one together, I
used
>an MFJ SWR analyzer and cut my cables with the method described in
"Low-Band
>DXing " by ON4UN where you use a transmitter and a dummy load to cut them
to
>length, with the MFJ taking the place of the transmitter.  I also cut my
>cables to 1/2 wavelength at double my desired frequency so I didn't have to
>continually short and un-short the cables, as I cut them. I don't know if
it
>plays any better, but I felt better about the results, because I could see
a
>definite dip on the meter, rather than a subjective "least noise" reading.
>73,
>Bob

Interesting that you should bring this up. At first, I tried the ON4UN
method with an MFJ SWR Analyzer and an Autek RF-1, but I just couldn't get
this to work. I had an old beat-up set of phasing lines cut by Colatchco for
7.150 MHz, and the SWR meters were all over the place -- I couldn't pin the
frequency down closer than plus or minus 50 Khz, and results were never
reproducible from one measurement to the next. As I recall, I had somewhat
better luck measuring them as 1/2 wavelength at twice the frequency, but the
results were still not very reproducible. Talk about being uncomfortable!

However, I got very good nulls on the S-meter of my TS950SDX with a simple
Heathkit noise bridge. These measurements were very reproducible, but
somewhat wide. I found that cranking the audio and listening for the
quietest spot allowed me to pinpoint the null within 10 KHz every time. It
really wasn't subjective at all. When I did that, all four lines were within
10 Khz of 7.150 MHz, exactly where they should have been. I proceeded to use
the noise bridge to cut the new phasing lines. Since I had similar trouble
with the SWR analyzers when I tried to tune my 40M elements, I suspect that
some sort of B/C interference was to blame (I get better results on my 80M
vee.) It's odd, because the closest AM station is 7 miles away and it's low
power. Perhaps it's something else. Any ideas?

73, Dick, WC1M



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