We got up the first of 4 Rohn-25 / Aluminum verticals this weekend. When
we first hooked up the SWR analyzers to the vertical it was showing a dip
around 3350Khz and another dip around 3650Khz. Changing how much stinger
was out of the top moved the lower dip frequency but only changed the SWR
reading of the higher frequency dip. After assurances that I wasn't
crazy from K3LR on the phone, I started looking closer.
The elevated radials were mounted below the insulated section. When I
tried to hook up the current balun to it, the shield side wire was not
long enough to reach the radials. I added about 1.5' of 14g wire to
reach the radials. I decided to move the radials up and eliminate the
extra wire on the shield side. Bingo... Now I had a resonant frequency
of 3725Khz and only one dip.
Now... Why would adding just 1.5' of wire on one side of that balun
produce such wierd results in both MFJ and Autek SWR analyzers?
The balun is one of those W2AU bead baluns. The verticals are 3 sections
of Rohn 25 regular sections and one top section. Then a 38' stinger made
of 6060-T6. The stinger is in the tower about 4' and the resonant
frequency now is 3675Khz.
PS... I'll be posting many pictures of the installation on the WWW. I'll
advise when they are available.
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