Thanks to Milt, N5IA; Jon, AA1K; and Pete, N4ZR, for their replies. It's
3 AM in the morning, and I have been laying awake thinking about the
possibilities. Jon and Milt suggested series feeding the tower by
isolating the cables and told how. Milt and Pete said the tower would
resonate well below 3.5 Mhz with Milt suggesting the insulators be moved
up to the 20' level for resonance on 80M. Moving the insulators probably
is not an option. He has 7/8ths hardline going up to the 6M beam. Adding
the coaxial chokes would involve cutting the coax going to the A-3 and the
hardline going to the 6M beam and inserting the chokes at the 10' level
where the insulators are. A coaxial choke would add significant
attenuation on 6M, which is a negative. There must be a better way. At
first blush, I thought maybe just cutting the hardline, the coax, and the
rotar cable at the 10' level where the insulators are, adding connectors
at both ends of each cable so that they can manually be disconnected and
connected might be a possibility, though it would be a nuisance. The
thought came that coaxial relays could be used, but no, the braid would be
continuous. I wish I could be sleeping, but ideas were swirling in my
head. My friend has a second 70' Rohn 45 tower about 100' to the north.
Why not run some dacron rope from the tops of the towers: 70' and 50' with
about 65' of vertical wire coming down from between the two towers. If
the dacron rope sagged a little or he wanted elevated radials, we could
make it a T antenna with a few feet of horizontal wire at the top. Then,
restless and unable to sleep (because my mind is obsessed with all of
this) I thought- why not avoid the radials by puting up a full wave loop
in the shape of a trapezoid between the two towers? Thr top and bottom
legs could be about 75', the north vertical leg would be 66 feet, fed at
33' for vertical polarization, and the south vertical leg would be about
45'. I realize that we are introducing some horizontal polarization with
the short, south vertical leg, but that is not all bad as my friend is
more a contester than a DXer, and this full wave loop broadside E-W might
be a killer antenna for SS. What do you think? Since I'm now wide awake,
I'll tell you might 160M solution in a seperate email and see if you think
it's a good one.
Doug W0AH
> A friend of mine, near Denver, had a profession installation of a 50'
> Rohn 25 tower with insulators on the 3 legs at the 10' level and
> phillystrand guys. He has a mast extending 15'above the top of the
> tower with a 28' boom 7el M2 6M beam at the 62' level and a CC A-3 just
> above the top of the tower at the 51' level. Of course,he has two runs
> of coax and one rotar cable going up the inside of the cable. The
> professional tower installer, a ham, told him that the tower could be
> used on 80M, but with the coax and rotar cable, he can not resonate it.
> Is there an easy way to isolate the cables going up the inside of the
> tower OR would it be easier to bypass the insulators so that the tower
> is grounded- and then shunt feed it. If shunt feeding is the way to
> go, can someone please model the gamma-wire tap height for 80M and the
> gamma or omega matching capacities. I promised to help get him on 80M
> (and 160M, but that's another story) before the snow flies.
> Doug W0AH
>
>
>
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