Gerry,
Even without the TH-11DX on top, a 72 foot tower should look
like at leat a 1/4 lambda on 80 meters. With the antenna installed, my
guess is that the structure will be resonant somewhere around 2.5 to 3 MHz.
What did you use to measure the "dip"? and how for up the tower is the
shunt arm tap? I am not sure that the shunt arm parameters (spacing,
tap point) will overcome the the electrical shortness of your tower/
antenna, but I would still expect that you should be able to get the match
point down to around 3 MHz without adding more toploading.
I once used a 55' TV tower gamma fed for 160. Although I probably had more
top loading (it had 40 and 80 meter dipoles plus a A3 tribander on top),
the antenna worked okay considering the limited number of radials that
I could get down (match was good, efficiency - probably moderate).
Mike, W4EF...............
----------
From: gbay[SMTP:gbay@odi.com]
Sent: Thursday, November 26, 1998 1:53 PM
To: TowerTalk
Subject: [TowerTalk] Tower Loading
I have a 72 foot motorized tower, TX-472MDP, with a TH-11DX on top of it. I
am attempting to load it with a gamma match for 160 per the books.
Unfortunately, when I dip it, the frequency is about 6.5 Mhz. Thus, it
would appear that the TH-11DX is not providing any top hat loading. I
recognize that the driven elements are insulated and thus, presumably,
provide no effect. However, I had hoped that the boom with the directors
and reflectors would provide adequate loading.
Has anyone tried to load a tower with a TH-11DX on top? If so, with what
results? Has anyone tried to model this setup? I'm pretty mystified as to
what is going on so any help would surely be appreciated.
73 and happy Thanksgiving de Gerry, W1XY
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