Topband: 160m Ant on 80m?

[David Gilbert]

Tom,

I believe your antenna should simply look like a vertical halfwave dipole,
except electrically off-balanced by the coil at the top end.  In theory, any
simple vertical one half wavelength or less and close to the ground should
have a single lobe at a takeoff angle determined by radials and ground
characteristics.  I made an approximate model in EZNEC and got a takeoff
angle of about 14 degrees and magnitude similar to a normal vertical, but
that will off course depend upon whether you use buried radials or tuned
radials above ground or whatever.  For simplicity, I did the model with two
elevated 80m tuned radials 2m above "average" ground just to give a decent
result for the feedpoint, which is of course high impedance.  If you can
send me (directly to my home email) the actual dimensions of the pole, the
location and inductance of the coil, and a description of the ground system
I could try to do a better model for you.

By the way, it is also possible to make an elevated ground plane out of the
160m vertical by connecting two 80m tuned quarter wave radials, which could
be sloped, to the proper point on the pole such that there was an electrical
quarter wave for 80m above that point.  The quarter wave radials act as a
kind of choke to make only the top section active as long as you feed the
vertical above that point.  The feed system could be a gamma match or you
could run a wire spaced along the pole to form a "folded vertical", but
you'd probably want to have some way of shorting out the top coil if you do
this.    It would probably have at least one low lobe and maybe a high lobe
as well.  I can easily model this for you if you'd like, including possible
impact on 160m operation.

Dave   AB7E



----- Original Message ----- 
From: "Thomas Hoeppe" <thomas.hoeppe at asamnet.de>
To: "Topband Reflektor" <Topband at contesting.com>
Sent: Thursday, April 15, 2004 10:15 PM
Subject: Topband: 160m Ant on 80m?


Hello,

has anyone tried to use a 160m Vertical on 80m? The antenna is mechanical
33m high, electrical about 37m (Top-coil). So it will be nearly 1/2 lamda on
80m, and I want to feed it by a parallel-resonant cirquit. I hope to have
improvements, as the trees around are about 20m high, so our 1/4 lamda on
80m ist just between the trees. A photo from the air is at QRZ.com, callsign
DL0AO.
Whitch radiation behaviour can I expect?

Tom, DJ5RE/DL0AO
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