Topband: electrically shortening a tall grounded tower
wb6tza at socal.rr.com
Fri Sep 25 22:55:02 PDT 2009
I operated for a number of years with a 250 ft tower with sloper 1/4 wave
lines attached 1/4 wave from the top of the tower. I simply hooked coax
between the tower and the sloper like you would feed a dipole. This made
the sloper and the tower appear a little like a half wave vertical. The
tower was of quite large cross section so I ran three slopers with separate
feedlines, one from each corner of the tower and selected them for some
directivity on transmit. The fact that it WAS directive means that it was
not at all a pure half wave vertical.
It worked very well. I am sure there were lots of odd currents flowing
here and there, especially in the solid guy anchor lines about 50 feet
below the slopers, but it performed well.
If you have the luxury, you CAN stop the stray currents. Come down a
quarter wave from the top and install at least 4 quarter wave radials of
serious size conductors - each could be a cage of 4 wires simulating
something faintly like the cross-section of the tower itself. Then come
down ANOTHER quarter wave and do it again, with these sloping much more
than the high ones.
This approach is much like the sleeve collinear antennas for VHF and UHF.
Feed the tower against the top set of radials - probably the best way, but
mechanically awkward, would be to gamma match the top quarter wave & tie
the set of radials hard onto the tower. If that's not convenient, make a
ring around the tower and attach all the radials to it and feed the power
in between the radials and the tower.
If you REALLY want to work hard on it, make it into a J pole. This will
require that the short part of the J be of similar cross section to the
main tower, and spaced appropriately.
----- Original Message -----
From: "Bob Kupps" <n6bk at yahoo.com>
To: "topband" <topband at contesting.com>
Sent: Friday, September 25, 2009 20:50
Subject: Topband: electrically shortening a tall grounded tower
Is there any effective way to electrically shorten a tall grounded tower to
1/4 wave? I tried modeling a 1/4 wave cage surrounding the tower but it
still shows a lot of current in the protruding section with resulting high
angle radiation. Then I tried modeling a 1/4 wave stub above the cage
grounded at the top to the central tower which appeared to turn it into a
dipole and still significant current above the stub. I was also thinking of
using an external parallel resonant trap/choke but would this actually work
when situated at a zero current point at the far end of a 1/4 wave
Thanks and 73 Bob N6BK/HS0ZIA
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