A little more information.
The tower is a free standing tubular crank up 75 footer with a top extension
and mast for a total height of about 90 feet. The tower has two Force12 C3S and
one shorty-40 on it. We haven't grounded the tower with any ground rods or
radials. The only contact with ground is the cement base.
I am using this tower to hold up a 160-meter ¼ wave inverted L antenna. The
length is unknown do to some trimming to tune. The wire runs along side the
tower from the ground to a tie point on the tower about 75 feet off the ground.
The distance is consistent between the tower and the wire and is about 15 feet.
I connect my coax line at the point closes to ground with a current balun. The
far end of the antenna wire is held up with the use of a tree. The base of the
inverted L is on the edge of a wet land preserve with eight ¼ wave radials
running into the swamp.
I believe the tower is coupling into the antenna. When trying to tune the
antenna by changing the length of the wire this is what I get. At 1.93 MHz I
get a very sharp dip in SWR to about a 1:1. Trying to lower the frequency of
the antenna I add some wire. The best SWR now becomes a 1.5:1 around 1.87 MHz
and the dip becomes very broad. By adding additional wire the antenna will dip
at 1.8 MHz but the dip remains very wide and never below a 1.5:1 SWR. I don't
know the exact frequency where the tower starts reacting with the inverted L
antenna.
Going over our logs I notice good performance N/E which is the side of the
tower the inverted L is mounted and direction the inverted wire runs. Contacts
in the S/W direction are difficult to make. We alway thought this was do to the
elevation of the station which is at 300 feet.
Our next thought is to increase the spaceing of the vertical wire from the 15
feet to about 30 feet.
Is there anyway to use the tower as the radiator without connecting the wire to
it? Feeding the wire as we are and increasing the coupleing into the tower
causeing it to act as the antenna. Simular to the open sleeve feeding Force12
uses?
I have ON4UN's books both First and Third edition. Also the ARRL Antenna Book
17th edition. I haven't been able to find anything on decoupleing towers. Tom,
W8JI's write up is very informative but on a tubular crankup hard to construct.
At one point we had the same inverted L in the center of the wet land preserve.
The distance between the tower and antenna was over a 100 feet. There was no
interaction.
Thanks,
Rich AA2MF
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