Merry Christmas to all, and to all, antennas at new heights!
I have been playing with EZNEC, which is dangerous since I really do not know
what I am doing. But here is what I have gotten so far. I have a two section
aluma tower 50' crank up that is nested at about the 30 foot level, a rotator
on top, a 10 foot mast on which 2 feet above the top of the rotator is mounted
a Cushcraft A3S, then at the top of the mast is a Cushcraft vhf/uhf 4 element
beam. At the 23 foot level up the side of the tower I ran a wire down to X=
-30 and 5 foot off the ground, there it turns and runs an additional 50 feet
along fence toward X= 20. I actually have a dip in the swr on 160 of 2:1 with
this arrangement. The tower has 10 radials on its base fanned out in a
semicircle toward the back fence line, laying just on top the ground, which I
am not able to model due to the 20 segment limitation on the demo EZNEC.
When I model this on EZNEC with the source right where the wire attaches to the
tower (coax center to the wire, braid bonded to tower), I get the 3D pattern
EZNEC makes, BUT what is interesting is raising the attachment point on the
tower lowers the gain of the antenna structure, and lowering this point raises
the gain on the patterns.
Lowering it makes the pattern more a donut with gain around 13-19dbi, raising
the feed point on the tower turns the donut into a bottom flattened sphere and
the top of this sphere begins to grow upwards as the feed point is raised
lowering the gain drastically. Looking at the currents on the model shows the
entire tower as a vertical with current highest at the base and tapering up the
entire tower structure just like you would expect on a vertical, there is also
some current distributed on the wire.
I would have expected the opposite results.
Am I doing something wrong?
Thanks
Gene Bigham
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|