I have been following the posts for the "T" antenna and hope that the
lengths given for the tophat wires don't discourage some from building this
very excellent antenna. By adding a third wire and manipulating the angle
of the slope on those wires it is possible to construct a really efficient
vertical with a much smaller footprint than some of the posts have
indicated.
I have two of these antennas, one at home and one at the summer place. Each
is made of lattice tower topped with 1.75" pipe to which are attached the
loading wires. Radials number 64 at the home place and 41 at the summer
QTH, ranging in length from about 35 to 140 ft at both places.
In each case I employ three top-load wires, so the antennas are not strictly
"T" style but more towards the "umbrella" style. The antenna at home is 81
ft with three top-load wires approximately 34 ft in length. At the summer
place the vertical is 61 ft high with three wires approximately 44 ft in
length. On each antenna there is one wire which droops at a 45 degree angle
(simply because I have never installed a support higher than about 10 ft for
it). The other two wires on each antenna slope a few degrees.
The late K6SE did an analysis on the shorter of the two antennas and
indicated that it was quite efficient, (above 70% compared to a full 1/4
wave vertical if I remember his e-mail), a fact born out by the amount of
"good" DX I seem to be able to work on both 160 and 80 meters. The longer
antenna at home is somewhat better than its shorter cousin. Both antennas
are self-resonant somewhere close to the 160M band, so the third top-loading
wire certainly lowers the overall length needed for tophat wires. Small-lot
owners take heart!
(There was a very good article a few years ago by AC7A (QST October, 1998)
which showed how one might employ folding the top-load wires back on
themselves, a practice which, the author felt, did not affect the efficiency
of the antenna very much. Again, small lot owners take heart.)
On both of my antennas matching is via inductance to ground, tapped down for
resonance and up from ground for 50 ohms. Matching on 80M is via series
variable capacitance and a different tap on the coil.
I have noticed that changing the angle of the top-load wires to the
horizontal does shift resonance somewhat. The longer antenna has a 2:1 SWR
bandwidth of about 90 khz while for the shorter it is about 120 khz.
Bill, VE3CSK/VE3NH
_______________________________________________
Topband mailing list
Topband@contesting.com
http://lists.contesting.com/mailman/listinfo/topband
|