With all the recent antenna modeling & RX antenna interest, I finally
decided to download the EZNEC Demo program. Thanks to K6SE's sending me
some Eznec models of Flags, etc, I now am beginning to have a little faith
in my modeling attempts.
My burning interest was to try combining the Fig. 8 output of an Adcock
antenna, 180 out of phase with the omni pattern of a small vertical wire, in
hopes of canceling out everything in both patterns but two pencil-thin
nulls, which would become thin, low-gain RX beams.
Due to phasing complications which I cannot fathom, this didn't work, but in
trying different phase/current combos, a pattern finally came up, which is
very close to a 525 ft Beverage elevation plot, & a 400 ft Beverage azimuth
plot, both for 1.83 mHz.
Having already built & tested a 14 ft X 20 ft Adcock, I know this antenna
will null DX sigs completely out. A recent null test on a super sig from
GA, nulled the sig down to S-9 from +40. The model for the new "3 element,
endfire phased array" is a 14 ft high X 30 ft long Adcock with a third
vertical dipole inserted in the center. The long end dipoles are fed 180
degrees out of phase in Adcock fashion by reversing the lead-in wires at the
center junction box. The third element phase dipole is only 4 ft long for
correct current, .25A, & is fed at 90 degr phase angle. The two signals are
combined.
I have only sourced the three vertical dipoles at their centers & realize
that connecting them with real coax may affect these patterns somewhat.
Also there are only 20 segments to play with in EZNEC Demo, so my results
may be limited. (Thanx W7EL for EZNEC Demo!)
An amazing feature of this antenna is that it's pattern is almost constant
over the full width of both 160m & 80m. The 160m version's phase
dipole current can be doubled by increasing it's length to 8 ft, (.5A),
yielding near identical patterns for the whole 80m band. I can envision
switching in one resistor for easy bandswitching.
Another item of interest is that my models show -3.17 dBi gain at 22 degr
elevation/160m , and +2.38 dBi gain at 22 degr/80m. If these figures are
anywhere near correct, preamplifiers would NOT be required.
This must be a dream & "real" modelers may find mistakes or problems, but I
will nevertheless, proceed to build & test this array, just in case. The
major problem most hams have on TopBand is in the low-noise Receiving
department, due to no Beverage space.
Any & all comments are welcome, even the dreaded "No Free Lunch".
73, Doug / Nx4d
|