RE: NEC-LIST: Enhancing low-angle radiation

Dean Straw N6BV <rdstraw_at_arrl.org> replied to my original post:
> I suggest you look at the article "An Adventure in Antenna Modeling"
> by Brian Beezley, K6STI, in The ARRL Antenna Compendium, Vol 4.

Excellent suggestion, Dean, thank you! I had that book on my shelf
but had never read that article, until you called it to my attention
today.

I avoided all the pitfalls described by Brian Beezley. All of my
modeling was done with copper wires of 2E-4 wl diameter, and I used
the Somerfeld-Norton ground calculation (what EZNEC calls "High
Accuracy"). I also avoid difficulty feeding the dipoles (or
whatever), as follows:

Center-feed each dipole with a balanced open-wire transmission line of
electrical length = 1/4 wl, with the opposite ends of these feedlines
connected together in parallel. (Note that the total length of these
feedlines equals the spacing between the dipoles.) To this junction
connect a feedline from the transmitter. The quarter-wave lines have
characteristic impedances differing by a factor of two, with the
lower-Z line going to the upper antenna.

This simple arrangement (which works well in practice without trimming
or tuning) produces feedpoint currents that are in phase and have the
desired 2:1 ratio of magnitudes, independent of the (unequal)
feedpoint impedances of the two dipoles (or whatever). This result
does depend on feedline losses being negligible, which is an excellent
assumption in this case. The input impedances of my dipoles were
about

    upper dipole Z = 65.78 - J 11.5 ohms
    lower dipole Z = 45.53 - J 60.7 ohms

... so the SWRs are modest. (Yes, my dipoles look a little bit short.
I made them 0.484 wl long and didn't trim them; there was no reason
to.)

73 de Chuck W1HIS
Received on Fri Oct 10 1997 - 08:59:38 EDT