Mininec neophyte question

Please pardon a neophyte question.

Recently I was trying to model a simple ham antenna, using 80 and 40 meter
dipoles at right angles to one another, fed at a common point. In Elnec
2.2, I set up the wire table so that the 4 wires all met at a common point
-- wires 1 and 2 were the 80-meter dipole (actually inverted vee, since the
ends were drooped for an included angle of about 90 degrees), wires 3 and 4
the 40-meter one. Heeding the manual, I specified wire 2,end 1 as the
feedpoint (center of the 80-meter dipole). Now the odd part:

The pattern and feed-point impedance looked fine on 80, but on 40 they were
'way out of line -- hundreds of ohms and lots of -j, and a big asymmetry in
the pattern that shouldn't have been there. So I moved the feedpoint to
wire 3, end 1 (the 40-meter dipole) -- now the 40-meter numbers and pattern
looked fine, but the 80-meter ones were 'way off.

Finally, I "moved" one of the antennas 6 inches below the other one,
removing any interconnection, and reran at the two source locations above,
and of course both (separate) antennas looked fine.

And to prove the point, I put up the antennas with the common feedpoint, and
the measured SWR on the two bands tracked closely with the model for the
antennas separately.

On reflection I think this may be a general problem, since I also tried 80
and 40-meter full wave vertical loops fed from a common feedline, and got
similarly anomalous results on one of the two.

What am I doing wrong? Or is this a limitation of MININEC? I don't recall
reading about it before.

Pete Smith (N4ZR) n4zr_at_ix.netcom.com

73,

Pete N4ZR (n4zr_at_ix.netcom.com)
Received on Tue Nov 07 1995 - 17:24:00 EST