Topband:Parallel vertical antennas and 160M amps

[Gary Breed]

> A cautionary note - NEC-2/EZNEC can give unreliable results when wires are
> close together (in WL).  In my models of the Force 12 C-3, for example, as
> well as the factory's, the feedpoint values are 'way off.  I think this is
> a case where the ony reliable way is to give it a try.
> 73, Pete N4ZR

NEC modeling of closely spaced wires can be accurate. I have done extensive
modeling of closely-spaced wires at less than 2 inches at HF, including
multiband feeds similar Force 12's. When doing this, it is essential to use
enough segments (add segments until the results converge), and even more
important to align pulses. This is done by having the number of segments
exactly proportional to the lengths of the wires. In EZNEC, you can zoom in
on the antenna view and visually check alignment.

MININEC is actually a bit more forgiving than NEC for modeling
closely-spaced wires, but the near-ground limitations of MININEC negate
this advantage when antennas have horizontal wires less than 1/4 wave above
ground.

Re VK6VZ's question about 3-inch spacing for a 80/160 Tee or inverted-L --
It should work fine at the close spacing, because the 160 portion will be a
half-wave on 80 and its high impedance will contribute little to the net
feedpoint impedance, which will be dominated by the 1/4 wave resonant wire
on 80.  I've had several dual-inverted-Ls for 160/80, and now only use spacing
of 12 inches or more. Closer spacing worked, but typically resulted in narrower
bandwidth and more critical pruning.

73, Gary
K9AY


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