NEC-LIST: Modeling short dipoles

Folks:

I know that this topic was discussed back in September under the topic

   Re: NEC-LIST: How to get actual far field value??

The modeling of short (compared to a wavelength) antennas is something
near and dear to my heart. We use short antennas, in active
configurations in a number of applications. As to the original
question: how does one use NEC to get an estimate of the actual gain
of the antenna? NEC will not give the correct gain of the short
antenna. I tried Mr. DeSchweinitz's suggestion of just sticking in a
series 50 ohm resistor but the gain didn't change. The gain was
identical to that calculated for a short dipole - see Kraus. NEC does
not know how to take into account impedance mismatches and losses to
calculate the efficiency. I have thought that one way to estimate the
efficiency is to do the simulation of the short dipole then repeat for
the resonant antenna and then compare the feed segment currents. The
ratio of currents might then be the efficiency. This is fine if you
are only running the simulation at 1 frequency but could be tedious
across a band for frequencies.

So, I as the group, is there a better way to calculate the gain of the
short antenna not worrying about a matching network or trying to guess
what the loss is? Or, is there a way to use NEC to get the real
efficiency? My use of the short antenna may in fact be a direct
connection to a 50 ohm receiver with no matching. I want to be able to
estimate the antenna's true (I will eventually measure it - but know
early will save some time) gain.

c.rhodes
Received on Mon Nov 17 1997 - 09:45:50 EST