Re: NEC-LIST: NEC-WIN Pro, Antenna Gain modeling [more]

Max Schmitt <mjschmit_at_bellatlantic.net> wrote:
>
> I modeled a biconical dipole for 25 - 1000 MHz with very short
> dimensions only 12 inches high. The power gain showed 1.03 dBi
> max. gain at 25 MHz! The input impedance was only 0.2 ohms, which
> means very small radiation resistance, i.e., efficiency. The power
> gain at 25 MHz should be extremely small. How come the power gain at
> 25 MHz for a small antenna still showed 1 dBi??
>
> Even when I used directive gain, not power gain, I got the same
> results. I modeled the antenna as a wire cage, 3.5 inch diameter. I
> remembered, when I used ELNEC for similar models, I would obtain
> very small antenna gains.
>
> What is wrong, any answers to this problem?

In my first reply I thought that you were wondering why the computed
gain wasn't 0 dBi. (I assumed that your model was perfectly lossless,
so that directive gain equals power gain.)

Now I suspect that you were wondering why the gain wasn't *very*
small, like minus some big number of dBi.

If this is what you're wondering, the basic answer is that your model
had no losses. For a very short dipole in the real world, input
resistance due to ohmic losses would greatly exceed the input
resistance due to radiation, so that most input power would be
dissipated. ELNEC may have given you a "very small antenna gain"
because you had wire resistance turned on, and/or it may have given
you a "very small antenna gain" because you were violating its
segment-length rules. Different versions of ELNEC, EZNEC, NEC, etc.,
have different rules and will fail differently if you violate the
rules. Remember the "GIGO" rule. (Garbage in, garbage out.)

73 -Chuck, W1HIS
Received on Tue Aug 19 1997 - 22:52:40 EDT