Re: NEC-LIST: NEC-WIN Pro, Antenna Gain modeling

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?

If I understand you correctly, you're wondering why the computed gain isn't
0 dBi. (I assume that your model is perfectly lossless, so that directive
gain equals power gain.)

An infinitesimal dipole, which is what you have at 25 MHz, is not
omnidirectional; its pattern has nulls on-axis. Thermodynamics requires
the surface-integral of the directive gain over a unit sphere to equal
4*pi, so if the gain is less than unity (i.e., less than 0 dBi) in some
direction, then it must be greater than unity (i.e., above 0 dBi) somewhere
else.

The directive gain of an infinitesimal dipole is proportional to
[sin(theta)]^2, where theta is measured from either end of the axis of the
dipole. I'm too lazy to carry out the spherical-surface integral of
[sin(theta)]^2 right now, but it's not difficult; I've done it; and the
result is something like [8/(3*pi)]. In any case, if you divide the result
into (4*pi) you'll get the constant of proportionality, which equals the
(peak) directive gain (occurring where theta = pi/2, and sine = 1). I
believe that I recall that the constant = (3/2), which in dBi would be
about 1.5. The result of 1.03 dBi that you got seems low; this probably
reflects numerical error in NEC (or whatever). At 25 MHz you were probably
violating some of the rules regarding segment size and/or spacing measured
in wavelengths.

The derivation of the gain of an infinitesimal dipole is in most antenna
textbooks, BTW. If I weren't away from home in a hotel room right now, I
would have pulled a book down from my shelf and could give you a reference.

73 -Chuck, W1HIS
Received on Mon Aug 11 1997 - 09:59:52 EDT