Topband: Radiation from ends of Beverage

[Tom Rauch]

This is kind of esoteric, so maybe someone can help double check my logic.

There is a way to estimate sensitivity of the ends. Radiation resistance of
short uniform current antennas is very easy to define using the formula
1580* (he/H)^2 . A 6 ft vertical drop with uniform current would have 1580*
(6/550)^2 = .188 ohms Rr.

Assuming current is limited by the 500 ohm Beverage impedance top-loading
the drop, the efficiency of a 6 ft vertical drop would be Rr/Rr+500 or
.0376%. or -34.2dB. This would assume zero ground loss (perfect soil) for
the vertical drop, so the actual radiation would be less due to earth losses
in the near field and Fresnel zone (certainly about 6dB or so less). The
feeds would be about -40dB radiation efficiency. We'd have to add some small
gain because the radiation would be concentrated from the pattern nulls that
go straight up and down from the vertical drop.

If the peak gain of the Beverage is -14dB, the combined end-drops would
certainly be at least 20 to 35dB less sensitivity than the antenna peak.

Models show a 6ft vertical T with 500 ohm series loss over lossless soil has
about 22 db less gain than a Beverage over lossy earth. Earth losses would
reduce the T gain by another several dB in the real world, but then the
horizontal wires (even when balanced) radiate more than the vertical area of
the T. The only way the horizontal wires won't radiate is over perfect
earth. (The purpose of the T is only to show the radiation of the vertical
section agrees with the estimate, and the horizontal section does radiate
significantly.)

It looks like this all crosschecks to me. As long as the peak gain of the
Beverage is not pushed down lower than -20dBi, and as long as you don't need
deeper than -20 to -30dB nulls, we could ignore the drops. (Which seems to
be what people find, that it is unimportant.) If you had  a really sensitive
Beverage (lossy earth) then the ends would mean less. Over high conductivity
soil they would mean more because the antenna sensitivity would be less.

If anyone sees anything I missed could you e-mail me?

73 Tom