sci.physics.electromag

Hello, and thanks for the reply. Please see below...
"rge11x" wrote in message
news: @ ...
> On Sep 12, 1:22 pm, "Bill Miller"
> wrote:
> [...]
>> In "real" antennas (disregarding "long" antennas that are multiples of a
>> wavelength) radiation is broadside to the conductor and near zero off the
>> ends. I'm not familiar with the calculation you refer to.
>
> There are many kinds of real antennas that radiate towards the ends.
> These are called end-fire antennas, the most familiar probably to most
> people is the Yagi-Uda. There is nothing that prevents a metallic
> structure to radiate into any particular direction if the current
> distribution over it is properly designed.

The Yagi-Uda (thank you for giving it the correct name) is an ARRAY of
dipoles aligned parallel to each other. The driven element is located
between an array of dipoles descriptively called "directors" and
"reflectors." The driven element radiates broadside. The directors and
reflector(s) act to channel the radiation in a quasi-unidirectional fashion
toward the directors. But the radiation overall is broadside to the
individual elements.

Related antennas include multiple driven dipoles that are phased and spaced
for either unidirectional or bi-directional broadside radiation. Another
similar device ids the log-periodic that looks somewhat like a Yagi-Uda, but
consists of multiple elements that are driven in a controlled phase manner.
Such antennas can provide excellent gain AND bandwidth. Radiation is
broadside to the elements.


>
>
> [...]
>
>> Generally, balls on the end of an antenna seem most useful in reducing or
>> eliminating corona discharge -- an important issue at higher power
>> levels.
>> They seem to have little or nothing to do with the function of radiating
>> signals.
>>
>
> Balls or other metallic stuff on the ends can act as lumped element,
> usually inductive, loading that reduce the size of the antenna while
> maintaining its resonant frequency.
>

Well... close, but no cigar.

If the balls or other metallic stuff are large enough, then they add the
equivalent of capacitance at the ends. Added capacitance implies reduced
capacitive reactance. And THAT allows an antenna to resonate with lower
values of inductive reactance. Since practical inductors are nuch more lossy
than practical capacitors, and since inductive reactance is directly
proportional to frequency, reducing the value of inductive reactance
dramatically improves the antenna's operating efficiency.

BUT most antennas are located in outdoor environments where wind is likely
to be a factor. Balls exhibit high wind resistance, and are usually eschewed
in favor of flat structures that are usually located for minimum resistance
to expected wind sources. They are often called "Capacitive Hats" because of
their physical shape.

Bill