[Elecraft] Fw: antennas for Susan

[Guy Olinger K2AV]

An antenna is usually called by its ELECTRICAL properties, and if
tophats, loading, etc present current magnitudes and phases associated
with a halfwave, it will usually be called a halfwave.  Some will
refer to a halfwave with a lot of loading as a loaded or shortened
halfwave.

The pattern, however, as well as the feed impedances and efficiency
will vary as an antenna is physically shortened but loaded by some to
maintain electrical length.  This is where caveat emptor should apply.
 There is a good deal of snake-oil dBi specification out there for
shortened antennas.  When radiating length is shortened, radiation
resistance is lowered, and higher current and voltage must be handled
properly or performance goes in the tank.  So there is a science in
making shortened antennas work properly.  Conductor joints, conducting
surfaces and dielectric materials used for insulation, support, etc,
have to be engineered for the application.  This last is where you
will find all the lossy shortcuts.

Lastly, shortened antennas will experience narrower bandwidth than
their full size versions.  Some snake oil antennas, with extra loss
just about everywhere, will display decent bandwidth, but only because
it is loaded down by all the extra loss.  This bandwidth is sometimes
advertised as an "advantage".  They are depending on hams' persistent
fascination with SWR as an indication of antenna health.  And they
make money from it, as convenience and simplicity of "explanation" win
out over physics and the headache of understanding the more difficult
set of rules actually in play.

Force 12, happily, is one place where the issues of shortening, and an
honest specification of gain, can be had for predictable results that
you do not have to "un-fudge" to make a reasoned decision.  However
you may like or dislike their particular physical construction
philosophy and "cell" multiband designs, at least you have a specific,
reproduceable and publicized method for annotating gain.

73, Guy

On Mon, Aug 2, 2010 at 11:47 AM, Steve Ellington <n4lq at carolina.rr.com> wrote:
>
>
>
> I know it's off topic but I have a question.
>
>  How is an antenna that is shorter than 1/2 wavelength called a half
>  wavelength antenna? Does adding loading coils really turn it back into a
> 1/2
>  wavelength antenna?
>
>  Seems to me that the current distribution and resulting radiation pattern
>  would change if it's short regardless of loading. I keep seeing these
>  antennas show up on the market and just want to be sure I'm right.
>
>  Steve
> N4LQ
>
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