[Antennas] small high gain 80m antennas

[David Kelley]

Hi, Mark

Unfortunately, there is no way to build an antenna (for any frequency)
that both is electrically small and has high gain.  The main reason is
that gain comes from being able to concentrate the radiated energy
into a narrow range of angles.  Yagis have higher gain than dipoles
because they force most of the radiated energy into a main lobe.
Many people on this reflector in the past have used the analogy of a
round balloon.  You can cause the balloon to bulge in one direction
but only at the expense of shrinking it in another direction.

Another inescapable law of physics is that an antenna has to be
electrically large in order to concentrate radiation.  The larger the
antenna, the more the radiation can be concentrated.  It's kind of
difficult to visualize why this is unless you've studied antenna theory
for a while, but it boils down to the fact that radiation is concentrated
by taking advantage of constructive and destructive wave interference.
That is, waves that travel in the same direction and that are in phase
CONstructively interfere (add together), but waves that are out of
phase DEstructively interfere (cancel out).  Waves that are neither
totally in phase nor out of phase partially interfere.

In the case of a large antenna, the radiation that comes from the
different parts of the antenna interfere with each other (constructively,
destructively, or somewhere in between).  However, a significant
fraction of a wavelength between wave sources is necessary to
achieve constructive interference in some directions and destructive
interference in other directions.  Consequently, in order to achieve
high gain, you need a large antenna.  Imagine using your two hands
to create water waves in a pool.  If your hands are close together,
the waves rippling away from your hands will either mostly cancel
or mostly add in all directions.  However, if you separate your
hands so that they are several feet apart, you can cause wave
addition in some directions and wave cancellation in others.

Interestingly, just about all antennas that are roughly a 1/4 to 1/2
wavelength in size or smaller have essentially the same gain, if
conductor and ground losses are neglected.  This is because
almost all small antennas have a toroidal (or half-toroidal, in the
case of a monopole) radiation pattern.  Ground reflection effects
usually cause some lobing of the toroidal pattern, but they don't
affect the gain much, if gain is defined as the peak of the main
lobe.  When losses are accounted for, some small antennas are
found to perform better (i.e., have higher gain) than others, even
though their radiation patterns have essentially the same shape.
The differences in gain are due to the differences in losses
experienced by the various designs.  That's why short monopole
antennas are often designed with top hats and large loading
coils.  These devices have a negligible effect on the radiation
pattern, but a profound effect on the efficiency.

73,
Dave NB4J


>Hi
>     Are there any small, discreet, high gain 80 meter antennas?
>                     God bless,
>                     Mark WZ0K
>God loves you!