[Antennas] Vertically Polarized Loops

2002年1月27日 13:11:14 -0500

I don't think this response made it to the list when I first
sent it, so I'm trying again...
At 05:12 PM 1/13/2002 -0500, Doc wrote:
>A loop with vertical polarization is just a horizontal loop fed on
>its side, no magic.
>>Now if your have the loop in the horizontal plane, laying flat, I
>am not certain you can cause it to radiate vertically, unless
>perhaps you treat part of the feedline (traveling vertically at a
>right angle to the loop) as part of the antenna, then you might
>get some vertical polarization.

A small loop can be thought of as a "magnetic dipole," that is,
you can visualize it as a magnetic current flowing perpendicularly
to the plane of the loop. Just as the electric field part of an
electromagnetic wave is coplanar with the electric current that
flows along a conventional half-wave dipole, the magnetic field
part is coplanar with the equivalent magnetic current of the loop.
Thus, when the loop is horizontal, the equivalent magnetic current
is vertical, so the magnetic field is oriented vertically on the
horizon away from the antenna. Since the electric field is always
perpendicular to the magnetic field in an electromagnetic wave,
the radiation from a horizontal loop is horizontally polarized
along the surface of the earth.
If you turn the loop on its side, in effect you are creating a
horizontal "magnetic dipole" antenna. The radiated magnetic
field will have a null off the "ends" of the "magnetic dipole"
(perpendicular to the loop) and will have a maximum in all
directions perpendicular to the dipole (in the plane of the
loop). The pattern of the loop is the same as the classic
doughnut shape of a conventional dipole, with the plane of
the loop having the same orientation as the plane of the
doughnut.
In the directions around the antenna pointing toward the
horizon, there will be minimum radiation off the ends of
the "magnetic dipole" and maximum radiation broadside to
it. The radiated magnetic field will be parallel to the ground,
so the electric field will be perpendicular to the ground.
Thus, a vertically-oriented loop is vertically polarized with a
bidirectional null.
Note that the tiny AM loop antennas that usually come with
new stereo receivers are designed to be used in the vertical
orientation. This is necessary because AM broadcast
stations use vertical polarization. Try turning one of these
little loops by hand and you'll discover their bidirectional
nulls as the station you're listening to drops in and out. Lay
the loop on its side and you'll probably lose the station if it's
local because of polarization mismatch. You should also
notice a significant drop in noise from the receiver with the
loop on its side.
Hope this info is helpful,
73,
Dave
--
Dave Kelley NB4J
State College, PA
Centre County