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Asymmetrically Feeding
Long - Wire
Antennas
John J. Schultz. W2EEYIl
40 Rossie Street
Mystic. Connecticut 06355
Cha nging the location of the feed- point on
long-wire antennas can make major changes
in t he radiation pattern - changes that can be
US l'L! advantageously when the antenna placemc
nt must remain fix ed.
Illustrative patterns and methods to match
and determine the feed point impedance are
presented.
Many amateurs have room enough to erect
an antenna that runs only in a specific direction.
This situation may be due to points
hcing available to support the antenna in only
specific locations. or an antenna may have to
be run in a specific direction because of obstacles,
safe t y requirements, etc. When a
wire antenna (doublet feed with a coaxial or
a resonant line) is used on the lower frequency
bands, the height in wavelengths is normally
not very great, and the antenna radiation
pattern is very broad-such that stations can
almost be worked equally well whether situated
"broadside" or "off the ends" of the
antenna. On higher frequency bands, however,
due to the increased electrical height
and length of the antenna, the radiation pattern
becomes quite sharp, both in the horizontal
and vertical planes. On bands such as
20 meters and lower, one may have room
enough to run an antenna that is several wave-­
lengths long. But, if the line of the antenna
must lie in a direction that coincides with
the direction to a desired area, the signal
radiated to that area will be many db below
what it would be if a simple Y, A dipole could
be erected at right angles to the long antenna.
Assuming that one can only run a wire
ante nna in a fixed direction, one has to find
so me means of changing the radiation pattern
to favor a desired area other than that of
ph ysically reorienting the antenna. One
met hod that ca n be used is to asymmetrically
feed the antenna. There is so me change in
radiation pattern when a wire antenna is eit
her center o r end fed . but the change is not
extremely grea t (when end-fed. the radiation
tends to be emphasized towards the unfed
end). However, asymmetrically feeding a
long antenna can produce a variety of tailored
radiation patterns. One can't completely
rotate the radiation pattern to any desired
direction, but it is possible to at least develop
useful radiation in directions that aren't covered
by a symmetrically fed antenna or to
produce a reduction in the response of the
antenna towards a direction from which interference
originates.
Effect of Asymmetrical Feed
The horizontal radiation pattern of a horizontally
placed wire antenna is determined
by the current/phase relationships in various
sections of the antenna. When the antenna is
symmetrically fed , a symmetrical horizontal
radiation pattern results, such as is shown in
Fig. I. The cloverleaf-type pattern shown in
Fig. 1 results whenever the total antenna
length is more than about 3/4 A ; ot herwise,
the main radiation is broadside to the line of
the antenna. As the antenna is made longer
in terms of wavelength, the lobes of the
cloverleaf pattern become sharper and have a
peak intensity at an angle closer to the line of
the antenna. Sharp secondary responses also
appear, some of which can have the radiated
intensity of a dipole at its maximum orientation.
If one had a 3 A long antenna which produced
the horizontal radiation pattern shown
by the solid line in Fig. I and found that this
pattern produced poor results in certain directions,
Fig. 2 sho ws some of the solutions
I I ... I
1I
( ~ • I
Fig. 1 . The horizontal pattern of a 1Along
symmetrically fed antenna (dotted line!'l) and
a 3 A long symmetrical antenna.
62
73 MAGAZINE