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The VHF TiLcation Special
TSgt Robert H. WHd er, W2ZRX/4
Bo x 23, 693 Radar Sqdn
Dauphin Island AFSTA
Dauphin 15, Alabama 36528
For years I have been associated with the
theory of using a simple whip or wire
antenna and pumping as much power into it
as you can to work as far as needed. This, of
course, is a necessity if high mobility
required for military communications is to
be achieved.
With this background I found that for
amateur operations this usually didn't work
if you want a highly portable, cheap but
efficient vhf antenna system.
The whole idea was to build an antenna
from materials at hand in the average home.
Using this premise a covey of antennas
ranging from ground planes, verticals and
simple beams were constructed out of metal
coat hangers; collinears from aluminum
clothes line and even the TV log periodic
when re-runs were the only thing available to
watch.
The latest, which is discussed here, was
built out of some aluminum foil I scro u nged
from the XYL. (She is WA2YXE and it sure
helps when you start taping the foil to the
walls).
The "slot" antenna in its basic configuration
of a plane surface is not usually used in
either amateur or commercial applications.
Commercially the slo t antenna has been in
use for several years, but, as in the case of
navigational systems such as TVOR, four
cylindrical slo ts are rotated electrically to
obtain an extremely accurate circular radiation
pattern.
Some basic theory of the slot will be
needed if a practical antenna is to be built.
The slot can be considered a length of
shorted open transmission line one-half
wavelength long, so the standard current,
voltage and impedance curves apply as seen
in Fig. I.
Fig. 3.
'--
,-
>-
,
A.< b.
,
Fig. 1.
_'E"l'IELOS
Fig. 2 .
Vertical polarization.
'J \
.> z.~
Basic slot antenna.
The simple slot is nothing more than a %
wavelength by . I 5 wavelength rectangular
hole cut in a sheet of metal. The width of
the slot is small at the higher frequencies,
but with current flowing over the entire
surface of the sheet (both faces) and not
restricted to the 5101 edges, the capture area
of the antenna is extremely large.
The interesting point is that if the slot is
cut vertically to the ground reference (Fig.
2), the antenna will be horizontally
polarized and conversely horizontal slots
(Fig. 3) are vertically polarized due to the
development of the "E" fields of the
antenna.
-
- - - - -
- •
-.
,
• • •
I
I
Horizontal polarization.
RG-8 or RG-58 coaxial cable can be used
to feed the plane surface "slot" antenna
even though the nominal center terminal
impedance of a rcsor- nt 1f2 wavelength slot
in a large metal sheet is in the neighborhood
of 500 ohms. Feeding the slot with the SO
10 73 MAGAZINE