Seven-Tens-Wavelength-Ground-Plane-for-Two-Meters.pdf

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seven-tenths-
wavelength
ground
plane
for two meters
The
subject
of
gain
antennas will often
produce a varied response when taken un-
der discussion by amateurs. This
is
es-
pecially true in vhf mobile applications
where the system uses a grounded vertical
as the radiating element.
The purpose of this article is to point out
some of the basic characteristics of the
ground plane, and
to
show how th.ese
properties were used to adapt an existing
antenna to 147 MHz.
lines.
A
three-quarter-wavelength radiator
is an ideal match for 50-ohm transmission
line, but low-angle lobes are secondary.
With more energy being directed toward
the
higher angles,
direct-wave
field
strength, at zero elevation angle,
i s
less
than that of the quarter-wave antenna.
Most vhf amateurs recognize the five-
eighths-wavelength antenna as the ulti-
mate in a single-element radiator, but
many don't know the true gain of the de-
vice. This is sometimes brought about by
misleading literature that uses the dipole
or isotrope as the reference source. The top
curve of
fig.
2
is convenient for determining
true zero elevation angle gain under ideal
conditions (perfectly conducting ground).
characteristics
Commercial mobile antennas designed
for the frequency spectrum of 140-170 MHz
have certain structural characteristics that
distinguish them from their lower-band
counterparts. At these higher frequencies,
the heavy-duty spring mount base
is
not
practical. Gain antennas within this range
are generally limited to half-wave or five-
eighths-wave radiating elements that are
base loaded to extend the electrical length
to three-quarter wavelength. The matching
network is sealed within a thin cylinder
that forms the mounting base for the en-
tire antenna.
The electrical properties of a grounded
vertical are shown in
fig.
1
and
2.
The
common quarter wave is widely accepted
because the feed point is not reactive and
presents an impedance of suitable value.
The lobe pattern of a half-wave antenna
provides some gain, but the element end
presents a current null that can't be
matched directly to common transmission
construction
These facts presented an interesting
challenge. I wanted to evaluate some
Motorola equipment i n the fm portion of
the two-meter band. However, the only
permanent antenna system on the car was
a quarter-wave six-meter whip, using a
232-Series Master Mobile tapered spring
base.
I
couldn't see any way to utilize this
system as a gain antenna in any conven-
tional manner.
The only practical place to locate a
matching network was at the coax termi-
nal point inside the trunk of the car. As
stated before, the purpose of the loading
network is to extend the electrical length of
the system to three-quarter wavelength.
This means that the junction point of the
40
march
1969
- '
ANTENNA
1
.+b.
A/4
(a)
CURRENT DISTRIBUTION
( b )
VERTICAL PLANE RADIATION PATTERNS
fig.
1.
Chanctwistics
of
proundod vertical antennas.
loading coil and the radiating element
will exhibit a high impedance with a half-
wave radiator, and the impedance will de-
crease as the radiator length
i s
extended.
Seven-tenths wavelength appeared to be
a good compromise. The zero elevation
march 1969
41
angle gain was
ter-wave whip,
junction point
over-all length
base)
i s
52-112
better than that of a quar-
and the impedance at the
approached minimum. The
of the antenna (including
inches for 147 MHz. The
provides the shunt impedance required to
cancel approximately
72.5
ohms of capac-
itive reactance.
Part of the capacitive compohent of this
reactance i s due to the mechanical struc-
ture of the Master mount and its relation-
ship to the automobile body, when proper-
ly installed. This small amount of
X,
can
be overlooked at the lower frequencies but
must be compensated for at vhf. Some addi-
tlonal strays are introduced with the in-
stallation of the matching network.
For a period of time, the development of
the antenna overshadowed the original
task of equipment evaluation. Practical
fig.
3.
Mechanical and electrical
dc
tails of the antenna.
L1
is 4 turns no.
14
bus wire,
3/4"
long, 1/2" diametar.
tapped
2-314
turns from ground w d
(total inductance
=
0.16
pH).
UHF
CONNECTOR
1
"
fig. 2. Radiation resistance and field intan-
sity as functions of antenna height.
gain slope i n this area
i s
very steep
(fig.
21,
and a difference of one or two inches
will alter the characteristics considerably.
Fig.
3 shows the electrical and mechan-
ical characteristics of the matching net-
work. The device is built on a three-inch
length of aluminum angle, with holes
spaced to accommodate two of the mount-
ing screws of the existing base. The coil is
wound around a %-inch form using num-
ber-14 bus wire.
A
teflon feedthrough, with
a 6-32 screw, serves as the junction point
for the coil tap and the antenna base
coupling wire. This wire should be cut so
the terminal lug center-to-center dimension
is 1-314 inches. Typical solder lugs provide
sturdy mechanical support of the coil at
the tap point and ground end.
Electrically,
L,
supplies the requirement of
two coils. The
1%
turn between the uhf
connector and the antenna is the series
loading inductance. The remaining portion
problems had to be discovered and cor-
rected by trial and error.
A
seven-tenths-
wavelength antenna
is
not conventional,
but i t does have useful applications. The
purpose in pursuing the project was to
prove that it could be done.
references
F.
E. Terman, "Electronic and Radio Englneering,"
McGraw-Hill, New York,
1955, p.
887.
I.
D.
Kraus,
"Antennas," McGraw-Hill, New
York,
1950,
pp.
315, 316.
.
ham
radio
42
march 1969

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