Mobile antennas present loading situations which must be
RELAY
CONTROL
carefully handled for each band. Because whip antennas
TO
must be kept short for mobile use, they represent only a
fraction of a wavelength on the lower frequency bands.
Thus, their radiation resistance is extremely low and their
reactance is capacitive. Therefore, loading coils must
be used
and the losses kept low to insure a minimum
loa of radiated
pwver in the form of heat in the loa resistances.
A
god quality antenna will have low resistance losses, and
with a
hi*
"(1"
loading coil, its bandwidth on 75 meters
could
be
less than the IF bandwidth of many receivers used
for AM reception. A typical loading coil with a
"0"
of 300
would have a bandwidth of 13 kHz to the half-power points
at
3.9
MHz.
Because of this sharp tuning, deviation from the center
frequency of the antenna will quickly introduce enough
reactance to present an
impossible loading situation to the
transmitter. The antenna should be carefully adjusted for a
Figure
2-7
low
SWR
before placing the transmitter in operation.
The following is a list of antenna considerations for each
band of the Transceiver.
LUGS
11, 1.
BATTERY
DC
RELAY
/
WITH
25-30
AMP CONTACT
RATING
ISTARTER RELAY1
112
A FUSE
LUGS
5&6
IN 58-104
PWR SOCKET
(ON-OFF
SWITCH1
Figure
2-8
If
you are not using the Heath mobile mount, refer to Figure
2-7
for a typical mobile installation, and to Figure 2-8 for a
wiring diagram.
MOBILE
ANTENNAS
Mount the antenna according to the manufacturer's
instructions. Be sure to make a good ground
conneftion
between the shield of the coaxial cable and the car body at
the antenna base.
3.5-4
MHz
80
VA
This band presents the greatest problem. The normal tuning
range of a good antenna on this band is about 10 kHz on
each side of the antenna's
resonanr frequency.
Actual
measured resistance at me bas of an antenna at
these frequencies is 15 to 20 ohms; this represents an
SWR
of nearly 3 to 1. In order to get proper matching to the 50
ohm line, a 1000
pF mica capacitor should be connected
between the inner conductor and shield of the coaxial line at
the base of the antenna. Some antennas may require a
different value, somewhere between 300 and 1500
pF.
The antenna tuning must be checked after the capacitor is
installed. This capacitor is part of an
L
network that is used
to get a 50 ohm match. The inductive portion of this
network is formed by
a
portion of the loading coil.
7-7.3
MHz
&
LA..
This band ordinarily does not need a correcting network,
and has a useful bandwidth of about 50 kHz.
14
MHz
a
2
.s;
:;
No network
needed.
Bandwidth is approximately 100 kHz.
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