ACAR HAS STALLED in the left
lane at the traffic light in
downtown St. George. Traffic moving north through the city should
use the right lane in the downtown area and proceed with caution to avoid unnecessary delay.
This is Clarence Jones in Mobile
Unit One, returning you to QUIZ
action central news."
The news flash you just read
came from a live broadcast on
WQIZ, a small 500w daytimer located in St. George, S. C. St.
George is a town of only 1800
• people, but WQIZ radio is known
all over the low country of South
Carolina as "The little station
with a big voice." It is not located
within any large metropolitan
area; the town has only one traffic
'light! Yet, it is the crossroad of
two principal highways—a 4-laner
winds through the town for about
a mile.
Finding a Frequency
Phone the FCC in Washington (202/EX 3-3620) person-to-person. Ask
the operator for: "The person who can tell me what frequency is available
for my city in the 26-mc remote pickup broadcast band." Be sure the
operator places your call exactly that way to avoid excessive phone charges.
Tell the FCC employee that you are applying for a 26-mc RPB operation and want to know what frequencies are not being used in your area—
starting at the 26-mc end of the band and working down. He will ask you
if you are near certain towns, so it will help if you have a map of your
state handy to check distances.
Crystal Information
Third overtone transmit and receive crystals may be ordered from any
reputable crystal manufacturer. The receiver oscillator operates at 455
kc below your specified frequency. When ordering, simply order crystals
for your assigned frequency and specify the make and model of the transceiver. It's a good idea to send along a copy of the schematic, too.
proximately $133. It can be powered from either 110v AC or 12v
DC. Faced with a choice of fixed
frequency and continuously-tuned
models, we chose the latter. Variable tuning is of no value in the
26-mc RPB band, but this model
has two additional IF transformers, providing greater selectivity
and less adjacent channel interference.
Modification of the Units
The only modification of the
base station unit, in addition to
tuning, is the installation of a 3.2-
ohm output jack on the rear of the cabinet. To match the output
to our console, we used a transformer of the proper ratio with
cables of suitable length attached
to each winding. The cable from
the 3.2-ohm winding terminates
in a phone plug to match the jack
installed on the base station unit.
The cable from the other winding
can be terminated in a plug to suit
any console input. Direct tape recordings for delayed use may also
be made if a multi-tap matching
transformer is used.
three changes were made: (1)
modification of a phone jack
phono-plug adapter and mic plug;
(2) addition of a 4-conductor mic
jack; and (3) addition of a carrier defeat switch.
Mic Plug and Phone Jack Adapter
with this adapter we can use a phone plug input on the
mobile unit rather than the PTT mic. We used a
Switchcraft adapter (phone-jack input to phono-plug
output) with the phono-plug soldered to the end of an
Amphenol connector (91-PC4M). In making this piece
the wires in the Switchcraft unit must not be damaged;
and by leaving the ring on the Amphenol plug, the mic
will be well grounded to the chassis jack. A jumper
between pins two and four is necessary so the defeat
switch will operate.
Tuning
In achieving optimum operation
on your RPB frequency, the transmitter and receiver sections will
have to be tuned and adjusted.
The transmitter oscillator coil, which is in series with the crystal,
may require adjustment. (Ours
did not.) It should be tuned for
maximum oscillator grid voltage.
To prevent circuit loading, use an
RF choke in series with the
VTVM probe.
The oscillator output and power
amplifier circuits will require
peaking at your specified frequency. The oscillator plate coil, power
amplifier plate, and antenna coupling trimmer should be tuned for maximum power output (about
3w). An RF wattmeter (the type
normally used for CB units is
sufficient) connected to the antenna output, should be used as
the adjustment indicator. Tune
the oscillator plate coil under modulation. A simple procedure is to
whistle into the mic while tuning
for maximum output.
Remember that all tuning and
adjustment should be made while
operating into a dummy load. A
50-ohm 10w resistor will serve
quite well. The load should be
connected across the antenna coax
connector when the transmitter is
keyed. Frequency and modulation
must be checked by a technician
holding at least a 2nd-Class Radiotelephone license.
The receiving antenna coil and
RF circuit should be tuned to your
specific RPB frequency. The best
way to do this is to feed the output of a suitable frequency generator directly into the receiver,
via the antenna coax connector,
while monitoring the AVC voltage.
Use an unmodulated signal and
adjust the generator for the lowest usable output; too much output will cause overloading.
If an accurate signal generator
is not available, simply peak the
antenna and RF circuits for maximum noise output. Then transmit a test signal from the other
unit at a remote location, or operate it from another room, using a
dummy load to produce a very
weak signal. Retune the receiver
for maximum signal volume.
Nothing else needs to be retuned.
We found that optimum operation
of our units required only these
adjustments.
Increasing Power Output and
Modulation
We were able to increase modulation by using a .25-mfd capacitor
across the series resistor in the
output winding of the modulation
transformer. Power output can
be raised by shorting out this
same resistor. W1 increased ours
to approximately 7w. In our case,
we found modulation and power
increases to be of no real advantage since most of our operation is
within close range of the station.
Our units could not be operated
with both increased modulation
and power, but they operated with
no problem at all on continuous
duty at 3-5w.
The main thing is that our units
work, and they work well. Our
dream of a remote pickup and
mobile news unit has come true
because we put wheels on our
dream . . . mobile news for under
$400!