Codan Radio Radio Repeater Systems Training Guide - Manual (Page 33)

RADIO REPEATER SYSTEM | TRAINING GUIDE
Chapter 3: Repeater System Equipment Page 25
POWER SUPPLIES
a) AC Power
The low powered repeaters discussed are basically 12 volt units. Converters are available from various
manufacturers or from the repeater supplier. Depending on the reliability of AC power to your proposed
mountain-top site, use of the above is one alternative. Another suggestion when AC is available, is to
run the repeater equipment from its usual battery supply (to be discussed in the next sections) with an
AC operated battery charger attached. If power fails, your repeater will continue to operate up to the
number of hours to which the battery supply is designed; and on resumption of power, the batteries will
be recharged.
b) Transmit Duty Cycle
Before discussing batteries, let’s fi rst think about how much our repeater system will be used. Many
radio systems are inactive more than active; in fact, a usage or transmit duty cycle of 3 or 4% is quite
often the case. There are 8,760 hours or 525,600 minutes in a year. If your business is conducted
during daylight or normal working hours, you would use your system possibly for 8 or 9 of each 24 hours.
During that time you may make only a dozen or so calls. Most radio calls will be short, a maximum of
three minutes. Although your repeater receivers will be operating all of the time, the repeaters will only
be transmitting, in the above case, for 36 minutes per day, or 13,140 minutes per year. This works out
to a duty cycle of 2 1/2%. Even a very busy system with 20 one-minute calls per hour for 16 hours totals
only 22%. In 90% of cases an 8% transmit duty cycle will be more than adequate. If your system is
likely to be used more at one time of the year than another, you may wish to increase this slightly.
c) Batteries
A reliable rechargeable battery is the nickel-cadmium (Ni-Cad) cell. Ni-Cad cells (used to make up the
battery of your voltage requirement) have low internal resistance, and are capable of high peak loads
over the temperature range -40 °C to +50 °C (-40 °F to +122 °F). They have excellent performance
when cycled, are self regulating, and are undamaged by short circuits and polarity reversals. The
gasses that are produced by Ni-Cads are neither toxic nor corrosive, and they have an exceptionally
long life. Ni-Cads come in 1.5 Volt cells and are approximately 69 mm X 134 mm X 394 mm (2.75” X
5.25” X 15.5”) and weigh 6.8 kg (15 lbs.).