Motorola CDR500 Programming Manual Download Page 21

Page: 21 / 104

March, 2001

6864110R66-O

1-5

CDR500 & CDR700 Repeater Service Manual

Introduction to Repeaters and Components

Duplexers, Cables, and Antenna Spacing

Series radios, the power handling capability would
have to be increased to 100 Watts.

• Insertion loss: 3 dB maximum

Less insertion loss of the duplexer means the receive
radio will be able to discern weaker signals and the
transmit radio will deliver more power output to the
antenna. The 3 dB specification will result in coverage
range being reduced approximately 30%. Typical
insertion losses quoted in catalog sheets are 1.5 dB.

• Frequency spacing: Band dependent

Frequency spacing” is the frequency difference
between the operating frequencies of the receiver
and the transmitter. Frequency spacing less than
3 MHz can be achieved but the physical size of the
duplexer increases dramatically. If a VHF repeater is
being assembled, be aware of the minimum fre-
quency spacing that a duplexer can provide when
choosing the operating frequencies for the repeater.
The VHF duplexers available from Motorola stock
are specified at 4.5 MHz minimum spacing. In the
United States, the spacing in the 450 MHz to
470 MHz UHF band is 5 MHz and the spacing in the
470 MHz to 512 MHz band is 3 MHz.

If the proper equipment necessary to tune a duplexer is
not available, then the duplexer must be pretuned by
the manufacturer. Be ready to provide the exact
receiver and transmitter frequencies at the time of pur-
chase. Also indicate that the duplexer will be used in a
CDR500 or CDR700 repeater.

• Connector type-N

The type BNC connector may be used on the
receiver and transmitter inputs but must be avoided
for the antenna. The BNC is prone to mechanical
movement which can generate noise when the
transmitter is operating. Type UHF connectors will
suffice for VHF but should be avoided for UHF. The
mini-UHF connector, if available for the duplexer, is
very good. Other connectors, such as the SMA and
the TNC, are very good but may be more expensive,
fragile and rather difficult to assemble in the field.
The best general performance comes from the
type-N.

The cables provided with the CDR500 and CDR700
repeaters mate with a type-N at the duplexer end. Any
other type of connector will require you to assemble
cables.

Cables

The coaxial cables that connect the radios to the
duplexer are fabricated from RG58A/U (the cable that
is used with most of the mobile antenna kits). Since
RG58A/U does not have a perfect shield, the routing of

the cables should allow a physical separation of
approximately 1 inch.

Do not use RG58A/U as the coaxial cable that connects
the antenna connector of the repeater to the antenna.
The rather small size of RG58A/U can introduce exces-
sive losses in the system that will decrease the effective
range of the repeater.

Substitute RG400/U for short lengths and RG214/U or
1/2-inch “hardline” for the longer lengths. If the
“hardline” is used, connect the antenna connector of
the repeater to the end of the “hardline” with a flexible
jumper cable. This will avoid undue stress on the con-
nectors of the cables and the repeater.

In planning a system, make sure that the various con-
nectors found on the duplexer, feedlines, feedline
jumpers and antenna are the correct mating pairs.

Table 1-5 can be used to determine which feedline to
choose for a given frequency band and line length to
maintain 1.5 dB (approximately 30%) or less power
loss. Table 1-6 is a list of Motorola part numbers for the
various connectors and cables.

Antenna Spacing

Isolation between the output from the transmitter and
the input to the receiver may be obtained with physical
distance. Instead of using a duplexer, two antennas may
be spaced apart and connected to the receive radio and

Table 1-5.

Maximum Feedline Length in Feet (Meters)

Band

RG400/U

RG214/U

Hardline

Low (50 MHz)

55 (17)

100 (30)

300 (90)

VHF (150 MHz)

25 (7.5)