Codan Radio Transportable Radio Systems User Guide - Manual (Page 5)

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2.1.1 Simplex Communications
To understand how a repeater actually works, it is best to fi rst start with portable to portable communications. In
this scenario, communications is on a single common frequency from one user’s radio to another user’s radio directly.
The transmitting radio is transmitting its signal on a single frequency, which we will refer to as F1. While it is trans-
mitting on F1, only the transmitter is connected to the antenna, and therefore it cannot receive at the same time.
On the other end of this scenario is the receiving radio, whose receiver is connected to its antenna (the default sce-
nario for any radio), and is always listening to F1 as long as its user does not try to make their own transmission.
This direct method of communication is referred to as Simplex Communication. A single frequency is used to allow
for single direction communications. Users of this system can talk back and forth between each other, but they must
take turns talking. This is how all standard portable and mobile subscriber radios work.
2.1.2 Duplex Communications
Duplex Communications on the other hand allow for simultaneous transmitting and receiving of two different
signals. This scenario requires 2 distinct frequencies, and obviously a radio that is capable of both transmitting and
receiving at the same time. While this is not how portable and mobile radios work, this is the general concept behind
the operation of a repeater.
2.2 How does a Repeater Work?
A repeater receives a signal at a particular location, and at the exact same time re-transmits that signal, at a much
higher strength, to the immediate vicinity. This is very handy for getting a signal to go around or over large objects
such as mountains, or to take a weak signal and boost it for penetration into a building or other structures.
To do this a repeater makes use of duplex communications. A repeater requires a pair of frequencies; a Receive
Frequency (F1 for example) and a separate Transmit Frequency (in this case, F2). When the repeater receives a signal
on F1, it simultaneously transmits the received signal on F2. All of the radio users in the coverage area of the repeater
that are listening to F2 will hear the signal that was originally sent out on F1.
For all users in the fi eld who want to communicate through the repeater, they will transmit on their portable radios
using F1 and receive on F2. While the portable radios in the fi eld are still making use of Simplex Communications, the
repeater is making use of Duplex Communications to provide the added coverage.
2.3 Other Types of Repeaters
2.3.1 Bi-Directional Amplifier (BDA)
A Bi-Directional Amplifi er (BDA) is a common solution for in-building or tunnel communications. A Bi-Directional Am-
plifi er system consists of one or more amplifi ers located inside the confi ned environment and in turn connected to
an external antenna network. The external antenna, usually located on the roof of the building, or mouth of the tun-
nel, receives the signal coming from the external radio site. The BDA then amplifi es the signal and retransmits it into
the building or tunnel. A subscriber unit within the building can use the BDA to extend their portable radio coverage
and communicate with the external system. The BDA listens for incoming traffi c inside the confi ned space, amplifi es
it and retransmits it to the external system, hence bi-directional. A BDA can be relatively inexpensive, however, the
supporting infrastructure of cabling, antennas, fi lters and power supplies quickly add to the total installed cost. Fur-
thermore, unless BDAs are adjusted correctly, they can create interference issues with themselves, with other BDAs,
or with the existing radio system.
Repeater Site
VHF F2 VHF F1
Portable to Portable
Communications
Duplex Communications
Repeater System