Section 3. Operation
3-11
REVERSE FREQUENCY WORKING (OPTION)
Reverse Frequency Working or “RFW” is a method of controlling a repeater from a control
station or fixed location mobile. The purpose is to replicate the service previously only
available using a direct audio connection from the radio operator to repeater site. This
connection would usually be a leased line for EIA tone or DC remote control.
The document from the (UK) Radio Agency RA263 titled “Reverse frequency working and
talkthrough for PMR (standard) licenses” fully details what is available and who can use it.
The following is a description of an implementation of this method of operation using the
Zetron Model 38.
The Model 38 equipped with the RFW option will operate as standard or RFW mode. RFW
mode allows the unit to act as a cross tone encoder and decoder as shown below.
RFW mode
Description
Dispatcher radio setup
Model 38
Mobile radio setup
Dispatcher to mobile call
Tx CTCSS 01
→
Rx 01 Tx 02
→
Rx CTCSS 02
Mobile to dispatcher call
Rx CTCSS 01
←
Tx 01 Rx 02
←
Tx CTCSS 02
In this mode the dispatcher can communicate with the fleet, but the fleet can not hear each
other.
In order for talkthrough to be used, the Model 38 is sent a 4 or 5 digit code in DTMF which
changes the mode of operation. In talkthrough mode, the radio fleet can talk to each other. In
order for the dispatcher to also communicate it must change its CTCSS encode and decode
tone to the same as the fleet. As it would be usual for only the dispatcher to have the ability
to switch talkthrough then this would be done at the same time.
Some radios have the ability to send a DTMF sequence when the channel is changed. This
would then completely automate operation. If the dispatch radio does not have a DTMF
capability, then a Zetron ZMX microphone could be used. This could have two speed dials
programmed to allow switching modes from a single button press.
Talkthrough mode
Description
Dispatcher radio setup
Model 38
Mobile radio setup
Mobile to mobile call
Tx 02 Rx 02
Tx / Rx CTCSS 02
Dispatcher to mobile call
Tx CTCSS 02
→
Rx 02 Tx 02
→
Rx CTCSS 02
Mobile to dispatcher call
Rx CTCSS 02
←
Tx 02 Rx 02
←
Tx CTCSS 02
