UHF Repeaters - How They Operate.

- Stolen from - 

What is meant by Simplex and Duplex?.

Simplex: You transmit and receive on a single channel frequency.

Duplex: You transmit on a repeater input frequency, channels 31 to 38, and receive on a repeater output frequency, channels 1 to 8.

The Anatomy of a Repeater
Put very simply, imagine, if you will, two UHF CB transceivers sitting back to back.  Now we are going to use this arrangement (in theory) to make a channel 1 repeater.  We will make the left hand set a receive only on channel 31 (frequency 477.175 Mhz) and connect it to an antenna.  The right hand one we will make the transmit only on channel 1 (frequency 476.425 Mhz) and connect it to a separate antenna. Making an appropriate interface between the two and coupling up a suitable power supply, we are now ready to take our "repeater" and place it at some geographically high location (Such as Kurrajong Heights). 

Having mounted the antennas on a suitable mast and housed the equipment we are ready to offer the extended range of the repeater to the CB fraternity. Having the height advantage of being on top of Kurrajong Heights, our antennas are able to look at a much larger distance to the horizon (distances of over 100 kilometres is not uncommon) so that it can pick up a transmission from say 50 kilometres to the west and re-transmit that incoming signal instantaneously to another station 80 kilometres to the east (total distance covered 130 kilometres).  You can now see the advantage and value of a UHF CB repeater, when compared to the normal reliable simplex range of UHF of up to 20-25 kilometres.

It goes without saying I think, that if you are going to use the UHF band, you at least understand the function of repeaters and their allocation of set input and output frequencies within the 40 channel UHF band. These frequencies are set aside by the Australian Communications Authority to allocate to repeaters, and as such should be avoided for all simplex transmissions. Below is an extract from the CB Class License which deals with mobile or base stations operating through a repeater:


Conditions relating to CB stations that operate through CB repeater stations

7. A person must not:

(a) except for the purpose of operating through a CB repeater station, operate a CB station on a channel specified in column 2 in item 5 (see note below) within the operational range of a repeater station.

(b) transmit to a CB repeater station on a channel specified in column 2 in item 5 in the Schedule by a number from 1 to 8 (inclusive); or

(c) receive a transmission from a CB repeater station on a channel specified in column 2 in item 5 in the Schedule by a number from 31 to 38 (inclusive).


Note: The channels specified in "column 2 in item 5" are 1 to 8 inclusive, repeater output channels, and 31 to 38, repeater input channels.


How do you work through a repeater?

All modern UHF CB transceivers have the function of being able to select duplex on channels 1 to 8.  What this does is configure the transceiver to transmit on a frequency 30 channels higher (called the repeater offset), every time you press your microphone button to transmit, but still receive on the channel you have selected. Therein lies the clue to the way repeaters operate - receiving your transmissions on the higher frequency, say in the case of a channel 4 repeater, channel 34, and retransmitting your transmission on the lower frequency, channel 4, so that all transceivers tuned to channel 4 in the footprint of the repeater will hear your transmission, as relayed from the repeater.

Now it follows from this that with the large number of Citizens Band UHF repeaters now Australia wide, many millions of dollars have been spent by individuals, organizations, clubs and associations, in setting up this vast network.  Please be aware that if you choose to operate on any of the repeater input or output channels in simplex mode you stand a good chance of upsetting the harmony of repeater operations somewhere. The Class License above states that you shall not operate on repeater frequencies "within operational range of repeater stations". How are you going to know when you are in operational range of a repeater station?. All operators should be aware of repeaters in their local areas, however when travelling, unless some homework has been done before setting out, not many would be aware of what repeaters were "in operational range" of their intended route. The point made here is that you will be doing a great service to the UHF fraternity by avoiding repeater input and output channels for all simplex operations.

To illustrate the point perhaps an example. Two mobile stations enter the footprint of a local repeater (let us say channel 2 repeater) travelling along one of the national highways. They stay in touch through the use of their UHF, pointing out things to each other along the way, and just engaging in general chit chat as they motor along. Fine, there is nothing wrong with that, as that is what their radios are for.  However they have chosen to communicate with each other on the local repeater input channel (channel 32). All their "overs" are being relayed through the local repeater on its output frequency (channel 2) for all to hear that monitor the local repeater. Why?. They certainly do not require the assistance of a repeater to maintain the contact between the two mobiles. Perhaps they have used this channel at home for years, perhaps they are not aware of the implications of using repeater channels for simplex, or is it because they do not care. 

I do not believe its the latter as people in general like to do the right thing. Now it can be argued that if the local repeater is not in use what's the problem?. The problem is that there are 22 other perfectly good channels that are simplex only channels that the two mobiles could use. And after all, does not item 7(a) of the Class License say that repeater channels must not be used for simplex operations. The point I make once again is that we all have to exist together in harmony, and keeping clear of repeater channels will go a long way to contributing to that harmony.

It is unfortunate that under the Class License system, no one has taken on the responsibility of making users aware of its contents. Under the old licensing system a copy of the regulations applicable to the license were part of the documentation issued with each license. But now UHF Transceivers are sold without this information deemed to be necessary. The Instruction Manual for a recently purchased Australian made UHF transceiver contains no mention of a Class License being applicable for that transceiver. Once again I ask why?. I have more to say on this subject in "Changes to UHF- a Personal Opinion".

The downside of repeaters is that they tend to attract the "moronic" element due to their large footprint. Control of this element is in the hands of the organization or individuals behind a particular repeater, and also everyone who uses the UHF CB band. As the Authorities seem little interested to contribute to any policing of the band, it is largely up to the UHF fraternity to conduct themselves in a proper manner on the air and in doing so set a good example to others. There will always be that rebel element who, when behind the anonymity of a microphone cause trouble. Just remember that to respond to these individuals is to reinforce their very existence. Getting responses back can only increase their satisfaction of what they do. So the moral of the story is DO NOT RESPOND. It is the only weapon the responsible operator has of maintaining some degree of control over the quality of the UHF CB band.

So stop the fucken commercial shit on 2...K!  God.