Amateur Radio Repeater Channel Offsets

To be able to transmit and receive simultaneously the repeater must transmit & receive on different channels

Ham radio repeaters includes:
Repeater basics     Repeater channel offsets     CTCSS repeater access     How to use a repeater    

Repeaters use a scheme called full duplex where they can transmit and receive at the same time.

In order to be able to achieve this, the repeater transmitter signal must not interfere with the receiver so that it is sufficiently sensitive to receive transmissions at the same time it is transmitting.

To achieve this the receiver and transmit channels are offset sufficiently to enable this to occur. The difference between the repeater transmit and receive channels is called the “offset.”

Amateur radio repeater in a rack mounting case
Commercially manufactured amateur radio repeater in a rack mounting case

Repeater offset requirement

Most repeaters operate using the same location for transmit and receive, and normally they use the same antenna.

In order that the transmitter signal does not enter the receiver and overload the input and desensitise it, the signal from the repeater passes into a duplexer.

 Typical amateur radio repeater RF block diagram
Typical amateur radio repeater RF block diagram

The purpose of this is to provide a filter for the receiver section of the repeater that only passes signals on the receive channel and rejects those on the transmit channel of the repeater. Often this consists of a band-pass filter for the receive channel and a notch filter for the transmit channel.

Typical filter response for repeater receiver
Typical filter response that may be required for repeater receiver filter

Also for the transmitter, filtering must ensure that no spurious signals are radiated on the receive channel as this would also impair the sensitivity of the repeater receiver.

The degree of the offset between the transmit and receive signals must be such that sufficient filtering can be achieved, whilst also taking into consideration the aspects of the band to be used: factors like its bandwidth, band-planning and the like. Also, in view of the way filters operate, spacing in terms of an absolute frequency difference can be less at lower frequencies than at higher frequencies.

Standardisation of repeater offsets

It is also important that the receive / transmit offsets for amateur radio repeaters are standardised so that they are easier to use. Accordingly there are a variety of offsets standardised for the different amateur radio bands in different areas of the world.

Some are given below, but it is worth checking any other standards with the local websites and data if they are not included.

Amateur Radio Band Europe and UK Repeater Offset USA Repeater Offset
10 Metres   - 100kHz
6 Metres   -500 kHz
4 Metres No repeaters in UK and other countries where there is an amateur radio allocation No allocation
2 Metres 600 kHz 145 MHz = -600 kHz
146 MHz = + or - 600 kHz
147 MHz = +600 kHz
1.25 Meters No allocation -1.6 MHz
70 Centimetres Europe 7.6 MHz
UK mainly 1.6 MHz although some repeaters now 7.6 MHz
-5.00 MHz*

* positive offsets are sometimes used, esp in N Calif

More Ham Radio Topics:
What is ham radio     Callsigns     Morse code     Voice modes     Digital data modes     QRP operating     Operating awards     Codes & abbreviations     Ham bands overview     Operating via differnet propagation modes     Repeaters     Callsigns     Contact formats     Setting up a shack & buying equipment    
    Return to Ham radio menu . . .