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.”
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.
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.
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
* positive offsets are sometimes used, esp in N Calif
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