Radio Repeater - Frequency Separation: Input To Output

Frequency Separation: Input To Output

There is no set rule about spacing of input and output frequencies for all radio repeaters. Any spacing where the designer can get sufficient isolation between receiver and transmitter will work.

The narrowest documented spacing found in preparing this article is a system dismantled in the 1980s. One channel used on the Riverside County, California Sheriff's Department system, which was replaced by an 800 MHz trunked system, used 158.760 MHz as input and 159.090 MHz as output, a spacing of 330 kHz. The station callsign was KMH971. It is unusual to see systems with input and output spaced this closely. It is believed working systems have been spaced as close as 175 kHz. There is currently a narrow spacing in use in Alamogordo, NM used for the city EMS dispatching. Input is 155.715 MHz and output is 155.385 MHz which is also a spacing of 330 kHz.

In some countries, under some radio services, there are agreed-on conventions or separations that are required by the system license. In the case of input and output frequencies in the United States, for example:

• Amateur repeaters in the US 144-148 MHz band usually use a 600 kHz (0.6 MHz) separation.
• Systems in the 450-470 MHz band use a 5 MHz separation with the input on the higher frequency. Example: input is 456.900 MHz; output is 451.900 MHz.
• Amateur repeaters in the US 420-450 MHz band use a 5 MHz separation with the input being either on the higher or lower portion of the 440-450 segment of the band (the standard changes regionally). Example: output is 444.400 MHz; input is 449.400 MHz.
• Systems in the 806-869 MHz band use a 45 MHz separation with the input on the lower frequency. Example: input is 810.1875 MHz; output is 855.1875 MHz.

These are just a few examples. There are many other separations or spacings between input and output frequencies in operational systems.