Duplexer and Repeaters: Basic Information May 21, 2015 09:33
Duplexers and Repeaters Some Basic Information
BY ROBERT A. LEHNING*, WA2YSJ
All across the country there are many amateur repeate stations operating on 2 meters, 220 and 440 MHz, and to some extent on 1.2 GHz. Several modes of operating such as FM, ATV, and packet (digital) have also become very popular. Many amateur radio operators use these repeaters but do not really understand the basics of duplexers or the role a duplexer plays in repeater operation.
It is common knowledge that if you “hit” a repeater with low‐level signal such as a mobile or handheld radio transmits, the repeater retransmits the information at a higher power level over a greater area. This is commonly referred to as the range of the repeater, the area within which you can activate the repeater with the transmitted signal. Antenna patterns can be adjusted so that a repeater range can cover a certain area or direction only, but a majority of repeater ranges are intended to be omnidirectional (see fig. 1). The repeater does this receiving on one frequency and retransmitting on another frequency. This occurs simultaneously and is called duplex operation. The frequency separation between the TX (transmit) and the RX (receive) is sometimes referred to as the split or repeater pair of frequencies. For example, on 2 meters the split is .6 MHz (600 kHz). The TX can be the low frequency and the RX the high, or vice versa. At 220 MHz the split is 1.6 MHz, and at 440 MHz it is 5.0 MHz. At 1.2 GHz the frequency separation can be 12 MHz or 20 MHz, depending on the area of the country where the repeater will be in operation. Therefore, it follows that if the repeater receives on a high frequency and transmits on a low frequency, the mobile or handheld radio will conversely transmit high and receive low. If you reverse the repeater, then the mobile or handheld radios must also be reversed to match it. Many repeaters both receive and transmit with a single antenna rather than operating with separate TX and RX antennas (see fig. 2). This is where the duplexer is employed. The duplexer enables the repeater to transmit and receive simultaneously on a single antenna and feedline without interference to each function by providing the necessary “isolation” between the TX and the RX frequencies. This isolation is measured in deciBels (dB). If separate TX and RX antennas were to be used, they would have to be physically spaced a certain distance apart, either vertically or horizontally, to provide the necessary isolation.
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