We've seen bigger duplexers.
A duplexer is a specialized filter that allows a repeater to transmit and receive on the same antenna at the same time. Without one, the repeater's transmitter — putting out 25-50 watts — would overload the receiver, which is trying to hear signals measured in millionths of a watt. The duplexer is often the most critical (and most expensive) component in a repeater system.
A GMRS repeater must transmit on one frequency while simultaneously listening on another frequency 5 MHz away. If both the transmitter and receiver are connected to the same antenna without filtering, the transmitter's powerful signal floods the receiver and makes it deaf to incoming signals. The receiver simply can't hear a faint distant signal when 50 watts of RF is blasting right next to it.
A duplexer uses tuned cavities — precision-machined metal cylinders that act as extremely sharp bandpass and band-reject filters. A typical GMRS duplexer has six or more cavities arranged in two groups:
The combined rejection — called isolation — needs to be at least 80-90 dB to keep the transmitter from desensitizing the receiver. Each cavity provides 15-25 dB of rejection, which is why multiple cavities are needed in series.
Why duplexers are expensive: the cavities must be precisely machined and tuned to your exact frequencies. At GMRS frequencies with only a 5 MHz split, achieving adequate isolation is particularly challenging because the transmit and receive frequencies are relatively close together. A quality GMRS duplexer typically costs $300-800.
A duplexer must be tuned to your specific frequency pair using specialized test equipment (a spectrum analyzer or network analyzer). Most duplexers ship untuned or roughly tuned and must be precisely adjusted before use. Getting this wrong means poor performance or a completely non-functional repeater.
If the cost or complexity of a duplexer is prohibitive, there are alternatives:
For the full picture on building a repeater, see Setting Up a Repeater.