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Now get carried away and place many straight wire Ls in parallel with the capacitor
that you form a solid cylinder around the capacitor. You now have a cavity resonator.
All you need is coupling loops and a method to adjust the internal C to tune the
cavity.

Figure 32-10. How a cavity filter works

REPEATER DUPLEXER

A repeater duplexer is shown in Figure 32-11. The purpose of the duplexer is to
prevent the repeater's transmit signal from overloading, desensitising, or blocking
the repeater's receiver. This is called transmitter-receiver isolation and is measured
in decibels. The transmitter has two cavities to allow the transmitter frequency signal
to pass through with minimum attenuation and cause all other signals to be
attenuated. Similarly, the receiver has two cavities that pass the received signal and
block everything else.

This duplexer is fairly simple. How much filtering is required depends on the RF
environment where the repeater is located. If it is a multi-transmitter site, then more
cavity filters may be required and a device called an isolator may need to be installed
to stop other transmitters from getting into the repeater's transmitter and causing
transmitter intermodulation. Likewise, on a multi-transmitter site, there are other
transmitters that may get through the duplexer and cause receiver intermodulation
interference.

There are many ways to tune a duplexer using simple equipment. The best way to
tune cavities in a duplexer is to use a spectrum analyser with a tracking generator.
This produces an easy-to-understand graphical display of the bandpass and
bandstop characteristics of the duplexer.

The duplexer will introduce an insertion loss of about 0.5dB. Increasing the insertion
loss is sometimes done deliberately to achieve sharper bandstop and bandpass
characteristics. It is a bit of a trade-off between insertion loss and duplexer
characteristics. Insertion loss can be chosen with consideration to RF environment
where the repeater is located.
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