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Figure 32-9. A four cavity duplexer
A repeater station is a receiver and transmitter connected to either a single, or two
antennas with the purpose of extending the range of mobile, base or handheld
transceivers. Whatever is received by the receiver is re-transmitted on the
transmitter, relaying the radio operator's message. The repeater is usually placed in
a prime location to give extended coverage and may run at high-power. The problem
is keeping the repeater's transmitter from interfering with its own receiver. The
transmit and receive frequencies may only be 600kHz apart. A combination of
bandpass and notch filters made from cavities can easily achieve the amount of
receiver-transmitter isolation (in decibels). By the way, it is often necessary for the
receiver and transmitter of a repeater to share the one antenna.

Cavity filters are rarely used below 30MHz as below this frequency they are
physically too large. The arrangement of 4 cavity filters shown in Figure 32-9 is
called a duplexer. A duplexer is just a system of cavities consisting of notch and
bandpass filters, to enable a transmitter and receiver on close frequencies to share
a single antenna.

The cavities shown in Figure 32-9 are about 300mm in diameter and about 700mm
long and operate in the VHF band. The most common shape of a cavity filter is that
of a cylinder, as shown. However, any shape can be used, such as squares or
rectangles.

Imagine for a moment a parallel tuned circuit with a conventional L and C, Figure
32-10. Now remove the L and replace with a single wire. That single wire has some
inductance - though not much and you would expect the resonant frequency to be
very high. What if you paralleled more straight wire inductors across the capacitor?
Well, inductances in parallel cause the inductance to decrease so the resonant
frequency increases.
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