RF Electronics Chapter 7: RF Filters Page 270
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
So that the maximum Q is obtained when B/A = 3.6.
Typical Q values are in the range of 2000 to 10000. Because cavity filters have such a
high Q, they are used in many applications requiring a low insertion loss. Air filled coaxial
filters have a high power handling capability are typically used for filters and combiners
for radio and TV transmitters and in mobile radio and mobile phone base stations.
Figure 7.97 shows a typical coaxial filter used in a mobile radio talk-through repeater
operating at 80 MHz. The total height of the filter is 1.2 m and the cavities are 200 mm
in diameter. The cavities used in this filter have a Q of 5000. The round cylinders at the
top of the cavities are isolators, which ensure that the input impedance to the filter is
perfectly matched over the entire bandwidth of the transmitter.
Some coaxial cavities are much smaller. Older mobile (cell) phones contain diplexers
with ceramic filled coaxial resonators. Figure 7.98 shows a surface mount, ceramic 3-
resonator coaxial filter for such applications. Using a high dielectric constant material for
the cavity gives a significant reduction in filter size. The centre frequency of this filter is
881 MHz. The filter is 14 mm long.
Figure 7.98. Coaxial cavity filter (UM).
Figure 7.99. Cavity filter diplexer for mobile radio applications [17].
RF Electronics: Design and Simulation
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