RF Electronics Chapter 7: RF Filters Page 244
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
The left filter in figure 7.54 shows a 3-resonator 142 MHz helical filter with a bandwidth
of 6 MHz from Temwell [9]. Each resonator is 8mm wide and 12mm high. Temwell
specialise in helical filters and make commercial filters from 45 MHz to 2.6 GHz. The
right filter in figure 7.54 is an older, 3-resonator, 137 MHz helical filter with a bandwidth
of 2 MHz originally made by Toko in Japan. The resonators are 12mm wide and 18mm
high. Temwell [9] now supply both filters in figure 7.56. The measured dimensions of
these filters are the outside dimensions of the cavities. The ratio of the internal dimensions
thus satisfies equation 7.17.
Interdigital Filters
With interdigital filters, the resonators are grounded at alternate ends. This reduces the
coupling between the resonators and as a result, a compact filter can be constructed that
has no walls between the individual resonators. A drawing of an interdigital filter is shown
in figure 7.57a.
H
L
E C
Tuning Screw
Resonator
Coupling
Loop
Figure 7.57a. Interdigital filter dimensions, top view.
Figure 7.57b. Interdigital filter dimensions, side view.
For a small bandwidth (<10%) M. Dishal [11] provided the following equations for
designing interdigital filters:
6 . 0
h
E
Eqn. 7.25
048 . 0 91 . 0 37 . 1
h
d
h
C
K Log
Eqn. 7.26
RF Electronics: Design and Simulation
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