Page 446 - Maxwell House

426 Chapter 8

line serially. The additional analysis shows that the manufacture of such filters is practically
satisfactory for the bandwidths less than about 20%. Wider bandwidth usually requires very
tight-coupled lines, i.e. the extremely narrow gap between coupled lines which is hard to
fabricate [16, 19].
8.4.8 Combline and Interdigital Filter

Both filters belong to the same family with distributed coupling between resonators as it can be
seen in Figure 8.4.9a (combline
filter) and 8.4.9b (interdigital
18
filter) . The various metal
19
(occasionally dielectric) screws
are designated for high-
resolution and temperature-
compensated tuning. The main
idea behind the screws is to insert
the small variable reactance
(capacitive or inductive) thereby
Figure 8.4.9b Interdigital filter slightly adjusting the resonance
frequency of resonators and
sometimes the coupling between them. Fabrication of screws and resonator bodies from invar
alloy, i.e. a material with very low thermal
expansion coefficient, substantially stabilizes the
characteristics of exceptionally narrow banded
filters over an extensive temperature range.

The contemporary tendency is the high precision
manufacturing and following post-production
inspection that eliminates once and for all costly
screws and time-consuming tuning procedure. The
central element of these filters is the quarter-
Figure 8.4.9c Combline and wavelength resonator. To get it one end of the
interdigital filter equivalent circuit metal post is grounded while the small gap
between the probe tip and filter casing is left. We
have considered the behavior of
such single resonance element in
Chapter 7 and illustrated the EM
field structure around it in Figure
7.2.5. Now, we should take into
consideration that all such posts are
not isolated and closely located
(relative to wavelength) to each
other that creates coupling trough
their join fields. As such, the
equivalent circuit of a combline Figure 8.4.9d Folded combline filter

18 Public Domain Image, source http://www.allenavionics.com/NewProducts/Microwave/CMBF.htm
19 Public Domain Image, source: http://www.pe1rki.com/23cmfilters.html