T EVIS i - AmericanRadioHistory.Com

COAXIAL CAVITY TUNING ELEMENT
Fig. 1. A view of two completely
assembled coaxial cavity elements
for use at ultra high frequencies.
This element, which may be used in TV tuners and
test instruments, covers the 470 -890 mc. TV band.
THE announced intention by the FCC
of opening up the u.h.f. band of frequencies
to television broadcasting
prior to the Korean War was the goahead
signal for one of the most intensely
competitive developments in
electronic history -the development of
a commercially feasible front end or
tuner to cover this new u.h.f. band.
Every interested company of any consequence
was relatively well staffed
with competent engineering talent; each
was armed with microwave techniques
gleaned from war development radar
contracts. This experience, coupled with
prior radio and v.h.f. television experience,
appeared to be quite adequate to
cope with the practical problems posed
by the new u.h.f. band. But just as FM
and v.h.f. television forced the radio engineer
to re- examine all of his experiences
based on lumped parameters, so
did the new u.h.f. band force television
and microwave engineers to acclimate
themselves to a new band of frequencies
that was too high for lumped constants,
too low for true cavities, and too wide
for transmission lines because of their
clumsiness.
Basically, the problem was one of
cost. U.h.f. techniques had already been
developed so that the "know -how" for
doing the job was in the public domain.
This "know -how" included awareness of
the solutions that were possible through
the use of sliding contacts on lines,
plunger type variable frequency cavities,
and many variations of the same
technique. These solutions proved to be
quite satisfactory for the elaborate military
equipment on which they were
used, but for home television receivers,
components requiring costly machining
operations and precision moving contacts
and the like become prohibitively
expensive. Thus, the problem was as
much mechanical as it was electrical.
Television manufacturers were further
confronted with a dearth of suitable
test equipment to cover this new
band. Not one completely satisfactory
u.h.f. sweeper was available. Existing
grid -dip meters and wavemeters stopped
at 400 mc., the end of the lumped constant
range, and signal generators were
available only in limited numbers and
were extremely costly. Nine out of ten
television manufacturers are even today
without adequate u.h.f. equipment, and,
of course, virtually no television technician
has any u.h.f. equipment.
Coaxial Cavity Development
Development of new equipment to
meet the industry's requirements represented
a major undertaking because a
considerable part of the development
lay in devising a suitable resonant
tuned circuit capable of being tuned
over the 470 -890 mc. range of frequencies.
Sliding contacts on transmission
lines were tried as the first approach to
the solution of this problem as far back
as 1947; this approach has resulted in
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LUMPED CI9CUIT
By
the Engineering Dept.
Granco Products Inc.
tuners that are now commercially available
in limited quantities. Tuning elements
have also been developed that
use lumped constants as the variable
elements. For some time now the members
of the engineering staff of Granco
Products Inc. have been involved in this
development and have thoroughly investigated
a large variety of circuit arrangements
to accomplish satisfactory
tuning over the frequency range. The
circuit arrangement chosen as the optimum
design is one that can be thought
of either as a lumped circuit, or as a
cavity. The tuning element is shown in
Fig. 1. It is evolved by starting with a
simple lumped parallel -tuned circuit, as
shown in Fig. 2, and then reducing the
inductance by adding more inductive
elements until they completely enclose
the capacitor. The inductance consists
of a metal cylinder, and a coaxial conducting
sleeve broken at the center acts
as the variable capacitor of the parallel -
tuned circuit. Inside these two sleeves,
separating and supporting them, is a
precision low -loss dielectric tube.
metallic plunger rides in the dielectric
tube and acts as the tuning member of
the tuned circuit.
As the plunger rides inside the smooth
precision -bored dielectric, the capacitance
is varied from C.,. to C.,. + C/2
where C.,. and C/2 are self- defined in
Fig. 4. Effective ratios of C.,. + C/2
to Curl,, better than 5 to 1 are possible,
including trimmer capacitance when the
cavity is used as a passive preselector
tuned circuit, or tube capacitance when
Fig. 2. Various steps in the evolution of the tuning element.
1
T
Ist (C) (D)
ACTUAL CIRCUIT CIRCUIT WITH MUL- INFINITE NUMBER OF PARALLEL
WITH ONE INOUC- TIPLE PARALLEL INDUCTANCE ELEMENTS OR CAV-
TANCE ELEMENT INDUCTANCE EL- try WITH CAPACITY LOADING
EMENTS
A
APRIL, 1953
RADIO- ELECTRONIC ENGINEERING 15