Full resolution (JPEG) - On this page / på denna sida - 1958, H. 7 - Microwave Load Isolators and Related Components, by Per Erik Ljung
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cepts with the lines of different saturation
magnetization at different values of resonance frequency. The
resultant attenuation properties of an isolator
employing this principle are obtained by
superposition of the different resonance curves. The average
reverse to forward loss ratio will be lower than that
of a narrow-band isolator, yet this might well be
tolerated in cases where large bandwidth is at a
premium.
Fairly recently, attempts to construct absorption
isolators in TEM-mode transmission structures
(coaxial and strip lines) have met with success. The
problem of generating circularly polarized H-fields
in these lines has been solved in various ways. One
method employing a partially dielectric-filled
coaxial line is illustrated in fig. 9, which also shows the
arrangement of the ferrite elements (thin rods) and
the magnetic field.
Components of this type can be constructed for
comparatively low microwave frequencies and
considerable bandwidths, e.g. 2—4 kMc/s. Matching can
Fig. 8.
Gap flux density
required for
resonance as a
function of
frequency for
different values of
ferrite saturation
induction.
Fig. 9.
Cross section of
a coaxial line
resonance isolator.
Fig. 10.
Cross section of
a single-slab [-field-displace-ment-]
{+field-displace-
ment+} isolator.
Fig. 11.
Phase-shift circulator.
be somewhat critical and the power handling
capacity is about the same as for Faraday rotation
devices or a few watts, increased by a factor of 10 if
special cooling is provided.
Field-displacement Isolators
This type of isolator utilizes the difference in
effective ferrite permeability as seen by two waves
travelling in opposite directions in a rectangular
waveguide. To achieve non-reciprocity, it is necessary to
use one asymmetrically located ferrite slab,
magnetized parallel to the rf E-field, or two symmetrically
placed slabs, magnetized in opposite directions. The
geometry used in the single-slab case is shown in
fig. 10. The right-hand side of the ferrite slab is •
partly covered by a resistive sheet.
With proper design a wave travelling in one
direction has a very high electric field component in the
plane of the resistive sheet, while the other has
practically none. Consequently, waves are strongly
attenuated in the former case, whereas in the latter
the effect of the resistive sheet is hardly noticeable.
The normal field distribution over the waveguide
section is in both cases displaced, although in
different ways.
Very good results have been reported with
isolators of this kind. One of its main features seems to
be that reverse and forward loss as well as VSWR
vary quite slowly with frequency within the useful
bandwidth of usually about 10 %.
The absolute values of loss depend on the length
of the isolator and loss ratios well in excess of 100
can be obtained. Good matching can be secured over
the useful band.
Power ratings of field-displacement isolators are
necessarily fairly low. A few watts can be regarded
as a practical limit, since the ferrite is not in direct
contact with the waveguide walls and the resistive
coating has a limited power handling capacity.
Nonlinear effects may influence the performance
considerably.
From a constructional point of view the
field-displacement devices offer the following advantages:
They can be constructed in standard rectangular
waveguide and the ferrite elements have a simple
shape. The static magnetic field required is less than
that of corresponding absorption isolators.
Furthermore once a unit has been constructed for one
frequency band a scaling process can be applied which
enables one to design devices with similar data for
other frequencies.
It appears, though, that a number of parameters
are very critical. Dielectric constant of the ferrite,
shape and resistivity of the lossy material and all
the linear dimensions of the arrangement seem to
have pronounced influence upon the performance
of the isolator. The interconnections between these
parameters are not easily understood, which is
probably the reason why good results are not always
obtained in experimental constructions.
Isolators Employing Non-reciprocal Phase-shift
The Faraday rotation isolator is an example of how
a circulator is used to provide non-reciprocal
attenuation. It is quite clear, that any type of circulator can
perform this function. A very common type of cir-
ELTEKN I K 1958 ]()7
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