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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conducting material mounted on the ferrite rod. A
VSWR of less than 1.1 can be maintained over
considerable bandwidths.
The power handling capacity of the arrangement
in fig. 4 is low, mainly because the resistive vanes
have a limited power rating. A few watts may be
taken as a good approximate limiting value. If,
however, ports 3 and 4 are constructed as waveguide or
coaxial side-arms connected to high-power
terminations the power threshold can be raised by a
factor of about 10. The limit is in this case set by
nonlinear effects and heating in the ferrite element.
The bandwidth is dependent on the variation of the
Faraday rotation with frequency. Within a 10
percent band this variation is usually tolerable. By
proper design, for instance dielectric sleeve- or
ridge-loading of the waveguide very satisfactory
improvements are obtainable.
Typical performance data of a rotation isolator arc
shown in fig. 5.
Resonance Absorption Isolators
The resonance absorption isolator utilizes the
difference in absorption between oppositely rotating
circularly polarized waves at gyromagnetic
resonance. Thus the problem is to find a location in a
Fig. 5.
Reverse and
forward loss of a
typical Faraday
rotation isolator.
Fig. 6.
Typical
resonance absorption
isolator cross
sections (a) low
power (b) high
power.
Fig. 7.
Reverse and
forward loss of a
resonance absorption isolator.
transmission line where a wave propagating in one
direction appears to be circularly polarized in one
sense whereas a wave in the other direction is seen
to be rotating in the opposite sense. As far as the
H-field is considered this is true for points in two
planes at a certain distance from the side walls in
a rectangular waveguide supporting the normal
mode. Ferrite slabs mounted in these locations and
magnetized parallel to the rf E-field vector therefore
exhibit non-reciprocal attenuation of waves
propagating in the guide.
Typical ferrite-loaded waveguide sections are shown
in fig. 6. Dielectric inserts, placed adjacent to and
of about the same size as the ferrite slabs are
sometimes used, mainly to enhance the coupling of the
wave to the ferrite.
Basically the field strength required for resonance
is derived from the formula a = y B0. The effective
internal field is, however, in general dependent on
the shape and magnetization of the ferrite specimen,
and different from the flux density in the magnet
gap in absence of the ferrite. For a certain
configuration similar to that of fig. 6 b, the diagram
in fig. 8 gives the relation between magnet gap flux
density, frequency and saturation induction of the
ferrite.
The reverse and forward loss of this type of
isolator can be chosen at random, as their absolute values
are proportional to the length of the isolator. The
loss ratios, however, are different for the various
designs and can vary from 10 to about 50. Typical
data are shown in fig. 7.
The reflections can be kept low (VSWR about 1.1)
over substantial bandwidths. This is particularly
simple when the geometry of fig. 6 b is adopted.
The somewhat worse loss ratio of isolators of this
type compared to that of Faraday rotation devices
is greatly outweighed by the excellent
heat-dissipating capability of the resonance isolator. Neither is
the construction complicated by any rectangular to
circular waveguide transitions or side-arms that
tend to impair the matching of rotation isolators.
The problem of making suitable permanent magnets
may be cumbersome, but can be solved in most
practical cases.
Peak average power levels may be as high as
kilowatts without forced cooling, as the heat is
conducted away from the ferrite by the waveguide walls.
In applications where the ambient temperature is
high it may be necessary to provide part of the
guide walls with fins or some water cooling device
to prevent excessive heating of the assembly.
The useful bandwidth of ordinary absorption
isolators is somewhat larger than that of rotation
isolators, or 10—15 %. There are, however, methods to
increase the bandwidth to cover the entire
recommended frequency range for the waveguide used. The
methods are based on the idea of making different
parts of the ferrite resonant at different frequencies
within the band. This can be achieved by
successively changing the ferrite geometry along the
isolator or by using ferrite elements made up of a
number of slabs of materials having different saturation
magnetization. Fig. 8 shows clearly the effect of the
latter method. A horizontal line, corresponding to
the constant gap flux density of the magnet inter-
1 106 ELTEKN I K 1958
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