Full resolution (JPEG) - On this page / på denna sida - 1958, H. 7 - Microwave Load Isolators and Related Components, by Per Erik Ljung
<< prev. page << föreg. sida << >> nästa sida >> next page >>
Below is the raw OCR text
from the above scanned image.
Do you see an error? Proofread the page now!
Här nedan syns maskintolkade texten från faksimilbilden ovan.
Ser du något fel? Korrekturläs sidan nu!
This page has never been proofread. / Denna sida har aldrig korrekturlästs.
Microwave Load Isolators and Related
Components
Per Erik Ljung, Research Engineer,
Research Institute of National Defence, Stockholm
Introduction
During the last few years a considerable amount
of work has been done in the field of non-reciprocal
microwave ferrite components. In conjunction with
the component development an associated ferrite
material research has resulted in successively
improved performance of these components. The range
of non-reciprocal devices utilizing the microwave
properties of ferrites can be divided in the following
sub-groups.
Circulators
Multi-port (usually four-port) devices in which
energy is passed from one port to the next according
to a particular schedule. Energy entering port 1
leaves at port 2 and so on. The most important use
of this component is probably as a passive duplexer.
The only transmitter power present at the receiver
input is in the idealized case due to reflections
caused by imperfect matching of the antenna. In
practice, this isolation seldom exceeds 30 db.
Isolators
Non-reciprocal four-pole networks having
negligible loss for one direction of propagation and
negligible transmission in the opposite direction. This
type of component has come into very wide use. Its
general property of separating a signal source from
its load presents an elegant solution of many
transmission problems of which perhaps the most serious
one is the so-called "long-line effect".
An isolator placed at the oscillator output
effectively prevents this phenomenon from occuring as
practically no reflections from the load will reach
the oscillator cavity. The output power is left
essentially unchanged which will not be the case if a
reciprocal resistive pad is being used.
Phase Shifters
These components can be either reciprocal or
non-reciprocal depending on the design. In both
modifications the amount of phase shift can be controlled
electrically. The non-reciprocal phase shifter is a
four-pole with different phase constant for the two
possible directions of propagation. The difference
in electrical length is termed the differential phase
shift. The most important case is that where the
differential phase shift is equal to ji radians. Such
a device is called a gyrator, a name that has at times
621.318.1 : 621.37.029.6
been misused for other ferrite components. The
gyrator forms the basic element for one of the most
popular types of circulator.
In the more general form phase shifters find use
in applications such as antenna beam scanning.
Other Ferrite Components
A number of components designed for special
purposes have also been developed. These include
amplitude stabilizers, variable power dividers, waveguide
switches, amplitude modulators, single sideband
modulators, directional couplers, antennas and tunable
resonant cavities.
Another study which has been pursued to a
considerable extent is the use of ferrites for
amplification, mixing and frequency doubling. Both
theoretical and experimental work has been carried out in
order to establish the significant parameters of these
applications.
For the time being, load isolators seem to be the
most frequently used non-reciprocal ferrite
components. Commercially available units are constructed
in a variety of ways taking advantage of different
properties of the ferrite medium. As the various
construction methods offer different advantages and
disadvantages they will be discussed in some detail
in the following section, after a brief review of the
propagation phenomena occuring in gyromagnetic
media.
Load Isolators
Properties of Ferrites at Microwave Frequencies
At microwave frequencies the ferrite medium is
generally characterized by a high resistivity and a
high dielectric constant. These properties can be
controlled to some extent by variations in the
chemical composition and method of preparation. The
magnetic permeability is depending on the
gyro-magnetic properties of the material. Ferrites are
ferrimagnetic, and the magnetic behaviour is
dependent on the existence of unpaired spinning
electrons, posessing a magnetic moment due to the
rotation of its charge and an angular momentum due
to the rotation of its mass. The quotient between
these momenta, y, is equal to the specific charge of
the electron, e/m. In connection with microwave
ferrites it is frequently called "gyromagnetic ratio"
and is expressed in more convenient units e.g.,
ELTEKNIK 1958 1 1 9
<< prev. page << föreg. sida << >> nästa sida >> next page >>
