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Fig. 6. Large electromagnet
for currents up to several amperes.
rite material at u.h.f. These curves are
somewhat similar to those of Fig. 3B
except that lower values of residual
losses were obtained. This is partly due
to the low input VSWR of 1.06 when
the ferrite material reached magnetic
saturation.
Additional data exhibiting ferromagnetic
resonance is shown in Fig. 7A
for the Croloy No. 20 ferrite material.
Resonance may be seen to occur at
higher magnetic field strengths as the
frequency is increased. The highest observed
absorption peak occurs at a frequency
of approximately 3200 mc.
In some cases, rotating the coaxial
14)
3700 Me
3200
3000
2400
1800
1000
KEY
AMPERES
2
line section containing the ferrite material
with respect to the electromagnet
produces a change in attenuation. The
effect is quite small or absent in most
cases except when ferromagnetic resonance
occurs. This phenomenon is
shown in Fig. 7B for a particular attenuator
using a Ferramic B ferrite.
It is to be presumed that the particular
sample of dissipative material used is
anisotropic.
Effects of dimensions, power level,
temperature, and hysteresis on the attenuation
characteristics are summarized
briefly in the following paragraphs.
The attenuation at zero electromagnet
current generally increases linearly
with the length of the insert. The diameter
of the insert and the relative diameter
of the inner and outer conductors
have their principal effect on the input
impedance. The insert chosen for the
u.h.f. magnetic attenuator was of such
a size as to give a fairly good impedance
match and efficient control of attenuation
with most materials used.
At power levels up to 10 watts, some
heating was observed but no noticeable
change in the attenuation characteristics
was observed. At 2000 mc., and
with no external magnetic field applied,
an attenuator using a Ferramic H ferrite
as the dissipative material was
heated to a temperature above 105 °C
35
3700 Mc
AREA REPRESENTS RANGE OF
ATTENUATION OBTAINED BY O-
TATMO ATTEMaTOR ELEMENT
(B)
RRTN RESPECT
AMPERES
TO ELECTROMA4KT
Fig. 7. (A) Attenuation vs. electromagnet current (showing ferromagnetic resonance)
for Croloy No. 20. (B) Effects of rotating the magnetic field near resonance.
Table 1.
Dissipative
Material
Ferramic B
,,
List of ferrite materials, with attenuation at 1000 mc., and manufacturers.
G
Zero Field Attenuation
(db /in.) at 1000 mc.
22
19
Manufacturer
General Ceramic and Steatite Corp.,
Keasbey, N. J.
H 35
Croloy 20 29 H. L. Crowley & Co., Inc.,
" 70 40 West Orange, N. J.
" BX113 22
Lavite F27 35 D. M. Steward Manufacturing Co.,
" F15 34 Chattanooga, Tenn.
" F4 72
XE2826 38 Stackpole Carbon Co.,
St. Marys, Pa.
2
and no noticeable change in its attenuation
characteristics was observed. One
hour was required for a complete heating
cycle.
There was practically no noticeable
hysteresis effect from increasing or decreasing
the external magnetic field.
Application
Electric control of the loss characteristics
of ferrite materials in wave
guides and coaxial lines immediately
suggests some interesting engineering
applications. As previously mentioned,
an electrically controlled variable attenuator
with low minimum loss for
obtaining a smooth output control of
u.h.f. and microwave generators can be
constructed. A control device for amplitude-
modulating or automatically stabilizing
the output of r.f. and microwave
generators may also be assembled.
The magnetic attenuator may be used
to amplitude -modulate a u.h.f. generator.
It is simply inserted into any part
of the transmission line network, and
when an a.c. field is applied to the electromagnet
windings, an amplitude -
modulated wave results. To eliminate
distortion in the modulated wave and to
increase the sensitivity of the attenuator,
a d.c. biasing field must be used.
The significance of the d.c biasing
field can be understood by referring to
Fig. 4. It can be seen that approximately
50 per cent of the curve is linear.
Therefore, to minimize distortion in
the modulated wave, it is necessary to
operate over the linear portion by
proper biasing. Moreover, the sensitivity
of the attenuator to external fields
is greatest in this region, in this instance
being approximately 4 db per
ma. of electromagnet current.
Modulation frequencies from d.c. to
above 10,000 cycles have been used successfully
with the attenuator assembly
shown in Fig. 1. Using a single attenuator
unit, amplitude modulation has
been obtained over the frequency range
of 10 to 10,000 mc.
The magnetic attenuator has also
been used as a control device in a degenerative
feedback network to stabilize
power output of a u.h.f. generator.
Automatically a small amount of
r.f. power taken from the coaxial transmission
line is detected, amplified, compared
against a d.c. reference voltage,
and used to control the output voltage
of a regulated power supply which, in
turn, controls the r.f. power level
through the attenuator.
It can be seen that the magnetic attenuator
offers excellent possibilities as
a transmission switch. When no external
d.c. field is applied, the device
offers its maximum attenuation. When
the ferrite dissipative material in the
wave guide is saturated with a d.c.
(Continued on page 24)
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R A D I O - E L E C T R O N I C E N G I N E E R I N G APRIL, 1953