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NOV. 26, 1945.
s, RAMO
RADIO FREQUENCY POWER MEASUREMENT
Filed Jan. l, 1943
MICROWAVE
POWER BRI DGE
RESISTANCE
NEASURING
DEVICE
2,41 1,553
‘2,411,553
Patented Nov. 26, vv1946
mm1
[STAT Es PATENT] ’ oli-"FICE ~
i
2,411,553;
' RADIO-’FREQUENCY POWER MEASUREMENT
to Gen
vvSimon Ramo, Ñiskayuna, N. Y.,`assignor
corporation of New
‘ >eraVEIectric Company, a
York l
v Application January 1, 1943, Serial No. 470,976
z'oianns.i
(o1. r11-ssi
‘
ing, and those vfeatures of -the invention which are
My invention relates to radio frequency appa
ratus and concerns particularly arrangements for
measurement and comparison of power.
'
believed to be novel and patentable will be pointedl
out inthe claims appended hereto.
The single figure of the drawing is a schematic
diagram of one embodiment of my invention.
For the sake of illustration the invention will
be described as used for the determination of the
,
It is an object of my invention to provide ar
rangements for determination of the magnitude
of alternatingcurrent power, especially radio fre
quency power in the microwave region although
power input at a low level to a device such as a
radio receiver II, for example, or an element of
Another object of my invention is to provide ` a radio receiver represented by one of the rec
10
arrangements for determination of the noise level
tangles in the larger rectangle II. The manner
thejpower level to be measured is extremely low.y
in radio apparatus.>
of measuring the noise level in such a receiver
or receiver component will al-so be explained.
For the purpose of making determinations a
source of energy input of the desired frequency
,
Still another object of my invention is to pro
vide arrangements for comparing relatively high
level and low level power.
,
It is also an object of my invention to provide
.is provided. This is represented by the box I2
arrangements Afor measuring low level power with
captioned. A comparator attenuator I3 is pro
w‘ded which is in the form of a pair of identical
measured at higher levels. `
attenuators I4 and I5 arranged to have power
Other and further objects and advantages will
from the RF signal source I2 supplied thereto
,20
become apparent as the description proceeds. '
through suitable connections such as a coaxial
My invention finds its principal use in the ñeld
line I6. The attenuators I4 and I5 are -so con
of microwave measurements, that is, measure
‘structed that the ratio of attenuation may be
ments of radio frequency energy where the wave
very accuratelyA known, although the absolute
length is of the order of centimeters, since in
value of attenuation need not f be determined.
substantially the same accuracy as power may be
this region conventional wattmeters , cannot be
25
employed and previously known power measuring
Likewise it is unnecessary to be able to determine
the value -of the power output of the RF signal
devices adapted to measurements in this range -
of frequencies do not give accurate results in case
of low levels of power which must frequently be
source.>
,_
One of the attenuators; viz., the attenuator I5,
30 has output connections in the form of a coaxial
measured.
"
line I1, for example, to the device II, the power
In carrying‘out my invention in its preferred
input to which is to be determined, or the noise
form in connection with the measurement of the .
level of which'is to be determined.
power input of vmicrowave alternating current
`
A suitable power'measuring device I8 is pro
energy, I provide a radio frequency signal source,
vided which need not be capable of functioning
a pair of adjustable attenuators towhich the 35 or measuring accurately, except at a relativelyv
radio frequency signal source is coupled and Aout
high level of power, in compari-son with that _sup
put connections from the attenuators,._ one of
plied to the device II.,y For thesake of conven
which leads to a device the power characteristics
ience, therefore, I shall refer to the device I8 in
of which are to be measured at low level and the
the description and claims, as a high level power
other of which leads to a power measuring device 40 measuring device. ~ It will be understood that by
which operates on microwaves but >which is `de
this expression I mean a power measuring device
signed primarily for relatively high power meas
urements in order to obtain the desired accuracy.
which mayv operate at a high> level in comparison
with the level of the power or the noise which is
The attenuators are so adjusted that the power
to be measured or determined. The output of
input to the device to be measured or tested is 45 the attenuator I4 is supplied tov the high level
much more greatly attenuated thanfthe power
input to the high level power measuring device.
power measuring device I8 through suitable con
nection-s such as a coaxial line I9. The attenu
ators I4 and I5 are shown as being of the wave
The attenuators are made as nearly as possible
identical in construction and arrangementV so that
guide type in the form of hollow pipes which may
the ratio of attenuation may be read from the 50 be circular or‘ rectangular, but for the sake of
settings of the attenuators althoughthe actual
illustration, will be assumed to be circular. The . .
attenuation may be unknown.
f
attenuators I4 and I5v are referred to as being of
A better understanding of the invention will
the wave guide type because no central conductor
be afforded by the following description consid
ered in connection with the accompanying draw
55
is provided but the dimensionssare such that they
2,411,553
do not actually serve as wave guides but as atten
uator-s instead, the transverse dimensions being
so chosen that the cutoff wave length is less than
the wave length of the energy supplied by the RF
signal source I2. Under such conditions of op
eration the degree of attenuation is very high,
the energy being attenuated to as little as one
millionth of the input energy ifdesired.
Since the attenuators I4 and I5 are identical,
it will be suilicient to describe one of them in
à detail. The attenuator. I5, for example, com
prises a tube or pipe 20 of considerable length in
comparison with its radius and having a radius
supplying a direct current to the resistor 40 and
for measuring the resistance of the resistor 40.
The direct current circuit comprises the device
4I, the conductor a, the resistorv40, the internal
conductors b and c, the stub cylinder d, the cyl
inder e, and the return conductor f. Inasmuch
as the current ilow through the resistor produces
variations in its temperature and resistance, a
measurement of the resistance of a resistor
serves as a measurement of the DCwer input
thereto. The high frequency power input to the
resistor 40 may therefore be determined by com
paring the resistance variations produced by the
such that the cutoif frequency of the pipe 20
considered as a wave guide is less than the wave 15 alternating current input power with the resist~
ance variations produced by direct current input.
length of the source I2. For adjustment of the
Ordinarily a null arrangement, such as a Wheat
attenuation of the attenuator I3 a telescoping
stone bridge or the arrangement described in
internal unit 2I is provided which is adapted to
Patent No. 1,501,663, Hoxie, is employed.
slide axially within the tube 20 and has a iiat
annular end member 22 serving as an electrical 20 sired, the microwave power bridge may be ad
justed for a predetermined power measurement,
termination for the pipe 20. 'I'he coaxial line I'I
deviations from which will be indicated by devi
is extended into the member 2| so as to form a
ation from a null indication in the device 4I.
cylinder 23 and an inner conductor 24‘mounted
The fuse 40 and its holder are
within the slidable member 2I so as to move with
justable in tube 39.
it when the member 2| is slid to the right or the 25
When it is desired to make a measurement of
left for adjustment of attenuation. An attenu
the power input to the device II or of the power
ation scale 25 is provided-and there is a pointer
26 cooperating with the scale 25. The pointer
26 is mechanically connected to the member 2I
so that it moves back and forth therewith and
required to produce a predetermined effect in the
device- II, the apparatus is connected as shown
and the attenuator I4 is so' adjusted that bal
the position of the pointer 26 with respect to the
ance is obtained in the device 4I or a satisfac
scale 25 serves as an indication of the position
of the electrical end point 22 of the attentuator
tory measurement of power is obtained in the
microwave power bridge I8. The power input
to the device I8 is then known. The power input
to the device II is determined by comparing the
attenuations of the attenuators I4 and I5. Such
pipe 20. The end of the internal conductor 24
may be extended to form a probe if desired, but
this is not necessary since the source I2 ordi
narily has suilîcient power so that the power
pickup of the concentric line elements 23 and
24 will be adequate.
For transferring power from the coaxial line 40
I6 to the attenuators I4 and I5, a pair of probes
scales 25 and 35.
If these scales are uniformly graduated, the dif
2'I and 28 may be provided‘which are-electrically
connected to the internal conductor 29 of the
to the decibel difference in attenuation. It will
coaxial line I6. The dimensions of the probes
be understood that the attenuation measured in
2'I and 28 are not critical, but if it is desired `to
n
to ten times the
make use of a signal source I2 of the minimum
logarithm of the ratio between the input power
necessary power output, the probes 21 and 28
may be made one-fourth of a wave length. Fur
thermore, if it is desired to conserve power out
put, a conventional matching stub 30 may be
provided having a movable end member 3I mak
ing electrical contact both with the continuation
ofthe coaxial line internal conductor 29 and the
external conductor 33.
'
‘I'he attenuator I4, as explained, is as far as 55
possible identical with the attenuator I5 and has
corresponding élements including a pointer' 34
cooperating with a ‘scale 35.
The highflevel power measuring arrangement
I8 may take the form of any suitable device for 60
tion may be obtained from the following formula:
measurement of power in the microwave region
such as a calibrated crystal, for example, con-A
nected to a deñecting instrument or may take
OI‘
the form of a bridge device of the type known
as a microwave power bridge. Such microwave 65
8. 686 2
’
power bridges do not constitute my invention.
a =$rN/l - <
54.6
and carefully made end constructions 38 and
39, such that for practical purposes all of the
where
power input received from the coaxial line I9 is 70
transmitted to a resistor 40 at the end 39 of the
element 36. The resistor 40 may take the form
of a Littel fuse, for example. A suitable device,
= Tx/1 _ <
decibels/meter
3
decibels/meter
a is attenuation per unit length
:wave length in meters
>`c=cuto?l wave length
represented by the box 4I, is provided for both 75 ic=1.640a
a=pipe radius in meters
2,411,553
The methods and the apparatus hereinbefore
6
reading of the device I8 and the attenuation
described may be used for making various types
of measurements involving determination of
power. For example, if it is desired to determine
ratios of the attenuators I4 and I5 then equals
the value of the noise level of the converter 43.
It will be understood that for noise level
the noise level of a radio receiver II having a
power output meter 42 or connections to which a
suitable measuring device such as a cathode ray
attenuators I4 and I5 is such that attenuation of
the unit I5 is much greater than that of the unit
oscilloscope may be connected for measuring
power output, the noise level may be determined '
in the following manner. With no input signal,
the reading of the device 42 is taken. An input
measurement, particularly, the adjustment of the
I4, thus permitting the measurement of very low
noise levels although the device I8 may read
satisfactorily only when very much higher levels
of power are applied to it.
I have herein shown and particularly described
signal is then applied, and the attenuator I5 is
certain
embodiments of my invention and certain
adjusted to give ~double the former reading on
methods
of operation embraced therein for the
the power output meter 42, the attenuator I4
being simultaneously adjusted to obtain a satis 15 purpose of explaining its practice and showing
~its application, but it will be obvious to thosel
factory reading on the microwave power bridge
skilled in the art that many modifications and
I8. The power input through the coaxial line I1
variations are possible, and I aim therefore to
is obviously equal to the noise level of the device
cover all such modifications and variations as
Il. This power input is calculated from the
fall within the scope of my invention which are
20
absolute value of power input measured by the
defined in the appended claims.
microwave power bridge I8 and the ratio of at
What I claim as new and desire to secure by
tenuations measured in decibels and determined
Letters Patent of the United States is:
`
by the difference between the readings of the
1. Apparatus for measuring low level power
scales 25 and 35.
which comprises a pair of attenuators of like con
In case the device in which noise level is to be 25
struction such that the ratio of attenuation is de
determined consists of a converter, or a converter
termined by the differences in adjustment al
and radio frequency ampliñer 43, the unit 43
though the absolute attenuations in each may be
will be combined with certain other units in a
unknown, means for simultaneously supplying
conventional manner so* that a power output
power to both of said attenuators, means for con
reading may be obtained in the device 42. In the 30 veying power output from one attenuator to a
case assume'd a local oscillator 44 is provided,
device in which low level power is to be measured,
and other elements such as an intermediate fre
a high level power measuring device, and means
quency pre-amplifier 45, an adjustable gain
for conveying power output from the other at
amplifier 46 and a main intermediate-_frequency
tenuator to said high level power measuring de
amplifier 41 may be interposed between the con 35
vice.
verter 43 and the power output meter 42. In this
2. Microwave low level power measuring ap
case a three-decibel loss switch 48 may be inter
paratus comprising a pair of wave guide type
y posed between the units 46 and 41 or may be in
attenuation pipes with means for supplying
microwave energy thereto, said pipes having such
dimensions that the cutoff wave
reading for a predetermined power output from
length is less than the wave length at which the
the main IF amplifier 41.
power measurement is to be made, said attenua
With the loss switch 48 disconnected and no
tors having movable elements for varying the
input signal source applied to the line I1, the
effective electrical lengths thereof to vary the
45
adjustable gain amplifier 46 is adjusted to obtain
attenuations and for determining the ratio of
_a null reading in the meter 42. Then the signal
attenuation, a relatively high level power measur
source is applied through the coaxial line I1, the
ing device, means for conveying power output
loss switch 48 is thrown to the position in which
from one of said attenuators to a device in which
it introduces a loss of three decibels, that is, a
low level power is to be measured, and means for
power reduction of one-half and the attenuator
conveying power output from the other of said
I5 is adjusted so that the meter 42 again reads
attenuators to a relatively highv level power
zero. The attenuator I4 is simultaneously ad
measuring device.
justed to obtain a satisfactory reading in the
SIMON RAMO.
microwave power bridge I8. The power input
through the coaxial line I1 determined from the 55
corporated in the unit 41. Then the device 42
may be a null device which gives a null or zero 40 transverse
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