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Nov. 5, 1946. _
Filed May 1e, 1944
our.> 5, 1946
William E. Bradley, Swarthmore, and Nels `lohn-y
son, Penn Wynne, Pa., assignors, by mesne as- -
signments, to Philco Corporation, Philadel
phia, Pa., a corporation of Pennsylvania
Application May 18, 1944, Serial No. 536,146
8 Claims.
(Cl. 171-95)
This invention relates to instruments for
Fig. 3 is a fragmentary elevation'viewed from
measuring electric power at ultra high frequencies Y
line 3-3 of Fig. 1; and
and is directed more particularly to the provi
sion of a stable and reliable instrument of that
¿_Q of Fig. 2.
character which is especially well adapted to`
measurement of what may conveniently be called
intermediate power values-by which is meant ‘
wattages ranging from say ten milliwatts up to
something like two watts. We do not intend to
imply, however, that our herein-described instru
ment is useful only within the power range above
indicated, but merely that it meets a need within
Fig. 4 is a cross-sectional view taken on line
Before undertaking to describe in detail the
instrument shown in the drawing, it is thought
best to point out some of the major essentialities _
and explain in a general way its mode of opera
F‘irst of all, let it be noted that the dot
identified by reference numeralf'i represents a
Y tion.
hot thermocouple
of constantan,
formed between
and la secondv
that range of Wattages which, so far as we are
fine wire 3, preferably of copper. The objective
aware, is not as well met by any prior art ultra
is to raise the temperature of thermocouple junc
high frequency power measuring device.
15 tion l proportionately to the magnitude of the
Another object is to provide a power measuring
device of the kind referred to which is small and
readily portable.
Still another object is to provide an ultra high
U. H. F. power dissipated by the instrument and
to translate the temperature rise intoa D. C.
“ voltage which is impressed upon the meter d-the
latter being usually a low resistance millivolt
frequency power measuring device which is satis 20 meter designed to respond to voltages of the mag
lfactorily accurate.
nitude expected to be generated by the thermo
The subject of the present application is an out
growth of the invention described` _and claimed in
The right hand end of the instrument, as
our co-pending application Serial No. 533,226,
viewed in Figs. 1 and 2, constitutes -the input end.
ñled April 28, 1944. In that application there is 25 Reference numeral 5 designates, generally, a
disclosed a microwave frequency power meter
comprising a non-resonant co-axial line whereof
the axial conductor consists of an elongate small
diameter resistance wire and in which a thermo
resonant co-axial line which is tunable precisely
to the frequency of the incoming wave-the line
being tunable over some predetermined wave
band such,l for instance, as 9.5-10.5 centimeters.
couple junction is'disposed at the input end of 30 The purpose is to generate Within the resonant
the resistance wire and is subjected to heat gener
ated in the resistance wire-the thermocouple
being arranged to actuate a meter proportionately
line 5 a standing wave having a current antinode
at thermocouple junction i. That is to say, it is
the intention to cause the maximum possible
to the power of the input U. H. F. wave.
current to ñow along wire 2 through thermo
The. device of the present invention is dis 35 couple junction l sc that'the temperature rise
tinguished from that of our aforementioned prior
thereat will be as large as it is possible to achieve
application and is characterized in that the co
with a given amount o_f input power at the fre
axial line is resonant and preferably tunable .
quency to which the line is tuned. We are able
throughout a prescribed frequency band.
_in this way to build up at the thermocouple hot
From the fact of resonance it follows that there 40 junction 12R values which are nearly twice as great
is engendered in the co-axial line a standing wave
as could be achieved in an untuned line with the
with incidental current nodes and antinodes; and
same amount of power.
by placing a thermocouple hot junction at a cur
Co-axial line 5 comprises an outer tubular con
rent antinode we are able to realize a much
ductor `6 threaded externally at l to engage a
greater temperature change at said junction'and, 45 coupling, not shown, by means of which the in
concomitantly, amuch greater meter' deflection
strument may be connected to an ultra high fre
quency source. Usually the instrument is con
nected to one end of a standard co-axial line, as
for example, a ñfty ohm line or a seventy ohm
50 line. Since line 5 usually has an input impedance
per unit of power input change than can bev
achieved vwith _a non-resonant line such as we
employ in our prior device.
In the drawing:
Fig. 1 is an elevational view of a power meter
in accordance with a preferred embodiment of
the invention;
Fig. 2 is a longitudinal sectional view taken on
line 2--2 of Fig. 1;
different from the internal impedance of the
source to which the instrument is to be conducted,
it ordinarily is equipped with a matching trans
former 8. Such a transformer will enable direct
connection to a source having some definite pre
determined internal impedance; and if it is nec
essary to effect connection with a source having
some other internal impedance a’match can be-
from plug Il at circumference 2I and if the con- ,
tact were not good the indicator of meter I would
member of a ñtting, not shown, on the end of the
co-axial feed line. Next in order is a conductor
length only at one frequency and differs there~
from considerably at each end of the operating
fluctuate erratically and thus introduce some
difllculty in the matter 'of determining the point
achieved by interpolating a second transformer
5 at which the instrument is accurately tuned to the
between the source and transformer 8.
frequency of the incoming wave. In addition, the
Beginning at the right hand end the axial
along plug I4 from surface 20 toits right
conductor comprises ñrst a. terminal member 9
hand end is exactly equal to a quarter-wave
designed to engage the complementary contact
portion I0 which forms the axial conductor of
the matching transformer 8. This transformer,
it will be notedhas a length equal to a quarter
wave-length in polystyrene-_the bushing I I being
' preferably of polystyrene.
Of course, if an al- .
ternative dielectric material is employed the
length of the transformer should be altered ac
Tubular outer-’conductor 6 is provided at its
left hand end (see Fig. 3) with a longitudinal slot
24 through which extends-a threaded stud 25
which is attached at its inner end to plug I4. Stud
25 is provided with 'a locking nut 26 which serves '
to secure plug III in any position to which it may
be adjusted.A Upon loosening nut 26 the tuning
cordingly. It is to be understood that transformer
plug can be moved axis-wise along wire 2, thus ,
8 should have a characteristic impedance equal
altering the distance from surface 20 to point I2
to the geometric mean between the characteristic
to conform to the wave length of the incoming
impedance of the feed line and the input imped
Wave. With a fixed matching transformer it is
ance of the resonant line to the left of the trans
not practicable to move tuning plug I4 a very
great distance because to do so would result in
Constantan wire 2 is attached at point I2 to the
so much of a mismatch that the meter readings
left hand end of conductor Il) and extends axially 25 would cease to be suillciently accurateV Moreover,
along line I5 and through a small center aperture
too great e. movement of plug I4 would cause the
I3 in a tuning plug I4 which presently will be de
dimension from surface 20 to junction I to depart
scribed. Wire 2 then passes along one side of an
oñ'set pin I5 carried by tuning plug I4 and is at.
excessively from a half-wavelength, which would
result in removing junction I too far from the
tached-at I6 to one end of a coil spring I1 which 30 center of the current antinode; This could, of
is anchored on a lug IB forming an integral part of
course, be offset by making allowance for the de
outer conductor B. Spring Il holds wire 2 under
viation in calibrating meter 4, but ordinarily it is
tension and„by virture of pin I5, pulls said wire
not necessary to design the instrument to cover a
laterally into .ilrm contact with one side of aper
band of such width as would give rise to errors of `
35 important magnitude. In an actual casethe total
ture I3.
Tuning plug I4 is adiustably movable length
permissible movement of plug I4 was determined
wise along the interior of tubular outer conductor
to be one centimeter., In that particular case the
B and the purpose of so doing is to tune the reso
wave band extended from 9.5 to 10.5 centimeters.
nant line to the frequency of the incoming wave.
This provided a tuning band equal to about 10%
Plug Il has a bore I9 which is terminated by a 40 of the center frequency and involved a calculated
transverse surface 20. The distance between this
~error of about 1%.
surface and the point I2 should be an integral
The direct current circuit for meter 4 includes
multiple of a half-wave-length at the frequency
thermocouple hot junction I, copper wire 3, ter
of the .received wave. In this particular case the
minal conductor 27, conductor 28 and conductor
distance referred to has been made equal to two
29, which is shown grounded at 30. Outer-tubu
half-wave-lengths. Because of the fact that
lar conductor 6 is also grounded through bind
transverse surface 20 is located at a current anti
ing post 3l. Alternatively, conductor 29, may
node, it is desirable, in `order to avoid larcing
be connected directly to binding post 3|. That
troubles, to so design plug Il that contact between
portion of wire 2 which is to the left of junction
it and the inner surface of tubular conductor 6 I
I is also included in the direct current circuit.
is made at e. point one quarter-wave-length in
It is necessary to confine the radio frequency
advance of surface 20; that is to say along the
energy within the resonant co-axial line and to
circumference identi?ed by reference numeral 2 I.
exclude the same from meter 4 and that part
Plug Il is accordingly designed to extend for
of the direct current circuit which is outside the
wardly, as shown, and to terminate in an external 55 resonant line. To that end wire 3 is brought out
flange 22 designed to effect good contact with the
from junction I through a filter stub 32. This
- bore of tubular conductor 6. In order to further
comprises an outer tubular conductor 33, pref,
ensure good contact along circumference 2 I, plug
erably of the same diameter as tubular conduc
I4 is provided with a number of longitudinal slits
tor 6, and an inner cup-like conductive member
23 (see Fig. 4) extending for some distance from 60 34 which is insulated from outer conductor 33
the right hand end. The external diameter of
»by means of a layer of insulation 35--preferably
flange 22 should be made somewhat larger than
polystyrene. The upper interior end of cup-like
the gore into which it fits so that when con
’ tracted to ñt' said bore a good contact will result.
` member 34 is positioned a quarter-wave-length
(in air) from junction I. This causes the filter
Inasmuch as there is theoretically no current 65 stub, when viewed from junction I, to look'like
flow between outer conductor t and flange 22 when
an open circuit or high impedance. But it is
‘ plug Il is adiusted to its proper position it may
necessary to insulate member 34 from outer con
be asked why a good contact is needed along
ductor 33 in order to prevent short circuiting
circumference 2I. The answer is that circum
the D. C. current and that is the reason for the
ference 2l is located at a current node only when
insulation 35. It is necessary to prevent leakage
the tuning plug has been aljusted to the point
of high frequency wave energy through the space
where the distance between surface 20 and point
surrounding member 34 and occupied by insula
I2 is precisely equal to a wave length During
tion 3E. For ythat purpose member 34 is made of
the time that the plug is being _moved to effect
a length equal to a, quarter-wave-length in poly
such an adjustment there is current flow to and 76
styrene. By so doing the gap at 36 is caused
for same amount of low frequency power input.
to look like a short circuit. Obviously, all dimen
W=wattage employed in determining value of T,
sions in terms of wave-length units and fractions
per second step, infra. This is an arbitrarily
of wave-lengths can be rigorous only at one spe
selected value, such as one watt.
cific frequency-which should be the center fre
d1=diameter of wire 2, in cms.
quency from which the device is tunable an equal
da=diameterof wire 3, in cms.v
amount in either direction.
¢n=thermal conductivity of wire 2 which, for
As previously indicated it is necessary to design
constantan=0~26 Joule/sec. cm. per deg. cent.
transformer 8 to effect an impedance match
n_:thermal conductivity of wire 3 which, for .
between the resonant line 5 and the co-axial feed 10
copper=3.8 joules/sec. cm. per deg. cent.
line, not shown. The ordinary procedure in
A=wave length in centimeters
matching co-axlal lines having different char
acteristic impedances is to use a coupling sec
tion or transformer one quarter-wave-length
' long and having a characteristic impedance equal 15 and record the meter scale readings correspond
ing to known input wattages. The latter are
to the geometric mean of the characteristic im
simply the values of 12R, where Ris substantially
pedances to 4be coupled. Where, however, the co
equal to the resistance of wire 5 between surface
axial line at >the output end of the transformer
20 and point l2.
is short circuited, as in the present case, it is not
The second step consists in determining at a
permissible to use the characteristic impedance 20
given input power W, say 1 watt, the tempera
of said line in computing the required charac
ture (T) above room temperature at thermo
teristlc impedance of the coupling transformer.
couple junction I. This'can be ascertained from
Instead, we must use what is termed the input
tables such as will be found in a, publication of
impedance of line 5 as distinguished from its
characteristic impedance. However, we must 25 the American Institute of Physics entitled “Tem
perature-Its Measurement and Contro .”
` first determine the characteristic impedance of
The third step is to determine the value of To
line E in order to compute the input impedance.
from the formula
The characteristic impedance of line 5 may be
computed from the formula
i L am Í
The fourth step is to compute the ratio
b=inside diameter of conductor 6
a=diameter1of wire 2
35 from the formula
Having Adetermined the characteristic imped
ance of line `5 we may then determine its input
impedance Z1 from the formula
1 - e-P‘Ä
. n
Zo=characteristic impedance
Z1=nZo mi
the ratio
c=hâse of natural logarithms
The high frequency power value for any meter
scale reading is then determined by multiplying
the corresponding low frequency power value by
p=_l`ength of resonant line in half-wave-length
units-which must be an integral number.
It is found from computations based upon the
a=attenuatlon constant of the resonant line in
foregoing formulae that when wires 2 and 3 are '
nepers per centimeter=R/2Z0, where R is the
each of .003" diameter and the wave length vis 10
resistance per unit length of the inner conduc 50 centimeters the valueof
tor in ohms per centimeter at the received
micro-wave frequency
. TL
_)\=wave length in centimeters
is approximately 1.90.
Having determined the input impedance of Zi
of line 5 from the above formula, the character 55 From the foregoing description, it will be seen
that the invention has provided an improved de
istic impedance of transformer 8 can be com
puted from the formula
vice for the measurement of electric power atl
ultra high frequencies. It will be understood, of
Zr is the characteristic impedance ofthe trans
Z: is the characteristic impedance of the feed
In relating the procedure for calibration of the
herein-described instrument the following sym
bols will be employed:
T=temperature rise in deg. cent. of junction l
for a given input wattage
course, that the invention is not limited to the I
l `
60 specific disclosure but only by the scope of the
appended claims.
We claim:
1. In a high frequency electric power measur
ing device, a co-axial resonant line having an in
65 put end, said line being equal in length to an in
tegral multiple of a half-wave-length, where the
wave length is that of the wave whose power is
to be measured, said line comprising a tubular
outer conductor and an axial conductor consist- ,
70 ing of a small diameter high resistance wire, a
To=asymptotic temperature rise of wire in deg.
hot thermocouple junction disposed within said
cent. computed from Formula 4 infra.
resonant line in intimate heat transfer relation
Trf=temperature rise at thermocouple junction yl
to said axial conductor and located at a current
for any given amount of microwave power input.
antlnode situated substantially an integral num
Tt=temperature rise at thermocouple Junction l ’l5 ber of half-wave-lengths from the nearest end `of
_ ,
said line, and a direct current circuit including
said Junction together with a meter responsive
to the D. C. voltage generated by said junction.
2. In a high frequency electric power measur
ing device, a. co-axial resonant line having an in
put end, said line being at least approximately.
equal in length to an integral multiple of a half
with a hot thermocouple junction at a. current
antinode situated substantially an integral num
ber of half-wave-lengths from the nearest end
of said line, and a direct current circuit includ
ing said junction and said second wire together
with a meter responsive to D. C. voltage gener- .
A ated by‘said junction.
5. A device according to claim 4 having a quar
ter~wave iilter stub connected to the outer tubu
being short-circuited at the end remote from said 10 lar conductor and through which said second
wire emerges from the resonant line, said filter
input end, said line comprising a tubular outer
stub being operative to isolate the meter from
conductor and 'an axial conductor consisting of a
>the high frequency wave within theresonant
small diameter high resistance wire, a hot thermo
waye-length, where the wave-length is that of the
wave whoseY power is to be- measuredf said line '
couple junction disposed within said resonant
. line in intimate heat transfer relation to said 15
axial conductor and located at a current antinode
situated substantially an integral number of half
6. A device according to claim 4 wherein the
plug contacts the Ainterior surface of the outer
tubular conductor along a circumference which
is approximately a quarter-wave-lensth distant
from the short-circuited termination of the reso
and a direct current circuit including said junc
tion together with a meter responsive to D. C. 20 nant line and to that extent nearer the input end
than is said short-circuìted termination.
' voltage generated by said junction.
7. A device according to claim 4 wherein the
3. In a high frequency electric power meas
tuning plug is bored axiswise from its input end
uring device, a co-axial resonant line having an
to a depth equal substantially to a quarter-wave
input end, said resonant line comprising a tubular
length at the center frequency of the device, said '
»outer conductor and an axial conductor consist
plug contacting circumferentialiy the interior
ing of a small diameter high resistance wire, a
surface of the tubular outer conductor at the
_tuning plug disposed within said tubular outer
input end of the plug.
conductor and adjustably movable therein
8. In a high frequency electric power measur
lengthwise oi said line, said plug forming a short
ing device. a co-axial line having a resonant por
circuited termination for said resonant line at tion terminating at its input end in an imped
a point which, when the device is tuned to a
ance-matching transforme f, said resonant por
speciñc wave-length, is substantially an integral
tion including a tubular outer conductor and an
number oi? half-wave-lengths removed from said
axial conductor consisting of a small diameter
input end, a hot thermocouple junction disposed
resistance wire, a. tuning plug disposed within
within said resonant line immediately' adjacent
wave-lengths from the -nearest end of said line.
said axial conductor at a current antinodelo
cated at an intermediate point between the ter
said tubular outer conductor and adjustabiy -
movable axiswise therein, said plug being bored
axiswise at the end thereof nearest the input end
mini of- the resonant line, said intermediate point
of said resonant portion and to a depth approxi
being substantially an integral number of half
wave-lengths from the nearest end of said line, 40 mately equal to a quarter-wave-lenath- at the
center frequency of the device, said plu'g having
and a direct current circuit including said junc
an axial aperture through which said resistance
tion ltogether with a meter responsive to D, C.
wire passes, the bottom of thebore of said plug
voltage generated by said junction.
being defined by a transverse wall which forms
4. In a high frequency electric power meas
uring device. a coi-axial resonant line having an 45 a short-circuited termination of said resonant
portion, said transverse wall being situated, when
input end, said resonant line comprising a tubu
lar outer conductor and an axial conductor con
sisting of a small diameter resistance wire, a tun
the device is tuned to a speciilc wave-length, an
integral number of half-wave-lengths distant
from said input .end of said resonant portion, a
ing plug disposed within said tubular outer con
ductor and adiustably movable therein length 50 second wire joined to said resistance wire at an
intermediate current antinode to form a thermo
wise of said line', said plug having an axial aper
couple hot Junction, and a direct current circuit
ture through which said resistance wire passes.
said plug forming a'short-circuited termination ~ including said Junction and said second wire to
gether with a meter responsive to D. C. voltage
for said resonant'line at a point which, when the
device is tuned to a specii‘lc wave-length, is sub 55 generated by said junction.
stantially an integral number of -half-wave
lengths removed from'said input end, a second
wire joined to said resistance wire to form there
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