Note: Descriptions are shown in the official language in which they were submitted.
HOPFER ~ ~
_
~ackground of the Inve tion
This invention relates to radiation de-tection
devices for electromagnetic radiation in free space, and
particularly to such devices which are useful for a broad-
band of radio frequencies.
A detector for free space radiation over a
broadlb~nd ofradio frequencies, particularly in the microwave
spectrum, is described in applicant's paper 7 "Design of
Broad-Band Resistive Radiation Probes," IEE~ Tr. Instr. and
Meas., November, 1972, Vol. IM-21, No. 4, pp. 416-421, and
in his patent U.S. 3,931,573. Such a detector is desirably
achieved by an arrangement of resistive strips for extracting
and absorbing radiant energy from the free space field rather
than by an antenna. The resistive strips keep the effects of
reflection and diffraction to a minimum, and the resistive
strip arrangement has sufficient transparency so as not to
disturb the field being measured and detected. The fractional
absorption of the radiant energy by the resistive strips
is substantially invariant with frequency, so that an ultra
broadband radiation detecting device is thereby produced.
This resistive strip detector is unlike an antenna which
uses metallic surfaces of negligible resistance, for the
resistive strips do not disturb the near field being measured.
Applicant's aforementioned paper and patent describe a specific
embodiment using thermocouples for coverting the absorbed r-f
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.
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~IOPFER
energy to a d-c measurement signal.
Other radiation detectors use antennas (i.e., trans-
ducers having conventional highly conductive surfaces). These
detectors tend to produce disturbance in the field being
measured due to the conductive materials, and to be narrow
band by reason of the frequency sensitivi~y of the antenna.
However, by using a "short" dipole antenna (i.e., a small
fraction of a wavelength at the high frequency end of the
band), and with a high reactance relative to the character-
istic free-space impedence, somewhat broader bands have been
achieved; see "New Near-Zone Electric-Field-Strength Meter"
by F.M. Greene, NBS J. of Research - C. Eng. and Instr.,
Vol 71C, No. 1, Jan.-Mar. 1967, pp. 51-57; "Near Field
Instrumentation," report of A.W. Rudge, et al, July, 1970,
Bureau of Radiological Health of U.S. Dep~. H.E.W.; and
U.S. Patents 3,750,017 of Bowman et al and 4,008,477 of
Babij et al. These detectors use diodes for converting
the r-f currents induced in the antennas to a d-c measurement
signal, which have the advantage of higher sensitivity.
Practical constraints may limit such detectors to frequencies
up to a few gigaHerz.
Known probes using short dipole antennas with
loads of Schottky barrier diodes are constructed to have a
broad bandwidth by compensating for the frequency depenclent
diode. The diode impedance in the frequency range of opera-
tion, is largely characterized by its barrier capacity.
The capacitance is in series with that of the short dipole
whose coupling action to free space is also essentially
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HOPF'ER
represented by an equivalent capacitance. For as long as
the total circuit reactance is large relative to free space
impedance, a condition which sets the upper frequency limit,
the induced voltage for constant power density remains con-
stant with frequency. As a consequence of the capacitive
divider network, the induced voltage across the diode is a
constant. This accounts for the probe's flat characteristic
of output with frequency. The lower frequency limit is
reached when the barrier capacitive reactance becomes com-
parable to the barrier resistance.
In applicant's U.S. Patent 4,207,518, a radiation
monitor is disclosed in which a broadband operation in the
microwave spectrum is maintained, and a high degree of
sensitivity is achieved. A diode monitoring circuit is
used, which includes means that ensures a constant operation
with frequency over that frequency band, notwithstanding the
capacitive reactance of the diodes that are used. This opera-
tion holds true generally in the microwave frequency band;
however7 at low frequencies, the behavior changes to that
of a short dipole characterized by a substantial space coupling
capacitance, which thereby limits the broadband characteristic
at those low frequencies.
The broadband radiation monitor described in
U.S. ~atent 4,207,518 is limited somewhat itl its operating
range by reason of capacitive effects at radio frequencies
somewhat below the microwave re.gion, and these capacitive
effects increase substantially as the frequency decreases.
In particular, at very low frequencies, it has been found
that the capacitive ef:Eects are the dominant ones of the
antenna, and the device acts similar to a conventional short dipole.
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HOPFER
Summary of the Invention
It is among the objects of this invention to
provide a new and improved radiation detector for free
space radiation.
Another object is to provide a new and improved
detector for free-space radiation which is effective over
a broadbandof radio frequencies, both microwave and lower
frequencies.
Another object is to provide a new and improved
free-space radiation detector which has a high degree of
sensitivity.
In one form of the invention, a radiation detector
uses thin resistive strips having a substantial d-c resist-
ance greater than the characteristic impedance of free-space
for producing r-~ currents whose amplitudes are generally
constant over a band of microwave frequencies, and which
vary with frequency over a band of lower frequencies. A
monitoring circuit, including a diode, is provided for
converting the r-f currents to corresponding d-c signals.
This circuit is formed of elements havi.ng resistance and
reactance, andparticu].arly a negligible reactance over the
broad ba~d of microwave frequencies, and a substantial reactance
over a broadband of lower frequencies. The resistive strips and
resistance o:E the converting circuit form a resistive
divider over the microwave band, and the reactances of the
resistive strip and of the converting circuit form a react-
ance dlvider over the lower frequencies, whereby the d-c
signals are substantially constant with frequency over
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~OPFER
both the microwave and lower f-requency bands.
In a particular embodiment of this inventlon,
an isotropic radiation monitor is built with three loops,
each leg of which contains a thin film resistive strip
fabricated by vacuum deposition on opposite sides oE a
thin dielectric substrate, which is preferably only a few
thousandths of an inch thick. Thereby, the resistive
strips on the same side of the substrate are deposited at
the same time, and the effective loop area that interacts
with the H-field is minimized, so that the interaction is
essentially limited to the E-field. Three such resistive
legs, spaced by equal angles, are formed on three planar
legs of the substrate that are correspondingly spaced. The
three legs are mounted on three planar surfaces similarly
inclined with respect to the axis of a wand handle to form
three resistive strips extending in three mutually orthogonal
directions for isotropic operation. A gap in the strip in
one leg of each loop is bridged by a dlode and other elements
of the monitoring circuit; the strip in the other leg provides
d-c continuity. The strips of the three loops are inter-
connected, so that only two terminals are required to con-
nect to resistive leads in a long probe tube that, in turn,
are connected to a metering circuit. A filtering circuit is
formed by a capactior across the two resistive legs of each
loop, which provides a time constant larger than the period
at the lowest operating :Erequency.
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HOPF_
Brief Description _f the Draw
.
The foregoing and other objects of this invention,
thevarious features thereof,as well as the invention itself,
will be more fully ~mderstood from the following description,
when read together with the accompanying drawing in which:
Fig. 1 is a schematic, equivalent circuit
diagram of an isotropic broadband radiation monitor embody-
ing this invention;
Fig. 2 is a plan view of a radiation detection
element used in the assembly of the lsotropic probe of
Fig. l;
Fig. 3 is a perspective view of a conical probe
assembly mount with the thin film element mounted thereon;
Fig. 4 is a cross-section view of a portion of
one arm of the thin film element of Fig. 2 with the diode
monitoring circuit assembly shown greatly enlarged;
Fig. 5 is a schematic equivalent circuit of one
leg of the probe in its relationship to free-space radiation;
and
Figs. 6A, B and C are equivalent circuit diagrams
similar to the circuit o:E Fig. 5 used to explain the operation
at different frequency ranges.
HOPFER
Description of a Preferred Embodimen-t
. _ _
In the drawing corresponding parts are
referenced by similar numerals throughout.
In the schematic circuit diagram of Fig. 1, -the
radiation monitor probe lO of this invention is made up of
a rigid dielectric wand 12 shown in broken lines which may
include a handle remote from an opaque dielectric housing 14,
(such asa styrofoam sphere) that encloses the probe element
16. As shown schematically within the housing 14, the probe
element consists of three loops 18, 20 and 22. Each of th~
three loops is constructed in a similar fashion and mounted
(for example, in ways known in the art) to extend in three
orthogonal directions to interact respectively with three
orthogonal components of the E-field, and which together
operate for isotropic monitoring of free-space radiation.
The loop ].8 consists of two resistive half-strips
24, 26 spaced by a small gap that is bridged by a diode cir-
cuit 28 (represented in simple form by a single diode 29)
The return leg is a highly resistive strip 30 which provides
d-c continuity. Connected across the inner terminals 31,
33 of the two legs of loop 18 is an integrating capacitor 32,
that forms a circuit with the high resistances of strips 24,
26, 30 of that loop 18 that provides a time constant that
is larger than the period of the lowest r-f frequencies to be
monitored. Each of the other loops 20 and 22 is similarly
constructed and corresponding parts are re~erenced by similar
numerals.
These three l.oops 18, 20 and 22 are connected in
ser:ies ~or d-c continuity with the diodes arranged in
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HOPFER
series-aiding relationship. Each diode operates in its
square-law region and produces d-c voltages proportional
to the square of the component of the ~-field along the
associated leg. The voltages from the three loops are
summedlalgebraically to provide a total d-c voltage represent-
ative of the sum of the squares of the individual d-c
voltages, and thereby representative of ~he E-field along
the associated leg.
The outer terminal 31 of loop 18 and terminal 33
of loop 22 are connected through suitable contacts 34, 36
(e.g., of a known bellows type~ to very high resistance
leads 37, 39 that extend along the length of the wand 12
and are connected to external terminals 38, 40 which may
provide connection to a suitable coaxial connector to an
amplifier 42 and metering circuit 44, on which the wand may
be mounted for use. Large by-pass capacltors 46 and 48 are
connected at either end of the resistive leads 37 and 39
to provide an isolating loop for any r-f or a-c currents
induced in the resistive leads, and thereby isolate that
loop from the amplifier 42 and from the probe.
All connecting elements that may effect the
radiation field being detected (e.g., those to amplifier 42)
are preferably formed of highly resistive material to avoid
disturbing that field, and any highly conductive connectors
(used, for example, where contacts are formed) are made
very small dimensionally compared to the wavelength at the
highest frequency in the :Erequency range being measured,
in a manner well known to those skilled in the art.
_9_
;
HOPFER ~-~
The probe element 16 in one form of the invention
includes a conical assembly mount 52 (Fig. 3) which comprises
a large circular ring 54 and a small cylindrical base 56
interconnected by three inclined planar arms 58, 60 and 62.
The probe sensing element 61 (Fig. 2) is formed on a sub-
strate 62 having three arms 64, 66 and 68 which are generally
planar, and which are mounted on the respective arms 58, 60
and 62 of the assembly mount 52. Cutouts 70 at the hub
of the three-arm substrate permit the bending of the element
thereat, and the use of an attachment plate 71 to hold the
element 61 in position on the mount, as shown in Figs. 2 and
3. The mount 52 is attached at the end of wand 12 and
coaxially therewith, so that the arms form equal angles
therewith.
On one face of each arm 64, 66 and 68 of the
substrate, there is deposited two spaced resistive strips
24, 26 of resistive material such as michrome. The thick-
nesses of the resistive films 24, 26 are small compared
to the skin depth at the highest operating frequency to
insure uniformity of resistance over the operating frequency
band, and to achieve the desired broadband operation; the
term "thin film" is used herein to describe that mode of
construction, which is achievable by vacuum deposition tech-
niques. At the hub of the substrate, there are two passages
through the substrate which are formed as probe ten~nals 72, 74 by suit-
able conducting material, such as silver film. Similarly, at
the outer end of each arm 64, 66 and 68 of the substrate, is
located through terminal 76, 78 and 80, respectively. At the
adjacent ends of-the resistive strips 24 and 26 are t~ contacts te.g.,
formed as a deposition of silver film) 82, 84 which define
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HOPFER
the gap for mounting the diode monitoring circuit thereat.
Simi.lar contacts 82 ancl 84 are formed in the other legs
as well. On substrate arm 64, the resistive strip 26 is
formed to extend from contact 84 at the gap to its inner
end where it joins the inner end oE strip 26 on arm 68;
the resistive strip 24 is formed between the terminal 76
and the contact 82; and on the reverse face of the substrate
arm 64, the resistive strip 30 (Figs. 1 and 4) is formed
to extend from ~he terminal 76 to the inner end where it
joins the inner end of strip 30 on arm 66.
A similar construction is provided ~or the other
substrate arms 66 and 68 and Eor the corresponding resistive
strips thereon, except that the inner end of strip 30 on
arm 68 is connected to the probe terminal 72, and the inner
end of strip 26 on arm 66 is connected to probe terminal 74.
A d-c probe circuit is formed between terminals 72 and 74:
from terminal 72 through strip 30 on the back face of arm
68, back through terminal 80 to the outer face of arrn 68,
and thereon through the strip 24, diode circuit 28 and strip
26 to its inner end where it Joins the inner end of strip 26
on arm 64; then along the strips 26 and 24 and diode circuit
28 on the outer face of arm 64, through terminal 76 to the
back face and along strip 30 of arm 64 to the inner end of
strip 30 of arrn 66; then along the strip 30 on the back face
of arm 66 through terminal 78 to the front :Eace, and along
strips 2~ and 26 and diode circuit 28 to the other probe
terrninal 7~. The diode on arm 64 is oriented reversely from
those on arms 66 a.nd 68 (as shown schematically in Fig. 1)
for the series~aiding rel.ation.
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HOP~ER
The diode monitor circuit assembly is shown in
Fig. 4 and the corresponding equivalent circuit is shown
as part of Fig. 5. This monitor circuit 28 includes a
shunt network resistor 90 and coupling capacitors 92, 94
on either side thereof. This series combination is con-
nected as a shunt circuit to diode 29, which is shown (Fig. 5)
in its schematic form, with a junction resistance Rv; and
a junction capacitance Cj; due, :Eor example, to the barrier
capacitance of a Schottky barrier diode.
As shown in the cross-sectional view of Fig. 4
through the center of diode circuit 28, the mechanical
assembly of this circuit 28 is made between the t~o spaced
silver contact areas 82, 84. The Schottky diode 29 in the
form of a thin rectangular box of small dimensions has its
extending leads 96 secured to those silver contacts by a
suitable conducting epoxy layer 98 on each contact 82, 84.
The shunt capacitors 92, 94 also in the form of small
rectangular boxes are secured respectively to the contacts
82 and 84 by means of the conductive epoxy layers 98. The
shunt ressitor 90 is deposited on a dielectric substrate
100. This resistive strip has a resistance somewhat less
than the d-c resistance of the diode (as explained in U.S.
Patent 4,207,518). At each end of the resistive strip,
a silver contact 102 is provided and the silver contacts
are electrically bonded to the terminals of the shunt capac-
itors 92 and 9~ by a suitable conducting epoxy 104; the
epoxy also ensures a good mechanical bond Eor all of the
elements.
The t:ime-constant capacitors 32 may be mounted on
~ 3~f~
HOPFER
each substrate arm between the inner encls of the resistive
strips 26 and 30 on the same arm. For this purpose, a
through terminal is formed on each arm at those points, and
the capacitor electrically bonded thereto and connected (by
a conductive ribbon or other suitable connector) to the
inner end of resistive strip 26 on the outer face; a suit-
able connector is formed on the back face betweeen this
through terminal and the inner end of strip 30. A single
diode circuit for r-f to d-c conversion is used, which tends
to improve the integration at low frequencies. That is,
the phase imbalances at very low frequencies are thereby
avoided. However, a diode circuit in each leg 30 may also
be used.
When the probe elemen-t assembly 52 is mounted on
the wand 12, the contacts 34 and 36 are closed, and the
circuit completed as shown in Fig. 1, ~or operation of the
probe. The resistive strips 2h, 26 in one arm are at right
angles to those in each of the other arms, and form separate
measuring planes for the E-field; the angles of inclination
of the assembly arms 58, 60, 62 are chosen in this way.
The crossed strips that define each measuring plane also
define the unit square of radiation which the probe element
interacts with.
The operation of this circuit in the microwave
region is explained in detai.l in the aforementioned U.S.
Patent 4,207,518. That is, the resistive s-trips 24, 26 in
each arm and i.n each direction determine the interaction
with the ~ree~space radiation on a frequency independent basis. These
half strips 24, 26 in each leg have a substantial length
(e.g., about 1 inch) which defines the unit square of inter-
action with the E-field over the frequency range.
The free space radiation field that is detected has a
-, . ...
! - ~ 3-
~ 3~
HOPFER
characteristic impedance (377 ohms) much smaller than the
equivalent surface resistance of the resistive strips 2~,
26, 30 and is not disturbed, or otherwise adversely affected
by those resistive strips. By using a dimensionally small
barrier device, such as a diode, as the element producing
the d-c measurement signal, the r-f field is generally not
affected. Though the diode's impedance is frequency sensi-
tive, it is small compared to the series resistance of the
strips 24, 26 in the same leg. Therefore, the strip resist-
ance, essentially on a frequency-independent basis in the
microwave region, establishes the induced r-f current corre
sponding to the energy extracted from free space; this r-f
current for corresponding intensity levels of radiation is
constant in arnplitude with frequency. The shunt-network
strip 90, which is resistive, and thus constant with fre-
quency, establishes the r-f voltage applied across diode 29.
Therefore, the voltage across each diode for particular
levels of intensity of free-space radiation, is constant
with frequency.
In the equivalent circuit diagram of Fig. 5, Ro
represents the characteristic impedance of free space, which
impedance 110 is shown as that of the equivalent generator
or source of radiation as well as of the free space which
normally serves as the load 112.
In the microwave or mid-band region, the reactance
of the stxips is small compared to that oE the resistance of
the strips 2~, 26 and the latter resistance is large compared
to the characteristic impedance of free space, and preferably
some thous;ands of ohms. In addition, the antenna resistances
Ra are large compared to the shunt resistance 90, the latter
-1~-
3~
HOPFER
being typically th~ order of 50 ohms. ln this microwave
region, the equivalent circuit of Fig. 5 reduces essentially
to that of Fig. 6A. As can be seen, the monitor of each
leg reduces essentially to a resistor divider composed of
the strip resistances 24, 26 in series with the shunt re-
sistance 90. Under those conditions, the voltage across
the diode resistance Rv remains constant with frequency.
This resistance divider condition is limi~ed at the high
frequency end. The diode reactance must be greater than
or equal to the shunt resistance. When the diode reactance
is less than this value> it becomes an effective part of
the circuit, and the shunt resistance is no longer effective
to maintain a constant voltage across the diode. For a
suitable Schottky cdiode, a typical junction capacitance is
0.08 pf; the reactance of that diode may be maintained
greater than the sh~mt resistance for suitable sensitivity
up to about 26 GHz.
At low frequencies, for example, in the range of
megahertz and down to hundreds of kilohertz, the space-
coupling capacitance Ca produces a reactance that is much
larger than the resistance Ra. In addition, ~a is fairly
constant in this region, Cs is lumped and also small. Under
these conditions, the equivalent circuit reduces to that of
Fig. 6B, in which the circuit acts as a capacitor divider.
Therefore, the voltage across the diode resistance Rv remains
reasonably constcmt with frequency in this range. The ratio
of the capacitances being relatively unaffected by frequency,
the proportional ~oltage across the shunt capacitance (which
determines the voltage across the diode re~sistance) also
~ 3
HOPFER
is relatively constant with frequency. This holds true
down to the low frequency limit where the shunt reactance
becomes greater than the diode video resistance. A-t that
point, the circuit no longer acts as a capacitor divider,
and the voltage across the diode is no longer constant with
frequency. The diode video resistance for a suitable
Schottky diode may be typically 300 kilohms. Thus, for the
coupling capacitors, 92, 9~, comb:ined of about 1~ pf, it
has been found possible to maintain a three db broadband
characteristic down to 100~200 kHz.
In the region of transition between the mid
and low bands, the circuit takes the form of Fig. 6C. In
this region, both Ra and Ca play a substantial role. The
circuit maintains a substantially constant voltage across
the diode where the changes in the resistance and reactance
of the shunt circuit correspond to those of the antenna
circuit. That is where the ratio of the shunt resistance
to the shunt reactance is approximately equal to the ratio
of the antenna resistance to the antenna reactance. Under
these conditions, the ratio of the antenna impedance to the
shunt impedance is approximately constant, and the impedance
divider of Fig. 6C operates to maintain the voltage across
the diode resistance substantially constant.
Thus, qualitatively, the extreme broadbanding
is achieved by matching the characteristics of the resistive
strip to those of the Schottky diode. Realizing that the
detected, open circuited voltage of the diode depends only
on the r-f voltage across it, it i.s evident that for any
given external field intensity E, this r-f voltage must be
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35~
HOPFER
kept constant, over the entire frequency spectrum. Th~s
is accomplished through ~he RC network shunting the diode.
Thus, in the high frequency portion of the range, the pre-
dominantly resistive property of the resistive strip is
matched by the predominantly resistive characteristic of
the shunt network. Similarly, in the low frequency portion
of the range, the predominantly capacitive nature of the
strip (operating as a short dipole) is matched by the pre-
dominantly capacitive characteristic of the shunt network.
In the transition region, their complex charactPristics
track each other with reasonable accuracy. The high and
low frequency limits of this design depend on the inherent
diode characteristics as well as the desired sensitivity.
In the present case, a radiation density of 1 mw/cm2 any-
where between 200 kHz and 26 GHz generates an output voltage
of approximately 8 mV, corresponding to a 100 fold improve~
ment in sensitivity over previous broadband probes.
In the transition region (e.g. 100 MHz to 1 GHz),
there may be a decrease in sensitivity due to a frequency
effect of Ra. By choosing Cs ~ be somewhat lower than
the nominal ~alue called for by the similar ratios of re-
sistance to reactance for the antenna and shunt networks,
it is found that the sensitivity decrease at low frequency
is offset.
It has been found that by forming the resistive
strips on opposite faces of an extremely thin dielectric
,substrate 62, the area of interaction with the H-field is
minimized. H-field effects are also minimized by fabrica-
tion of the shunt circuit, which also minimizes the area of
interaction with the H-field.
~''
3~ Z
HOPFER
For high powered applications, it is possible to
extend the dynamic range of the diodes by operat;ng them
outside of their square-law region. When this is done, a
separate compensating network for each leg would be pro-
vided to produce square-law signals, which are then summed
to provide the signal propor-tiona:L to E-field intensity.
Alternatively, the shunt resistor can be made still smaller
to shunt the larger propor-tion of the current, so that the
diode does operate in the square-law region.
The following are typical values of a radiation
probe constructed in accordance with the preferred embodi-
ment. The resistance Ra of the strip 24-26 is 6 kilohms,
the resistance of strip 30 is 9 kilohms. The resistive
strips are 1" long and less than 0.01" wide, and are thin
film deposlti.ons of nichrome on a 2 mil Kapton substrate.
The Schottky diode has a junction capacitance Cj of 0.1 pf
maximum, and a video resistance Rv of 300 kilohms. The
shunt resistance Rs is 50 ohms. The capacitance Cs of each
shunt capacitor 92 is 25 pf (or where only a single shunt
capacitor is used, it is 12 pf). The filteri.ng capacitors
32 are each 100 pf chosen for a low frequency end of about
200 kHz. The capacitor 48 is .044 microfarads; the resis-
tive leads 37, 39 are 27.5 kilohms, the capacitor ~6 is
0.022 microfarads. The input impedance of the d-c amplifier
42 preferably is about 100 megohms. Sensitivity of the probe
ranges from 0.01 to 20 mW/cm2.
This invention may be used with probes extending
in one or two directions, i.n the same fashion as in the
isotropic unit described above, and the probe operates in
the same way in each direction. Various typesof barrier devices
3~2
HOPFER
for r-f to d-c conversion may be used. The shunt network
assembly may be simplified by using a single coupling
capacitor 92 and connecting the shunt resistor directly
to the contact 84 by bonding or by a conductive ribbon.
Other modifications within the spirit of the following
claims will be apparent to those skilled in the art.
Thus, applicant has provided a new and improved
microwave detector tha~ is effective for measuring free-
space radiation over a broad band of radio frequencies,
both in the microwave region and in lower frequencies.
This radiation detector has a high sensitivity to measure
extremely low thresholds of free-space radiation over a
wide frequency band.
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