chapter9-Radiation sensor Radiation sensor.ppt

Radiation sensor

1. Radiation Sensors
Chapter 9

2. Introduction
nWe have discussed radiation in Chapter 4 when
talking about light sensors.
nOur particular concern there was the general range
occupied by the infrared, visible and ultraviolet
radiation.
nHere we will concern ourselves with the ranges below
and above these.
nRange above UV
nRange below IR.

3. Introduction
nRange above UV is characterized by ionization –
nFrequency is sufficiently high to ionize molecules based on
Plank’s equation.
nThe frequencies are so high (above 750 THz) that many forms of
radiation can penetrate through materials and therefore the
methods of sensing must rely on different principles than at lower
frequencies.
nOn the other hand, below the infrared region, the electromagnetic
radiation can be generated and detected by simple antennas.
nWe will therefore discuss the idea of an antenna and its use as a
sensor.

4. Introduction
nAll radiation may be viewed as electromagnetic
radiation.
nMany of the sensing strategies, including those
discussed in chapter 4 may be viewed as radiation
sensing.
nWe will however follow the conventional
nomenclature
nWill call low frequency radiation “electromagnetic” (electromagnetic
waves, electro-magnetic energy, etc.)
nWill call high frequency radiation, simply “radiation” (as in X-
ray, or cosmic)

5. Introduction
nOne important distinction in radiation is based on the Planck
equation and uses the photon energy to distinguish between them:
e = hf
h = 6.6262x10


[joule.second] is Plank’s constant
f is the frequency in Hz
e is called the photon energy.

6. Introduction
nAt high frequencies, where particles are concerned,
one can view them either as particles or as waves.
nThe energy in these waves is given by the Planck
equation.
nTheir wavelength is given by de Broglie’s equation
(p=mv is the momentum of the particle):
λ =
h
p

7. Introduction
n
The higher the frequency the higher the photon energy.
n
At high frequencies, the photon energy is sufficient to strip
electrons from atoms –ionizing radiation.
n
At low frequencies, ionization does not happen and hence
these waves are called non-ionizing.
n
The highest frequency in the microwave region is 300 Ghz.
The photon energy is 0.02 eV. This is considered non-
ionizing.
n
The lowest frequency in the X-ray region is approximately
3x10
16
and the photon energy is 2000 eV. Clearly an ionizing
radiation.

8. Introduction
n
Some view radioactive radiation as something different than,
say X-ray radiation or microwaves
n
It is often viewed as particle radiation.
n
One can take this approach based on the duality of
electromagnetic radiation, just as we can view light as
electromagnetic or as particles – photons.
n
We will base all our discussion on the photon energy of
radiation and not on the particle aspects.
n
In some cases it will be convenient to talk about particles.
(Geiger-Muller counter, for example)

9. Introduction
nMany of the radiation sensors based on ionization are
used to sense the radiation itself (detect and quantify
radiation from sources such as X-rays and from
nuclear sources (and  radiation).
nThere are however exception such as smoke
detection and measurement of material thickness
through radiation.
nIn the lower range, the sensing of a variety of
parameters through microwaves is the most important
nSensing of the microwave themselves is not

10. Units
nUnits for radiation, except for low frequency
electromagnetic radiation are divided into three:
nUnits of activity,
nUnits of exposure
nUnits of absorbed dose.
nAlso - units for dose equivalent.
nThe basic unit of activity is the Becquerel [Bq]
nDefined as one transition (disintegration) per
second.
nIt indicates the rate of decay of a radionuclide.

11. Units
nAn older, non-SI unit of activity was the curie (1
curie=3.7x10
10
becquerel).
nThe Becquerel is a small unit so that the [MBq],
[GBq] and [TBq] are often used.
nThe basic unit of exposure is the coulomb per
kilogram [C/kg]=[A.s/kg].
nThe older unit was the roentgen (1
roentgen=2.58x10

C/kg].
nThe [C/kg] is a very large unit and units of [mC/kg],
C/kg] and [pC/kg] are often used.

12. Units
nAbsorbed dose is measured in grays [Gy] which is [J/kg].
nThe Gray is energy per kilogram and 1[Gy]=1[J/kg].
nThe old unit of absorbed dose was the rad (1 rad = 100
[Gy]).
nThe units for dose equivalence is the sievert [Sv] in [J/kg].
nThe old unit is the rem (1 rem = 100 [Sv]).
nNote that the sievert and the gray are the same.
nThis is because they measure identical quantities in air.
nHowever the dose equivalent for a body (like the human
body) is obtained by multiplying the absorbed dose by a
quality factor to obtain the dose equivalent.

13. Radiation sensors
nWill start the discussion with ionization sensors
nThen will discuss the much lower frequency
methods based on electromagnetic radiation
nThree basic types of radiation sensors:
nIonization sensors
nScintillation sensors
nSemiconductor radiation sensors
nThese sensors are either:
nDetectors – detection without quantification or:
nSensor - both detection and quantification

14. Ionization sensors (detectors)
nIn an ionization sensor, the radiation passing
through a medium (gas or solid) creates electron-
proton pairs
nTheir density and energy depends on the energy of
the ionizing radiation.
nThese charges can then be attracted to electrodes
and measured or they may be accelerated through
the use of magnetic fields for further use.
nThe simplest and oldest type of sensor is the
ionization chamber.

15. Ionization chamber
nThe chamber is a gas filled chamber
nUsually at low pressure
nHas predictable response to radiation.
nIn most gases, the ionization energy for the outer electrons is
fairly small – 10 to 20 eV.
nA somewhat higher energy is required since some energy may
be absorbed without releasing charged pairs (by moving
electrons into higher energy bands within the atom).
nFor sensing, the important quantity is the W value.
nIt is an average energy transferred per ion pair generated.
Table 9.1 gives the W values for a few gases used in ion
chambers.

16. W values for gases
Table 9.1. W values for various gases used in ionization chambers (eV/ion pair)
Gas Electrons (fast) Alpha particles
Argon (A) 27.0 25.9
Helium (He) 32.5 31.7
Nitrogen (N2) 35.8 36.0
Air 35.0 35.2
CH4 30.2 29.0

17. Ionization chamber
nClearly ion pairs can also recombine.
nThe current generated is due to an average rate of
ion generation.
nThe principle is shown in Figure 9.1.
nWhen no ionization occurs, there is no current as
the gas has negligible resistance.
nThe voltage across the cell is relatively high and
attracts the charges, reducing recombination.
nUnder these conditions, the steady state current is a
good measure of the ionization rate.

18. Ionization chamber

19. Ionization chamber
nThe chamber operates in the saturation region of the
I-V curve.
nThe higher the radiation frequency and the higher the
voltage across the chamber electrodes the higher the
current across the chamber.
nThe chamber in Figure 9.1. is sufficient for high
energy radiation
nFor low energy X-rays, a better approach is needed.

20. Ionization chamber - applications
nThe most common use for ionization chambers is in
smoke detectors.
nThe chamber is open to the air and ionization occurs in
air.
nA small radioactive source (usually Americum 241)
ionizes the air at a constant rate
nThis causes a small, constant ionization current
between the anode and cathode of the chamber.
nCombustion products such as smoke enter the chamber

21. Ionization chamber - applications
nSmoke particles are much larger and heavier than air
nThey form centers around which positive and negative charges
recombine.
nThis reduces the ionization current and triggers an alarm.
nIn most smoke detectors, there are two chambers.
nOne is as described above. It can be triggered by humidity, dust and
even by pressure differences or small insects, a second, reference
chamber is provided
nIn it the openings to air are too small to allow the large smoke
particles but will allow humidity.
nThe trigger is now based on the difference between these two
currents.

22. Ionization chambers in a residential
smoke detector

23. Ionization chambers - application
nFabric density sensor (see figure).
nThe lower part contains a low energy radioactive isotope
(Krypton 85)
nThe upper part is an ionization chamber.
nThe fabric passes between them.
nThe ionization current is calibrated in terms of density (i.e.
weight per unit area).
nSimilar devices are calibrated in terms of thickness (rubber for
example) or other quantities that affect the amount of radiation
that passes through such as moisture

24. A nuclear fabric density sensor

25. Proportional chamber
nA proportional chamber is a gas ionization chamber
but:
nThe potential across the electrodes is high enough to
produce an electric field in excess of 10
6
V/m.
nThe electrons are accelerated, process collide with
atoms releasing additional electrons (and protons) in a
process called the Townsend avalanche.
nThese charges are collected by the anode and because
of this multiplication effect can be used to detect lower
intensity radiation.

26. Proportional chamber
nThe device is also called a proportional counter or
multiplier.
nIf the electric field is increased further, the output
becomes nonlinear due to protons which cannot move
as fast as electrons causing a space charge.
nFigure 9.2 shows the region of operation of the
various types of gas chambers.

27. Operation of ionization chambers

28. Geiger-Muller counters
nAn ionization chamber
nVoltage across an ionization chamber is very high
nThe output is not dependent on the ionization energy but
rather is a function of the electric field in the chamber.
nBecause of this, the GM counter can “count” single
particles whereas this would be insufficient to trigger a
proportional chamber.
nThis very high voltage can also trigger a false reading
immediately after a valid reading.

29. Geiger-Muller counters
nTo prevent this, a quenching gas is added to the noble gas
that fills the counter chamber.
nThe G-M counter is made as a tube, up to 10-15cm long
and about 3cm in diameter.
nA window is provided to allow penetration of radiation.
nThe tube is filled with argon or helium with about 5-10%
alcohol (Ethyl alcohol) to quench triggering.
nThe operation relies heavily on the avalanche effect
nUV radiation is released which, in itself adds to the
avalanche process.
nThe output is about the same no matter what the ionization
energy of the input radiation is.

30. Geiger-Muller counters
nBecause of the very high voltage, a single particle
can release 10
9
to 10
10
ion pairs.
nThis means that a G-M counter is essentially
guaranteed to detect any radiation through it.
nThe efficiency of all ionization chambers depends
on the type of radiation.
nThe cathodes also influence this efficiency
nHigh atomic number cathodes are used for higher
energy radiation ( rays) and lower atomic
number cathodes to lower energy radiation.

31. Geiger-Muller sensor

32. Scintillation sensors
nTakes advantage of the radiation to light conversion
(scintillation) that occurs in certain materials.
nThe light intensity generated is then a measure of the
radiation’s kinetic energy.
nSome scintillation sensors are used as detectors in
which the exact relationship to radiation is not
critical.
nIn others it is important that a linear relation exists
and that the light conversion be efficient.

33. Scintillation sensors
nMaterials used should exhibit fast light decay
following irradiation (photoluminescence) to allow
fast response of the detector.
nThe most common material used for this purpose is
Sodium-Iodine (other of the alkali halide crystals may
be used and activation materials such as thalium are
added)
nThere are also organic materials and plastics that may
be used for this purpose. Many of these have faster
responses than the inorganic crystals.

34. Scintillation sensors
nThe light conversion is fairly weak because it involves
inefficient processes.
nLight obtained in these scintillating materials is of light
intensity and requires “amplification” to be detectable.
nA photomultiplier can be used as the detector
mechanism as shown in Figure 9.5 to increase
sensitivity.
nThe large gain of photomultipliers is critical in the
success of these devices.

35. Scintillation sensors
nThe reading is a function of many parameters.
nFirst, the energy of the particles and the efficiency of
conversion (about 10%) defines how many photons are
generated.
nPart of this number, say k, reaches the cathode of the
photomultiplier.
nThe cathode of the photomultiplier has a quantuum efficiency
(about 20-25%).
n
This number, say k
1 is now multiplied by the gain of the
photomultiplier G which can be of the order of 10
6
to 10
8
.

36. Scintillation sensor

37. Semiconductor radiation
detectors
nLight radiation can be detected in semiconductors
through release of charges across the band gap
nHigher energy radiation can be expected do so at
much higher efficiencies.
nAny semiconductor light sensor will also be sensitive
to higher energy radiation
nIn practice there are a few issues that have to be
resolved.

38. Semiconductor radiation
detectors
nFirst, because the energy is high, the lower bandgap materials
are not useful since they would produce currents that are too
high.
nSecond, high energy radiation can easily penetrate through the
semiconductor without releasing charges.
nThicker devices and heavier materials are needed.
nAlso, in detection of low radiation levels, the background
noise, due to the “dark” current (current from thermal sources)
can seriously interfere with the detector.
n Because of this, some semiconducting radiation sensors can
only be used at cryogenic temperatures.

39. Semiconductor radiation
detectors
nWhen an energetic particle penetrates into a semiconductor,
it initiates a process which releases electrons (and holes)
nthrough direct interaction with the crystal
nthrough secondary emissions by the primary electrons
nTo produce a hole-electron pair energy is required:
nCalled ionization energy - 3-5 eV (Table 9.2).
nThis is only about 1/10 of the energy required to release an ion
pair in gases
nThe basic sensitivity of semiconductor sensors is an order
of magnitude higher than in gases.

40. Properties of semiconductors
Table 9.2. Properties of some common semiconductors
Material Operating
temp [°K]
Atomic
number
Band gap [eV]Energy per electron-
hole pair [eV]
Silicon (Si) 300 14 1.12 3.61
Germanium (Ge) 77 32 0.74 2.98
Cadmium-teluride
(CdTe)
300 48, 52 1.47 4.43
Mercury-Iodine (HgI2)300 80, 53 2.13 6.5
Gallium-Arsenide
(GaAs)
300 31, 33 1.43 4.2

41. Semiconductor radiation
detectors
nSemiconductor radiation sensors are essentially diodes
in reverse bias.
nThis ensures a small (ideally negligible) background
(dark) current.
nThe reverse current produced by radiation is then a
measure of the kinetic energy of the radiation.
nThe diode must be thick to ensure absorption of the
energy due to fast particles.
nThe most common construction is similar to the PIN
diode and is shown in Figure 9.6.

42. Semiconductor radiation sensor

43. Semiconductor radiation
detectors
nIn this construction, a normal diode is built but with a
much thicker intrinsic region.
nThis region is doped with balanced impurities so that
it resembles an intrinsic material.
nTo accomplish that and to avoid the tendency of drift
towards either an n or p behavior, an ion-drifting
process is employed by diffusing a compensating
material throughout the layer.
nLithium is the material of choice for this purpose.

44. Semiconductor radiation
detectors
nAdditional restrictions must be imposed:
nGermanium can be used at cryogenic temperatures
nSilicon can be used at room temperature but:
nSilicon is a light material (atomic number 14)
nIt is therefore very inefficient for energetic radiation such
as  rays.
nFor this purpose, cadmium telluride (CdTe) is the most
often used because it combines heavy materials (atomic
numbers 48 and 52) with relatively high bandgap
energies.

45. Semiconductor radiation
detectors
n
Other materials that can be used are the mercuric iodine (HgI
2)
and gallium arsenide (GaAs).
nHigher atomic number materials may also be used as a simple
intrinsic material detector (not a diode) because the background
current is very small (see chapter 3).
nThe surface area of these devices can be quite large (some as high
as 50mm in diameter) or very small (1mm in diameter) depending
on applications.
nResistivity under dark conditions is of the order of 10
8
to 10
10

.cm depending on the construction and on doping, if any
(intrinsic materials have higher resistivity).
n.

46. Semiconductor radiation detectors -
notes
n
The idea of avalanche can be used to increase
sensitivity of semiconductor radiation detectors,
especially at lower energy radiation.
n
These are called avalanche detectors and operate
similarly to the proportional detectors
n
While this can increase the sensitivity by about two
orders of magnitude it is important to use these only
for low energies or the barrier can be easily breached
and the sensor destroyed.

47. Semiconductor radiation detectors -
notes
nSemiconducting radiation sensors are the most sensitive and
most versatile radiation sensors
nThey suffer from a number of limitations.
nDamage can occur when exposed to radiation over time.
nDamage can occur in the semiconductor lattice, in the package
or in the metal layers and connectors.
nProlonged radiation may also increase the leakage (dark)
current and result in a loss of energy resolution of the sensor.
nThe temperature limits of the sensor must be taken into
account (unless a cooled sensor is used).

48. Microwave radiation sensors -
introduction
nMicrowaves are often employed in the sensing of
other quantities because of the relative ease of
generating, manipulating and detecting microwave
radiation.
nUse in speed sensing, in sensing of the environment
(radar, doppler radar, mapping of the earth and
planets, etc.) are well known.
nAll of these applications and sensors are based on the
properties – especially the propagation properties of
electromagnetic waves.

49. Electromagnetic waves
nProperties of waves were discussed in ch. 6.
nElectromagnetic waves differ from acoustic waves in
two fundamental ways
nThe electromagnetic wave is a transverse wave (acoustic
waves are longitudinal)
nThe electromagnetic wave is the variation in space and
time of the electric and magnetic field.
nThe electric field intensity E and the magnetic field
intensity H are transverse to the direction of
propagation of the wave and to each other.

50. Electromagnetic waves
nThe electric and magnetic field can propagate in
matter as well as in vacuum.
nA visual interpretation of how an electromagnetic wave
propagate is shown in Figure 9.7.
nThe properties of the electromagnetic wave are
significantly different than those of the acoustic wave
numerically.
nThe most important is the speed of propagation of the
wave (also called phase velocity).

51. Propagation of electromagnetic
waves

52. Electromagnetic waves
nThe phase velocity is given as
v
p
=
1
εμ
 is the permittivity and  the permeability of the
medium in which the wave propagates.
The wavelength  and wavenumber k which depend
on phase velocity also change.
The phase velocity of electromagnetic waves in
vacuum is 3x10
8
m/s but is lower in all other media

53. Electromagnetic waves
nAttenuation of electromagnetic waves, is exponential and material
dependent
nIt is zero in vacuum
nIt is low in low conductivity materials such as dielectrics.
nIt is high in conducting materials.
nThe whole spectrum of electromagnetic waves, from very low to
very high frequencies may be used for sensing
nMicrowaves are particularly well suited for this purpose.
nThe microwave spectrum is defined broadly from about 300 MHz
to 300 GHz (wavelengths from 1m to 1mm).
nThe band above this is sometimes called millimeter waves and
overlaps with the low infrared band.

54. The electromagnetic spectrum

55. Microwave sensing
nSensing with microwaves is based on four
distinct methods, some more useful than
others:
n1. Propagation of waves
n2. Reflection of waves
n3. Transmission of waves
n4. Resonance
nThese may be combined in a sensor to affect a
particular function.

56. Microwave sensing - RADAR
n
RADAR - RAdio Detection And Ranging.
n
Best known method of microwave sensing
n
In its simplest form it is not much different than a
simple flashlight (source) and our eye (detector)
n
Shown schematically in Figure 9.9.
n
The larger the target and the more intense the source
of waves, the larger the signal received back from the
target.

57. Scattering of electromagnetic
waves

58. Microwave sensing - RADAR
nReception may be by the same antenna (pulsed-echo
radar), or (a-static radar)
nReception may be continuous by a separate antenna
(bi-static radar)
nBoth are shown in Figure 9.10.
nThe operation of radar is based on the reflection of
waves by any target the incident waves encounter.

59. A-static and bi-static radar

60. Radar
nFor any object in the path of electromagnetic waves, the
scattering coefficient, called the scattering cross-section or radar
cross-section 
σ = 4πR
2
P
s
P
i
P
s is the scattered power density
Pi the incident power density
R isthe distance from source to target

61. Radar
n
The power received is calculated from the radar
equation
 is the wavelength
 the radar cross-section
P
r
the total received power
P
rad
the total radiated power
D is the directivity of the antenna.
P
r
= P
rad
σ
λ
2
D
2
4π
3
R
4

62. Radar
nDirectivity is a property of the antenna
nIt is an indication of how directive the radiation is
nDepends on the type and construction of antennae.
nRadar is a short range device because of dependency on 1/R
4
.
nIt is one of the most useful sensing systems capable of sensing
distance as well as size (radar cross-section) of objects.
nIn more sophisticated systems the position (distance and
attitude) may be sensed as well as the speed of the target but
these are obviously as much a function of the signal
processing involved as they are of the radar itself.

63. Doppler Radar
n
A different approach to radar sensing is based on the
doppler effect.
n
In this type of radar, the amplitude and power
involved are not important (as long as a reflection is
received).
n
Rather, the doppler effect is taken advantage of.
n
This effect is simply a change in the frequency of the
reflected waves due to the speed of a target.

64. Doppler Radar
nConsider a target moving away from a source at a velocity v as
shown in Figure 9.11.
nThe source transmits a signal at frequency f.
nThe reflected signal arrives back at the transmitter after a delay
2t where t=S/v.
nThis delay causes a shift in the frequency of the received signal
as follows:
f ' =
f
1 +
2v
c

65. Doppler radar - principle
(1)
(2)
source
vehicle
v
t1
source
vehicle
v
t2
ΔS

66. Doppler Radar
nThe returning wave’s signal is lower the higher the
velocity of the vehicle.
nIf the motion is towards the radar source, the
frequency increases (negative velocity).
nMeasuring this frequency gives an accurate
indication of the speed of the vehicle.
nUsed in police speed detectors
nThe same can be used to detect aircraft or
tornadoes – all relying on speed detection.
nDoppler radar is totally blind to stationary targets.

67. Doppler Radar - notes
nDoppler radar is also actively pursued for anti
collision systems in vehicles (rudimentary systems
exist in trucks for side collision detection) and for
active cruise control.
nRadar relies heavily on good antennas and on
directivity of these antennas.
nPractical radar sensor operate at relatively high
frequencies – from about 10GHz to 30 GHz
nSystems for collision avoidance operate in excess
of 80 GHz

68. Radar - notes
nThere are many other types of radar.
nOne is the into the ground radar (also called
ground penetrating radar).
nOperates at lower frequencies for the purpose of
penetrating and mapping underground objects.
nFor space exploration and for mapping of planets,
- SAR (Synthetic Aperture Radar)
nThis method makes use of moving antennas and
signal processing to increase the range and
apparent power of radar.

69. Reflection sensors
nThe basic approach is to send an electromagnetic
wave and sense the reflected waves but,
npropagation aspect is negligible since the distance
is very short (different from radar) This is
nShown schematically in Figure 9.12.
nReflection coefficient of an electromagnetic wave
depends on the wave impedance of the materials
involved.

70. Reflection sensors
nAssuming that the source is in air, and it propagates into a
material, denoted as (1).
nThe wave impedances of the materials are
η
0
=
μ
0
ε
0
, η
1
=
μ
1
ε
1
< η
0
The reflection coefficient is
Γ =
η
1
− η
0
η
1
+ η
0

71. Reflection sensors
nReflection coefficient varies between –1 to +1
nDepends on the properties of the materials
n
For amplitude E
0, the reflected amplitude is E
0.
nThis is measured and can be directly linked to the permittivity in
material (1).
nReflection coefficient depends on permittivity - it depends on many
parameters, the most obvious is moisture.
nThe sensor in Figure 9.13 is in fact a moisture monitor: it can be
calibrated in terms of material density, material thickness, etc. since
all of these affect permittivity.
nThis particular sensor is calibrated in terms of water content in solids
(0 to 100%)

72. Microwave moisture sensor

73. Transmission sensors
nA transmission sensor may be built equally easily and is
shown in principle in Figure 9.14.
nThe transmission between source and detector is a
function of the material intervening (T = 1 + ).
nThe sensor can be calibrated in terms of any of the
properties of the material.
nMoisture content is most often the stimulus since water
has a high permittivity and can be sensed easily and
because water content is important to a wide range of
industries (paper, fabrics, foods)

74. Transmission sensing

75. Resonant microwave sensors
nA third important method of sensing with microwave is
based on microwave resonators.
nA microwave resonator may be thought of as a box, or
cavity with conducting walls that confines the waves.
nStanding waves are generated (provided that energy is
coupled into the structure) in each dimension of the
cavity.
nThe standing waves the cavity can support must be a
multiple integer of half wavelengths in any dimension
or a combination of these.

76. Resonant microwave sensors
nThese are the resonant frequencies of the cavity
nFor a rectangular cavity of dimensions a,b,c, the resonant
frequencies are:
f
mnp
=
1
2πμε

m
a
2
+
n
b
2
+
p
c
2
m, n, p are integers (0,1,2,….) can take different values.
These define the modes of the cavity.

77. Resonant microwave sensors
nThese define the modes of the cavity.
nFor example in an air filled cavity, for m=1, n=0, p=0,
the 100 mode is excited. Its frequency in a cavity of
dimensions a=b=c=0.1m is 477.46 MHz.
nNot all values of m,n,p result in valid modes but for
simplicity’s sake the discussion here should be
sufficient.
nCavities do not need to be rectangular – they may be
cylindrical or of any complex shape in which case the
analysis is much more complicated.

78. Resonant microwave sensors
nAt resonance the fields in the cavity are very high
nOff-resonance they are very low.
nThe cavity acts as a sharp band-pass filter
nResonant frequency depends on the electrical properties of the
material in the cavity – its permittivity and its permeability, in
addition to physical dimensions.
nAny material inserted in the cavity will reduce its resonant
frequency (since air has the lowest permittivity).
nBecause resonance is sharp, the change in resonant frequency
is easily measured and can be correlated to the sensed
quantity.

79. Resonant microwave sensors
nTo produce a cavity resonator sensor, there are two
basic conditions necessary:
nFirst, the property sensed must somehow alter the
material permittivity in the cavity or its dimensions.
nSecond, a means of coupling energy into the cavity
must be found.
nThe resonant frequency is then measured and,
provided a transfer function can be established the
stimulus is sensed directly.

80. Resonant microwave sensors
nEnergy can be supplied to a cavity in many ways
nThe simplest is to simply insert a probe (a small
antenna) which radiates fields in the cavity.
nThis is shown in Figure 9.15.
nThose fields at the right frequency are amplified by
the standing waves, the others are negligible.

81. Coupling to a cavity resonator

82. Resonant sensors
nTo sense a quantity, the permittivity must change
with this quantity.
nThis can be accomplished in a number of ways.
nFor gases, it is sufficient to provide holes in the walls
of the cavity to allow them to penetrate
nIn this form, the cavity can sense gases emitted by
explosives, fumes from chemical processes, smoke,
moisture and almost anything else that has a
permittivity larger than air.

83. Gas sensing cavity resonator

84. Cavity resonator sensors
nThese “sniffers” can be extremely sensitive but
nIt is difficult to separate the effects of say, smoke and
moisture
nThe measurement of resonant frequency at the
frequencies involved is not a trivial issue.
nNevertheless, these methods are some of the most useful
in evaluation of gases.
nSolids may be equally sensed for variations in
permittivity provided they can be inserted into the
cavity.

85. Cavity resonator sensors
nThe change in resonant frequency is usually small
nTypically on a fraction of a percent
nSince the frequencies are high, it is sufficient for
detection.

86. Open cavity resonator sensors
nTo allow measurements on solids,
nThe idea of the cavity can be extended by partially
opening the cavity and allowing the solids to move
through the cavity.
nAn example is shown in Figure 9.17.
nResonance is established by the two strips acting as a
transmission line between the two plates.
nResonance depends on the lengths of the strips as
well as location and size of the plates.

87. Stripline resonator

88. Open cavity resonator sensors
nThe material to be sensed for variations in permittivity passes
between the strips.
nThis method has been successfully used to sense moisture
content in paper, wood veneers, plywood and to monitor the
curing process in rubber and polymers.
nTo improve performance, the plates are bent down to partially
enclose the cavity.
nThis improves sensitivity and reduces influences from outside.
nFigure 9.18 shows an open cavity resonator operating at 370
MHz in air and designed to monitor the water content in
drying latex in a continuous industrial coating process.

89. Open cavity resonator

90. Open cavity resonator
n
The change in resonant frequency is only about 2
MHz (from wet to dry)
n
This represents about 0.5% change in frequency.
n
Using a network analyzer, changes of the order of
less than 1 kHz are easily measured
n
This makes for a very sensitive device.

91. Open cavity resonator - notes
nA variation of the open resonator is the transmission
line resonator shown in Figure 9.19.
nMade of two strips at fixed distance from each other and
shorted at both ends.
nConnections are made to each strip
nThe resonant frequency depends on dimensions and
locations of the feed wires and, of course, on the
permittivity of the material.
nA similar device is commonly used to sense the
thickness of asphalt on roads.

92. Transmission line resonator

93. Antennas as sensors
n
Antennas are unique devices
n
Not normally thought of as sensors since they are usually
associated with transmitters and receivers.
n
Antennas are true sensors – sensing the electric or the
magnetic field in the electromagnetic wave.
n
One can say that the receiver or transmitter are in fact
transducers and the antenna is the sensor (in a receiver ) or the
actuator (in a transmitter).
n
In microwaves, antennas are often referred to as “probes”
because of their use as sensors and actuators.

94. Antennas as sensors
nAntennas are based on the operation of one of two
related fundamental or elementary antennas.
nThe electric dipole and
nThe magnetic dipoles
nThe electric dipole is a very short antenna, made as
shown in Figure 9.20a.
nIt consists of two short conducting segments (in the
ideal case they are differential in length), fed by a
transmission line.

95. Elementary electric and magnetic
dipole antennas

96. Antennas as sensors
nThe magnetic dipole, shown in Figure 9.20b is a loop
of small diameter fed by a transmission line.
nTheir names are related to the fields they produce
which look like the fields of an electric dipole and a
magnetic dipole respectively.
nIn all other respects, the two antennas are very similar
and, in fact, the two produce identical field
distribution in space except that the magnetic field of
the electric dipole is identical (in shape) to the
magnetic field of the magnetic dipole and vice versa.

97. Antennas as sensors
nThe field radiated from a small dipole (electric or
magnetic) is shown in Figure 9.21.
n It shows, that near the antenna, the field is essentially
the same as for an electrostatic dipole.
nIt is called the electrostatic field or the near field.
nWhen antennas are very close to a source (less than
about one wavelength), they behave more or less like
capacitors.
nAt larger distances, the antennas radiate (or receive
radiation) in what is called the far field.

98. Radiaton from an electric dipole

99. Antenna relations
nThe electric field intensity and magnetic field intensity of a
dipole in the far field are
H =
Il
2λR
e
−j2πλ /R
sin θ
I,R
E = ηH
l is the length of the dipole
 the wavelength
R the distance from antenna
 is the angle between the antenna and the direction of
propagation of the wave
 is the wave impedance in space

100. Antenna relations
n  is called the wave impedance and in vacuum (air)
is equal to 377 .
nThe ratio between the electric field and magnetic field
is constant and equal the wave impedance.
nThis impedance is only dependent on material
properties and equals:
η =
E
H
=
μ
ε

101. Antenna properties
nThe electric field and magnetic field are
perpendicular to each other
nBoth are perpendicular to the direction R, in which
direction the wave propagates.
nMaximum fields are obtained when =90, that is,
perpendicular to the current.
nA plot of the relation will reveal that the fields
diminish as the angle becomes smaller or larger and
at =0 the field is zero.

102. Antenna properties
nThis plot is called the radiation pattern of the antenna
nIt gives the distribution of the field over a plane that contains the
dipole (other planes may also be selected and similarly
described).
nThe radiation pattern changes with the length and type of
antenna.
nAnother important quantity is the directivity of the antenna
nIt simply indicates the relative power density in all directions in
space.
nAntennas are dual elements – they are equally suitable
for transmission and for reception

103. Antenna as a sensor
nThe electric dipole, may be viewed as an electric field
sensor.
nThe magnetic dipole senses a magnetic field
nFigure 9.22 shows a propagating wave at the location
of the antenna, making an angle  with it.
nThe electric field intensity in the wave is E and is
perpendicular to the direction of propagation of the
wave.

104. The electric dipole as a sensor

105. Antenna as a sensor
nThe voltage of the antenna due to this field (assuming
l is small) is:
V
d
= Elsin θ
A linear relation between the electric field and the
voltage is obtained.
Only true for very short antennas while for longer
antennas the relation is not liner.

106. Antenna sensors - notes
nMore practical antennas are made of various lengths
(or diameters)
nMay have different shapes and may in fact be an
array of antennas
nIn general, the “larger” the antenna, the higher the
power it can transmit or receive (not always and not
linearly).
nThe size of the antenna changes the radiation pattern
of the antenna but again

107. Antenna sensors - notes
nAntennas are very efficient sensors/actuators
nConversion efficiencies that can easily exceed 95%.
nIn practical applications, certain antennas have been shown to be
better than others in some respects.
nMost applications try to use a /2 antenna if possible:
nIts input impedance can be shown to be 73
nThe antenna has a good radiation pattern,
nOther antennas are higher or lower in impedance.
nDipole antennas can sometimes be replaced by monopoles (half a
dipole – like the car antenna) with appropriate changes in
properties (half the impedance, half the total radiated power, etc.)