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Radiation Sensors
Chapter 9
Introduction
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We have discussed radiation in Chapter 4 when
talking about light sensors.
Our particular concern there was the general range
occupied by the infrared, visible and ultraviolet
radiation.
Here we will concern ourselves with the ranges below
and above these.
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Range above UV
Range below IR.
Introduction
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Range above UV is characterized by ionization –
Frequency is sufficiently high to ionize molecules based on
Plank’s equation.
The 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.
On the other hand, below the infrared region, the
electromagnetic radiation can be generated and detected by
simple antennas.
We will therefore discuss the idea of an antenna and its use as
a sensor.
Introduction
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All radiation may be viewed as electromagnetic
radiation.
Many of the sensing strategies, including those
discussed in chapter 4 may be viewed as radiation
sensing.
We will however follow the conventional
nomenclature
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Will call low frequency radiation “electromagnetic” (electromagnetic
waves, electro-magnetic energy, etc.)
Will call high frequency radiation, simply “radiation” (as in Xray, or cosmic)
Introduction
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One 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.
Introduction
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At high frequencies, where particles are concerned,
one can view them either as particles or as waves.
The energy in these waves is given by the Planck
equation.
Their wavelength is given by de Broglie’s equation
(p=mv is the momentum of the particle):
 = hp
Introduction
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The higher the frequency the higher the photon energy.
At high frequencies, the photon energy is sufficient to strip
electrons from atoms –ionizing radiation.
At low frequencies, ionization does not happen and hence
these waves are called non-ionizing.
The highest frequency in the microwave region is 300 Ghz.
The photon energy is 0.02 eV. This is considered nonionizing.
The lowest frequency in the X-ray region is approximately
3x1016 and the photon energy is 2000 eV. Clearly an ionizing
radiation.
Introduction
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Some view radioactive radiation as something different than,
say X-ray radiation or microwaves
It is often viewed as particle radiation.
One can take this approach based on the duality of
electromagnetic radiation, just as we can view light as
electromagnetic or as particles – photons.
We will base all our discussion on the photon energy of
radiation and not on the particle aspects.
In some cases it will be convenient to talk about particles.
(Geiger-Muller counter, for example)
Introduction
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Many 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).
There are however exception such as smoke
detection and measurement of material thickness
through radiation.
In the lower range, the sensing of a variety of
parameters through microwaves is the most
important
Sensing of the microwave themselves is not
Units
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Units for radiation, except for low frequency
electromagnetic radiation are divided into three:
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Units of activity,
Units of exposure
Units of absorbed dose.
Also - units for dose equivalent.
The basic unit of activity is the Becquerel [Bq]
Defined as one transition (disintegration) per
second.
It indicates the rate of decay of a radionuclide.
Units
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An older, non-SI unit of activity was the curie (1
curie=3.7x1010 becquerel).
The Becquerel is a small unit so that the [MBq],
[GBq] and [TBq] are often used.
The basic unit of exposure is the coulomb per
kilogram [C/kg]=[A.s/kg].
The older unit was the roentgen (1
roentgen=2.58x10 C/kg].
The [C/kg] is a very large unit and units of [mC/kg],
C/kg] and [pC/kg] are often used.
Units
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Absorbed dose is measured in grays [Gy] which is [J/kg].
The Gray is energy per kilogram and 1[Gy]=1[J/kg].
The old unit of absorbed dose was the rad (1 rad = 100
[Gy]).
The units for dose equivalence is the sievert [Sv] in [J/kg].
The old unit is the rem (1 rem = 100 [Sv]).
Note that the sievert and the gray are the same.
This is because they measure identical quantities in air.
However 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.
Radiation sensors
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Will start the discussion with ionization sensors
Then will discuss the much lower frequency
methods based on electromagnetic radiation
Three basic types of radiation sensors:
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Ionization sensors
Scintillation sensors
Semiconductor radiation sensors
These sensors are either:
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Detectors – detection without quantification or:
Sensor - both detection and quantification
Ionization sensors (detectors)
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In an ionization sensor, the radiation passing
through a medium (gas or solid) creates electronproton pairs
Their density and energy depends on the energy of
the ionizing radiation.
These charges can then be attracted to electrodes
and measured or they may be accelerated through
the use of magnetic fields for further use.
The simplest and oldest type of sensor is the
ionization chamber.
Ionization chamber
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The chamber is a gas filled chamber
Usually at low pressure
Has predictable response to radiation.
In most gases, the ionization energy for the outer electrons is
fairly small – 10 to 20 eV.
A 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).
For sensing, the important quantity is the W value.
It is an average energy transferred per ion pair generated.
Table 9.1 gives the W values for a few gases used in ion
chambers.
W values for gases
Table 9.1. W va lues for var ious gases used in ionization chambers (eV/ion pair)
Gas
Electro ns (fast)
Alpha particles
Argo n (A)
27.0
25.9
Helium (He)
32.5
31.7
Nitroge n (N2)
35.8
36.0
Air
35.0
35.2
CH4
30.2
29.0
Ionization chamber
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Clearly ion pairs can also recombine.
The current generated is due to an average rate of
ion generation.
The principle is shown in Figure 9.1.
When no ionization occurs, there is no current as
the gas has negligible resistance.
The voltage across the cell is relatively high and
attracts the charges, reducing recombination.
Under these conditions, the steady state current is a
good measure of the ionization rate.
Ionization chamber
Ionization chamber
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The chamber operates in the saturation region of the
I-V curve.
The higher the radiation frequency and the higher the
voltage across the chamber electrodes the higher the
current across the chamber.
The chamber in Figure 9.1. is sufficient for high
energy radiation
For low energy X-rays, a better approach is needed.
Ionization chamber - applications
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The most common use for ionization chambers is in
smoke detectors.
The chamber is open to the air and ionization occurs
in air.
A small radioactive source (usually Americum 241)
ionizes the air at a constant rate
This causes a small, constant ionization current
between the anode and cathode of the chamber.
Combustion products such as smoke enter the
chamber
Ionization chamber - applications
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Smoke particles are much larger and heavier than air
They form centers around which positive and negative charges
recombine.
This reduces the ionization current and triggers an alarm.
In most smoke detectors, there are two chambers.
One 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
In it the openings to air are too small to allow the large smoke
particles but will allow humidity.
The trigger is now based on the difference between these two
currents.
Ionization chambers in a residential
smoke detector
Ionization chambers - application
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Fabric density sensor (see figure).
The lower part contains a low energy radioactive isotope
(Krypton 85)
The upper part is an ionization chamber.
The fabric passes between them.
The ionization current is calibrated in terms of density (i.e.
weight per unit area).
Similar 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
A nuclear fabric density sensor
Proportional chamber
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A proportional chamber is a gas ionization chamber
but:
The potential across the electrodes is high enough to
produce an electric field in excess of 106 V/m.
The electrons are accelerated, process collide with
atoms releasing additional electrons (and protons) in a
process called the Townsend avalanche.
These charges are collected by the anode and because
of this multiplication effect can be used to detect
lower intensity radiation.
Proportional chamber
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The device is also called a proportional counter or
multiplier.
If the electric field is increased further, the output
becomes nonlinear due to protons which cannot move
as fast as electrons causing a space charge.
Figure 9.2 shows the region of operation of the
various types of gas chambers.
Operation of ionization chambers
Geiger-Muller counters
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An ionization chamber
Voltage across an ionization chamber is very high
The output is not dependent on the ionization energy
but rather is a function of the electric field in the
chamber.
Because of this, the GM counter can “count” single
particles whereas this would be insufficient to trigger
a proportional chamber.
This very high voltage can also trigger a false reading
immediately after a valid reading.
Geiger-Muller counters
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To prevent this, a quenching gas is added to the noble gas
that fills the counter chamber.
The G-M counter is made as a tube, up to 10-15cm long
and about 3cm in diameter.
A window is provided to allow penetration of radiation.
The tube is filled with argon or helium with about 5-10%
alcohol (Ethyl alcohol) to quench triggering.
The operation relies heavily on the avalanche effect
UV radiation is released which, in itself adds to the
avalanche process.
The output is about the same no matter what the ionization
energy of the input radiation is.
Geiger-Muller counters
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Because of the very high voltage, a single particle
can release 109 to 1010 ion pairs.
This means that a G-M counter is essentially
guaranteed to detect any radiation through it.
The efficiency of all ionization chambers depends
on the type of radiation.
The cathodes also influence this efficiency
High atomic number cathodes are used for higher
energy radiation ( rays) and lower atomic number
cathodes to lower energy radiation.
Geiger-Muller sensor
Scintillation sensors
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Takes advantage of the radiation to light conversion
(scintillation) that occurs in certain materials.
The light intensity generated is then a measure of the
radiation’s kinetic energy.
Some scintillation sensors are used as detectors in
which the exact relationship to radiation is not
critical.
In others it is important that a linear relation exists
and that the light conversion be efficient.
Scintillation sensors
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Materials used should exhibit fast light decay
following irradiation (photoluminescence) to allow
fast response of the detector.
The 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)
There are also organic materials and plastics that may
be used for this purpose. Many of these have faster
responses than the inorganic crystals.
Scintillation sensors
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The light conversion is fairly weak because it
involves inefficient processes.
Light obtained in these scintillating materials is of
light intensity and requires “amplification” to be
detectable.
A photomultiplier can be used as the detector
mechanism as shown in Figure 9.5 to increase
sensitivity.
The large gain of photomultipliers is critical in the
success of these devices.
Scintillation sensors
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The reading is a function of many parameters.
First, the energy of the particles and the efficiency of
conversion (about 10%) defines how many photons are
generated.
Part of this number, say k, reaches the cathode of the
photomultiplier.
The cathode of the photomultiplier has a quantuum efficiency
(about 20-25%).
This number, say k1 is now multiplied by the gain of the
photomultiplier G which can be of the order of 106 to 108.
Scintillation sensor
Semiconductor radiation
detectors
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Light radiation can be detected in semiconductors
through release of charges across the band gap
Higher energy radiation can be expected do so at
much higher efficiencies.
Any semiconductor light sensor will also be sensitive
to higher energy radiation
In practice there are a few issues that have to be
resolved.
Semiconductor radiation
detectors
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First, because the energy is high, the lower bandgap materials
are not useful since they would produce currents that are too
high.
Second, high energy radiation can easily penetrate through the
semiconductor without releasing charges.
Thicker devices and heavier materials are needed.
Also, in detection of low radiation levels, the background
noise, due to the “dark” current (current from thermal sources)
can seriously interfere with the detector.
Because of this, some semiconducting radiation sensors can
only be used at cryogenic temperatures.
Semiconductor radiation
detectors
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When an energetic particle penetrates into a
semiconductor, it initiates a process which releases
electrons (and holes)
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To produce a hole-electron pair energy is required:
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through direct interaction with the crystal
through secondary emissions by the primary electrons
Called ionization energy - 3-5 eV (Table 9.2).
This is only about 1/10 of the energy required to release an
ion pair in gases
The basic sensitivity of semiconductor sensors is an
order of magnitude higher than in gases.
Properties of semiconductors
Table 9.2. Properties of some common semiconductors
Material
Operating Atomic
Band gap [eV]
temp [K] number
Silicon (Si)
300
14
1.12
Germanium (Ge)
77
32
0.74
Cadmium- teluride
300
48, 52
1.47
(CdTe)
Mercury- Iodine (HgI2) 300
80, 53
2.13
Gallium-Ars enide
300
31, 33
1.43
(GaAs)
Energy per electronhole pair [eV]
3.61
2.98
4.43
6.5
4.2
Semiconductor radiation
detectors
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Semiconductor radiation sensors are essentially
diodes in reverse bias.
This ensures a small (ideally negligible) background
(dark) current.
The reverse current produced by radiation is then a
measure of the kinetic energy of the radiation.
The diode must be thick to ensure absorption of the
energy due to fast particles.
The most common construction is similar to the PIN
diode and is shown in Figure 9.6.
Semiconductor radiation sensor
Semiconductor radiation
detectors
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In this construction, a normal diode is built but with a
much thicker intrinsic region.
This region is doped with balanced impurities so that
it resembles an intrinsic material.
To 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.
Lithium is the material of choice for this purpose.
Semiconductor radiation
detectors
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Additional restrictions must be imposed:
Germanium can be used at cryogenic temperatures
Silicon can be used at room temperature but:
Silicon is a light material (atomic number 14)
It is therefore very inefficient for energetic radiation
such as  rays.
For 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.
Semiconductor radiation
detectors
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Other materials that can be used are the mercuric iodine (HgI2)
and gallium arsenide (GaAs).
Higher 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).
The 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.
Resistivity under dark conditions is of the order of 108 to 1010
.cm depending on the construction and on doping, if any
(intrinsic materials have higher resistivity).
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Semiconductor radiation detectors notes
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The idea of avalanche can be used to increase
sensitivity of semiconductor radiation detectors,
especially at lower energy radiation.
These are called avalanche detectors and operate
similarly to the proportional detectors
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.
Semiconductor radiation detectors notes
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Semiconducting radiation sensors are the most sensitive and
most versatile radiation sensors
They suffer from a number of limitations.
Damage can occur when exposed to radiation over time.
Damage can occur in the semiconductor lattice, in the package
or in the metal layers and connectors.
Prolonged radiation may also increase the leakage (dark)
current and result in a loss of energy resolution of the sensor.
The temperature limits of the sensor must be taken into
account (unless a cooled sensor is used).
Microwave radiation sensors introduction
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Microwaves are often employed in the sensing of
other quantities because of the relative ease of
generating, manipulating and detecting microwave
radiation.
Use in speed sensing, in sensing of the environment
(radar, doppler radar, mapping of the earth and
planets, etc.) are well known.
All of these applications and sensors are based on the
properties – especially the propagation properties of
electromagnetic waves.
Electromagnetic waves
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Properties of waves were discussed in ch. 6.
Electromagnetic waves differ from acoustic waves in
two fundamental ways
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The electromagnetic wave is a transverse wave (acoustic
waves are longitudinal)
The electromagnetic wave is the variation in space and time
of the electric and magnetic field.
The electric field intensity E and the magnetic field
intensity H are transverse to the direction of
propagation of the wave and to each other.
Electromagnetic waves
The electric and magnetic field can propagate in
matter as well as in vacuum.
A visual interpretation of how an electromagnetic
wave propagate is shown in Figure 9.7.
The properties of the electromagnetic wave are
significantly different than those of the acoustic wave
numerically.
The most important is the speed of propagation of the
wave (also called phase velocity).
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Propagation of electromagnetic
waves
Electromagnetic waves
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The phase velocity is given as
vp = 1

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 3x108 m/s but is lower in all other media
Electromagnetic waves
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Attenuation of electromagnetic waves, is exponential and
material dependent
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It is zero in vacuum
It is low in low conductivity materials such as dielectrics.
It is high in conducting materials.
The whole spectrum of electromagnetic waves, from very low
to very high frequencies may be used for sensing
Microwaves are particularly well suited for this purpose.
The microwave spectrum is defined broadly from about 300
MHz to 300 GHz (wavelengths from 1m to 1mm).
The band above this is sometimes called millimeter waves and
overlaps with the low infrared band.
The electromagnetic spectrum
Microwave sensing
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Sensing with microwaves is based on four
distinct methods, some more useful than
others:
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1.
2.
3.
4.
Propagation of waves
Reflection of waves
Transmission of waves
Resonance
These may be combined in a sensor to affect a
particular function.
Microwave sensing - RADAR
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RADAR - RAdio Detection And Ranging.
Best known method of microwave sensing
In its simplest form it is not much different than a
simple flashlight (source) and our eye (detector)
Shown schematically in Figure 9.9.
The larger the target and the more intense the source
of waves, the larger the signal received back from the
target.
Scattering of electromagnetic
waves
Microwave sensing - RADAR
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Reception may be by the same antenna (pulsed-echo
radar), or (a-static radar)
Reception may be continuous by a separate antenna
(bi-static radar)
Both are shown in Figure 9.10.
The operation of radar is based on the reflection of
waves by any target the incident waves encounter.
A-static and bi-static radar
Radar
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For any object in the path of electromagnetic waves, the
scattering coefficient, called the scattering cross-section or
radar cross-section 
=
Ps
2
4 R
Pi
Ps is the scattered power density
Pi the incident power density
R isthe distance from source to target
Radar
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The power received is calculated from the
radar equation
2D 2

Pr = Prad
4 3 R 4
is the wavelength
the radar cross-section
Pr the total received power
Prad the total radiated power
D is the directivity of the antenna.
Radar
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Directivity is a property of the antenna
It is an indication of how directive the radiation is
Depends on the type and construction of antennae.
Radar is a short range device because of dependency on 1/R4.
It is one of the most useful sensing systems capable of sensing
distance as well as size (radar cross-section) of objects.
In 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.
Doppler Radar
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A different approach to radar sensing is based on the
doppler effect.
In this type of radar, the amplitude and power
involved are not important (as long as a reflection is
received).
Rather, the doppler effect is taken advantage of.
This effect is simply a change in the frequency of the
reflected waves due to the speed of a target.
Doppler Radar
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Consider a target moving away from a source at a velocity v as
shown in Figure 9.11.
The source transmits a signal at frequency f.
The reflected signal arrives back at the transmitter after a delay
2t where t=S/v.
This delay causes a shift in the frequency of the received
signal as follows:
f'=
f
1 + 2v
c
Doppler radar - principle
source
v
vehicle
(1)
t1
source
v
(2)
vehicle
t2
S
Doppler Radar
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The returning wave’s signal is lower the higher the
velocity of the vehicle.
If the motion is towards the radar source, the
frequency increases (negative velocity).
Measuring this frequency gives an accurate
indication of the speed of the vehicle.
Used in police speed detectors
The same can be used to detect aircraft or
tornadoes – all relying on speed detection.
Doppler radar is totally blind to stationary targets.
Doppler Radar - notes
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Doppler 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.
Radar relies heavily on good antennas and on
directivity of these antennas.
Practical radar sensor operate at relatively high
frequencies – from about 10GHz to 30 GHz
Systems for collision avoidance operate in excess
of 80 GHz
Radar - notes
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There are many other types of radar.
One is the into the ground radar (also called
ground penetrating radar).
Operates at lower frequencies for the purpose of
penetrating and mapping underground objects.
For space exploration and for mapping of planets,
- SAR (Synthetic Aperture Radar)
This method makes use of moving antennas and
signal processing to increase the range and
apparent power of radar.
Reflection sensors
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The basic approach is to send an electromagnetic
wave and sense the reflected waves but,
propagation aspect is negligible since the distance
is very short (different from radar) This is
Shown schematically in Figure 9.12.
Reflection coefficient of an electromagnetic wave
depends on the wave impedance of the materials
involved.
Reflection sensors
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Assuming that the source is in air, and it propagates into a
material, denoted as (1).
The wave impedances of the materials are
0 =
0
0 ,
1 =
The reflection coefficient is
1  0
=  +
1
0
1
1 < 0
Reflection sensors
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Reflection coefficient varies between –1 to +1
Depends on the properties of the materials
For amplitude E0, the reflected amplitude is E0.
This is measured and can be directly linked to the permittivity
in material (1).
Reflection coefficient depends on permittivity - it depends on
many parameters, the most obvious is moisture.
The 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.
This particular sensor is calibrated in terms of water content in
solids (0 to 100%)
Microwave moisture sensor
Transmission sensors
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A transmission sensor may be built equally easily and is
shown in principle in Figure 9.14.
The transmission between source and detector is a
function of the material intervening (T = 1 + ).
The sensor can be calibrated in terms of any of the
properties of the material.
Moisture 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)
Transmission sensing
Resonant microwave sensors
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A third important method of sensing with microwave
is based on microwave resonators.
A microwave resonator may be thought of as a box,
or cavity with conducting walls that confines the
waves.
Standing waves are generated (provided that energy is
coupled into the structure) in each dimension of the
cavity.
The standing waves the cavity can support must be a
multiple integer of half wavelengths in any dimension
or a combination of these.
Resonant microwave sensors
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These are the resonant frequencies of the cavity
For a rectangular cavity of dimensions a,b,c, the resonant
frequencies are:
fmnp =
1
2 
m
a
2
p2
2
n
+
+ c
b
m, n, p are integers (0,1,2,….) can take different values.
These define the modes of the cavity.
Resonant microwave sensors
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These define the modes of the cavity.
For 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.
Not all values of m,n,p result in valid modes but for
simplicity’s sake the discussion here should be
sufficient.
Cavities do not need to be rectangular – they may be
cylindrical or of any complex shape in which case the
analysis is much more complicated.
Resonant microwave sensors
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At resonance the fields in the cavity are very high
Off-resonance they are very low.
The cavity acts as a sharp band-pass filter
Resonant frequency depends on the electrical properties of the
material in the cavity – its permittivity and its permeability, in
addition to physical dimensions.
Any material inserted in the cavity will reduce its resonant
frequency (since air has the lowest permittivity).
Because resonance is sharp, the change in resonant frequency
is easily measured and can be correlated to the sensed quantity.
Resonant microwave sensors
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To produce a cavity resonator sensor, there are two
basic conditions necessary:
First, the property sensed must somehow alter the
material permittivity in the cavity or its dimensions.
Second, a means of coupling energy into the cavity
must be found.
The resonant frequency is then measured and,
provided a transfer function can be established the
stimulus is sensed directly.
Resonant microwave sensors
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Energy can be supplied to a cavity in many ways
The simplest is to simply insert a probe (a small
antenna) which radiates fields in the cavity.
This is shown in Figure 9.15.
Those fields at the right frequency are amplified by
the standing waves, the others are negligible.
Coupling to a cavity resonator
Resonant sensors
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To sense a quantity, the permittivity must change with
this quantity.
This can be accomplished in a number of ways.
For gases, it is sufficient to provide holes in the walls
of the cavity to allow them to penetrate
In 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.
Gas sensing cavity resonator
Cavity resonator sensors
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These “sniffers” can be extremely sensitive but
It is difficult to separate the effects of say, smoke and
moisture
The measurement of resonant frequency at the
frequencies involved is not a trivial issue.
Nevertheless, these methods are some of the most
useful in evaluation of gases.
Solids may be equally sensed for variations in
permittivity provided they can be inserted into the
cavity.
Cavity resonator sensors
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The change in resonant frequency is usually small
Typically on a fraction of a percent
Since the frequencies are high, it is sufficient for
detection.
Open cavity resonator sensors
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To allow measurements on solids,
The idea of the cavity can be extended by partially
opening the cavity and allowing the solids to move
through the cavity.
An example is shown in Figure 9.17.
Resonance is established by the two strips acting as a
transmission line between the two plates.
Resonance depends on the lengths of the strips as
well as location and size of the plates.
Stripline resonator
Open cavity resonator sensors
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The material to be sensed for variations in permittivity passes
between the strips.
This method has been successfully used to sense moisture
content in paper, wood veneers, plywood and to monitor the
curing process in rubber and polymers.
To improve performance, the plates are bent down to partially
enclose the cavity.
This improves sensitivity and reduces influences from outside.
Figure 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.
Open cavity resonator
Open cavity resonator
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The change in resonant frequency is only about 2
MHz (from wet to dry)
This represents about 0.5% change in frequency.
Using a network analyzer, changes of the order of less
than 1 kHz are easily measured
This makes for a very sensitive device.
Open cavity resonator - notes
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A variation of the open resonator is the transmission
line resonator shown in Figure 9.19.
Made of two strips at fixed distance from each other
and shorted at both ends.
Connections are made to each strip
The resonant frequency depends on dimensions and
locations of the feed wires and, of course, on the
permittivity of the material.
A similar device is commonly used to sense the
thickness of asphalt on roads.
Transmission line resonator
Antennas as sensors
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Antennas are unique devices
Not normally thought of as sensors since they are usually
associated with transmitters and receivers.
Antennas are true sensors – sensing the electric or the
magnetic field in the electromagnetic wave.
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).
In microwaves, antennas are often referred to as “probes”
because of their use as sensors and actuators.
Antennas as sensors
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Antennas are based on the operation of one of two
related fundamental or elementary antennas.
The electric dipole and
The magnetic dipoles
The electric dipole is a very short antenna, made as
shown in Figure 9.20a.
It consists of two short conducting segments (in the
ideal case they are differential in length), fed by a
transmission line.
Elementary electric and magnetic
dipole antennas
Antennas as sensors
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The magnetic dipole, shown in Figure 9.20b is a loop
of small diameter fed by a transmission line.
Their names are related to the fields they produce
which look like the fields of an electric dipole and a
magnetic dipole respectively.
In 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.
Antennas as sensors
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The field radiated from a small dipole (electric or
magnetic) is shown in Figure 9.21.
It shows, that near the antenna, the field is essentially
the same as for an electrostatic dipole.
It is called the electrostatic field or the near field.
When antennas are very close to a source (less than
about one wavelength), they behave more or less like
capacitors.
At larger distances, the antennas radiate (or receive
radiation) in what is called the far field.
Radiaton from an electric dipole
Antenna relations
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The electric field intensity and magnetic field intensity of a
dipole in the far field are
H = Il e j2 /RsinI,R
2R
E = H
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
Antenna relations
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 is called the wave impedance and in vacuum (air)
is equal to 377 .
The ratio between the electric field and magnetic field
is constant and equal the wave impedance.
This impedance is only dependent on material
properties and equals:
= E =
H


Antenna properties
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The electric field and magnetic field are
perpendicular to each other
Both are perpendicular to the direction R, in which
direction the wave propagates.
Maximum fields are obtained when =90, that is,
perpendicular to the current.
A plot of the relation will reveal that the fields
diminish as the angle becomes smaller or larger and
at =0 the field is zero.
Antenna properties
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This plot is called the radiation pattern of the antenna
It gives the distribution of the field over a plane that contains
the dipole (other planes may also be selected and similarly
described).
The radiation pattern changes with the length and type of
antenna.
Another important quantity is the directivity of the antenna
It simply indicates the relative power density in all directions
in space.
Antennas are dual elements – they are equally
suitable for transmission and for reception
Antenna as a sensor
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The electric dipole, may be viewed as an electric field
sensor.
The magnetic dipole senses a magnetic field
Figure 9.22 shows a propagating wave at the location
of the antenna, making an angle  with it.
The electric field intensity in the wave is E and is
perpendicular to the direction of propagation of the
wave.
The electric dipole as a sensor
Antenna as a sensor
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The voltage of the antenna due to this field (assuming
l is small) is:
Vd = Elsin
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.
Antenna sensors - notes
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More practical antennas are made of various lengths
(or diameters)
May have different shapes and may in fact be an
array of antennas
In general, the “larger” the antenna, the higher the
power it can transmit or receive (not always and not
linearly).
The size of the antenna changes the radiation pattern
of the antenna but again
Antenna sensors - notes
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Antennas are very efficient sensors/actuators
Conversion efficiencies that can easily exceed 95%.
In practical applications, certain antennas have been shown to
be better than others in some respects.
Most applications try to use a /2 antenna if possible:
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Its input impedance can be shown to be 73
The antenna has a good radiation pattern,
Other antennas are higher or lower in impedance.
Dipole 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.)