1. Radiation Sensors
Chapter 9

2. Introduction
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.
Range above UV
Range below IR.

3. Introduction 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.

4. Introduction 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
Will call low frequency radiation “electromagnetic” (electromagnetic waves, electro-magnetic energy, etc.)
Will call high frequency radiation, simply “radiation” (as in X-ray, or cosmic)

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

6. Introduction
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):

7. Introduction 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 non-ionizing.
The lowest frequency in the X-ray region is approximately 3x1016 and the photon energy is 2000 eV. Clearly an ionizing radiation.

8. Introduction
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)

9. Introduction
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

10. Units
Units for radiation, except for low frequency electromagnetic radiation are divided into three:
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.

11. Units
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.

12. Units 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.

13. Radiation sensors 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:
Ionization sensors
Scintillation sensors
Semiconductor radiation sensors
These sensors are either:
Detectors – detection without quantification or:
Sensor - both detection and quantification

14. Ionization sensors (detectors)
In an ionization sensor, the radiation passing through a medium (gas or solid) creates electron-proton 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.

15. Ionization chamber 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.

16. W values for gases

17. Ionization chamber 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.

18. Ionization chamber
Fig 9.1

19. Ionization chamber
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.

20. Ionization chamber - applications
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

21. Ionization chamber - applications
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.

22. Ionization chambers in a residential smoke detector
Fig 9.1x

23. Ionization chambers - application
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

24. A nuclear fabric density sensor
Fig 9.1y

25. Proportional chamber
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.

26. Proportional chamber
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.

27. Operation of ionization chambers
Fig 9.2

28. Geiger-Muller counters
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.

29. Geiger-Muller counters
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.

30. Geiger-Muller counters
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.

31. Geiger-Muller sensor
Fig 9.3

32. Scintillation sensors
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.

33. Scintillation sensors
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.

34. Scintillation sensors
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.

35. Scintillation sensors
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.

36. Scintillation sensor
Fig 9.5

37. Semiconductor radiation detectors
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.

38. Semiconductor radiation detectors
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.

39. Semiconductor radiation detectors
When an energetic particle penetrates into a semiconductor, it initiates a process which releases electrons (and holes)
through direct interaction with the crystal
through secondary emissions by the primary electrons
To produce a hole-electron pair energy is required:
Called ionization energy 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.

40. Properties of semiconductors

41. Semiconductor radiation detectors
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.

42. Semiconductor radiation sensor

43. Semiconductor radiation detectors
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.

44. Semiconductor radiation detectors
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.

45. Semiconductor radiation detectors
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). .

46. Semiconductor radiation detectors - notes
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.

47. Semiconductor radiation detectors - notes
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).

48. Microwave radiation sensors - introduction
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.

49. Electromagnetic waves
Properties of waves were discussed in ch. 6.
Electromagnetic waves differ from acoustic waves in two fundamental ways
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.

50. 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).

51. Propagation of electromagnetic waves
Fig 9.7

52. Electromagnetic waves
The phase velocity is given as
 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

53. Electromagnetic waves
Attenuation of electromagnetic waves, is exponential and material dependent
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.

54. The electromagnetic spectrum
Fig 9.8

55. Microwave sensing
Sensing with microwaves is based on four distinct methods, some more useful than others:
1. Propagation of waves
2. Reflection of waves
3. Transmission of waves
4. Resonance
These may be combined in a sensor to affect a particular function.

56. Microwave sensing - RADAR
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.

57. Scattering of electromagnetic waves
Fig 9.9

58. Microwave sensing - RADAR
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.

59. A-static and bi-static radar
Fig 9.10

60. Radar
For any object in the path of electromagnetic waves, the scattering coefficient, called the scattering cross-section or radar cross-section 
Ps is the scattered power density
Pi the incident power density
R isthe distance from source to target

61. Radar The power received is calculated from the radar equation
 is the wavelength
 the radar cross-section
Pr the total received power
Prad the total radiated power
D is the directivity of the antenna.

62. Radar 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.

63. Doppler Radar
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.

64. Doppler Radar
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:

65. Doppler radar - principle

66. Doppler Radar
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.

67. Doppler Radar - notes
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

68. Radar - notes 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.

69. Reflection sensors
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.

70. Reflection sensors
Assuming that the source is in air, and it propagates into a material, denoted as (1).
The wave impedances of the materials are
The reflection coefficient is

71. Reflection sensors 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%)

72. Microwave moisture sensor
Fig 9.13

73. Transmission sensors
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 + G).
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)

74. Transmission sensing
Fig 9.14

75. Resonant microwave sensors
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.

76. Resonant microwave sensors
These are the resonant frequencies of the cavity
For a rectangular cavity of dimensions a,b,c, the resonant frequencies are:
m, n, p are integers (0,1,2,….) can take different values.
These define the modes of the cavity.

77. Resonant microwave sensors
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 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.

78. Resonant microwave sensors
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.

79. Resonant microwave sensors
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.

80. Resonant microwave sensors
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.

81. Coupling to a cavity resonator
Fig 9.15

82. Resonant sensors
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.

83. Gas sensing cavity resonator
Fig 9.16

84. Cavity resonator sensors
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.

85. Cavity resonator sensors
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.

86. Open cavity resonator sensors
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.

87. Stripline resonator

88. Open cavity resonator sensors
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.

89. Open cavity resonator
Fig 9.18

90. Open cavity resonator
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.

91. Open cavity resonator - notes
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.

92. Transmission line resonator

93. Antennas as sensors 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.

94. Antennas as sensors
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.

95. Elementary electric and magnetic dipole antennas

96. Antennas as sensors
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.

97. Antennas as sensors
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.

98. Radiaton from an electric dipole

99. Antenna relations
The electric field intensity and magnetic field intensity of a dipole in the far field are
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
 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:

101. Antenna properties
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.

102. Antenna properties
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

103. Antenna as a sensor
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 q with it.
The 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
The voltage of the antenna due to this field (assuming l is small) is:
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
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

107. Antenna sensors - notes
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:
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.)