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Antennas: from Theory to Practice 1 Antennas: from Theory to Practice 5. Popular Antennas Yi HUANG Department of Electrical Engineering & Electronics The University of Liverpool Liverpool L69 3GJ Email: [email protected]

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Antennas: from Theory to Practice 5. Popular Antennas. Yi HUANG Department of Electrical Engineering & Electronics The University of Liverpool Liverpool L69 3GJ Email: [email protected]. Objectives of this Chapter. - PowerPoint PPT Presentation

### Transcript of Antennas: from Theory to Practice 5. Popular Antennas

Antennas: from Theory to Practice 1

Antennas: from Theory to Practice

5. Popular Antennas

Yi HUANGDepartment of Electrical Engineering & Electronics

The University of LiverpoolLiverpool L69 3GJ

Email: [email protected]

Antennas: from Theory to Practice 2

Objectives of this Chapter

To examine and analyse some of the most popular antennas using relevant antenna theories, to see why they have become popular, what their major features and properties (including advantages and disadvantages) are, and how they should be designed.

Antennas: from Theory to Practice 3

Classification of Antennas

Wire-Type Antennas Aperture-Type AntennasDipoles Horn and open waveguideMonopoles Reflector antennasBiconical antennas Slot antennasLoop antennas Microstrip antennasHelical antennas

Linearly polarised antennas Circularly polarised antennasElement antennas Antenna arrayNarrow-band Broad-bandTransmitting Receiving

Antennas: from Theory to Practice 4

5.1 Wire Type Antennas

Evolution of a dipole of total length 2l and diameter d

Dipole Antennas

Antennas: from Theory to Practice 5

Current distribution along an open transmission line is:

Thus the current distribution on the dipole is

Current distribution of dipoles

Antennas: from Theory to Practice 6

In the far field:

Antennas: from Theory to Practice 7

Antennas: from Theory to Practice 8

Input impedance of dipoles

Antennas: from Theory to Practice 9

Electrically short dipoles

• When the dipole length is much shorter than a wavelength (</10), it can be called an electrically short dipole

• The input impedance can be approximated as

• Radiation pattern is E() = sin

• The directivity is D = 1.5 (1.76dBi)

Antennas: from Theory to Practice 10

Half-wavelength dipole

• The most popular dipole– Radiation pattern: E() = cos[(/2)cos ]/sin– Radiation resistance: 73 – Directivity: 1.64 (2.15 dBi)– The input impedance is not sensitive to the radius

and is about 73 Ω which is well matched with a standard transmission line of characteristic impedance 75 Ω or 50 Ω (with a VSWR < 2).

– Its size and radiation pattern are suitable for many applications

Antennas: from Theory to Practice 11

Example 5.1

A dipole of the length 2l = 3 cm and diameter d = 2 mm is made of copper wire (= 5.7 107 S/m) for mobile communications. If the operational frequency is 1 GHz,

a). obtain its radiation pattern and directivity; b). calculate its input impedance, radiation resistance and

radiation efficiency;c). if this antenna is also used as a field probe at 100 MHz

for EMC applications, find its radiation efficiency again, and express it in dB.

Solution on pages 135 - 137

Antennas: from Theory to Practice 12

Some popular forms of dipole antennas

Antennas: from Theory to Practice 13

Monopole Antennas• Half of a dipole antenna mounted above

the earth or a ground plane• Normally one-quarter wavelength long

– almost the same feature as a dipole, except the 37 radiation resistance, higher gain, a

shorter length, and easier to feed!• Based on the Image Theory

l

Ground

Antennas: from Theory to Practice 14

Antennas: from Theory to Practice 15

Effects of the ground plane

Its size and material property of can change the radiation pattern (hence the directivity) and input impedance.

Antennas: from Theory to Practice 16

An example

Antennas: from Theory to Practice 17

Some popular forms of monopole antennas

Antennas: from Theory to Practice 18

Duality Principle

Duality means the state of combing two different things which are closely linked. In antennas, the duality theory means that it is possible to write the fields of one antenna from the field expressions of the other antenna by interchanging parameters:

System 1

System 2

Antennas: from Theory to Practice 19

Antennas: from Theory to Practice 20

Loop Antennas

• For a short dipole

• Thus for a small loop

Antennas: from Theory to Practice 21

• Directivity of a loop

Antennas: from Theory to Practice 22

• Current distribution of a resonant loop

Antennas: from Theory to Practice 23

• Radiation pattern of a one wavelength loop – this is very different from that of a short loop!

Antennas: from Theory to Practice 24

Radiation patterns of loops with various circumferences

Antennas: from Theory to Practice 25

• Input impedance of loops

Antennas: from Theory to Practice 26

Helical Antennas

It may be viewed as a derivative of the dipole or monopole, but it can also be considered a derivative of a loop.

Antennas: from Theory to Practice 27

• Normal mode Helix– It may be treated as the superposition of n elements,

each consisting of a small loop of diameter D and a short dipole of length s, thus the far fields are

– They are orthogonal and 90 degrees out of phase;– The combination of them gives a circularly or

elliptically polarised wave. – The axial ratio:

Antennas: from Theory to Practice 28

– When the circumference is equal to

the axial ratio becomes unity and the radiation is circularly polarised.

Antennas: from Theory to Practice 29

• Axial Mode Helix– The axial (end-fire) mode occurs when the

circumference of the helix is comparable with the wavelength (C = D ≈ ) and the total length is much greater than the wavelength.

– This has made the helix an extremely popular circularly-polarised broadband antenna at the VHF and UHF band frequencies

– The recommended parameters for an optimum design to achieve circular polarisation are:

Antennas: from Theory to Practice 30

Half power beamwidth:

1st null beamwidth:

Antennas: from Theory to Practice 31

– The directivity:

– The axial ratio

Antennas: from Theory to Practice 32

Example 5.2

Design a circularly polarised helix antenna of an end-fire radiation pattern with a directivity of 13 dBi. Find out its radiation resistance, HPBW, AR and radiation pattern.

Solution on pages 150 - 152

Antennas: from Theory to Practice 33

Which is better?

Antennas: from Theory to Practice 34

Yagi-Uda Antennas

Antennas: from Theory to Practice 35

• The driven element (feeder) is the very heart of the antenna. It determines the polarisation and centre frequency. For a dipole, the recommended length is about 0.47to ensuring a good input impedance to a 50 Ω feed line.

• The reflector is longer than the feeder to force the radiated energy towards the front. The optimum spacing between the reflector and the feeder is between 0.15 to 0.25 wavelengths.

• The directors are usually 10 to 20% shorter than the feeder and appear to direct the radiation towards the front. The director to director spacing is typically 0.25 to 0.35 wavelengths,

• The number of directors determines the maximum achievable directivity and gain.

Antennas: from Theory to Practice 36

Log-periodic Antennas

Antennas: from Theory to Practice 37

– The antenna is divided into the so called active region and inactive regions.

– The role of a specific dipole element is linked to the operating frequency: if its length, L, is around half of the wavelength, it is an active dipole and within the active region; Otherwise it is in an inactive region and acts as a director or reflector as in Yagi-Uda antenna

– The driven element shifts with the frequency – this is why this antenna can offer a much wider bandwidth than the Yagi-Uda. A travelling wave can also be formed in the antenna.

– The highest frequency is basically determined by the shortest dipole length while the lowest frequency is determined by the longest dipole length (L1).

Antennas: from Theory to Practice 38

Antenna design

• This seems to have too many variables. In fact, there are only three independent variables for log-periodic antenna design.

the scaling factor:

the spacing factor:

the apex angle:

Antennas: from Theory to Practice 39

Antennas: from Theory to Practice 40

In practice, the most likely scenario is that the frequency range is given from fmin to fmax, the following equations may be employed for design

Another parameter (such as the directivity or the length of the antenna) is required to produce an optimised design.

Antennas: from Theory to Practice 41

Example 5.3

Design a log-periodic dipole antenna to cover all UHF TV channels, which is from 470 MHz for channel 14 to 890 MHz for channel 83. Each channel has a bandwidth of 6 MHz. The desired directivity is 8 dBi.

Solution on page 160

Antennas: from Theory to Practice 42

5.2 Aperture-Type Antennas

• They are often used for higher frequency applications (> 1GHz) than wire-type antennas.

Antennas: from Theory to Practice 43

Antennas: from Theory to Practice 44

Directivity:

Antennas: from Theory to Practice 45

Near field and far field

Antennas: from Theory to Practice 46

Example 5.4

An open waveguide aperture of dimensions a long x and b along y located in the z = 0 plane. The field in the aperture is TE10 mode and given by

Find i). the radiated far field and plot the radiation pattern in both

the E and H planes;ii). the directivity.

Solution on pages 166 - 168

Antennas: from Theory to Practice 47

Antennas: from Theory to Practice 48

Horn Antennas

• Horn antennas are the simplest and one of the most widely used microwave antennas – the antenna is nicely integrated with the feed line (waveguide) and the performance can be easily controlled. • They are mainly used for standard antenna gain and field measurements, feed element for reflector antennas, and microwave communications.

Antennas: from Theory to Practice 49

Antennas: from Theory to Practice 50

Pyramidal Horn Design

To make this horn, we must have

i.e.

Antennas: from Theory to Practice 51

The directivity:

We can therefore obtain the design equation

This equation in A can be solved using numerical methods. For the optimum design, use a first guess approximation

Antennas: from Theory to Practice 52

Example 5.5

Design a standard gain horn with a directivity of 20 dBi at 10 GHz. WR-90 waveguide will be used to feed the horn.

Solution on pages 172 - 173

Antennas: from Theory to Practice 53

Reflector Antennas

• Reflector antennas can offer much higher gains than horn antennas and are easy to design and construct.

• The most widely used antennas for high frequency and high gain applications in radio astronomy, radar, microwave and millimetre wave communications, and satellite tracking and communications.

• The most popular shape is the paraboloid – because of its excellent ability to produce a pencil beam (high gain) with low sidelobes and good cross-polarisation characteristics

Antennas: from Theory to Practice 54

Paraboloidal and Cassegrain reflector antennas

Antennas: from Theory to Practice 55

Antenna design

• The reflector design problem consists primarily of matching the feed antenna pattern to the reflector. The usual goal is to have the feed pattern about 10 dB down in the direction of the rim, that is the edge taper = (the field at the edge)/(the field at the centre) ≈10 dB.

• Directivity:

• Half-power beamwidth

Antennas: from Theory to Practice 56

Offset parabolic reflectors

It reduces aperture blockage while maintaining acceptable structure rigidity

Antennas: from Theory to Practice 57

Radiation patterns in E and H planes

Antennas: from Theory to Practice 58

Slots AntennasThey are very low-profile and can be conformed to basically

any configuration, thus they have found many applications, for example, on aircraft and missiles.

Antennas: from Theory to Practice 59

Slot waveguide antenna array: widely used for radar

Antenna Equivalent circuit

Antennas: from Theory to Practice 60

Equivalence Principle: for field analysis

• The radiated field by the slot is the same as the field radiated by its equivalent surface electric current and magnetic current which were given by

where E and H are the electric and magnetic fields within the slot, and n is the unit vector normal to the slot surface S

For a half-wavelength slot, its equivalent electric surface current JS = ˆn × H = 0, the remaining source at the slot is its equivalent magnetic current MS = −ˆn × E (it would be 2MS if the conducting ground plane were removed using the imaging theory).

Antennas: from Theory to Practice 61

Antennas: from Theory to Practice 62

Babinet’s Principle

• The field at any point behind a plane having a screen, if added to the field at the same point when the complementary screen is substituted, is equal to the field at the point when no screen is present.

• Apply this to antennas:

Since the impedance for a half-wavelength dipole is about 73 ohms, the corresponding slot has an impedance of

Antennas: from Theory to Practice 63

Self-complementary antennas – frequency independent

This type of antenna has a constant impedance of

Antennas: from Theory to Practice 64

Microstrip/Patch Antennas

• Ease of construction and integration, relatively low cost, compact low profile configuration and good flexibility

• Typical applications for 1 - 20 GHz

Antennas: from Theory to Practice 65

Operational principles

• To be a resonant antenna, the length L should be around half of the wavelength. In this case, the antenna can be considered as a /2 transmission line resonant cavity with two open ends where the fringing fields from the patch to the ground are exposed to the upper half space (z > 0) and are responsible for the radiation.

• This radiation mechanism is the same as the slot line, thus there are two radiating slots on a patch antenna.

• As a resonant cavity, there are many possible modes (as waveguides), thus a patch antenna is multi-mode and may have many resonant frequencies.

Antennas: from Theory to Practice 66

Antennas: from Theory to Practice 67

Main properties

• Directivity

• Input impedance

• Bandwidth for VSWR < 2

Antennas: from Theory to Practice 68

Antenna design

Optimised width:

Resonant freq.:

Length:

Antennas: from Theory to Practice 69

Example 5.7

RT/Duroid 5880 substrate ( and d = 1.588 mm) is to be used to make a resonant rectangular patch antenna of linear polarisation;

a). Design such an antenna to work at 2.45 GHz for Bluetooth applications;

b). Estimate its directivity;c). If it is to be connected to a 50 ohms microstrip using the

same PCB board, design the feed to this antenna;d). Find the fractional bandwidth for VSWR < 2.

Solution on pages 189 - 191

Antennas: from Theory to Practice 70

5.3 Antenna Arrays

• Motivations: to achieve desired high gain or radiation pattern, and the ability to provide an electrically scanned beam.

• It consists of more than one antenna element and these radiating elements are strategically placed in space to form an array with desired characteristics which are achieved by varying the feed (amplitude and phase) and relative position of each radiating element;

• The main drawbacks are the complexity of the feeding network required and the bandwidth limitation (mainly due to the feeding network)

Antennas: from Theory to Practice 71

A typical antenna array of N elements

Antennas: from Theory to Practice 72

Pattern Multiplication Principle

Since the total radiated field for an array is the summation of the fields from each element

where An is the amplitude, n is the relative phase, Ee is the radiated field of the antenna element, and AF is called array factor.

Thus the radiation pattern of an array is the product of the pattern of individual element antenna with the (isotropic source) array pattern.

Antennas: from Theory to Practice 73

Example

element elementAF Totalarray AF Total

array

Antennas: from Theory to Practice 74

For a uniform array of a constant d and identical amplitude (say 1)

Thus

The normalised antenna factor of a uniform array:

Antennas: from Theory to Practice 75

AF: N = 20, d = and 0 = 0

Antennas: from Theory to Practice 76

AF: N = 10, d = and 0 = 0

Antennas: from Theory to Practice 77

AF: N = 10, d = and 0 = 0

Antennas: from Theory to Practice 78

AF for N = 10 and d = 2, and 0 = 450

Antennas: from Theory to Practice 79

Phased Array

The maximum of the radiation occurs at = 0

That is:

Normally the spacing d is fixed for an array, we can control the maximum radiation (or scan the beam) by changing the phase 0 and the wavelength (frequency) – this is the principle of phase/frequency scanned array

Antennas: from Theory to Practice 80

• An array is called a broadside array if the maximum radiation of the array is directed normal to its axis ( = 00); while it is called an end-fire array if the maximum radiation is directed along the axis of the array (= 900).

Antennas: from Theory to Practice 81

The radiation pattern, SLL, HPBW, and gain for four different source distributions of eight in-phase isotropic sources spaced by /2; there are trade-offs!

Antennas: from Theory to Practice 82

Element mutual coupling

• The interaction between elements due to their close proximity is called the mutual coupling which affects the current distribution hence the input impedance as well as the radiation pattern

• The voltage generated at each element can be expressed as

Antennas: from Theory to Practice 83

Self-impedance:

Mutual impedance:

Antennas: from Theory to Practice 84

5.4 Some Practical Considerations

• The differences between transmitting and receiving antennas– From the reciprocity theorem, the field patterns are

the same for transmitting or receiving.• Antenna feeding and matching

– Balun (a device to connect a balanced antenna to an unbalanced transmission line) may be required.

• Polarisation– Polarisation has to be matched from Tx to Rx.

• Radomes, housings and supporting structures.– Affecting the antenna performance (impedance,

pattern, …)