Unit 4
Antenna Design
For a horizontal grounding electrode, with a length , a diameter , and an embedded depth , six possible earth current divergence impedance models are presented in Table
Figure 1. Horizontal grounding electrode
The resistance of a horizontal grounding electrode with an embedded depth of zero can be expressed by the neutral-point potential method. This states that increasing the earth conductivity and the size of electrode can reduce its current divergence resistance. By placing several electrodes in parallel as one new electrode, the grounding impedance can be further reduced.
The new grounding impedance can be expressed as follows
Rd = RL / nղ ----------------------------(1)
where is the grounding impedance of a single electrode; is the number of electrodes; is the grounding electrode utilization factor, which is determined by the shape and number of electrodes and their relative positions.
The parallel resistance can be similarly calculated by introducing a hemispherical grounding equivalent model.
For a horizontal grounding electrode buried at the subsurface level, its equivalent radius expression can be obtained based on image theory and the neutral-point potential method as
r = L / 2 ln (2L/d)
4.1.1 Short Electric dipole
Figure 2. Dipole
The circuit diagram of a short dipole with length L is shown. The actual size of the antenna does not matter. The wire that leads to the antenna must be less than one-tenth of the wavelength. That is
L<λ/10
Where
Another type of short dipole is infinitesimal dipole, whose length is far less than its wave length. Its construction is similar to it, but uses a capacitor plate.
4.1.2 Fields of short dipole
I(z) = Io(1-2|z|)/L
The current distribution is plotted in Figure 2. Note that this is the amplitude of the current distribution; it is oscillating in time sinusoidally at frequency f.
Figure3 . Current distribution.
The fields radiated from the short dipole antenna in the far field are given by:
E
= jnkIoL e-jkr / 8πr sin 
Hɸ = E
/ 
Er = Hr= E ɸ = H
=0
4.1.3 Radiation resistance of short electric dipole
The radiation resistance of any antenna can be expressed as:
Rrad=2Prad|Io|−2 --------------------------(1)
where |Io| is the magnitude of the current at the antenna terminals, Prad is the resulting total power radiated.
For an ESD
Prad = ղ |Io| 2 (βL)2 / 48 π
Rrad ≈ղ (β L)2 / 24 π
We know that
Β L = 2π/λ L = 2π. L/λ
where L/λL/λ is the antenna length in units of wavelength
Rrad≈20π2(L/λ)2
L = λ/2 the magnitude of current distribution is given by
I mag = Io sin[ 2π/λ[ λ/4 +z]
= Io sin[ π/2 
2πz/4]------------91)
When z=0 I mag = Io [ ie maximum value of current at the centre]
When z = 
λ/2 I mag = 0
When L= λ/2 the pattern factor becomes
E = [ cos [ π/2 cos
/ sin 
]
The pattern is as shown in figure 3.9(a), it is slightly more directional than the pattern of infinitesimal of short dipole [which is given by sinθ]. The beam width between half power points of λ/2 antenna is 780 as compared to 900 for a short dipole
4.4.1 Broadside case and end fire case
Figure 5. Broad side array
The end fire array is very much similar to the broadside array from the point of view of arrangement. But the main difference is in the direction of maximum radiation. In broadside array, the direction of the maximum radiation is perpendicular to the axis of array; while in the end fire array, the direction of the maximum radiation is along the axis of array.
Figure 6. End fire array
Thus, in the end fire array number of identical antennas are spaced equally along a line. All the antennas are fed individually with currents of equal magnitudes but their phases vary progressively along the line to get entire arrangement unidirectional finally. i.e. maximum radiation along the axis of array. Thus end fire array can be defined as an array with direction of maximum radiation coincides with the direction of the axis of array to get unidirectional radiation.
4.4.2 Horizontal antennas above plane ground
A ground plane antenna is essentially one half of a dipole mounted vertically. The term monopole is also used to describe this antenna. The earth ground below the antenna, a conducting surface a least λ/4 in radius or a pattern of λ/4 conductors called radials, makes up the other half of the antenna (Fig.).
Figure 7. Above ground horizontal antenna
Ground plane monopole and Marconi antennas are one quarter wavelength high. The ground or radials are the other half of the antenna.
If the antenna is connected to a good earth ground, it is called a Marconi antenna. The ground structure serves as the other λ/4 half of the antenna. If the ground plane is adequately sized and conductive, the performance of the ground plane is equivalent to a vertically mounted dipole.
The length of a quarter-wave vertical is:
λ/4 = 246 K/fMHz
The K factor is smaller than 0.95 for verticals, which are usually made with wider tubing.
4.4.3 Vertical antennas above plane ground
It is found that for a monopole antenna like a quarter wavelength vertical, the ground acts as a plane to reflect the radio waves so that an image of the top half of the antenna is seen in the Earth. It is possible to simulate this function by replacing the real earth with a conducting plane. To function as an antenna ground plane, the conducting surface must extend for least a quarter wavelength from the base of the antenna.
\
Figure 8. Vertical antenna with ground plane.
Often four conducting radials are used and these often provide a sufficient simulation of the complete circular ground plane.
Figure 9. Conducting rails
4.5 Yagi-Uda antenna design
4.5.1 Construction
Figure 10. Yagi antenna components
There are three types of element within a Yagi antenna:
in reducing the levels of interference received.
Typically a reflector will add around 4 or 5 dB of gain in the forward direction.
Radiation Pattern
. 
Figure 11. Radiation Pattern of Yagi- Uda Antenna
Advantages
The Yagi antenna is directional enabling interference levels to be minimised for receiving and transmitting.
The Yagi antenna has gain allowing lower strength signals to be received.
The Yagi antenna is mechanically relatively straightforward when compared to other designs. It can be constructed using straight rods which are simple to use and robust for most instances.
The construction enables the antenna to be mounted easily on vertical and other poles with standard mechanical fixings.
Disadvantages
Key takeaways:
Long wire antenna characteristics
If a wire antenna is half a wavelength long, then it will perform very much like a dipole, the main difference being that it is end fed.
The radiation pattern for a dipole is given in the diagram below.
Figure 12. Long wire antenna
The same radiation pattern is exhibited for an equivalent end fed wire, however, as the length of the end fed wire is extended in terms of the number of wavelengths, it is found that the radiation pattern becomes more complicated.
A number of different lobes form and these move out towards the the line of the axis of the wire of the antenna, aligning further with this as the length is extended.
Figure 13. Patterns
It can be seen that the radiation pattern of this long wire antenna is very different to that of the half wave antenna. As the length of the antenna increases, so the major lobes align further with the axis of the antenna.
Figure 14. Basic dipole antenna
There are two main advantages for using a folded dipole antenna over a standard dipole:
When higher impedance feeders need to be used, or when the impedance of the dipole is reduced by factors such as parasitic elements, a folded dipole provides a significant increase in impedance level that enables the antenna to be matched more easily to the feeder available.
The folded dipole antenna has a flatter frequency response - this enables it to be used over a wider bandwidth with many transmissions utilising a variety of different selectable channels, e.g. television and broadcast radio, a wide bandwidth antenna is needed. The standard dipole antenna does not always provide the required bandwidth and the additional bandwidth of the folded dipole meets the requirements.
There are many ways in which folded dipoles can be used. They find uses in many applications:
Folded dipole antennas are sometimes used on their own, but they must be fed with a high impedance feeder, typically 300 ohms. This on its own can be very useful in certain applications where balanced feeders may be used.
However folded dipoles find more uses when a dipole is incorporated in another RF antenna design with other elements nearby. The issue is that incorporating a dipole into an antenna such as a Yagi where elements are closely coupled reduces the feed impedance. If a simple dipole was used, then the feed impedance levels of less than 20 Ω or less can easily be experienced.
Sometimes folded dipoles may be employed purely to give a greater bandwidth. When used to increase bandwidth, folded dipoles may be used on their own or within another antenna system.
Key takeaways:
References:
Antenna Theory: Analysis and Design Book by Constantine A. Balanis
Antenna and Wave Propagation Book by Deepak Handa and K. D. Prasad
Antenna and Wave Propagation Book by Ranjana Trivedi
ANTENNAS AND WAVE PROPAGATION Book by SACHIN D. DR RUIKAR
