2. • So far, we studied the radiation characteristics of some generic
antennas.
• Antennas are grouped such as wire antennas, aperture antennas,
array antennas
• These types of antennas were easy to study using analysis techniques.
• However, there are special antennas that cannot be easily classified
into these categories
3. • A Yagi–Uda array is made up of an array of dipoles, but only one of
the dipoles is directly excited and all other dipoles are excited
parasitically. The length of the directly excited dipole is different from
those of the parasitic elements.
• In the case of a log-periodic array, although the dipole elements are
excited using a serial feed, their lengths are all different. Therefore,
the pattern multiplication theorem cannot be applied to these
structures.
4. • A turnstile antenna, on the other hand, is constructed using a pair of
dipoles placed orthogonal to each other and fed by currents in phase
quadrature. This produces a pattern that rotates in time.
6. • The medium frequency (MF) band extends from 300 kHz to 3000 kHz
which corresponds to wavelengths of 1000 m to 100 m, and the high
frequency (HF) band is from 3 MHz to 30 MHz (wavelengths from 100
m to 10 m).
• a quarter-wave monopole antenna with a ground plane, operating at
1.25 MHz has a height of 60 m.
• the antenna is constructed as a metallic tower held in place by a set
of wire ropes, called guy wires
7. • Consider an antenna operating at 1250 kHz, which corresponds to a
wavelength of 240 m. Therefore, a quarter-wave monopole is 60 m
high. Assuming a perfectly conducting earth, we can use the image
principle to compute the radiation pattern of a quarter-wave
monopole above the ground.
8. the effect of the guy wires of 120 m long on the
performance of the antenna.
• Since the guy wires are half a wavelength long, they become resonant
structures and distort the radiation pattern of the monopole.
Therefore, it is important to ensure that the support structure used in
the construction of the monopole is not resonant at the frequency of
operation of the antenna.
9. • The guy wires can be made non-resonant by introducing more
insulators on each of the guy wires and the radiation pattern with this
modification is very close to that of a free-standing monopole.
• The current on a quarter-wave monopole above a ground plane has a
sinusoidal distribution. Making the antenna short, the current
distribution becomes triangular and the radiation resistance of the
antenna decreases.
• It is possible to increase the radiation resistance by controlling the
current distribution and this can be done using top-loading
10. For a dipole of length l = 0.2λ, the radiation resistance is 7.9 Ω. Therefore, the
radiation resistance of a monopole of length 0.1λ is half this value, i.e., 3.95 Ω. A
low value of Rrad is difficult to match and the radiation efficiency is also low. One
of the methods to increase the radiation resistance is to make the current
distribution on the monopole near uniform. For a dipole of length l supporting
uniform current distribution, the radiation resistance is given by
The radiation resistance of a short dipole of length l, with triangular current
distribution
11. What is the top-loading of monopole
• It is the process of introducing a flat disk, radial mesh, or a simple
cross at the top of a short monopole in order to make the current
distribution on the monopole more uniform and, therefore, increase
the radiation resistance.
12. How to increase the effective conductivity of
the earth below the monopole
• The earth is usually used as a ground plane for monopoles operating in the MF
and HF bands. It is observed that the conductivity of the earth is in the order of
millisiemens (mS) and the relative permittivity is in the range of 3 to 12.
Therefore, the Earth does not meet the requirements of a good electric
conductor.
• To increase the effective conductivity of the earth below the monopole,
conducting wires are laid on the ground, running radially from the base of the
monopole. These radial wires simulate a good conducting ground plane. For HF
antennas, generally 120 radials at an equal angular separation of 3◦, each at least
λ/4 long (preferably λ/2 long) are laid to realize a good ground
13. Monopole at VHF
• To design an antenna operating at 2 m wavelength which is to be
mounted on an automobile.
• The height of the antenna would be 0.5 m
• In order to increase the gain of the monopole, a 5λ/8 long monopole
is used. a 5λ/8 long monopole above an infinite ground plane has a
directivity of 5.2 dB and its input impedance has a large capacitive
reactance.
• Therefore, a matching section is required to efficiently transfer power
from a 50 Ω transmission line into this antenna
14. What is the air core inductor
• An air core inductor is used near the base of the antenna to tune out
the capacitive reactance and match the antenna to a coaxial
transmission line having a characteristic impedance of 50 Ω.
15. Long Wire Antennas
• The radiation characteristics of a wire antenna fed near one end with
the other end left open
• For convenience, let us again assume that the antenna is oriented
along the z-axis with the feed point in the region 0 < z ≤ λ/4
16. • Let L be the length of the wire and I0 be the maximum amplitude of
current on the dipole. The current at any point z on the wire is
assumed to be z-directed
The current on a one-wavelength-long wire antenna
17. • the far-field of the wire antenna by first computing the magnetic
vector potential
For a long wire antenna whose length is equal to an integral
multiple of a wavelength, i.e., L = Nλ, where N is an integer, the
magnitude of the electric field in the far-field region reduces
19. • The pattern has two main lobes, one is symmetric about the positive
z-axis and other with the negative z-axis.
• The direction of the maximum is at 29.3◦ from the wire axis (z-axis).
• The pattern also has four side lobes.
• The radiation pattern of a longer wire has more number of side lobes.
• For an Nλ long wire, the direction of the maximum, θm, is given by
solving the following equation (Prove that)
21. • A dipole array with only one excited dipole and all other dipoles
parasitically coupled to it.
• The induced currents in the parasitic elements and, hence, the
radiation characteristics, depending on the length of the dipoles and
the spacing between the dipoles.
• If a second dipole, with its terminals short-circuited, is brought near
the driven dipole element and kept parallel to it, a current is
established on the second dipole due to electromagnetic induction.
• The amplitude and phase of the induced current on the parastic
element depend on the length and the radius of the element as well
as the distance from the excited dipole.
22. • it can be treated as a 2-element array and apply the array theory
Let the current on the driven element be 1 A, and the induced current
on the parasitic element be 0.7 ˂140◦ A
23. • z 1 = 0, z 2 = −0.125λ, and θ is the angle measured from the z-axis.
The magnitude of the array factor along θ = 0◦ is |AF| = |0.94 − j0.7| = 1.172
and along θ = 180◦ is |AF| = |0.303 − j0.06| = 0.309.
25. • Now consider again a 2-element parasitic array with I1 = 1 A and I2 =
0.7 and angle is − 140◦ A, i.e., the current in the parasitic element lags
the current in the driven dipole. The array factor for this situation is
given by
27. • Consider a 3-element array with one driven element and two parasitic
elements on either side, one longer and the other shorter than the
driven element.
• In a Yagi–Uda array, the driven element is around 0.46λ to 0.47λ, the
reflector is longer than the driven element, and the director is
shorter. Typical dimensions of a 3-element Yagi–Uda antenna are:
• Length of the driven element: 0.47λ
• Length of the director: 0.442λ
• Length of the reflector: 0.482λ
28. • Optimum reflector-to-driven-element spacing (for maximum
directivity) is between 0.15λ and 0.25λ and driven-element-to-
director spacing is typically between 0.2λ and 0.35λ.
29. • The radiation patterns in the horizontal and vertical planes of the
array with both the spacings set to 0.2λ
E-plane pattern ( = 0 plane)
30. • The directivity of a Yagi–Uda array can be increased by adding more
directors. These are generally added at approximately regular
intervals of 0.18λ to 0.3λ spacing.
• The directivity of a Yagi–Uda array antenna is proportional to the
overall length of the array in terms of wavelength.
• only one reflector is used in a Yagi–Uda antenna because the field of
the antenna just behind the reflector (in the negative z-direction) is
small
• Question:
• Does a second parasitic element kept behind the reflector will be
effective?
• The answer is no since a very small current is induced on it.
