Antennas

An antenna is a structure that is generally a metallic object, often a wire or group of wires, used
to convert high-frequency current into electromagnetic waves, and vice versa. Apart from their
different functions, transmitting and receiving antennas have similar characteristics, which
means that their behaviour is reciprocal
Electromagnetic Radiation
When RF energy is fed into a mismatched transmission line, standing waves occur. Energy is
lost or radiated into the space surrounding the line. This process is considered unwanted in
the transfer of energy to the radiation device. If we examine this process and expand upon it
(Figure 9-2a),

We can see, by separating the ends of the transmission line, that more surface area of the wire
is exposed to the atmosphere and enhances the radiation process.
The radiation efficiency of this system is improved even more when the two wires are bent at
90' (right angles) to each other (Figure 9-2b). The electric and magnetic fields are now fully
coupled to the surrounding space instead of being confined between the two wires, and
maximum radiation results. This type of radiator is called a dipole. When the total length of
the two wires is a half wavelength, the antenna is called a half-wave dipole.
The Elementary Doublet (Hertzian Dipole)
The doublet is a theoretical antenna shorter than a wavelength (Figure 9-3a). It is used as a
standard to which all other antenna characteristics can be compared.
The field strength of this antenna can be calculated as follows
As shown in Figure 9-3b, the radiation is a double circular pattern, with maximum radiation
at 90° to the axis of the wire

Current and.Voltage Distribution

When an RF signal voltage is applied at some point on an antenna, voltage and current will
result at that point. Traveling waves are then initiated, and standing waves may be established,
which means that voltage and current along the antenna are out of phase.
The radiation pattern depends chiefly on the antenna length measured in wavelengths, its
power losses, and the terminations at its end (if any). In addition, the thickness of the antenna
wire is of importance. For this discussion such antennas may be assumed to be lossless and
made of wire whose diameter is infinitely small.
Figure 9-4 shows the voltage and current distribution along a half-wave dipole. We
can recognize the similarity to the distribution of voltage and current on a section of λ/4
transmission line open at the far end. These voltage and current characteristics are duplicated
every λ/2 length, along the antenna (Figure 9-5).

By referring to Figure 9-4, it will become apparent that to connect a transmission line to this
antenna configuration, we must observe the impedance at the connection points. The
impedance varies along the length of the antenna, being highest where the current is lowest,
and lowest where the current is highest (at the center). At the center of a half-wave antenna the
impedance is approximately 73Ω and increases to about 2500 Ω at either end.
TERMS AND DEFINITION
 Isotropic antenna An isotropic antenna is one which radiates equally in all direction.
 Resonant antenna: Length of the antenna is multiples of λ/2 [Half wave length]

1 Antenna Gain and Effective Radiated Power

Certain types of antennas focus their radiation pattern in a specific direction, as compared to
an omnidirectional antenna.
Another way of looking at this concentration of the radiation is to say. that some antennas
have gain (measured in decibels).
Directive gain
Directive gain is defined as the ratio of the power density in a particular direction of one
antenna to the power density that would be radiated by an omnidirectional antenna (isotropic
antenna). The power density of both types of antenna is measured at a specified distance, and
a comparative ratio is established

Two sets of characteristics can be obtained from the previous information:

1. The longer the antenna, the higher the directive gain.
2. Nonresonant antennas have higher directive gain than resonant antennas.

Directivity and power gain (ERP)

Another form of gain used in connection with antennas is power gain.
Power gain is a comparison of the output power of an antenna in a certain direction to that of
an isotropic antenna.
The gain of an antenna is a power ratio comparison between an omnidirectional and
unidirectional radiator. This ratio can be expressed as:

Radiation Measurement and Field Intensity

The voltages induced in a receiving antenna are very small, generally in the microvolt
range. Field strength measurements are thus given in· microvolts per meter.

Field intensity
The field strength (field intensity) of an antenna's radiation, at a given point in space, is equal
to the amount of voltage induced in a wire antenna 1 m long, located at that given point.
The field strength, or the induced voltage, is affected by a number of conditions such as the
time of day; atmospheric conditions, and distance.

Antenna Resistance
Radiation resistance is a hypothetical value which, if replaced by an equivalent resistor, would
dissipate exactly the same amount of power that the antenna would radiate.
Radiation resistance Radiation resistance is the ratio of the power radiated by the antenna to
the square of the current at the feed point.

Antenna losses and efficiency

In addition to the energy radiated by· an antenna, power losses must be accounted for.
Antenna losses can be caused by ground resistance, corona effects, imperfect dielectric near
the antenna, energy loss due to eddy currents induced into nearby metallic objects, and I2R
losses in the antenna itself. We can combine these losses and represent them as shown in
equation

From this expression we can now develop an equation for calculating antenna efficiency.

Bandwidth, Beamwidth, and Polarization

Bandwidth, beamwidth, and polarization are three important terms dealing respectively with
the operating frequency range, the degree of concentration of the radiation pattern, and the
space orientation of the radiated waves.
Bandwidth: The term bandwidth refers to the range of frequencies the antenna will radiate
effectively; i.e., the antenna will perform satisfactorily throughout this range of frequencies.
. When the antenna power drops to 1/2 (3 dB), the upper and lower extremities of these
frequencies have been reached and the antenna no longer performs satisfactorily.
Antennas that operate over a wide frequency range and still maintain satisfactory
performance must have compensating circuits switched into the system to maintain impedance
matching, thus ensuring no deterioration of the transmitted signals.

Beamwidth: The beamwidth of an antenna is described as the angles created by comparing

the half-power points (3 dB) on the main radiation lobe to its maximum power point.
In Figure 9-9, as an example, the beam angle is 30°, which is the sum of the two angles
created at the points where the field strength drops to 0.707 (field strength is measured in
·μJV Im) of the maximum voltage at the center of the lobe.
(These points are known as the half-power points.)
Polarization Polarization of an antenna refers to the direction in space of the E field
(electric vector) portion of the electromagnetic wave being radiated (Figure 9-10) by
the transmitting system.

Radiation pattern:
The radiation pattern of antenna is a representation (pictorial or mathematical) of the
distribution of the power out-flowing (radiated) from the antenna (in the case of transmitting
antenna), or inflowing (received) to the antenna (in the case of receiving antenna) as a function
of direction angles from the antenna
Antenna radiation pattern (antenna pattern): It is defined for large distances from the antenna,
where the spatial (angular) distribution of the radiated power does not depend on the distance
from the radiation source is independent on the power flow direction
It is clear in Figures a and b that in some very specific directions there are zeros, or nulls, in
the pattern indicating no radiation.
The protuberances between the nulls are referred to as lobes, and the main, or major, lobe is in
the direction of maximum radiation.
There are also side lobes and back lobes. These other lobes divert power away from the main
beam and are desired as small as possible.

Dipole Arrays
An antenna array is a radiation system consisting of grouped radiators, or elements
These are placed close together so as to be within each other's induction field. They therefore
interact with one another to produce a resulting radiation pattern that is the vector sum of the
individual ones.
Whether reinforcement. or cancellation takes place in any given direction is determined not
only by the individual characteristics of each element, but also by the spacing between
elements, as measured in wavelengths, and the phase difference (if any) between the various
feed points.
By suitably arranging an array, it is possible to cause pattern cancellations and reinforcements
of a nature that will result in the array's having strongly directional characteristics
Problem1
A half-wave dipole antenna is capable of radiating 1KWand has a 2.15dB gain over an
isotropic antenna. How much power must be delivered to the isotropic (omnidirectional)
antenna to match the field strength?
Solution:

2.15 =10 log 10(P2/1000) P2= 1640 W

Additional Problem
The radiation resistance and loss resistance of an antenna are 50Ω and 5Ω respectively.
Find the efficiency of the antenna.