Antennas

Lecture 9

Introduction An antenna is an electrical conductor or

system of conductors. An antenna is a device whose function is to radiate electromagnetic energy or to intercept electromagnetic radiation Transmission - radiates electromagnetic energy

into space Reception - collects electromagnetic energy

from space

Introduction A transmitting antenna can also be used for

reception and vice versa. In two-way communication, the same antenna can be used for transmission and reception.

This property of interchangeability is known as antenna reciprocity.

Introduction The received signal strength is described in

terms of electric field strength. If a signal induces a 15 mV in a receiving antenna 3m long, then the field strength is 15 mV/3m or 5 mV/m.

For maximum power transfer the antenna must be able to match the transmission and the load in terms of impedance.

The polarization must be the same in the receive and transmit antennas. The polarization is the direction of the electric filed.

Introduction At radio frequencies a wire can serve as an

impedance matching device.

The spacing, length and shape of the device are related to wavelength of the transmitter.

Introduction When RF energy is fed into a transmission

line standing waves occur.

Energy is lost or radiated into the space surrounding the line.

By separating the ends of the transmission

line, a greater surface area is exposed and this enhances the radiation process.

Introduction Greater efficiency is achieved if the two

lines are at right angles to each other.

The electric and magnetic fields are now fully coupled into the surrounding space instead of being confined between the wires. This type of radiator is called a dipole.

Radiation Patterns Graphical representation of radiation properties of an

antenna. A radiation pattern is a polar diagram showing field strengths or the power densities at various angular positions relative to the antenna.

Depicted as two-dimensional cross section The distance from the location of the antenna to a point on

the radiation pattern indicates the strength of the radiation in that direction.

This means that antennas do not perform equally well in all directions

Length Calculations The radiation pattern depends mainly on the length of the

antenna. The length of an antenna can be calculated using the following equation, the velocity factor of wire is 95% compare to air.

where c is the speed of light L is the length in meters f is frequency in Hertz and k is the velocity factor

kf

cL

Length Calculations

Example It is required that an antenna of the half-

dipole type be built to receive broadcast at 100 MHz, calculate the optimum length of the antenna.

Beam width It is sometimes necessary to compare the directivity of

antennas very quickly, the beam width provides such a quick comparison.

The beam width of an antenna is the angle within which the power radiated is above one half of what it is in the most preferential direction, or the angle when the voltage remains within 70.7% of the voltage developed when the antenna is aimed at the most preferential direction.Beam width (or half-power beam width)

Measure of directivity of antenna

Antenna resistance For power to get to an antenna it must be connected to a

transmission line. To prevent standing waves from occurring within the line and

for maximum power transfer, the resistance of the transmission line must be equal to the resistance of the antenna.

The antenna resistance is termed radiation resistance. This is defined as a fictitious resistance which would dissipate as much power as an antenna in question is radiating if it were connected to the same transmission line.

If an antenna is radiating 100 W when drawing a current of 2 A then its radiation resistance will be 25 ohm. (P=I2R).

Antenna resistance Not all energy absorbed by an antenna is radiated.

Losses can occur within the antenna (imperfect dielectrics, eddy currents etc), as such antenna efficiency is defined

Rr is the resistance of the antenna Rl is resistance due to losses

lr

r

input

dtransmitte

RR

R

P

P

Types of Antennas Isotropic antenna (idealized)

Radiates power equally in all directions Dipole antennas

Half-wave dipole antenna (or Hertz antenna) Quarter-wave vertical antenna (or Marconi

antenna) Parabolic Reflective Antenna

Half-wave dipole antenna (or Hertz antenna)

When the total length of the two wires is half the wavelength, the antenna is called a half wave dipole. This is also known as the Hertz antenna.

Quarter-wave vertical antenna (or Marconi antenna)

In this case, the total length of the two wires is a quarter of the wavelength.

This type of antenna is also referred to as the Marconi antenna.

It is normally used at frequencies below 2 MHz. This antenna requires a conducting path to ground. The ground is thus used as the other quarter

wavelength. Electrically therefore it acts as a half wavelength.

Antenna Gain Antenna gain

Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)

If an antenna is said to have a gain of 10dB, it means it improves upon the reference antenna in that direction by 10dB.

The increased power in one direction is at the expense of other directions.

Effective area Related to physical size and shape of antenna

The increased power in one direction is at the expense of other directions. This gain is a power ratio and can be expressed as:

1

2log10)(P

PdBG

The actual amount of power received by an antenna through free space can be predicted by use of the following equation

22

2

16 d

GGPP rtt

r

Antenna Gain Relationship between antenna gain and effective

area

G = antenna gain Ae = effective area f = carrier frequency c = speed of light (» 3 ´ 108 m/s) = carrier wavelength

2

2

2

44

c

AfAG ee

Example

Two half-dipole antennas each with a gain of 1.64 are separated by a distance 50 km. The transmitter feeds its antenna with 10 W at 144 MHz. Calculate the power received.

A half wave dipole antenna is capable of radiating 1 kW and has a gain of 2.15 dB over an isotropic antenna. How much power must be delivered to the isotropic antenna match the field strength of the directional antenna?

Reflectors and Directors It is sometimes necessary to focus

power in one particular direction. This can be done by the use of reflectors and directors.