Antenna Theory

Question 1

Define the term “Antenna”?

Answer 1

An antenna is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver.

Question 2

Explain why an Antenna can be considered as a Transducer?

Answer 2

An antenna is a transducer designed to transmit or receive radio waves which are a class of electromagnetic waves. In other words, antennas convert radio frequency electrical currents into electromagnetic waves and vice versa.

Question 3

What range of frequencies within the Electromagnetic Spectrum are Radio Waves?

Answer 3

Radio waves have frequencies as high as 300 gigahertz (GHz) to as low as 30 hertz (Hz).

Question 4

What is the speed of a Radio Wave?

Answer 4

Radio waves travel at the speed of light (approximately 300,000 km/s) in vacuum.

Question 5

Explain the structure of a Radio Wave?

Answer 5

All Electromagnetic Waves consist of two separate fields.
Fields are known as the “E Field” and “H” Field;
• E Field = Electric Field
• H Field = Magnetic Field
Electric + Magnetic = Electromagnetic

Question 6

Define how an Antenna produces Radio Waves?

Answer 6

Radio waves are produced when the electric field rapidly changes. There has to be an alternating current.
This field sends a wave outward from the wire.
When the current is removed, the field collapses which again sends a wave.

Question 7

State how wavelengths are measured?

Answer 7

Wavelength is measured in metres

The measurement is the distance between adjacent points in a series of periodic waves

Denoted by the Greek letter λ (Lambda)

Question 8

Define the term “Frequency”?

Answer 8

Frequency is the number of waves that pass a point in one second.

The unit of measure for frequency is Hertz (Hz).
1000 Cycles/Sec = Kilohertz – KHz
1,000,000 Cycles/Sec = Megahertz – MHz
1000,000,000 Cycles/Sec = Gigahertz - GHz

Question 9

What is the relationship between Wavelength and Frequency?

Answer 9

The lower the frequency, the longer the wavelength.
The higher the frequency, the shorter the wavelength.

Frequency and wavelength are said to be;
Inversely Proportional

Question 10

Calculate the wave speed (in m/s) for a wave with a frequency of 10,000,000Hz and a wavelength of 30m?

Answer 10

Velocity is a constant 300,000,000 meters per second

Velocity (V) = Frequency (f) x Wavelengh (λ). (m/s)

Question 11

• Calculate the wavelength (in metres) of a wave with a frequency of 440Hz travelling at 340 m/s?

Answer 11

Velocity is a constant 300,000,000 meters per second.

Frequency (f) = Velocity (V) x Wavelengh (λ). (Hz)

Question 12

Calculate the frequency (in Hz) of a wave travelling at 300,000,000 m/s with a wavelength of 0.15m?

Answer 12

Velocity is a constant 300,000,000 meters per second

Wavelengh (λ) = Velocity (V) Divided by Frequency (f). (m)

Question 13

Define the term “Propagation”?

Answer 13

The movement or spreading of waves.

Question 14

What are the three modes of Propagation?

Answer 14

• Ground Waves
• Sky Waves
• Space Waves

Question 15

What are the four factors that affect the Angle of Propagation?

Answer 15

• Height.
• Frequency.
• Angle of Elements
• Terminating Resistance.

Question 16

Define Ground Wave Propagation?

Answer 16

Ground Wave propagation is a method of radio wave propagation that uses the area between the surface of the earth and the ionosphere for transmission.

Question 17

Explain how Sky Waves can be transmitted over large distances?

Answer 17

By using the Ionosphere,

The lower frequency waves reflect off the Ionosphere and essentially “skip” around the earth.

Question 18

Explain Space Wave Propagation?

Answer 18

The space waves can travel through atmosphere from transmitter antenna to receiver antenna either directly or after reflection from ground in the earth’s troposphere region.

Question 19

What are Polar Diagrams used for?

Answer 19

Polar diagrams are used for plotting the radiation patterns of antennas as well as other applications like measuring the sensitivity of microphones in different directions.

Question 20

Explain Sky Wave Propagation?

Answer 20

The sky waves are the radio waves of frequency between 2 MHz to 30 MHz. These radio waves can propagate through atmosphere and are reflected back by the ionosphere of earth’s atmosphere.

Question 21

Explain the term “Gain” with regards to an Antenna?

Answer 21

As an antenna, the gain describes how well the antenna converts input power into radio waves headed in a specified direction or radio waves arriving from a specified direction into electrical power.

Question 22

How is Gain measured?

Answer 22

dBi (decibels relative to an isotropic antenna).

Question 23

State an example where a high gain antenna would be beneficial?

Answer 23

TV Antennas. If you mount a TV antenna on your roof and know the tv broadcast antennas are to the south, then it is preferred to have a high gain antenna.

Question 24

State two applications where a low gain antenna would be utilised?

Answer 24

Mobile Phone Antennas.

Vehicle Antennas.

Question 25

State the two ways an Antenna can radiate?

Answer 25

• Omni-Directional

- Directional (Uni/Bi)

Question 26

State the two categories of polarisation for Antenna Systems?

Answer 26

• Horizontal

- Vertical

Question 27

What is the frequency band for High Frequency?

Answer 27

3MHz to 30MHz

Question 28

What is the frequency band for Super High Frequency?

Answer 28

3GHz to 30GHz

Question 29

What is the frequency band for Ultra High Frequency?

Answer 29

300MHz to 3GHz

Question 30

What is the frequency band for Very High Frequency?

Answer 30

30MHz to 300MHz

Question 31

State one advantage and disadvantage of a Broadband Antenna?

Answer 31

Advantages;

• Broad Spectrum of available Frequencies.
• Reduces the need for multiple antennas
• Potentially ensures less cost, equipment and servicing required.

Disadvantages:

• Complex in design.
• Some Antennae need large ground areas.

Question 32

What type of propagation does High Frequency Antennas use?

Answer 32

Sky Wave Propagation.

Question 33

What type of propagation does Ultra High Frequency use?

Answer 33

Space Wave/Line of Sight Propagation

Question 34

What type of propagation does Super High Frequency use?

Answer 34

Space Wave/Line of Sight Propagation

Question 35

State three considerations that should be factored in when installing antenna systems?

Answer 35

Antenna Frequency
Polarity
Water Ingress Protection
Climate

Question 36

List four common faults with antenna systems?

Answer 36

• Incorrect Antenna for desired frequency
• Different polarities between Tx and Rx
• Broken/Missing elements
• Water Ingress.

Question 37

State the advantages and disadvantages of Bayed and Stacked Antenna Systems?

Answer 37

Advantages:

i. Performance & Gain of a system Gain” as a parameter measures the directionality of a given antenna. An antenna with a low gain emits radiation in all directions equally, whereas a high-gain antenna will preferentially radiate in particular directions. Specifically, the Gain, Directive gain or Power gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated at the same distance by a hypothetical isotropic antenna:

Disadvantages:

ii. More connectors I.e. more dB loss. Complex in design. Limitations of structure