KC04 - RF Antenna

Question 1

Define the following: Radiating element

Answer 1

The radiating element is the part of an antenna that is capable of radiating or receiving transmitted RF energy.

The simplest way to make an antenna is by:
Using a wire, spread the ends
The parallel feed-in wire prevents losses of any radiation
The ends radiate and receive well

Question 2

Define the following: Antenna Aperture

Answer 2

A surface, near or on an antenna, on which it is convenient to make assumptions regarding the field values for the purpose of computing fields at external points.

Question 3

Define the following: Antenna Gain

Answer 3

antenna’s gain is a key performance parameter which combines the antenna ‘s directivity and radiation efficiency.

Question 4

Define the following: Antenna Beamwidth

Answer 4

beamwidth is the aperture angle from where most of the power is radiated

Question 5

Define the following: Antenna Bandwidth

Answer 5

The bandwidth of an antenna is the range of frequencies over which it operates

Question 6

Define the following: Main Lobe and Side Lobes

Answer 6

Main Lobe - region of the radiation pattern containing the highest power or exhibiting the greatest field strength.

Side Lobe - lobes (local maxima) of the far field radiation pattern of an antenna or other radiation source, that are not the main lobe.

Question 7

Define the following: Effective Radiated Power (ERP)

Answer 7

The overall emitted PWR from the antenna
- PT = Transmitted Peak PWR
- LT = Transmitted Line Losses
- GT = Antenna gain

Question 8

Describe the use of Antenna Polarisation

Answer 8

1) Horizontal polarisation reflects well off horizontal structures
2) Circular polarisation reflects well off complex structures

Question 9

Describe the relationship between Antenna Gain, Aperture Size, Beamwidth and Frequency

Answer 9

1) Beamwidth size decreases when you
increase
antenna physical size and
increase
operating frequency

2) Antenna gain increases when you
increase
antenna physical size and
increase
operating frequency.

Question 10

Describe the Antenna Beam Pattern of the following: Half-Wave Dipole Antenna

Answer 10

Dipole Antenna:
Omni-direction - good coverage for receiving signal from all directions

Directional Antenna:
Signals front on will be concentrated into the receiver (strong signal)
Signals side on will be less concentrated into the receiver (weaker signal)

Question 11

Describe the Antenna Beam Pattern of the following: Slotted Wave Guide

Answer 11

• Slots arrange at 1/4 wavelength apart
• Each slot acts like a dipole
• Fan beam pattern
• Mechanically rotating for steering, beam forming is due to interactions of the phases emanating from the individual slots

Question 12

Describe the Antenna Beam Pattern of the following: Parabolic Antenna

Answer 12

Parabolic: Directional Pencil Beam

Stacked Beam: Directional pencil beam with changing elevation, possible through transmission using different outputs

Alternative Parabolic Antennas: Modifying the shape of the dish can turn the beam into a fan shape

Question 13

Describe the Antenna Beam Pattern of the following: Antenna Arrays

Answer 13

Some antenna arrays can be mechanically steered with the directed beam patterns mechanically and electrically controlled (i.e Planar Array antennas)

Other antenna system will use multiple antenna modules. Each module contains a receiver/transmitter and an antenna. The receiver collector module gathers all the return echoes and will process these as required (i.e Active Electronic Scan Array antennas)