antenna parameters part 2 - polarisation

Poynting Antennas (Pty) Ltd

5 April 2016 AntennaParameters Part 2

ON THE PROPERTIES OF AN ANTENNA This is part of an internal document that that gives an overview of the properties of

antennas for non-engineers.

We have divided the document into different posts where we discus each of the

parameters:

- Frequency bands, gain and radiation pattern

- Polarisation

- Input Impedance and VSWR

- Port to port Isolation and Cross-polarisation

- Power Handling ability

- Antenna “Specmanship”

Where applicable we have added some videos explaining the properties discussed. You

will find a link to a PDF below.

Introduction An antenna is a device that converts energy from one form to another. When used in

transmit mode, currents in the coaxial cable (feeding the antenna) flow into the antenna

and the energy is converted to electromagnetic radiation which propagates into space.

When an antenna is used in receive mode, electromagnetic radiation interacts with the

antenna inducing currents into its components. These currents flow along the coaxial

cable connected to the antenna to a receiver.

In some ways the antenna is analogous to a speaker in a sound system. A speaker

converts electrical energy (from the wires powering the speaker) into sound energy which

we can detect using our ears. When operated in the opposite mode a microphone is

created. This device detects sound wave and converts them to electrical energy. An

antenna works with electromagnetic radiation and electric currents rather than sound and

electric currents.

An antenna is described by a number of attributes including frequency bands of

operation, gain, radiation pattern, polarisation, VSWR, input impedance, coupling, power

handling ability and so on.

This document describes each of these parameters.

1. Frequency bands of operation See part 1

2. Gain and radiation pattern See part 1

3. Polarisation The radiation from an antenna comprises both electric and magnetic fields. Once the

radiated wave has established itself – (a distance greater than 2𝐷2

𝜆 from the antenna, where

D is the largest dimension of the antenna and λ is the wavelength), then the electric and



magnetic fields are perpendicular to each other (and both are perpendicular to the

direction of propagation – the so called Poynting vector).

Figure 1: Illustrating the plane of the electric field lines (red), magnetic field lines (blue) and

Poynting vector (green)

The polarisation specification for an antenna specifies how the electric field behaves in

the far field (where the radiated wave has established itself).

If the electric field stays in one plane as the wave propagates, then its polarisation is said

to be linear.

Figure 2: Illustrating a linearly polarised wave

On the other hand, if the electric field rotates about the axis of propagation (and does not

change size as it rotates) then the polarisation is said to be circular. Circularly polarised

antennas can be either left or right-hand circularly polarised. The polarisation is defined

as left-hand circularly polarised if the electric fields rotates anti-clockwise about the

direction of propagation.



Figure 3: Illustrating a circularly polarised wave

Linear and circular polarisations are the extremes of the polarisation specification and

typically all one will see in a brochure. In reality, however, the polarisation will be

anywhere between these two extremes and the electric field will trace an ellipse as it

rotates about the direction of propagation.

Elliptically polarised waves are typically specified in brochures as circularly polarised,

but they include an additional parameter called the axial ratio. The axial ratio defines the

shape of the ellipse.

Figure 4: Illustrating an elliptically polarised wave

Sometimes the polarisation specification is given as vertical or horizontal. In such cases,

the polarisation is linear and the electric field is vertical/horizontal with respect to the

earth when the antenna is mounted in the correct orientation.



When using antennas to form a radio link it is important that the antennas at each end of

the link are correctly oriented. The polarisations should be the same and both have the

same orientation with respect to the earth.

If two linearly polarised antennas are used to form a link and one is mounted vertically

and the other horizontally, then one will experience about a 20 dB drop in the signal

strength (compared to when they are both oriented in the same direction). The same 20

dB reduction in received signal strength will be experience if two circularly polarised

antennas are used with one antenna being left-hand circular and the other right-hand

circular.

If a circularly polarised antenna is used at one end of a link and a linear antenna at the

other end, then the orientation of either antenna will make no difference to the received

signal strength. The signal strength will, however, be 3 dB worse than if one used

antennas of the same polarisation (assuming the gain of the replacement antenna is

unchanged).

Andre Fourie explains polarisation in a video here:

https://youtu.be/1gXNJ5TOmOQ

https://youtu.be/1gXNJ5TOmOQ

antenna parameters part 2 - polarisation

Engineering