Info about Wireless Networks 2

2.0 Choosing an Antenna

2.1 Introduction

A good microwave antenna is well designed and manufactured to extremely tight tolerances, out of materials that have the correct electrical properties and will withstand its operating environment over many years.

The challenge is that it is not possible to determine how good an antenna is by just looking at it. Most antennas work to a degree. To fully test an antenna requires a considerable amount of time and sophisticated test equipment. Common problems are:

performance that differs markedly from the stated “specifications”.
“specifications” that have been judiciously chosen to paint the antenna in the best light possible.
wide performance variations between antennas of the same model.
use of materials that result in degradation of the performance of the antenna over time.

These problems are caused by:

antennas that are assembled from parts, often sourced from different countries, and simply assembled together to make up the antenna. In some cases the parts are not designed for use at the required operating frequency.
use of lower cost components that inevitably have high variability in electrical characteristics and dimensions. These differences are magnified in the performance of the end product.
Lack of testing and/or tuning of the finished product before it reaches the customer.

Given this, the reputation of the company you are buying from is important. If in doubt, talk to other people who are using a particular manufacturer’s antenna before you buy. Ask questions about overall satisfaction, reliability, local service, technical and warranty support, local stock of antennas, accessories and spares.

2.2 Antenna Basics

The concept of an antenna is quite simple:

the purpose of an antenna is to focus microwave energy
focusing the energy, also concentrates it

Thus the key attributes of an antenna are:

Beam width (how much the energy is focused)
Gain (how much the energy is concentrated)
Return loss (how well matched an antenna is)
Polarization

Other antenna attributes to consider are:

Size and Weight
Materials

2.3 Gain

The gain of an antenna is a measure of how much an antenna concentrates (or amplifies) an input or output signal. The gain of an antenna is typically measured relative to a point source, which will radiate uniformly outwards in a sphere. This is called an isotropic antenna and is defined as having a gain of 0dBi.


Point source has gain of 0dBi	Antenna concentrates energy to produce higher gain

A higher gain antenna will produce more power and thus will cover longer distances.
Higher gain is achieved by focusing the energy; therefore there is a trade-off between gain and beam-width. Higher gain is achieved at the expense of a narrower beam width.
An antenna is a reciprocal device – it has the same gain when transmitting and receiving.
There are legislated maximum power limits for each frequency band. In the case of the ISM bands (2.4GHz and 5.8GHz) used for 802.11 wireless networks, the maximum power from an antenna is 4 Watts EIRP (effective isotropic radiated power).

To determine the appropriate gain for your antenna you should consider the following factors:

For longer distances you require more gain.
Microwave propagates best when there is clear line of sight between the base station and subscriber. Antennas well clear of the ground will work best.
The terrain impacts signal strength. Propagation over water typically requires more power than over land.
High-traffic environments typically have higher noise levels and require a stronger signal.
Higher gain antennas are physically larger and more expensive.
Using the optimum gain will reduce interference for other users in your vicinity.
Radiated power is determined by four main factors:
- Radio output power
- Cable losses
- Amplifier gain (if fitted)
- Antenna gain
Any of these factors can be varied to increase gain

2.4 Beam Width

pacsat image

Beam width is a measure of how much the radiated energy is focused or concentrated.
Beam width is commonly measured at the point where power drops by half (the -3dB point). Thus a 24dBi grid with a 20 degree horizontal beam width will have a gain of 21dBi 10 degrees either side of its bore sight.
This means there is still significant power outside the beam-width, particularly close to an antenna.

To determine the appropriate beam width you should consider the following points:

For point-to-point applications a narrow beam-width is desirable as it maximises power and minimises interference for other users.
For point-to-multipoint applications a narrow beam width may not be desirable. As an example high gain omni antenna achieve their gain by narrowing their vertical beam width. This can create coverage problems if subscribers are at significant different vertical heights (elevation).

2.5 Return loss

If the antenna is not correctly matched at the operating frequency being used, much of the received signal will never reach the radio (instead being reflected back by the antenna) and much of the transmitted signal will never leave the antenna (instead being reflected back to the radio). Return loss is a measure of the match. Two measures of the quality of the match are used: Voltage Standing Wave Ratio (VSWR) and Return Loss. The VSWR should be a small number while the corresponding Return Loss should be a high number:

	Typical	Worse Case
VSWR	1 : 1.2	1 : 1.4
Return loss (dB)	20	15

2.6 Polarisation

Antenna are commonly vertically, horizontally or circularly polarised. To give you sense, a vertical dipole will radiate vertically polarised signals and a horizontal dipole will radiate horizontally polarised signals. For many antennas it is possible to change between vertical and horizontal polarisation by rotating the feed (excluding omnis). While there is significant debate about the relative merits of each type of polarisation, it is essential that send and receive antenna share the same polarisation. Vertical polarisation is most commonly used as it has been shown to offer the best propagation in general use.

2.7 Size and Weight

As a general rule, the larger the antenna the higher its gain. The exception is antennas that use a larger reflector to compensate for poor design in other areas.

The following factors should be considered:

The size of the antenna determines the wind loads placed on the antenna, its mounting and the supporting structure.
The size and weight of the antenna determines how easy it is to handle during the installation process
Aesthetics can be an important consideration in choosing an antenna. Patch panels and enclosed yagis are less visually obtrusive in some environments.

2.8 Materials

Antennas frequently operate in a hostile environment where they are continually exposed to the elements with no maintenance over long periods of time. The materials and construction techniques used in an antenna need to suit that environment.

Some points to consider are:

Steel components have a limited life particularly in a tidal zone. Even with galvanizing and painting, eventually the surface will erode and corrosion will occur.
Aluminium reflectors (should be corrosion resistant, surface treated aluminium alloy) are superior and normally give years of maintenance free operation.
Plastic components need to be UV stabilized and used carefully due to their vulnerability to attack by birds.
Sealed feeds are vulnerable to seal failure and diurnal pressure changes which can lead to build-up of moisture inside the unit. Gas-feed, positive pressure systems are best, although usually only used in very expensive systems.
Vented units exclude weather but are vented to the atmosphere to prevent moisture build-up. To operate correctly these units must be installed properly (drainage holes in lowest position).
Vented feeds may use a membrane that excludes water but passes gas to prevent pressure build-up. These are well-proven and used on many “sealed” devices.
Foam-filled feeds overcome the problems associated with either sealed or vented feeds and are particularly suited to extreme conditions.
Printed circuit boards used in feeds or microstrip antenna are vulnerable to corrosion and build up of deposits. Weatherproofing for these units is critical. Solid brass feeds are more robust.
Fitting and mounts should be aluminium or stainless steel for longest life. Where steel is used it should be galvanized or surface coated.

With the proper equipment you should expect to receive years of trouble-free service from your network. As discussed, the key is good equipment and network planning.

> go to 3.0 Implementing a Wireless Link
< return to 1.0 Introduction to Wireless Networks

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