Which Antenna

Choosing the correct antenna is one of the most, if not the most important aspect of wireless networking. There are some important decisions to be made - home-built or bought, radiation pattern, and fixings. If you're not sure what you need, the easiest solution is to join the list and quiz the people there.

BUILD OR BUY?

The simplest argument here is building=cheap, buying=easy. Of course, there is a lot more to it....

• Built antennas rarely perform as well as bought ones, but do perform well.

• Antennas are often very expensive to buy.

• It's easier to customise an antenna when you build it than to change it afterwards.

RADIATION PATTERN

This defines in which direction the signal is transmitted and received from the antenna. What pattern you require depends on where you want to communicate. Antennas are generally seperated into two main types named "directional" and "omni-directional".

Omni-directional antennas communicate with devices on any side. The higher the gain of the antenna, the less it is able to communicate above and below. To understand where the antenna communicates, imagine a large beach ball. The antenna is in the centre of the ball. The antenna can communicate with any device inside the ball, but nothing outside.

For an omni-directional antenna with no gain, the ball would be round. As the gain is increased, the ball is squashed. The ball is not as high as it was, but is much wider. The ball will go through the shape of a doughnut to the shape of a pancake. Whilst the pancake stretches the furthest distance, it can only communicate with devices on the same plane.

The main thing to remember about this is that antennas do not amplify signal, they just decide where it comes and goes.

A directional antenna communicates in one direction. There are various types of directional antennas such as panel antennas, and yagis. A highly directional antenna such as the famed whisky tin and Pringles can (otherwise known as a 'cantenna') is great for point-to point links, especially over long distances.

A less directional antenna, such as a panel or an omni-directional antenna is useful for connecting a number of devices together. This could be a mesh of nodes or a number of client machines. The ECC has a less directional antenna which communicates over about a 70 degree angle. Some of the nearby nodes use the fabled whisky tin to connect to this, whilst others use omni-directional or panel antennas depending on their uses and surroundings.

LEGAL ISSUES

Here in the UK, we are limited to a maximum of 100mW EIRP. In practice, with the 30mW cards that are so prominent, we are limited to a maximum gain of 5dB. This includes the cable; so an 8dB antenna using a connector with 1dB loss, connected to a cable with 1dB loss, and connected to a wireless card using a pigtail with 1dB loss is right on the legal limit.

This limit effectively imposes a one mile radius for links, provided we have perfect conditions and RLOS. However, it is possible to stretch the limits using diversity. This involves using a separate antenna purely for receiving.

One antenna is used to transmit (and generally receive), and is therefore limited to 100mW, whilst another is not used to transmit. Because there are no limits on receiving, you can use a highly directional antenna to make much longer links.

For example, one node 5 miles from another with RLOS between them can communicate. Each node outputs 100mW using a regular card and a 5dB omni-directional antenna. In addition to this, each node has a high gain whisky tin antenna used purely to receive signals, and aimed directly at the other node. The nodes can then easily communicate legally, whilst serving local clients.

So how do you implement diversity? Well, you need a card that is capable of turning off the transmitter on one of its antennas. Allegedly, some of the Cisco cards can do this. The Actiontec PCI card is capable of this. It has two internal antennas (easily replaced with SMA sockets) with the one closest to the PCI slot used for transmit and receive, and the other used purely for receive.

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