Mention WiFi and most people – at least those who know what WiFi is – think of a wireless link between a couple of rooms in the home or maybe a couple of offices in a building. That’s mainly because that’s all they are used to and all that they expect.

But WiFi can go much further than this: the current world record for a "naked" (ie unamplified) 802.11g (WiFi) terrestrial link is (we believe still!) 280km (see SILICON CHIP, February 2007).

But that was using some pretty esoteric gear including large, high-gain dishes, equipment that would probably be illegal to use here. Even so, you could use off-the-shelf and legal gear in Australia to set up a reliable, fast 2.4GHz link of perhaps tens of kilometres.

The link could be used for anything a Local Area Network (LAN) connection can be used: Internet access, file sharing, Voice over Internet Protocol (VoIP), Video Surveillance and many more applications.

But isn’t WiFi rated only up to 100m?

Most WiFi equipment has a comment somewhere that the effective range is something like 100m or less. That figure assumes a lot of worst case conditions, such as:

you only use the small antenna that comes with the unit

the other end of the link (a laptop?) has no external antenna and

this is all operating inside a building with walls and people in the way.

By changing some or all that, much greater ranges can quite easily be achieved.

Art – or Common Sense?

If you think you don’t already know how to do this, think again. What if someone asked you to use visible light to make a signalling beacon over 10km from rooftop to rooftop? We’re sure you would conclude you needed something like that in first column of Table 1.

The second column in the table shows what we need for our 10km WiFi link. Notice the similarities?

Let’s go through them individually:

Transmitter Power

The transmitter/receiver unit in the WiFi world is referred to by the all encompassing term Access Point (AP). Obviously the stronger the transmitted signal, the further it will go.

Power, at least as far as WiFi is concerned, is expressed in either milliwatts (mW) or dBm. dBm is often confusing to the novice but is simply a ratio of the power with respect to 1mW. A 1mW transmitter would therefore have an output of 0dBm; a 100mW transmitter would have an output of 20dBm.

Most mass-market APs have low transmitter power – as they are for the ‘50m’ market. Powers of 15 or 30mW (12 or 15dBm) are common but these are usually too low for long distance WiFi. Avoid them.

At the other end of the scale you can get high power APs with transmit powers of 100mW or more. Amplifiers can boost that even further but there are legal limits on how far you can go. See the "Keeping it Legal" box for more information.

The Freenet Antennas UltraWAP AP is available in a number of power levels from 60 to 200mW. The 90mW unit is a good all-rounder for long distance links that remain within the ACMA limits for directional antennas.

Antennas are analogous to lenses in optics. They neither create nor destroy energy but rather focus it into a smaller beam – giving the impression of more power.

The focusing power of an antenna is called "gain" and is measured in dBi. This abbreviation stands for gain (in decibels) over a theoretical isotropic (point source) antenna. But don’t let that worry you: simply remember that the higher the gain, the more focused the beam and the more accurately it must be pointed.

There are lots of commercially available WiFi antennas. There are even more home-brew designs available on the web (some excellent, some not!) and some great ones have been published in SILICON CHIP (see Stan Swan’s article in November 2002; Rob Clark’s in August 2003 and Stan Swan’s WiFry antenna in November 2004.)

Just like a dirty lens wastes valuable light, a lossy antenna cable wastes valuable WiFi signal. But there’s another wrinkle with antenna cable: the higher the frequency, the lossier a cable becomes.

Above: PC (PCMCIA) WiFi adaptor and (below) USB WiFi adaptor, both with external antenna connectors. These usually perform much better than the more usual adaptors which have the antenna "built in".

Cable that is perfectly acceptable for long runs at, say, 144MHz (the "two metre" amateur band) can be a poor performer at WiFi frequencies – 2.4GHz (2400MHz).

As a rule, we must use short, low-loss antenna cables. Less than 3m is a good rule. In some cases, this will necessitate installing the AP in a weatherproof enclosure close to the antenna, and running a weatherproof power/ethernet cable up to the external AP.

The ethernet and power cables (or sometimes one cable serving both) can be much longer than the 2.4GHz cable without appreciable loss.