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Antennas


Pacific Wireless 15dbi VagiThere is a wide, almost endless variety of antennas to choose from. Panels, grids, dishes, vagi's and yagi's are available in all shapes and sizes and choosing the one most suitable for your needs might seem pretty challenging. We have had pretty good results with comparitively short links (up to 10km) with the Pacific Wireless 15dbi Vagi pictured on the right of this text. 15dBi is not a massive amount of gain and is ideal for short links of a few hundred metres as well as medium range runs. They appear to be reasonably sensitive, have a fairly wide spread so that alignment is not incredibly critical and are quite modestly priced. For longer links more gain is necessary and in that instance we tend to favour a 26dBi grid, also available from Pac Wireless. Select an antenna that suits your purpose, there is no point in using a 26dBi Grid antenna to go across the road, they will overpower each other and you will end up with a lot of noise. A quality signal is more desirable than a strong one though the goal should be to have a good strong quality signal.

When you purchase an antenna there a few points to consider and these are determined by the distance of your link and any vegetation that may be in the way.

Antenna Gain is usually measured in decibels (dB) and is a standard of power output when compared to a reference antenna. There is a degree of confusion around antenna gain and it is true that a 26dB antenna will deliver a stronger signal accross a certain distance than a 13dB antenna. It doesnt do this by magic however and a higher gain antenna does not amplify the signal in any way but focuses it in a tighter beam. The higher the gain the lower the signal spreads and the more accurate your antenna alignment must be. A 13dB antenna might have a signal spread of 25 degrees where a 26dB might only spread 10 or 15 degrees, effectively concentrating the same amount of power over a smaller area.

Antenna Polarity is another consideration though one that is the subject of a lot of debate, there are differing opinions as to what polarity is appropriate under what conditions. Horizontal Polarity means that the wave of the signal is horizontal and vertical polarity means that the signal wave is straight up and down. In most instances it does not really make a massive impact on the quality of your connection which polarity you choose as long as both antennas on your link are orientated to the same polarity (otherwise they wont see each other very well!). If you have no choice but to run your link through vegetation horizontal polarity would be more suitable. Accross a lake or body of water vertical might do you in better stead. Links with clear line of sight can be either polarity with no ill effects. If we look at our picture of the 15dB Vagi we can see the white plastic element on the antenna is horizontal so the antenna is horizontally polarised. To change it to vertical we would rotate the whole antenna (not just the element) 90 degrees so that the element is vertical.

Amplifiers are another way of increasing the strength of the signal. I dont suggest using an amplifier unless it is truly necessary as they tend to amplify noise as well as signal. Care must be taken when selecting an amplifier as most countries have legal limits to the amount of power your wireless link can use and an amplifier with a high gain antenna can result in a combination that will exceed these limits quite markedly. Most amplifiers are installed on your mast with a short cable going to the antenna and the longer cable going through to the router and power injector inside the building. The order is generally -

Router --> Power Injector --> long cable to antenna mast --> Amplifier --> Antenna

Amplifiers can make a substantial difference on the quality of a long distance link however they are completely unnecessary on a shorter wireless link and can be quite detrimental to the signal quality.

If your unsure of which of the many different types of wireless antenna's will suit your needs take the time to speak to the salesperson and see what they recommend. Many of the more established wi-fi equipment suppliers have salespeople who are familiar with their products and can guide you as to what would be most suitable for your intended purpose.