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By David Benchoam BE (EE)
The antenna system you select for your phone is critical to the performance of your reception. It is
important to correctly match your antenna with the environment with which you will use your phone. This article will hopefully shed some light on an often-misunderstood feature of antennas, "Gain".
All communication systems include antennas in some way, whether they are hidden or exposed they are always present. Unfortunately most people know very little about antennas and how they work. Even the so-called experts that you may encounter at various phone distribution outlets, really don't understand the "ins and out's" of an antenna system. In fact, antenna theory is quite complex, it can be considered a science on its own (a science I might add, which is not even comprehensively taught at Universities), there are so many traps and pit falls for the regular user who simply wants a phone system that works reliably I
Consider your Hi Fi system at home, it is possible to spend thousands of dollars on a quality CD,
amplifier, and tape deck and still have poor sound, because you economised on the speakers. In a sound system each part of the system is important to achieving great quality sound. Similarly in a communications system, the antenna you select is as critical as the speakers in the sound system. There is no point in spending lots of money on a great phone if you buy a cheap and nasty antenna. Like most industries, you get what you pay for. There is also no point in buying what is considered the best antenna "with the most gain" if you can't take advantage of the gain, in your environment. A little thought into the antenna system can result in better cellular coverage throughout the life of your phone. An understanding of gain will help you select which antenna to use.
The gain of the antenna affects the way the signal is distributed. Its not possible to create more power from your phone by adding a higher gain antenna, however it is possible to send the signal to where it most counts. All mobile antennas, ie antennas fitted to vehicles, are omnidirectional, that means they radiate signal 360' around the antenna. The radiation pattern of the antenna describes how that signal is distributed in the 360' fashion. The diagram below, shows the side on view of the radiation pattern of the signal from a unity gain antenna.
The two circles represent the side view of a large doughnut shaped radiation pattern. Prom this shape you can see that the signal is spread evenly upwards, downwards and sidewards. This shape of radiation pattern suits the Sydney environment, where there are many hills and dips. The signal is spread in all directions meaning there is a better chance to reach a cell site from the bottom of a dip or from behind a building. This design of antenna should eliminate dropouts around the Sydney or other built-up areas. As we modify the design of the antenna and increase the gain, the doughnut shaped radiation pattern is squashed flat. The next diagram shows how the gain flattens the omnidirectional radiation pattern therefore widening the coverage of your signal.
The above diagram compares a Unity gain or 0dB antenna with 3 and 6 dB radiation patterns. From the diagram above you can see that it is possible to communicate with a cell site further away. There are a few traps however, the AMPS and GSM systems operate between 825-960MHz. These high frequency signals have a tendency to bounce off buildings in built up areas, and off mountains in a valley. This means a high gain antenna is best-used in country areas where wide flat plains are between you and the cell site. In mountainous areas or in city environments such as Sydney where the topography varies considerably, a high gain radiation pattern will not work as efficiently as a unity gain. Say a high gain antenna was used in a dip or valley and the vehicle was at the lowest point, the phone signal would he weaker, as the signal would bounce off surrounding hills or buildings and find it difficult to reach a cell site. Similarly if the vehicle was on a slope, the high gain antenna would be sending most of the signal into the sky. Therefore a high gain antenna can result in a poorer signal when used in the wrong environment. Melbourne however is fairly flat, therefore a 3dB antenna would be an advantage. Most antennas installed on cars are "On-Glass" antennas. This design is a compromise, as losses in signal strength are experienced in the transfer through the glass. An antenna which is fed by cable directly will outperform all "On-Glass" designs. In the city you would probably not experience too much difficulty, due to the dense concentration of the cell sites. This means that even a weaker signal will result in reasonable performance. There probably would be times however where you will experience drop outs, where the signal is weak, these may be avoided with the extra signal strength or a quality antenna and cable combination. If you were very serious about keeping in contact with a cell site and your travels included some country driving, I would recommend having two antennas installed. When your communicating in the city use the unity gain antenna and in the country use the high gain. BENELEC manufactures a Collinear antenna (Cat # 02457) with 7dBi of gain, it has been designed for country and freeway driving. From my own tests, this antenna provided full signal strength for 95% of the trip between Sydney and Brisbane (Down the Pacific Highway). Benelec also manufactures a 0dB and a 3dB gain antenna which can be mounted anywhere on the vehicle using conventional mounting methods. The antennas mentioned all offer ground plain independent designs. I will elaborate on this, in future issues. |
