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  • Antennas

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    Tim Kannegieter

    There are a range of antenna options such as reverse polarity SMA, which is a type of screw on connector commonly seen on small two way radios. RPSMA will typically be used where you want the antenna externally mounted on a case such as a vending machine. U.FL connectors are an alternative type of connectors which are internal. For price sensitive applications a wire antenna directly soldered to the module may provide sufficient performance. 

    The RF receiving rate of antennas needs to be considered in detail. A whole lot of design considerations come into play there. Is there strong sunlight? Is radar required? Is UV protection required? Is mobility required? Are there environmental considerations, such as exposure to water. Are there physical space constraints? What physical security is necessary, for example, to prevent and/or reduce the impact of theft and vandalism? One possible strategy might be to select a cheap antenna and implement maintenance plans to replace them as required, and accept the expense of lower performance. Another is adding redundancy such that damage to one antenna will not disable the whole system.

    The kind of connector is an another consideration. How do we physically screw the antenna in? How robust does it need to be? The antennas above left are stud mounted connectors. The entire assembly is waterproof.

    Some typical examples of antennas follow. The easiest resonant antenna is a 1.4 wave whip. The speed of light, denoted as c is 3x10^8 per second. Radio engineers think of a quarter wave as 300 meters divided by the frequency in MHz. A quarter wave is 75 meters divided by the frequency. For example, at 750 MHz, a quarter wave is .1 of a meter or 10 cm. A 920 MHz quarter wave whip might be about 9 cm. A 2.4 GHz one is about 3 cm.

    There are various types of antennas. Rubber duckies are a common one used for two way radios. We can use patch antennas put on ceramic element. Yagi's are another, which can have multiple elements and which have the characteristic that they are very directional. A purpose built antenna design needs to cover all the frequencies being used for the application, which may be more than one. A 920 MHz system, for example, may also require GPS, 2G/3G/4G or satellite communications, for which a custom antenna could be constructed to service all these required frequencies in one antenna.

    The effective isotropic radiated power, EIRP, is a theoretical concept. A point radiator is said to radiate out into a sphere in all three dimensions which is not a very accurate model of actual antennas. Practical scenarios must consider the antenna gain. Most importantly the direction of that gain results from restricting the angle of radiation. Restricting the angle of radiation to 90 degrees, for example, will create a high gain antenna. For a Yagi, the angle of radiation might be 10 degrees, and so an even higher the gain. High gain resulting from a reduced angle of radiation is beneficial, but the designer must also consider whether that is suitable for the terrain. When outdoors and in hilly country for example, a high gain antenna might miss the other antenna because the angle of the radiation is two restricted. Similarly for a yagi antenna. The designer must consider the tradeoff between higher the gain and the more restricted the angle of radiation to determine if the antenna is suitable.

    Antenna design must consider power restrictions. In a 920 MHz scenario with a maximum of one watt of EIRP and a really high gain antenna, it may be necessary to back the power off. Most radio modules will allow selection of the power output such that the power can be reduced as the gain of the antenna. It's really important is increased. It is the whole system (transceiver and antenna in combination) that is not allowed to radiate more than one watt EIRP.

    Sources: Material on this page has primarily been sourced from the following:

    Edited by Tim Kannegieter


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