Tel: 61 2 9829 4133
Fax: 61 2 9618 2990
E-mail: bernie@dynmast.com.au
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Tel: 61 2 9829 4133 Fax: 61 2 9618 2990 E-mail: bernie@dynmast.com.au |
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Choosing the proper antenna for any application requires careful consideration in regard to the following four major criteria's: Gain Physical Size Cost Appearance The gain required depends on system margins and how far the system must operate. Generally the more gain, the more cost and the longer the antenna. Systems where the antenna can be a lower performance type will have less cost. Installations where suitable ground plane is available will be less costly than installations without adequate ground plane. To determine the best mobile antenna type for your particular installation, you should first be acquainted with the electrical types available. Keep in mind that not all types are available in all mechanical configurations and frequencies.  The loaded 1/4 wave type antenna is electrically a 1.4 wave, while being shorter than a full size 1/4 wave antenna. This is accomplished with a loading coil that places a portion of the electrical length of the antenna in a coil located along the radiating element. The efficiency of the antenna depends on how much of the electrical length is inside the coil (and therefore not radiating).  The loaded 1/4 wave is a single radiation element 1/4 wavelength long. It is the simplest and least expensive type of antenna. 1/4 wavelength antennas require no loading or matching coils. Typical gain is 0dBd when mounted on a suitable groud plane.  The 1/2 wave antenna is a single radiating element 1/2 wavelength long. Because the end fed impedence of the antenna is not suitable for matching the radio, an impedance matching transformer is used at the base of the radiating element. The 1/2 wave antenna is suitable for use where no or little ground plane is available. The gain with no ground plane is 0dBd. Gain with a suitable ground plane is typically 2.4dBd.  The 5/8-wave antenna is a single radiating element 5/8 wavelength long. In single element antennas, the 5/8 wave antenna has best performance (3dBd) when mounted on a suitable ground plane. Since the end fed impedence of a 5/8-wave antenna is unsuitable for interfacing with a radio, an impedence matching transformer is used at the base of the antenna. The antenna must be mounted on a suitable ground plane.  These colinear designs have two elements separated by a phasing coil. The top element is a 5/8 wave and the bottom element is either a 1/2 wave or 1/4 wave. Gain is typically 5dBd for the 1/2 wave lower element and 3 to 4dBd with the 1/4 wave lower element when mounted on a suitable ground plane.  This colinear design has two elements separted by a phasing coil. Both top and bottom elements are 5/8 wavelengths. Gain is typically 5dBd when mounted with or without a ground plane. The end fed impedence does not match the transmitters impedence so again a transformer is used. The power gain of an antenna system is usually expressed in Decibel's (dB). The decibel is a practical unit for measuring power ratios because it is more closely related to the actual effects produced than the power ratios itself. One decibel represents a just detectable change in signal strength, regardless of the actual value of the signal voltage. A 20dB increase in signal represents 20 observable steps in increased signal. The decibel is a logarithmic unit and therefore gains and losses expressed in decibels can be added and subtracted arithmetically; for example 3dB = 2 times the poer, 6dB = 4 times the power etc. The electrical length of an antenna wire is not necessarily the same as it's physical length. The lower the frequency, the longer section of wire is required to achieve a wavelength, less wire is therefore required the higher the frequency becomes. Antenna radiation patterns are the electromagnetic field an antenna produces when transmitting. Generally, the lower the gain the more evenly the pattern is distributed. High gain antennas produce a more compressed pattern concentrating transmission power into a narrow beam, which makes the antenna more directional. Antenna patterns are seriously affected by the antennas surrounds, mounting position and ground plane effects. The majority of antennas commercially produced require ground to operate efficiently. The more ground provided; particularly for lower frequency antennas, the better the antennas will perform. Ground plane is generally a grounded metal surface area. Centre roof mounting an antenna to a vehicle is considered the ideal position. Ground independent antennas are antennas that have been designed to incorporate in its functionality a siimulated ground plane to allow it to perform efficiently with a minimum or no ground plane. These antennas are generally comprised of a minimum of a 1/2 wave element. All antennas, even the simplest types, exhibit directivity effects. Directivity is the property of radiation stronger in some directions than others. Directivity relates to the amount of power density radiated from an antenna in a give direction; eg Yagi style antennas are very directional, radiating all it's power in one direction. This antenna as designed exhibits a much higher gain value due to its directivity. Typically the higher the gain, the more the radiation pattern is compressed (squeezed) in a give direction. This provides higher transmission efficiency in a given direction. The gain of an antenna is closely related to its directivity. The more the pattern is compressed (squeezed) in a give direction, the more concentrated the power becomes therefore increasing the efficiency of the antenna to achieve a higher gain value. High gain and directivity is generally achieved by increasing the number of antenna elements. Each 3dB gain is normally associated by doubling the number of elements in the antenna. |
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