![]() In many amateur systems the requirement is for a highly directive pattern to achieve increased 'gain and interference reduction'. It is necessary to be able to physically rotate antennas in order to get maximum power in a required direction. A-1, typical radiation patterns for a 2-element beam are shown, and its horizontal field over 360° and its vertical angle of radiation. Radiation resistance is that value of resistance, if replaced, the radiation mechanism of antenna would dissipate exactly the same amount of power that the antenna radiated. Radiation resistance is defined as the ratio of power radiated by the antenna to the square of the current at the feed point. The directional antennas have directivity gain. A2) are both expressed as the number of decibels (dB) down with respect to the maximum radiation of the main beam. Side lobes (radiation in any direction other than that required) and the front to back ratio (F/B ratio) with directional beam antennas (the ratio of the energy radiated in the required direction to that radiated in the diametrically opposite direction - 180°, Ref. This is also known as the 'half-power beam width'. ![]() The beam width in a principal plane of the radiation pattern is defined by the pattern’s angular width at a level which is 3dB down from the beam maximum. ![]() However, for such systems like repeaters and mobile communication, it is often necessary to radiate energy in an omni directional pattern, in order to provide a broadcast type of coverage.With amateur antennas, which normally have radiation patterns of a simple shape, the important characteristics of the patterns can be specified by the beam width and side lobe level in the two principal planes, usually taken as the V (vertical) plane and the H (Horizontal) plane. ![]()
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