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Similar radial wires can be placed on the top of antennas (instead of at the base) that also promote more efficient distribution of current in the antenna, but the structure of radial wires added to the top end of the antenna is called a capacitance hat or top loading. Like ground radials, top radials are symmetrically arranged wires placed to ...
The top shows the directive pattern of a horn antenna, the bottom shows the omnidirectional pattern of a simple vertical dipole antenna. In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the directional (angular) dependence of the strength of the radio waves from the antenna or other ...
The vertical plane is used to plot an antenna's relative field strength perpendicular to the ground (which directly affects a station's coverage area) on a polar graph. Normally, the maximum of 1.000 or 0 dB is at the side (unless there is beam tilt), which is labeled 0°, to 90° at the top and −90° at the bottom.
The antenna's feed point is at the center of the disc. It is usually fed with 50-ohm coaxial cable, with the center conductor connected to the disc, and the outer conductor to the cone. The cone: The length of the cone should be a quarter wavelength of the antenna's lowest operating frequency. [2] The cone angle is generally from 25 to 40 degrees.
The antenna rod and its image together act like a dipole antenna of twice the length, so a monopole over an infinite, perfectly conducting plane has a radiation pattern identical to the top half of the pattern of a vertical dipole of twice the length. [1] [70] [71] For the quarter wave monopole, the antenna acts like a half wave dipole.
When the antenna is fed at some other point, the formula requires a correction factor discussed below. In a receiving antenna the radiation resistance represents the source resistance of the antenna, and the portion of the received radio power consumed by the radiation resistance represents radio waves re-radiated (scattered) by the antenna. [8 ...
A short antenna pole next to a house Multiple Yagi TV aerials. Antennas are commonly placed on rooftops and sometimes in attics. Placing an antenna indoors significantly attenuates the level of the available signal. [19] [20] Directional antennas must be pointed at the transmitter they are receiving; in most cases great accuracy is not needed ...
Buried in the ground under the antenna is an enormous radial ground system, which forms the bottom 'plate' of the capacitor with the overhead top load. The antenna must be very large at the VLF frequencies used; the supporting masts are 250–300 metres (820–980 ft) high, and the topload is about 1,900 metres (6,200 ft) in diameter.