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Antenna directivity is the ratio of maximum radiation intensity (power per unit surface) radiated by the antenna in the maximum direction divided by the intensity radiated by a hypothetical isotropic antenna radiating the same total power as that antenna. For example, a hypothetical antenna which had a radiated pattern of a hemisphere (1/2 ...
The frequency range or bandwidth over which an antenna functions well can be very wide (as in a log-periodic antenna) or narrow (as in a small loop antenna); outside this range the antenna impedance becomes a poor match to the transmission line and transmitter (or receiver).
As frequency increases, the directivity of an antenna of a given physical size will increase. In order to keep receiver antenna directivity constant in the formula, the antenna size must be reduced, and a smaller size antenna results in less power being received as it is able to capture less power with a smaller area.
This will often be a half-wave dipole, a very well understood and repeatable antenna that can be easily built for any frequency. The directive gain of a half-wave dipole with respect to the isotropic radiator is known to be 1.64 and it can be made nearly 100% efficient.
Note: Information in the chart has been superseded by the information in File:United States Frequency Allocations Chart 2016 - The Radio Spectrum.pdf, which was downloaded from the US Department of Commerce web site and archived at archive.org.
The latter factor is quantified by the antenna gain, which is the ratio of the signal strength radiated by an antenna in its direction of maximum radiation to that radiated by a standard antenna. For example, a 1,000 watt transmitter feeding an antenna with a gain of 4× (equiv. 6 dBi) will have the same signal strength in the direction of its ...
The electrical length of an antenna, like a transmission line, is its length in wavelengths of the current on the antenna at the operating frequency. [1] [12] [13] [4]: p.91–104 An antenna's resonant frequency, radiation pattern, and driving point impedance depend not on its physical length but on its electrical length. [14]
The Goubau antenna from 1976 has a size ratio of 1 and bandwidth of 80%. Q is 1.5 times the limit. [10] The Foltz drawing pin like antenna from 1998 size 0.62 and 22% bandwidth. The Rogers cone from 2001 is size 0.65 and right on the limit. Lina and Choo planar spirals in size ratios range from 0.2 to 0.5