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In transmitting antennas the impedance of the antenna must be matched to the feedline for maximum power transfer. A half wave whip antenna (length of ) has somewhat higher gain than a quarter wave whip, but it has a current node at its feedpoint at the base of the rod so it has very high input impedance. If it was infinitely thin the antenna ...
The horizontal gain continues to increase up to a maximum of about 6.6 dBi at a length of five-eighths wavelength 5 / 8 λ so this is a popular length for ground wave antennas and terrestrial communication antennas, for frequencies where a larger antenna size is feasible. The input impedance drops to about 40 Ohms at that length.
The most common type of dipole consists of two resonant elements, each just under a quarter wavelength long, hence a total length of about a half-wave. This antenna radiates maximally in directions perpendicular to the antenna's axis, giving it a small directive gain of 2.15 dBi (2.15 dBi means that in the direction of maximum radiation, signal ...
It is constructed of two telescoping rods attached to a base, which extend out to about 1 m (3.3 feet) length (approximately one-quarter wavelength at 54 MHz) and can be collapsed when not in use. For best reception, the rods should be adjusted to be a little less than 1 / 4 wavelength at the frequency of the television channel being ...
The spring antenna is further enclosed in a plastic or rubber-like covering to protect it. The technical name for this type of antenna is a normal-mode helix. [7] Rubber ducky antennas are typically 4% to 15% of a wavelength long; [7] that is, 16% to 60% of the length of a standard quarter-wave whip.
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 antenna pattern is the response of the antenna to a plane wave incident from a given direction or the relative power density of the wave transmitted by the antenna in a given direction. For a reciprocal antenna, these two patterns are identical. A multitude of antenna pattern measurement techniques have been developed.
If a small loop antenna's loss is 50 dB (as if the antenna included a 50 dB attenuator), then the electrical inefficiency of that antenna will have little influence on the receiving system's signal-to-noise ratio. In contrast, at quieter frequencies at about 20 MHz and above, an antenna with a 50 dB loss could degrade the received signal-to ...