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Very high frequency (VHF) is the ITU designation [1] for the range of radio frequency electromagnetic waves (radio waves) from 30 to 300 megahertz (MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF are denoted high frequency (HF), and the next higher frequencies are known as ultra high frequency ...
Before the rubber ducky, antennas on portable radios usually consisted of quarter-wave whip antennas, rods whose length was one-quarter of the wavelength of the radio waves used. [1] In the VHF range where they were used, these antennas were 0.6 or 0.9 m (2 or 3 feet) long, making them cumbersome. They were often made of telescoping tubes that ...
The length of a whip antenna is determined by the wavelength of the radio waves it is used with. Their length varies from compact electrically short antennas 1 / 10 wavelength long, up to 5 / 8 wavelength to improve directivity. The most common type is the quarter-wave whip, which is approximately 1 / 4 wavelength long.
Since a half-wave dipole has a gain of 2.19 dBi and a radiation resistance of 73 Ohms, a quarter-wave ( 1 / 4 λ) monopole will have a gain of 2.19 + 3.0 = 5.2 dBi and a radiation resistance of about 36.5 Ohms. [13] The antenna is resonant at this length, so its input impedance is purely resistive.
A 5 / 4 -wave dipole antenna has a much lower but not purely resistive feedpoint impedance, which requires a matching network to the impedance of the transmission line. Its gain is about 3 dB greater than a half-wave dipole, the highest gain of any dipole of any similar length.
[citation needed] The loading provided by the helix allows the antenna to be physically shorter than its electrical length of a quarter-wavelength. This means that for example a 1 / 4 wave antenna at 27 MHz is 2.7 m (110 inches; 8.9 feet) long and is physically quite unsuitable for mobile applications. The reduced size of a helical ...
Two half wave elements spaced a half wave apart and a quarter wave from a reflecting screen have been used as a standard gain antenna with about 9.8 dBi at its design frequency. [4] Common 4-bay television antennas have gains around 10 to 12 dB, [ 5 ] and 8-bay designs might increase this to 12 to 16 dB. [ 6 ]
A Yagi–Uda antenna, or simply Yagi antenna, is a directional antenna consisting of two or more parallel resonant antenna elements in an end-fire array; [1] these elements are most often metal rods (or discs) acting as half-wave dipoles. [2] Yagi–Uda antennas consist of a single driven element connected to a radio transmitter or receiver (or ...