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A quarter-wave impedance transformer, often written as λ/4 impedance transformer, is a transmission line or waveguide used in electrical engineering of length one-quarter wavelength (λ), terminated with some known impedance. It presents at its input the dual of the impedance with which it is terminated.
As the length of an antenna is made shorter than its fundamental resonant length (a half-wavelength for a dipole antenna and a quarter-wavelength for a monopole), the radiation resistance the antenna presents to the feedline decreases with the square of the electrical length, that is the ratio of physical length to wavelength, (/). As a result ...
The length of the antenna, therefore, is determined based on the wavelength of the desired radio waves. The most common form is the quarter-wave monopole, in which the antenna length is approximately one quarter of the wavelength of the radio waves.
Transmission lines become necessary when the transmitted frequency's wavelength is sufficiently short that the length of the cable becomes a significant part of a wavelength. At frequencies of microwave and higher, power losses in transmission lines become excessive, and waveguides are used instead, [ 1 ] which function as "pipes" to confine ...
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.
About 1/8 wavelength long: (left) 200 MHz stub is 19 cm, (right) 300 MHz stub is 12.5 cm 10 kW FM broadcast transmitter from 1947 showing quarter-wave resonant stub plate tank circuit. In microwave and radio-frequency engineering, a stub or resonant stub is a length of transmission line or waveguide that is connected at one end only. The free ...
Fig. 1 - Relationship between TL length and wavelength Fig. 2 - Frequency response (magnitude) measurement of drive unit and TL outputs. Phase inversion is achieved by selecting a length of line that is equal to the quarter wavelength of the target lowest frequency.
Setting ℓ = 1 / 2 λ despite its use not quite being valid for so large a fraction of the wavelength, the formula would predict a radiation resistance of 49 Ω, instead of the actual value of 73 Ω produced by a half-wave dipole, when more correct quarter-wave sinusoidal currents are used.