Their reactive properties are determined by their physical length in relation to the wavelength of the radio waves. The behaviour of stubs is due to standing waves along their length. Stubs may thus function as capacitors, inductors and resonant circuits at radio frequencies. Neglecting transmission line losses, the input impedance of the stub is purely reactive either capacitive or inductive, depending on the electrical length of the stub, and on whether it is open or short circuit. The free end of the stub is either left open-circuit, or short-circuited (as is always the case for waveguides). 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. 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 Short electrical transmission line Resonant stub tank circuits in vacuum tube backpack UHF transceiver, 1938.
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