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The gain in any given direction and the impedance at a given frequency are the same when the antenna is used in transmission or in reception. The electric field of an electromagnetic wave induces a small voltage in each small segment in all electric conductors. The induced voltage depends on the electrical field and the conductor length.
To transfer maximum power to the antenna, the transmitter and feedline must be impedance matched to the antenna. This means the feedline must present to the antenna a resistance equal to the input resistance and a reactance (capacitance or inductance) equal but opposite to the antenna's reactance. If these impedances are not matched, the ...
Instead of altering thickness or spacing, one can add a third parallel wire to increase the antenna impedance to 9 times that of a single-wire dipole, raising the impedance to 658 Ω, making a good match for open wire feed cable, and further broadening the resonant frequency band of the antenna.
SWR of a vertical HB9XBG Antenna for the 40m-band as a function of frequency. In radio engineering and telecommunications, standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line or waveguide.
The characteristic impedance () of an infinite transmission line at a given angular frequency is the ratio of the voltage and current of a pure sinusoidal wave of the same frequency travelling along the line. This relation is also the case for finite transmission lines until the wave reaches the end of the line.
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 radio wave power radiated by an antenna is proportional to the square of the antenna current, so an antenna fed at a resonant frequency radiates much more power than the same antenna fed with the same voltage at some other frequency. [61] An antenna only absorbs all the input power from the feedline when it is in a condition of resonance.
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).
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