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[5] (pp19.4–19.6) Thus, a better match of the antenna to the feed line, that is, a lower SWR, becomes increasingly important with increasing frequency, even if the transmitter is able to accommodate the impedance seen (or an antenna tuner is used between the transmitter and feed line).
Antenna measurement techniques refers to the testing of antennas in order to ensure that the antenna meets specifications or simply to characterize it. Typical antenna parameters are gain , bandwidth , radiation pattern , beamwidth , polarization , impedance ; These are imperative communicative means.
The Smith chart graphical equivalent of using the transmission-line equation is to normalise , to plot the resulting point on a Z Smith chart and to draw a circle through that point centred at the Smith chart centre. The path along the arc of the circle represents how the impedance changes whilst moving along the transmission line.
The impedance seen at the feedpoint of a dipole of various lengths has been plotted above, in terms of the real (resistive) component R dipole and the imaginary component j X dipole of that impedance. For the case of an antenna with perfect conductors (no Ohmic loss), R dipole is identical to the radiation resistance, which can more easily be ...
G: the antenna gain; is the magnetic constant; is the electric constant; For antennas which are not defined by a physical area, such as monopoles and dipoles consisting of thin rod conductors, the effective length (units: meter) is used to measure the ratio between voltage and electric field.
[40] [36] He credited Prof. Moisè Ascoli of Rome with first calculating in 1897 that the antenna radiated at a wavelength of four times its height. [38] An integral equation for the current in wire antennas was derived by Henry Pocklington in 1897, [38] [41] who showed the current was approximately a sinusoidal standing wave.
For a given frequency, the antenna's effective area is proportional to the gain. An antenna's effective length is proportional to the square root of the antenna's gain for a particular frequency and radiation resistance. Due to reciprocity, the gain of any antenna when receiving is equal to its gain when transmitting.
When the antenna is fed at some other point, the formula requires a correction factor discussed below. In a receiving antenna the radiation resistance represents the source resistance of the antenna, and the portion of the received radio power consumed by the radiation resistance represents radio waves re-radiated (scattered) by the antenna. [8 ...