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An antenna designer must take into account the application for the antenna when determining the gain. High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully in a particular direction. Low-gain antennas have shorter range, but the orientation of the antenna is inconsequential.
Antenna gain (G) is defined as antenna efficiency (e) multiplied by antenna directivity (D) which is expressed mathematically as: =. A useful relationship between omnidirectional radiation pattern directivity (D) in decibels and half-power beamwidth (HPBW) based on the assumption of a sin ( b θ ) / b θ {\displaystyle \sin(b\theta )/{b ...
When considering an antenna's directional pattern, gain with respect to a dipole does not imply a comparison of that antenna's gain in each direction to a dipole's gain in that direction. Rather, it is a comparison between the antenna's gain in each direction to the peak gain of the dipole (1.64). In any direction, therefore, such numbers are 2 ...
The half-power point is the angle off boresight at which the antenna gain first falls to half power (approximately −3 dB) [a] from the peak. The angle between the −3 dB points is known as the half-power beam width (or simply beam width). [4] Beamwidth is usually but not always expressed in degrees and for the horizontal plane.
Half width at half maximum (HWHM) is half of the FWHM if the function is symmetric. The term full duration at half maximum (FDHM) is preferred when the independent variable is time . FWHM is applied to such phenomena as the duration of pulse waveforms and the spectral width of sources used for optical communications and the resolution of ...
For the Arecibo antenna at 2.4 GHz, the beamwidth was 0.028°. Since parabolic antennas can produce very narrow beams, aiming them can be a problem. Some parabolic dishes are equipped with a boresight so they can be aimed accurately at the other antenna. There is an inverse relation between gain and beam width.
Before the advent of the CCD beam profiler, the beam width was estimated using the knife-edge technique: slice a laser beam with a razor and measure the power of the clipped beam as a function of the razor position. The measured curve is the integral of the marginal distribution, and starts at the total beam power and decreases monotonically to ...
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.