An antenna radiation pattern is drawn on a polar or rectangular graph to evaluate the component energy interactions of antennas, which are designed to transmit or receive an electromagnetic (EM) field via interaction with an alternating current. Frequency and polarization are plotted on a spatial plane, using decibels (dB) as the unit of measurement. The interactions between these elements create many different geometrical patterns that emit around different types of antennas. Graphs reveal their specific transceiving capabilities and relative field strength.
Factors affecting antenna radiation pattern can include input impedance, measured with a vector network analyzer (VNA), or standing wave ratio (SWR) meter. Bandwidth, efficiency, azimuth, and elevation round out analysis of the pattern in the field. Rotating the antenna 360 degrees in the horizontal and vertical planes, a polar plot of the EM radiation pattern can be constructed.
These aerial devices are typically designed for a particular frequency and use, in solitary or grouped placements. Different lengths approximate multiples or divisors of a target frequency. They transmit and receive electronic information encoded into EM frequencies, as in radio or television communications.
No antenna radiation pattern transmits equally in all directions. Theoretical isotropic antennas have patterns that are perfectly spherical; not directional, these models are used only as a basis of comparison for calculating an antenna's gain. Omnidirectional antennas exhibit symmetrical patterns. Polarizations in the frequencies create symmetrical fields: lobes and null spaces, extending out from the axis of the antenna.
Symmetrical monopole and dipole antennas exhibit an equally symmetrical antenna radiation pattern, owing to radiation emitting out of phase at certain angles, where it goes to zero. The axis of maximum radiation travels through a main lobe, while minor or side lobes extend in other directions. A lobe that extends opposite the main lobe's beam axis is called a back lobe.
Graphs reveal that the farther away from the axis a pattern extends, the more strength the wavelength possesses. These are depicted in print as two-dimensional cross-sections. An antenna radiation pattern can be shown in three dimensions using computer graphic modeling.
Directivity refers to whether an antenna must be aimed. Television satellite dishes have directivity, but cell phone antennas do not. Rod antennas service radios and cell phones. Yagi antennas are half-wavelength dipoles used for frequencies above 10 megahertz (MHz); these perform duty in Citizens Band (CB), ham radio, and television reception.
Aperture antennas are microwave transceivers fashioned into disks, dishes, and rectangular arrays, as seen on cell phone towers. The antenna radiation pattern appears as a torus or horn around the beam axis. Others can include helical, conical, parabolic and more; dozens of other antenna types and patterns exist in numerous geometrical variations.