Antenna terms, and definitions

Scatter Angle: The angle at which the receiving antenna must be aimed to capture the scattered energy of tropospheric scatter.

Scattering: The spreading or breaking up of electromagnetic waves when they encounter objects of different electrical properties than those in which the wave is traveling.

Schmidt Antenna: A style of scanning microwave antenna.

Selectivity: A measure of the ability of a receiver to discriminate between a wanted signal on one frequency and unwanted signals on other frequencies.

Shortened dipole: A dipole antenna made to resonate at a lower frequency by use of a coil.

Side Lobe: In a directional antenna radiation pattern, a lobe in any direction other than that of the main lobe. A radiation lobe in any direction other than that of the intended lobe.

Signal: A radio wave that contains the transmitted message.

Signal loss: The amount of signal power lost between the transmitter and receiver.

Signal-to-noise ratio: [SNR] The power intensity of the signal compared to that of the noise. The relative power levels of signal and noise on a communication line.

Single-polarized Antenna: An antenna that radiates or receives radio waves with a specific polarization.

Single, Stationary-Lobe System. Antenna (with a single, stationary beam) that is rotated to obtain 360-degree coverage.

Skip Distance: The distances on the surface fo the Earth between the points where a radio wave sky wave leaves the antenna and is successfully reflected and/or refracted back to Earth from the ionosphere. At a given azimuth, the minimum distance between the transmitting station and the closest point of return to the Earth of a transmitted wave reflected from the ionosphere.

Skip zone: The space or region within the transmission range where signals from a transmitter are not received, i.e., between the ground wave and the point where the refracted wave returns.

Sky wave: A radio wave that is reflected from the ionosphere. A radio wave that travels upward from the antenna.

Slewing: Rotating a directional antenna or transducer rapidly about one or more axes.

Sleeve Antenna: A broadband, vertically polarized, omnidirectional antenna, primarily used for receiving. A type half-wave antenna with the lower portion covered by a metal sleeve.

Sloping long-wire antenna: A wire antenna of length greater than one wave-length and supported in an inclined orientation with respect to the ground. A single wire antenna is shown, but a sloping Vee antenna is also used in some applications.

Sloping Wire Antenna

Slot Antenna: A radiating element formed by a slot in a conducting surface or in the wall of a waveguide.

Slotted Waveguide Antenna: A waveguide used as an antenna.

Space Wave: Waves that travel in a direct path between the transmitting antenna and receiving antenna, or that are reflected from the surface of the earth.

Spherical Coordinate System: Refer to the graphic below.

Spill-over: The part of the antenna's radiated energy from the feed that does not impinge on the reflectors.

Spiral Antenna: An RF antenna which is shaped as a spiral, and used in the microwave region. A Multi-arm spiral antenna has more than one arm.

Spoiler Antenna: An antenna used to change (spoil) the antenna pattern of a second antenna so as to reduce the nulls in the pattern of the second antenna. See also Null Fill.

Spreader: Insulator used with transmission lines and antennas to keep the parallel wires separated.

Spring Mount: A type of antenna mount or base used in mobile application. See the graphic below.

Stacked Dipole Antenna: An antenna configuration which increases its directivity by adding a number of identical dipole antenna elements. An array of dipole antennas mounted on the same antenna mast.

Standing-wave: The distribution of voltage and current formed by the incident and reflected waves which have minimum and maximum points on a resultant wave that appears to stand still.

Standing-wave ratio: [SWR] The ratio of the maximum to minimum amplitudes of voltage, or current, along a transmission line.

Standing waves: Waves that appear not to be moving as the result of power traveling in both directions along a transmission line.

Steerable Antenna. An antenna having a main lobe that may be shifted in direction. A directional antenna.

Stratosphere: The second layer of the Earths atmosphere, extending from 10 to 50 km.

Stub: Short section of a transmission line used to match the impedance of a transmission line to an antenna.

Superheterodyne: The process of translating a high frequency to a lower frequency.

Superturnstile Antenna: A stacked antenna with radiation elements shaped like a bat-wing. Each element in the array is a batwing antenna.

Surface Wave. Electromagnetic waves that travel along the surface of the earth from the transmitter to the receiver. One component of a ground wave that also consists of a space wave.