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Semiconductor : an element that is normally an insulator but which can, under certain physical conditions, be made to conduct.

Series circuit: Usually used to describe tuned circuits in which the capacitive and inductive elements are in series with each other.

Signal to Noise Ratio S/N : the ratio of signal power to noise power. The better the S/N ratio, the better the quality of the signal and the easier it is to decode.

Simplex : Transmit and receive on the same frequency - most often used when communicating directly (not used when operating via voice repeaters).

SK : Silent Key, a term used in memory of a deceased radio operator. The SK prosign in Morse code indicates the end of a message.

Skip : Skip is a radio phenomenon in which signals are reflected or refracted by the atmosphere and return to earth in unexpected places, far away from the normal reception zones. In between transmitter and receiver there is a zone where no transmissions can be heard

Skip Distance : The shortest distance, for a fixed frequency, between transmitter and receiver.

Skip Zone : The distance between the point of no reception and the closest point of reception.

S meter : A meter on a receiver that displays the relative strength of an incoming signal.

SNP, SNPF : Shared non-protected pair, shared non-protected frequency. In repeater coordination, an input/output frequency pair on which no fixed/permanent repeater stations have been deployed. These frequencies remain available for temporary use only within a very limited coverage area by portable public service, emergency, search and rescue operations or short experiments. Individual temporary repeaters on these frequencies are not regionally-coordinated, so are not protected from mutual interference with other temporary repeaters on the same frequency.^[1]

SOTA : Summits On The Air. An international event in which operators gain points from contacts to and from mountain summits over 500m.

Split operation : Direct communications where transmit and receive are offset from each other in frequency - most often used by DXpedition stations which must accommodate large numbers of incoming calls.

Sporadic E-Skip: Unexpected and unpredictable propagation using refraction in the E Layer.

SSB : Single Sideband. A transmission mode used mainly in the HF bands.

SSTV : Slow Scan Television - still picture ham transmissions.

Standing Wave: The vector sum of two waves - in ham radio, this most usually refers to the forward and reflected waves in a feedline

Sunspot : A region on the sun where an electromagnetic "storm" is happening. These have an effect on propagation

Sunspot Cycle : A cycle of approximately 11 years in which the number of sunspots increases and decreases. The broader sunspot cycle is 22 years as the polarity of the sunspots reverses every 11 years.

Susceptance : the reciprocal of Reactance, measured in Siemens; symbol S

SWL : Shortwave listener. While SWL's do not operate licensed transmitting stations, some are using equipment capable of receiving both radioamateur and broadcast signals. For both, SWL's may submit reception reports in order to receive QSL cards. On VHF/UHF and above, one who can receive (but not transmit) is a scanner operator.

SWR : Standing Wave Ratio. The ratio of the height of a standing wave on a transmission line to the height of an adjacent node.