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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).

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.

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.

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 electromangetic "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.

SWL : Shortwave listener. While SWL's do not operate licensed transmitting stations, in some cases they will be using equipment capable of receiving both radioamateur and broadcast signals. For both, SWL's may submit reception reports in order to receive QSL cards. A similar capability exists on VHF/UHF and above for scanner operators; one need not already be the operator of a licensed station to merely listen and observe on any of the radioamateur bands.

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