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Safety interlock
A switch that automatically turns off ac power to a piece of equipment when the top cover is removed.

Schematic symbol
A drawing used to represent a circuit component on a wiring diagram.

Selectivity
The ability of a receiver to separate two closely spaced signals.

Sensitivity
The ability of a receiver to detect weak signals.

Series circuit
An electrical circuit in which all the electrons must flow through every part of the circuit. There is only one path for the electrons to follow.

Shack
The room where an Amateur Radio operator keeps his or her station equipment.

Short circuit
An electrical circuit in which the current does not take the desired path, but finds a shortcut instead. Often the current goes directly from the negative power-supply terminal to the positive one, bypassing the rest of the circuit.

Sidebands
The sum or difference frequencies generated when an RF carrier is mixed with an audio signal. Single-sideband phone (SSB) signals have an upper sideband (USB - that part of the signal above the carrier) and a lower sideband (LSB - the part of the signal below the carrier). SSB transceivers allow operation on either USB or LSB.

Silent Key
SK. Euphemism for a deceased Amateur Radio operator. In the Western Union company's "92 code" used even before the American Civil War, the number 30 meant "the end. No more." It also meant "good night." In Landline Morse, 30 is sent didididahdit daaah, the zero being a long dash. Run the 30 together and it has the same sound as SK.

Simplex operation
Receiving and transmitting on the same frequency. See duplex operation.

Single Sideband (SSB) phone
A common mode of voice operation on the amateur bands. SSB is a form of amplitude modulation.The amplitude of the transmitted signal varies with the voice signal variations.

Single-pole, double-throw (SPDT) switch
A switch that connects one center contact to one of two other contacts.

Single-pole, single-throw (SPST) switch
A switch that only connects one center contact to another contact.

Skip zone
An area of poor radio communication, too distant for ground waves and too close for sky waves.

Sky-wave propagation
The method by which radio waves travel through the ionosphere and back to Earth. Sometimes called skip, sky-wave propagation has a far greater range than line-of-sight and ground-wave propagation.

SOS
A Morse code call for emergency assistance.

Space station
An amateur station located more than 50 km above the Earth's surface.

Specific absorption rate (SAR)
A term that describes the rate at which RF energy is absorbed into the human body. Maximum permissible exposure (MPE) limits are based on whole-body SAR values.

Splatter
A type of interference to stations on nearby frequencies. Splatter occurs when a transmitter is overmodulated.

Spurious emissions
Signals from a transmitter on frequencies other than the operating frequency.

Standing-wave ratio (SWR)
Sometimes called voltage standing-wave ratio (VSWR). A measure of the impedance match between the feed line and the antenna. Also, with a Transmatch in use, a measure of the match between the feed line from the transmitter and the antenna system. The system includes the Transmatch and the line to the antenna. VSWR is the ratio of maximum voltage to minimum voltage along the feed line. Also the ratio of antenna impedance to feed-line impedance when the antenna is a purely resistive load.

Station grounding
Connecting all station equipment to a good earth ground improves both safety and station performance.

Sunspot cycle
The number of sunspots increases and decreases in a predictable cycle that lasts about 11 years.

Sunspots
Dark spots on the surface of the sun. When there are few sunspots, long-distance radio propagation is poor on the higher-frequency bands. When there are many sunspots, long-distance HF propagation improves.

Switch
A device used to connect or disconnect electrical contacts.

SWR meter
A measuring instrument that can indicate when an antenna system is working well. A device used to measure SWR.

Tactical call signs
Names used to identify a location or function during local emergency communications.

Teleprinter
A machine that can convert keystrokes (typing) into electrical impulses. The teleprinter can also convert the proper electrical impulses back into text. Computers have largely replaced teleprinters for amateur radioteletype work.

Television interference (TVI)
Interruption of television reception caused by another signal.

Temperature inversion
A condition in the atmosphere in which a region of cool air is trapped beneath warmer air.

Temporary state of communications emergency
When a disaster disrupts normal communications in a particular area, the FCC can declare this type of emergency. Certain rules may apply for the duration of the emergency.

Terminal
An inexpensive piece of equipment that can be used in place of a computer in a packet radio station.

Third-party communications
Messages passed from one amateur to another on behalf of a third person.

Third-party communications agreement
An official understanding between the United States and another country that allows amateurs in both countries to participate in third-party communications.

Third-party participation
The way an unlicensed person can participate in amateur communications. A control operator must ensure compliance with FCC rules.

Ticket
A common name for an Amateur Radio license.

Time-out timer
A device that limits the amount of time any one person can talk through a repeater.

Transceiver
A radio transmitter and receiver combined in one unit.

Transistor
A solid-state device made of three layers of semiconductor material. See NPN transistor and PNP transistor.

Transmission line
The wires or cable used to connect a transmitter or receiver to an antenna. Also called feed line.

Transmitter
A device that produces radio-frequency signals.

Troposphere
The region in Earth's atmosphere just above the Earth's surface and below the ionosphere.

Tropospheric bending
When radio waves are bent in the troposphere, they return to Earth farther away than the visible horizon.

Tropospheric ducting
A type of VHF propagation that can occur when warm air overruns cold air (a temperature inversion).