S - Sierra - di-di-dit
S-Meter.
Safety Interlock.
A switch that automatically turns off ac power to a piece of equipment when the cover is removed.
SAR.
SARL.
South African Radio League, the national association for Amateur Radio in South Africa.
Scan.
Step through memory stored frequencies or a band of frequencies looking for a signal.
Scan Edge.
End and start frequencies for a scanning range.
Scatter.
Several factors may cause some energy from a radio signal to follow a path other than the normal "straight line". Scattering can take place from Earth's ionospheric and atmospheric layers as well as objects in the wave path.
Schematic Symbol.
A drawing used to represent a circuit component on a wiring diagram.
Sealed Lead Acid (SLA).
Search.
Secondary Winding.
The coil in a transformer that is connected to the load (output).
Selectivity.
The ability of a receiver to receive a weak signal on a frequency while rejecting strong signals on adjacent frequencies.
Semi Duplex.
An operation mode in which transmissions and receptions are accomplished on different frequencies alternatively.
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.
SHF.
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.
Short-Path Communication.
Communication made by pointing a beam antenna in the direction indicated by the shorter Great-Circle path.
Sidebands.
The sum or difference frequencies generated when an RF carrier is mixed with an audio signal. Single-sideband telephony (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. Convention is that LSB is used below 10Mhz (30m) and usb is used above 10Mhz.
Signal Diode.
A diode that is only rated upto about 25 Volts and just a few milliamps of current.
Signal Strength Meter (S-Meter).
A meter usually incorporated into a radio receiver to display the strength of a received signal. It may also display the relative output, modulation level, grid current or even SWR on transmit.Do not confuse it with the 'S' of the RST code, the meter is rarely accurate and should be used as a guide only.
Signal Tracer.
A test instrument that shows the presence of RF or AF energy in a circuit. The signal tracer is used to trace the flow of a signal through a multistage circuit.
Simplex Operation.
1. According to the ITU definition, a simplex circuit is one where all signals can flow in only one direction at a time.
.
2. According to the ANSI definition, a simplex circuit is one where all signals can flow in only one direction. These systems are often employed in broadcast networks, where the receivers do not need to send any data back to the transmitter/broadcaster.
To put it simply, you won't find your radio receiver actually sending out any information; all it does is receive, so information moves one way only from the radio broadcasting station to your radio receiver.
See also: Duplex Operation, Full Duplex Operation, Half-Duplex Operation.
SINAD.
Is the abbreviation for Signal Including Noise And Distortion. It is the ratio of the signal including noise and distiortion to the noise and distortion component only. This is calculated as one of:
- The ratio of (a) total received power, i.e., the received signal plus noise plus distortion power to (b) the received noise plus distortion power.
- The ratio of (a) the recovered audio power, i.e., the original modulating audio signal plus noise plus distortion powers from a modulated radio frequency carrier to (b) the residual audio power, i.e., noise plus distortion powers remaining after the original modulating audio signal is removed.
The SINAD is usually expressed in dB. The measurement is most widely used for measuring and specifying the sensitivity of a radio receiver, for example:
A typical specification might be that a receiver has a sensitivity of 0.25 μV [microvolts] for a 12 dB SINAD.
The lower the input voltage needed to achieve the given level of SINAD, the better the receiver performance.
Single Sideband (SSB).
A common mode of voice operation on the amateur bands. SSB is a form of amplitude modulation (AM). 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.
Schematic symbol: 
Single-Pole, Single-Throw (SPST) Switch.
A switch that only connects one center contact to another contact.
Schematic symbol: 
Skip zone.
An area of poor radio communication, too distant for ground waves and too close for sky waves.
Sky Waves.
Radio waves that travel from an antenna upward to the ionosphere, where they either pass through the ionosphere into space or are reflected back to Earth.
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.
SLA.
Slim Jim.
A type of antenna based on the J Pole design. It is a vertically polarised omnidirectional end-fed antenna having some "gain" over a quarter wave ground plane type antenna, up to about 50 percent better is claimed.
See: Constructors Corner Antenna Projects for further details.
Slow-Scan Television (SSTV).
A picture transmission method to transmit and receive static pictures via radio in monochrome or color. A technical term for SSTV is narrowband television. SSTV usually takes up to only 3 kHz of bandwidth. It is a slow method of still picture transmission, usually lasting from about eight seconds to a couple of minutes. SSTV systems operate on voice frequencies so it is used on HF, VHF and UHF bands.
SMA.
A type of antenna connector, often used in VHF and UHF portables.
Solar Flare.
A large eruption of energy and solar material from the surface of the sun.
Solar Flux.
Radio energy coming from the sun.
Solar Flux Index.
A measure of solar activity. The solar flux is a measure of the radio noise on 2800 MHz.
Solid State.
Often seen on radio equipment from the 1960s and 70s. It is a term that means it has no valves (tubes) and is all transistorisied.
See also: Hybrid.
SOS.
A Morse code call for emergency assistance.
Sound Card.
A computer expansion card that can input and output sound under control of computer programs. Typical uses of sound cards include providing the audio component for multimedia applications such as music composition, editing video or audio, presentation/education, and entertainment (games). Amateur Radio programs may use the sound card as the input, from the radio, of digital data, as sound, for decoding and display on the monitor. Many computers have sound capabilities built in, while others require these expansion cards if audio capability is desired.
SP.
'Shorthand' for Speaker.
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.
Spectrum.
Spectrum or more importantly the Radio Spectrum is simply the range of frequencies that radio signals may exist in. Just as visible light has a spectrum that moves from red to orange, yellow, green, blue, indigo, and violet; Radio Spectrum moves from VLF to LF, MF, HF, VHF, UHF, SHF and EHF. These stand for Very Low Frequency, Low Frequency, Medium Frequency, High Frequency, Very High Frequency, Ultra High Frequency, Super High Frequency and Extremely High Frequency.
Speech Processor.
A device used to increase the average power contained in a speech wave form. Proper use of a speech processor can greatly improve the readability of a voice signal.
SPKR.
'Shorthand' for Speaker.
Splatter.
A type of interference to stations on nearby frequencies. Splatter occurs when a transmitter is overmodulated or overdeviated.
Split.
A mode in which the transmit and receive frequency are different.
SPDT.
SPST.
Spurious Emissions.
Signals from a transmitter on frequencies other than the operating frequency.
SQL.
Squelch (SQL).
Mutes the speaker output from the receiver so that unwanted background noise is not heard.
SRAL.
The Finnish Amateur Radio League, the national association for Amateur Radio in Finland.
SSTV.
Standard Wire Gauge (SWG).
A standard method of denoting wire diameter for nonferrous, electrically conducting wire. Also known as British Standard, British Standard Gauge, Imperial Wire Gauge, Imperial Standard Wire Gauge.
See also: Wire Gauges.
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.
See also: SWR Explained & How To.
Station Grounding.
Connecting all station equipment to a good earth ground improves both safety and station performance. But care must be taken if the shack is connected to a PME electricial system.
Sub-Audible Tone.
Sudden Ionospheric Disturbance (SID).
A black out of HF sky-wave communications that occurs after a solar flare.
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.
Superheterodyne Receiver.
A method of recovering a radio signal from the airwaves. The signal is mixed with a locally generated frequency to create an intermediate frequency (IF). The IF is always the same frequency, the locally generated frequency being varied with the tuning control along with the receivers tuning. This method has many benefits, one of which is is that only one set of filters is required for a whole band of frequencies because it is the intermediate frequency that is processed. Another improved selectivity of the receiver because, as only one set of filters is required, better quality filters can be used cost effectively.
Super High Frequency (SHF).
The portion of the radio spectrum from 3 GHz to 30 GHz or 3,000,000,000 to 30,000,000,000 cycles per second. Four Amateur bands exist in this region.
| ITU Region 1 (Including UK) |
|---|
| Band | Frequency Range
|
|---|
| 9 cm | 3,400-3,475 MHz
|
| 6 cm | 5,650-5,850 MHz
|
| 3 cm | 10,000-10,500 MHz
|
| 12 mm | 24,000-24,250 MHz |
See also: EHF. HF, LF, MF, UHF, VHF.
Suppressor Capacitor.
A capacitor (often ceramic) connected across the transformer primary and/or secondary winding in a power supply. These capacitors are intended to suppress any transient voltage spikes, preventing them from getting through the power supply.
Switch.
A device used to connect or disconnect electrical contacts.
See also: DPDT, DPST, SPDT, SPST.
SWG.
SWL.
Short Wave Listener.
SWR (VSWR).
SWR Meter.
A
measuring instrument that can indicate when an antenna system is working well. A device used to measure SWR.
See also: SWR Explained & How To.
