D - Delta - dah-di-dit
D.
Sometimes used as 'shorthand' for Duplex, i.e. on a radio display.
D Region or Layer.
The lowest region of the ionosphere. The D region contributes very little to short-wave radio propagation. It acts mainly to absorb energy from radio waves as they pass through it. This absorption has a significant effect on signals below about 7.5 MHz during daylight.
D-STAR.
Data.
Computer-based communications modes, such as packet radio, which can be used to transmit and receive computer files, or digital information.
See also: Digital Modes page.
Data Communications.
The transfer of data between two or more locations.
dB.
dBd.
dBi.
dBm.
DC.
DC Ground.
A connection point directly to chassis or battery ground to prevent build-up of hazardous DC voltages.
DCS.
DE.
The Morse code abbreviation for "from" or "this is.".
Deci.
The metric prefix for 10-1, or divide by 10.
DeciBel (dB).
A dimensionless unit of ratio which is used to express the relationship between a variable quantity and a known reference quantity. Commonly seen are:- dBd = Decibels as compared to a ½ wave dipole
- dBi - Decibels as compared to an isotropic (theoretical dipole) antenna.
- dBW = Power relative to 1watt.
- dBm = decibels measure, 1 mW with a load impedance of 600Ω (0 dBm = 1 mW).
See also: information on dB.
Decode.
The process of returning encoded digital data to its original form.
Delta Loop Antenna.
A variation of the cubical quad with triangular elements.
Desense.
Desensitization (Desense).
The reduction of receiver sensitivity due to overload from a strong, nearby, transmitter.
Detector.
The stage in a receiver in which the modulation (voice or other information) is recovered from the RF signal.
Deviation (FM Deviation).
The amount of bandwidth your modulated signal occupies either side of the transmitted frequency.
See: Deviation page in the Appendix for a full explanation.
Deviation Ratio.
The ratio between the maximum change in RF-carrier frequency and the highest modulating frequency used in an FM transmitter.
DF.
Digipeater.
A packet radio station used to retransmit signals that are specifically addressed to be retransmitted by that station. Similar in operation to a repeater except that it retransmits on the same frequency as receiving.
See also: Digital Modes page.
Digital.
A digital system uses discrete (that is, discontinuous) values (usually 0 & 1, or 2 different tones) to represent information for input, processing, transmission, storage, etc. By contrast, non-digital (or analogue) systems use a continuous range of values to represent information. Although digital representations are discrete, the information represented can be either discrete, such as numbers, letters or icons, or continuous, such as sounds, images, and other measurements of continuous systems.
Digital Code Squelch (DCS).
This system is similar to CTCSS. It uses a 3-digit octal code [000~777], sent as a binary encoded 23-bit word at 134.3 baud. DCS permits semi-private operation between two stations on the same frequency by keeping the receiver muted until it receives the correct code.
Digital Communications.
Non speach based communications modes. This can include data modes like packet radio and text-only modes like radioteletype (RTTY).
See also: Digital Modes page.
Digital Signal Processing (DSP).
The goal of DSP is usually to measure or filter continuous real-world analog signals, the first step is usually to convert the signal from an analog to a digital form, by using an analog to digital converter. The signal is then processed to remove unwanted noise, enhance wanted frequencies etc. Then the signal is passed on to a digital to analog converter, then on to the speaker. If using a PC as a DSP for a digital mode then the digital to analog converter and speaker stages can be omitted, the cleaned up signal being passed to the software for decoding and display.
Digital Smart Technologies for Amateur Radio (D-STAR).
Digital mode introduced in 2001. For voice conversations. Popular in America, it is little used in the UK at the moment.
See also: Digital Modes page and D-STAR appendix entry.
Diode.
An electronic component that only allows current to flow in one direction. There are different types of diodes specifically designed for different purposes.
See also: Light emitting diode, rectifier diode, varactor diode, zener diode.
Diplexer.
Dipole Antenna .
See: Half-wave dipole. A dipole need not be ½ wavelength long. A ½ wave dipole is often used as a standard for calculating gain, dBd.
Schematic symbol: 
Direct Current (DC).
Electrical current that flows in one direction only. Direct current as supplied by a battery or power supply.
Direction Finding (DF).
The establishment of the direction from which a received signal was transmitted. This can refer to radio or other forms of wireless communication. By combining the direction information from two or more suitably spaced receivers, the source of a transmission may be located in space via triangulation. This is called a cross-cut or fix.
See also: Amateur Radio Direction Finding.
Directional Antenna.
An antenna that concentrates more of the transmitted energy in a particular direction. A directional antenna must be turned to provide the best signal coverage in various directions. Such antennas also receive signals better from the direction they are pointed.
See also: Beam Antenna.
Directional Wattmeter.
Director.
An element in front of the driven element in a Yagi and some other directional antennas.
Dish / Dish antenna.
Double-Pole, Double-Throw (DPDT) switch.
A switch that has six contacts. The DPDT switch has two center contacts. The two center contacts can each be connected to one of two other contacts.
Schematic symbol: 
Double-Pole, Single-Throw (DPST) switch.
A switch that connects two contacts to another set of contacts. A DPST
switch turns two circuits on or off at the same time.
Schematic symbol: 
Downlink.
A radio link from a satellite or the moon to a receiving site on earth.
DPDT.
DPST.
Driven Element.
The part of an antenna that connects directly to the feed line.
Dropping Out.
The situation, while using a repeater, when your signal does not have enough strength to keep the repeater triggered.
DSP.
DTMF.
Dual-Band Antenna.
An antenna designed for use on two different Amateur Radio bands.
Dual-Tone Multi-Frequncy (DTMF / Touch-Tone).
A standard set of 8 tones, used for paging, remote control or interconnect applications. The tones are the same as the ones you hear on a telephone when dialing.
Dual Watch.
The ability of a reciever or transceiver to receive two signals simultaneously.
Dummy Antenna.
A station accessory that allows you to test or adjust transmitting equipment without sending a signal out over the air. Also called dummy load.
Dummy Load.
A station accessory that allows you to test or adjust transmitting equipment without sending a signal out over the air. It converts radio energy to heat. Offers a match to the transmitter output impedance. Also called dummy antenna.
DUP.
Duplexer.
Frequency splitting/combining and isolation device suitable for 2 signals e.g. 2 transceivers utilising a single antenna or vice versa.
Some dual-band radios have the duplexer built in, these radios only have one antenna socket.
Sometimes called a Diplexer.
Duplex (D / DUP).
'D', 'DUPLEX' or 'DUP' is often displayed on a transceiver to denote that it is set to use different frequencies for transmiting and receiving.
For full details see: Full Duplex Operation, Half Duplex Operation, Simplex Operation.
Duty Cycle.
A measure of the amount of time a transmitter is operating at full output power during a single transmission. A lower duty cycle means less RF radiation exposure for the same PEP output.
DX.
Originally an early telephone term for distant exchange. General useage now means distance, foreign countries. List of country prefixes for DXing.
DXCC.
Award offered by ARRL for contacting and confirming 100 or more different countries first introduced in 1937.
DXpedition.
Radio expedition to remote and rare locations. Carried out by 'DXpeditioners'.