A Look At Common Emission Types
Radio transmissions begin with a steady radio frequency signal. An unmodulated radio wave (a steady signal with no information included) is called a test emission. This type of radio signal doesn't communicate any information.
To actually communicate with a radio signal we have to find a way to add information to the signal. The process of combining an information signal with a radio signal is called modulation. Turning the radio signal on and off is the simplest form of modulation. CW, (Morse Code), is transmitted by on/off keying of a radio frequency signal. You demonstrate this whenever you use a key to send CW. To send CW, you press a lever, and to stop sending you let up on the lever. Code is either on or off and by controlling the on/off patterns of the radio signal, you send information by Morse Code. It is that simple.
The FCC rules recognize any voice mode used for communication as a phone emission. AM, SSB, and FM voice are all phone emission types.
What does single sideband (SSB) mean? Begin with a steady radio frequency (RF) signal such as you would get by pressing the key of a Morse Code transmitter and just holding it down. This is called the RF carrier. Then combine this signal with a voice signal from a microphone. The process of combining an RF carrier with any information signal is called modulation. If you use amplitude modulation, the resulting signal has two sidebands, one higher in frequency than the carrier frequency and one lower in frequency. These are called the upper sideband and the lower sideband. For a SSB voice signal, the carrier and one of the sidebands is removed, and only one sideband is transmitted upon.
The RF carrier is the signal that you modulate to produce a radio telephone signal.
SSB is the most common voice mode on HF. To transmit an SSB signal you can use either the lower sideband or the upper sideband. Amateurs normally use the upper sideband for 10 meter voice operation. Most amateurs use frequency modulated (FM) phone more than any other mode. Nearly all VHF and UHF voice repeaters use FM phone. A practical FM transmitter varies the carrier frequency to produce the modulated signal.
Amateurs also use a variety of digital communication modes. Digital communications refers to a type of communication that uses computers to send and receive signals through a radio transceiver.
Packet radio, which sends the information in short bursts, or packets, is called a data emission. Receiving stations will acknowledge each packet. Your radio will transmit for a short time and then listen for a brief instant before it transmits again while you type on the keyboard. The term packet radio comes from the fact that packet radio can be used to send computer data files, or digital information.
Packet radio signals are produced by feeding an audio signal that switches between two audio tones into the microphone input of an FM transmitter. This results in a signal called audio frequency shift keying, or AFSK.
Radioteletype (RTTY) is another type of digital communication. RTTY signals consist of a constant transmitted signal as you type on the keyboard. After you stop typing and give the command for your station to stop transmitting, you can listen for the other station's reply. RTTY transmits and receives text only.
RTTY signals are produced by shifting the transmitter RF carrier frequency between two frequencies. Some HF transceivers allow a signal to automatically shift the carrier frequency for RTTY transmissions. Other transceivers use audio tones fed into the microphone input of an SSB transceiver. The end result is the same, the transmitted RF simply shifts between two carrier frequencies. In both cases, this system is called frequency shift keying, or FSK.
Packet is more commonly used on VHF and UHF, and RTTY is more often used on HF, although there is no technical reason why any of these modes could not be used on any band. In fact, there are forms of packet used on HF and some operators use RTTY on the VHF and UHF bands.
Modern data transmission techniques provide very reliable high speed communications. A computer system can be set up to automatically send and receive the data. Data communication systems use some form of error checking to ensure that the information is received correctly.