We are very fortunate and privileged to be able to use various types or modes of transmission, here is a brief explanation of them.
Modulation.
Modulation
Amateurs radio operators have a variety of modes to choose from when engaged in two way communication. A mode refers to the way the signal is modulated during transmission. Commonly used forms of modulation are AM, FM, SSB, and digital. In order for a signal to be transmitted and received in a readable manner it is modulated electronically. Both transmitter and receiver must be using the same form of modulation for the communication to be successful. Each of these modes will be discussed below. The table of preferred modes for voice communication gives some idea of what to expect when you use a particular band. Some modes such as Rtty use LSB for all bands.
Preferred Modes
The following voice modes are used by general agreement.
LSB 160, 80, 40 meters
USB 20, 17, 15, 12, 10 meters
FM 2, 1.25 m and 70 cm. Some USB is also used.
Each mode has its own unique characteristics. One of these is amount of bandwidth occupied by the signal. CW is quite narrow (less than 250 Hz) while FM is rather wide (15-20 kHz). A narrower signal means there is room for more signals and thus more activity on the band. On the other hand a narrow signal transmits less quality or information. CW requires the use of Morse code whereas FM results in a high quality signal for voice communication. In the following each of the more widely used modes is discussed briefly.
CW
CW (continuous wave) is a simple unmodulated signal unlike others which use some form of modulation. By interrupting the signal with a key, Morse code is sent. Thus Morse code is not a mode but, as the name implies, a code which is used to communicate by controlling the CW signal. Although it takes some time and practice to become proficient with the code using CW is one of the most reliable forms of communication as it can generally make it through the most difficult conditions where other signals can't.
AM
AM (amplitude modulation) was the early mode used by hams for voice transmission. In AM the signal is a carrier (like CW) that has upper and lower sidebands that are modulated by varying the amplitude (strength) of the signal. Most shortwave broadcast stations use this method. If you tune to the BBC or some such station using either USB or LSB on your receiver you can hear the carrier as a continuous tone as you move slightly away from the centre of the signal. If you listen around the upper end of the 80 meter band you may find some hams using this mode. However AM takes twice the bandwidth of SSB and so is not widely used in Amateur radio.
SSB
SSB (single sideband) is a mode where the carrier and one sideband of the AM mode has been suppressed. Whether using USB (upper sideband) or LSB (lower sideband) more of the transmitter's signal is focused in the sideband used as compared to AM. As a result the signal travels farther and is easier to copy under many unfavourable conditions. SSB is the phone mode of choice for Amateurs on the HF bands.
FM
FM (frequency modulation) is what you hear on 2 meters when using a hand-held and working through the club repeater. It is the mode where most hams begin. FM has exceptional quality for voice communication and there is generally no noise or fading that you hear on HF with SSB or
CW. However because of its wide bandwidth requirements it is usually limited to bands such as 2m or 70cm where there is lots of room. Some FM can also be heard on 10 meters around 29 MHz.
Digital Modes
Digital modes
Digital modes have been around since RTTY but really took off with the computer generation. To oversimplify digital modes use the off-on (binary 0-1) to send information. CW is really a form of this although quite rudimentary. Most digital modes require a computer to be interfaced with the radio to assist with sending and receiving the data. Most also require a TNC (terminal node controller) with a chip that supports the particular mode. You send by tying on a keyboard and receive by viewing the information received on the screen. Some of the more popular digital modes are:
- RTTY - Radioteletype (RTTY) uses a baudot (5 bits per character) or ASCII code (7 bits per character) to communicate. RTTY is almost as reliable as CW and there are many hams who use this mode on a regular basis on the HF bands.
- Packet - uses the complete ASCII character set which permits both upper- and lower-case characters in a transmission. Packet is error-free which is achieved by sending data in small packets with a check bit. If an error is detected by the receiving station it replies and requests that the packet be resent. This is repeated as needed to receive the packet correctly. When signals are good a packet rarely needs to be sent twice but under poor conditions the resending of error packets slows down the exchange of information.
- Tor Modes - TOR means "teleprinting over radio." These modes include AMTOR, Pactor, G-TOR and Clover. Basically they all use some variation of the technique mentioned in packet for ensuring error-free transmission. Each use specialized algorithms for transmission resulting in improved speed and accuracy.
- PSK-31 - is a relative newcomer to the digital scene and is fast becoming a primary digital mode. One reason for its appeal is that it uses the sound card in the computer to send and receive through the radio. No other special equipment is needed. PSK-31 uses very little bandwidth, less than CW and can function very well at low signal strengths. Unlike Packet and TOR it is not error- free.
FSTV and SSTV
FSTV & SSTV
Fast scan TV (FSTV) and slow scan TV (SSTV) are modes used to send pictures or images over the radio. SSTV is generally used on the HF bands and can only send a still picture due to its low data rate and bandwidth. FSTV on the other hand is generally used on the UHF bands and can send a moving picture. Recently several HT manufacturers have produced hand-held radios with built-in cameras and screens for use in this mode.
IRLP
IRLP
IRLP (Internet Radio Linking Project) is a method of linking the Internet with Amateur Radio. Usually the link is made through a local repeater so you can connect to someone with a hand-held. Basically you sign on to the local repeater and enter a code to connect you to the Internet link. From there you are connected to other repeaters who are also on the Internet. So with your hand-held you can be taking to hams many thousands of miles away with the signal quality of a local contact.
IRLP is a Canadian invention by VE7LTD and uses Voice over IP (VoIP) to instantly interconnect one or more repeaters around the world. Now with your basic license new radio amateurs are able to use an HT to communicate worldwide.
