Winter 2010 Issue

Digital Radio

Digital Frequency Coordination

By Mark Thompson, WB9QZB

Band plans have been present on the HF, VHF, and UHF bands for many decades. They were created to ensure that various modes and band uses didnít interfere with one another. Bands are regulated by the FCC, and it mandates band frequency usage by license class and/or mode. Band usage is also stipulated by ham organizations. National organizations such as the ARRL have created voluntary band plans to specify calling frequencies for each VHF/UHF band and designate band usage for segments of each band. Band plans can vary by geographical area to meet the specific needs of that area. Statewide and regional frequency-coordination organizations were created primarily to coordinate the frequency usage of FM repeaters in an orderly manner to ensure a minimum of interference among repeaters on the same frequency. Coordination organizations typically work with organizations in nearby states and areas to minimize interference.

On the HF bands there are recommended frequencies for the use of both digital data and voice modes such as RTTY, PSK-31, and WinDRM, etc.

Some History

During the 1970s and 1980s the number of 2-meter and 70-cm FM repeaters increased dramatically. In order to accommodate the need for more frequencies, many areas changed from 30-kHz channel spacing to 15-kHz pair spacing in the 146- and 147-MHz repeater sub-bands. Fifteen-kHz spaced repeater pairs were created between the 30-kHz pairs and used inverted receiver and transmit frequencies in order to minimize interference to existing 30-kHz spaced repeaters. When the 145-MHz repeater sub-band was opened, it typically was spaced at 20 kHz. In some areas the 146- and 147-MHz repeater sub-bands were respaced to 20 kHz as well.

In the early 1980s, when packet radio became popular on the VHF and UHF bands, band plans had to be modified to accommodate packet radio usage. The ARRL modified its national band plan and specified frequencies for packet radio usage.

Packet radio grew dramatically from the middle 1980s through the early 1990s. Most hams who used packet first set up their station on 2 meters on the typical calling frequency, 145.01 MHz. The frequency quickly became congested to the point of becoming unusable due to slow throughput on the frequency because of repetitive packet retries. Packet radio is a half-duplex mode. If a packet transmission isnít complete, the packet TNC will transmit again until it is successful. As usage grew, there wasnít enough time available on the calling frequency to handle all of the stations. Additionally, packet radio relies on all stations on a frequency to hear one another. If a station does hear others on the frequency, its transmissions could collide with them, causing packet retry transmissions and reducing throughput even further.

One of the solutions to frequency congestion was to create LAN (Local Area Network) frequencies for separate areas in a region to improve overall throughput. LAN frequencies were also designated to specific uses such as bulletin board systems (BBS), DX PacketCluster, and Keyboard-to-Keyboard.
Packet frequencies on 2 meters typically were in the 144.91Ė145.09 MHz range spaced every 20 kHz, like the repeater frequencies in the 145-MHz repeater sub-band. Packet radio also often used frequencies in the non-repeater sub-band from 145.51Ė145.79 MHz.

In some areas packet users put full-duplex packet FM repeaters on the air on the 2-meter and 70-cm bands. The purpose was not only to extend the range of packet stations, but more importantly to dramatically improve throughput by eliminating the hidden-transmitter effect caused by packet stations on a frequency not hearing one another.

Packet users also constructed high-speed backbone links to transmit data among BBSes. The backbone link frequencies often used the same bandwidth as analog FM, but sometimes they were wider when higher data speeds were used. The network backbone links on 70 cm often needed to be coordinated in order to not cause interference to other packet users and also to not interfere with repeaters and remote links. This required coordination with repeater coordinators who worked together, but often didnít publish, the frequencies of repeater system remote links.

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