digital connection
Some Practical Advice on Breaking the 1k2 Barrier
Most hams I speak with believe that packet on VHF and above is almost all 1,200-baud FM, and it is likely that you agree, since it is true. Way back in the day, we had links with higher speeds, including 56 kilobaud — the GRAPES modem — but this and others never really caught on because of several significant issues.
Rather than revisit history, this month we’ll take a practical look at running 9,600-baud (9k6) packet, regaining wider popularity as network backbones, TARPN links, on Winlink and, just recently, the 2022 Appalachian Trail Golden Packet, where it is reported to have worked even better than the 1,200-baud attempt. We’ll start with a look at the technical requirements for higher-speed packet, cover some details of getting and/or modifying a radio, and then how to check that it works.
The gold standard for higher-speed radio modems is arguably the G3RUH design. It is scalable to 38.4 kilobits per second (kb/s), but has some stringent requirements for the radio. First, any distortion of the audio signal, including pre or de-emphasis that is commonly used in voice radios, is unacceptable. Second, the transmitted and received signals need a frequency response down to DC — particularly the receive audio. Lastly, the “group delay” — the difference by audio frequency of the phase of the audio signal — needs to be as close to zero as possible. These significant requirements very much limit the range of radios that
