SDR for Amateur Radio: a Primer

[NOTE: This Primer is based on my notes for a series of presentations on the Linux User Net, during May and June of 2019. -KC7MM]

On the path to a software-defined ham shack
Understanding Software Defined Radio — and where it can lead.

I recently moved to a new home, and I'm in the process of setting up my station there. What I want to do is to locate my radios in the garage, where they can run off our solar power system's battery backup, and where there is good access to the outside for running antenna leads and grounding. But the garage isn't a good place to sit and operate — I have space available inside the house that is much more amenable to creature comfort, and that's where I want to be when operating.

To that end, I've been working to develop the idea that the functions of Software Defined Radio – and the equipment that performs them – can be distributed over a computer network. My term for it is DARS: Distributed Amateur Radio Station. I sometimes refer to it as Split Station, since the components can be split between different locations.

I've had to learn a lot about SDR in order to both clarify the concept in my mind and to identify what needs to be done to actually implement it. One thing I discovered is that SDR as it applies to Amateur Radio is a complex and often-subtle subject, and there's no single information source that explains it in its entirety.

This primer, then, is my attempt to lay out the fundamentals of SDR as they apply to Amateur radio operation, and to explore where they could take us. I will cover:

• What SDR is, what it can do, and how it does it. This includes:
  • Basic principles of operation, based on Information Theory.
  • How SDR differ from analog radio.
  • What software defined radios can do that analog rigs cannot.
• How is currently is being implemented by hams and manufacturers.
• What it could become: future possibilities raised by the technology.
• Implications for the use of Linux in SDR.

We already use software while operating our stations. How does the computer fit into the ham shack?

The short answer: it is employed to add certain capabilities to our radios. In computer terms, it functions as a peripheral of the radio. The station architecture is centered on radios, and computers connected to them perform additional functions.

Radio equipment manufacturers have long installed microcontrollers in their rigs, for control purposes. Their uses include:

• User interfaces: menu systems, touch screens.
• Memory storage of frequencies and settings.
• Serial interfaces for connecting to external computers.

Computers connected via serial lines are widely used for:

• Logging.
• Programming radio memories (e.g., Ham Radio Deluxe, CHIRP).
• Rig control and operation. Manufacturers sometimes produce proprietary software for controlling a rig via a computer. Also used by thirt-party software such as wsjt and fldigi.
• “Digital” modes on HF (e.g., FT-8, RTTY). These employ a PC sound card to generate tones to send and to interpret received tones.

For the most part, ham radios have not been designed to work over computer networks. The closest most come is some sort of serial port for connecting to a local PC. However, there are systems that can operate, at least in part, over the Internet.

• Radio over the Internet:
  • Internet gateway systems, such as APRS (and packet radio in general), IRLP, Echolink, Winlink, provide an interface between RF and the Internet that allow distant stations to communicate over distances not possible by RF alone. These gateways generally are run on repeaters, though small SDR devices that plug into USB ports have recently extended some of these to any computer that has Internet access.
  • Digital voice systems (D-STAR, System Fusion) include a formally-strucured Internet component for long-distance communication between VHF and UHF radios.Here again, these are designed around repeaters, but small plug-in SDR devices are extending them to individual computers.
  • Some manufacturers produce remote-control heads for specific models of radio, such as the Elecraft K3 and the Kenwood NX-5000. These allow remote control of those rigs over a network. Remoterig makes a generic interface designed for remote control of rigs that have detachable control heads.

[Under construction]