Sunday, June 29, 2008

Radio Sport

You know you’re a true tech geek when you play at what they pay you to do at work--or in my case, write about.

In Portable Design we spend a lot of time covering low-power wireless issues—from exploring evolving air interfaces to explaining how to design them into your next portable gadget. In my spare time I design and build low-power wireless transceivers that operate in the amateur radio (‘ham’) bands. Working exclusively below 30 MHz, I don’t worry whether my 5 GHz UWB signal can reach from my living room to my bedroom TV. I’m more concerned about whether Serge can hear my 5W 14 MHz signal in Tahiti or Joao in Brazil. Since local deed restrictions relegate me to using an attic dipole, that’s a neat trick.

“Daddy listens to static.” What’s that about? In an age when cell phones have made people blasé about international wireless communications, ham radio seems like a relic—and in some ways it is. Why would you spend time analyzing sunspot cycles, atmospheric ionization and the maximum usable frequency (MUF) for wireless communication between your home and Brazil when you can just pick up your phone can call someone there?

Cell phones are fine for point-to-point communications when you know who you’re calling. Ham radio is fun precisely because you never know who you’ll wind up meeting. It’s a mixture of science and serendipity—like sailing. Sure, you could fire up an engine and get somewhere faster and more predictably. But as in sailing you’re harnessing a force of nature—in this case the ionosphere—and working with it for the sheer joy of the adventure. The Germans refer to ham radio as “radio sport,” which seems a fitting term.

Still, there is a lot of science involved, and not just in analyzing propagation. Hams have long experimented with different data communications modes, inventing more than a few. I was only able to contact Serge and Joao with my tiny transmitter because I was using PSK31, a type of binary phase shift keying invented by Pete Martinez, G3PLX. PSK31 transmits 31.25 bits per second, using a binary code whose length varies with the popularity of the letter (‘e’ is two bits, ‘z’ is nine). This makes for a very efficient modulation protocol, well suited to low power stations.

If band conditions permit, you can switch to QPSK31—quaternary phase shift keying—which adds a second BPSK carrier that is 90 degrees out of phase with the first. The second channel carries redundant bits, so QPSK adds a convolutional encoder to generate one of four different phase shifts that correspond to patterns of five consecutive data bits. On the receiving end a Viterbi decoder sorts it all out. All of this magic is done using your computer’s sound card and Pete’s software. Plug your computer into a 5W transmitter, add a decent antenna, and you can get a lot farther than the nearest cell tower.

Hams have invented and are using a number of other interesting air interfaces. Whereas PSK31 uses anywhere from 2-12 symbols per text character, MFSK (multi-tone frequency shift keying) uses only one—but modulates an RF carrier with as many as 16 different tones; while slower than PSK31, MFSK signals are less affected by multipath errors. MT-63 uses 64 different tones, plus forward error correction; MT-63 is robust against selective fading. Olivia uses a two-layer code and Walsh Functions, making it readable even when the signal is 10 dB below the noise floor (“Can you hear me now?”). JT65 is a digital protocol optimized for the extremely weak signals found in earth-moon-earth (EME) communications on the VHF bands. When not bouncing signals off the moon, hams can communicate via one of several satellites—called OSCARs (Orbiting Satellite Carrying Amateur Radio)—that support VHF and UHF communications.

Ham radio has come a long way since I got started. I got my novice license just after my 11th birthday. I got on the air using a WWII surplus BC-654 transceiver (AM and CW) that ran off a battery and dynamotor. One of my first contacts was the postmistress of Vladivostok. I ran out and bought a world map and started sticking colored pushpins into places I worked, reading up about them in the encyclopedia at the local library. Ham radio really opened up the world for me. Now my kids can read all about Vladivostok on Wikipedia and call there on their cell phones. Still, all the instant information available on the Internet doesn’t begin to substitute for the thrill of the hunt and the unplanned meeting with a stranger.

While I enjoy experimenting with digital RF designs, I’m basically into ham radio for one reason—because it’s fun.

John Donovan, K6YLG, has been a licensed amateur radio operator for over 50 years. When not writing for Portable Design or skulking about trade shows, he can often be found on the digital portions of the 20-, 30- and 40-meter ham bands. He’s also active in the Amateur Radio Emergency Services (ARES).

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