Low Power Ham Radio, PSK31, and Making your Watts Count


Yesterday I had a wonderful contact of firsts. It was:

  • My first contact outside the US
  • My first contact with Canada
  • My first contact on 20 meters
  • My first contact on PSK31

Until later that day, my only radio is my beloved Yaesu FT-817ND. It's a low power rig that is intended for backpacking purposes, featuring a maximum output of 5 watts and an internal battery. It's an adorable little rig, but the total output wattage is tiny compared to typical Ham Radio rigs. 

Let's compare my FT-817 to my newest rig that arrived only hours later -- an Icom 707. This older rig was originally released in 1993, 11 years before the 817ND! Nevertheless, the power output for the 707 is common to many rigs manufactured today, 100 watts.Even with a modest antenna, you can go across the country easily on 100 watts. With luck and the Ionosphere on your side, you can go around the world! With only 5 watts, however, things become much more tricky. You need to consider your mode carefully.

A mode is the method of encoding information in a radio signal. The most basic one you're probably familiar with through movies, Morse Code. This method turns the transmitter (or more specifically, the power amplifier connected to the antenna) on and off. Like binary, it has two "digits", a short "dit" and a long "dah". By encoding each letter of the alphabet as a series of dits and dahs, you can exchange information. Since the signal is either fully on, or fully off, and the frequency is constant, Hams like calling this mode CW, or Continuous Wave. CW is the mode proper, while Morse is merely a method of encoding the information. 

Other modes you're already familiar with, AM and FM. These are voice modes. Explaining these requires a bit more imagination. Think of your voice as a sine wave. Radio waves can also be thought of as a sine wave. For your voice, the length between the peaks of wave is much, much bigger than the radio wave. So much so, you can superimpose your voice wave by varying the power output of the transmitter. This is how AM, or Amplitude Modulation works. FM, or Frequency Modulation works by varying the frequency of the radio signal proportionally with your voice sine wave. FM was created to avoid noise generated by the atmosphere, most of which causes spikes in amplitude. 

While voice modes are easily approachable, they are rarely used in QRP work. Voice modes have a hidden cost, Sidebands. The original, unmodulated radio signal produced by the transmitter still makes it to the antenna even when you're talking! This bit of the signal, the Carrier Wave, doesn't contain any information. Instead, by a quirk of combining waveforms, the signal spreads out below and above the carrier wave proportional to the spoken frequency. These are Sidebands. FM has huge sidebands, since it's frequency changes slightly. While AM's are smaller, the two sidebands are mirrors, containing the same vocal information. A more efficient modification of AM is called Single Side Band. This mode cuts out one of the sidebands and the carrier wave. This means that what is transmitted has much more punch and travels further than it would otherwise. This makes SSB a good mode for low power work.

The ultimate mode for QRP work is still CW. Since there is no superimposed audio wave, the sidebands are tiny. All the punch is put into a very narrow range of frequencies, meaning each watt carries the signal that much further. The problem is that CW is a dying art. It's no longer a required skill to get an Amateur Radio license. While this was done to lower the barrier to entry for prospective Hams, the ubiquity of computers has popularized Morse Code Decoders -- software that can listen to a CW signal and print out the text. Likewise, Morse Encoders allow you to use a keyboard to type letters, making the computer translate them into dits and dahs for you. 

The problem is, that feels like cheating to me. If you're going to do CW, you should break out a straight key, or an iambic paddle and cut out the silicon middleman. And while that is my plan, after I practice thoroughly, I'm still facing the problem of a poor antenna and a low power rig. Enter PSK31.

PSK31 stands for Phased Shift Keying, 31 Baud. In many ways, it's like Morse Code. It encodes letters in an equivalent of dits and dahs, but by shifting the phase of the signal. Going back to our imagined sine wave, if you're at a peak, PSK31 will suddenly shift to a valley, with no transition in between. By doing this rapidly, we can encode letters in a series of timed phase reversals. The nifty thing is that this is not done with the radio signal! Instead, PSK31 is encoded as an audio tone! Why do this? PSK31 was designed to be simple, and bare bones. Requiring new transmitters to be build just to support it would hamper adoption. Instead, PSK31 is produced (and received) by a computer, and sent to a standard SSB transceiver. Unlike transmitting a full human voice on SSB, PSK31 only results in a very narrow range of audio, this means the sidebands are very narrow. As narrow or more narrow than CW! This means all the power is packed into a tiny bit of frequency, making each watt count. 

Now here comes the really nifty part: PSK31 conversations are often all sent and received with the transceiver tuned to the same frequency. Multiple people, having multiple conversations, at the same time, on the same frequency. How? Recall that PSK31 is encoded as audio first. By telling the PSK31 software to produce audio at one tone or another, multiple people can occupy the same frequency. It's like a room full of people each with a whistle. Only two people in the room have the same kind of whistle that produces the same tone. Since they're only listening for the same tone they themselves are producing, they can ignore everyone else. 

Want to try it yourself? Really! No radio required! Download and install Digipan on your computer. If you're on Linux, it runs just fine in Wine! Now, plug in a microphone and start up the software. Go to Configure -> Sound Card and set things appropriately. Then put on this Youtube video while having the mic pointed at your computer speakers. Nifty!

In a typical Ham setup, however, a special interface is required to interconnect the computer in the transceiver in a safe way. Too much audio power from the receiver can damage your computer's sound card. Too much from the computer, can damage the transmitter. I recently purchased a SignaLink USB. The nice thing about this unit is it's only a USB sound card to the computer. It works in Linux just fine.

While I made my 560 mile contact fairly easily, there's a lot more I can do. My antenna was not properly tuned, resulting in poor efficiency. Not all of my 5 watts ended up "in the air". I think with some additional work and tuning, I can make it much, much further.