Getting started on data modes with the Elecraft K2
Narrow band data modes such as PSK31 and MFSK are excellent QRP modes, providing reliable copy at power levels of just a few watts. The Elecraft K2 works very well for these modes. If you have a computer with a sound card and a K2 equipped with the KSB2 module, you can try out these exciting modes for the price of a couple of audio cables.
It's easy to get started in data modes using the K2. You don't need an expensive interface or to make modifications to your K2. The instructions that follow are based on a posting to the Elecraft reflector by Ron D'Eau Claire, AC7AC, with additional material by me.
You need a PC, which you obviously have if you're reading this! The PC needs a sound card. You also need to download and install one of the freeware data programs that will enable you to try out data modes at low cost. I'd recommend Stream, by Nino Porcino, IZ8BLY or Hamscope . Both of these programs support both PSK31 and MFSK, which are the two most popular narrow band data modes used on HF.
If the PC is set up correctly, then with the speakers plugged in you should hear the data tones when you transmit using the software. The audio tones should sound pure and clean, like those that you hear on the air. If they don't, you may have a sound card with a "3D enhancement" feature: disable this, or use a different sound card if that isn't possible. Cheap external USB sound cards are not good enough quality and will give poor results.
You will of course use the speaker output to supply transmit audio to the K2, so you won't be able to use the PC speakers and work data modes at the same time!
First, you need to connect the audio output of your K2 to the sound card input. A simple male-male audio cable with 1/8 in stereo plugs will suffice for this. Connect it between the headphone output and sound card "line" input. If you wish, you can use a "Y" adapter available from Radio Shack etc. with a single male plug and two female jacks, which will allow you to plug in your headphones in parallel with the computer. This will allow you to monitor what's going on by ear.
Next, you need to connect the output of your sound card to the audio input of the K2. The audio input of the K2 is designed for a microphone, which has a very low output, and if you connect the speaker output to this you'll seriously overdrive the K2. So you need to attenuate this signal. Radio Shack and others sell a cable with a built-in attenuator, intended to connect a line output to the mic input of a recorder. Alternatively, you can make your own cable. You can't escape doing some soldering, anyway, because you'll need to convert the end of the attenuated Radio Shack cable to an 8-pin plug of the type used by the K2. So you might as well make this cable from scratch. If you use 1/8 watt resistors you can easily build the attenuator into the 8-pin plug shell, where it will be nicely screened against any possible RF pickup.
The attenuator requires two resistors. A 4.7K resistor should be wired between the mic input pin and ground. A100K resistor should be wired between the mic input pin and the center of the screened cable from the PC. The screen of the cable goes to ground, of course. The resistor values aren't critical, since fine adjustment of the level will be done using the speaker output slider on the Windows mixer panel.
Finally, you need to be able to switch the K2 from receive to transmit. You can't use the K2's VOX for this, because it isn't sensitive enough: an audio level high enough to trigger the VOX will invoke ALC action which is undesirable if you want to have the cleanest possible PSK signal. If you decide to become a regular operator of data modes, you can make up an interface to control the PTT using a serial port, or you can use a CAT program like MixW to control the K2 via the KIO2 computer interface using CAT commands. But to get on the air quickly so you can try data modes out, you can use a small toggle switch connected across the PTT pins of the 8-pin plug. Just remember to flip the switch to put the K2 into transmit when you start an over, and flip it back at the end to go back to receive.
Preparing to operate
In order to avoid intermodulation of the PSK31 signal products (which will make your signal much wider than it should be, wasting power and interfering with adjacent QSOs) the signal path through the K2 from mic input to antenna output should be as linear as possible. This means not overloading the mic input amplifier, not using any SSB compression, and not activating the ALC (which is also a form of compression.) However, you don't have to worry too much about ALC if you have a later model K2 (s/n 3000 or greater) or have performed the KI6WX mod to improve IMD for PSK31 on your KSB2.
You can operate data modes using USB or LSB. In this case, remember to disable compression from the menu before you start (SSBC 1-1). But it's better to enable the K2's RTTY mode from the secondary menu, which is an option available on all K2s except those with very old firmware. The RTTY mode is the same as SSB, but compression is automatically disabled, and you get a separate set of receive filter settings that allow you to set up narrow filters better suited to receiving narrowband modes. If you take up data modes seriously you'll find this last facility very useful.
The K2 will happily run at 5W output without getting too hot, as long as overs aren't excessively long. However. on 10m, where the PA efficiency is less, I limit the power to 4W.
The shape of the K2's SSB filter will affect the amount of output, depending on the frequency of the audio tone used. Therefore you should always aim to work close to the peak of the filter response. On my K2, that is about 1200Hz. Start your data mode software, and click in the waterfall at 1200Hz to set the transmit and receive frequency there. Connect a dummy load and slide the speaker output control to minimum.
Setting the audio drive level
Before venturing on the air, you need to determine the correct level of audio drive to produce a clean PSK31 signal. If you have an early, unmodded K2, you must use the speaker output slider on the PC mixer panel to regulate the audio level so as to achieve the desired RF output, without activating the ALC. Set the power control to 10W, then switch to transmit (both the K2 and the software) and increase the audio level until you're getting 5W output. Because the gain of the K2 will vary from band to band, you'll need to do this adjustment on every band. You'll need to keep an eye on the output power level, to ensure that you aren't using too little or too much power (the latter might overheat the PA) since you can't rely on the ALC to keep it constant for you.
If you have a newer K2, or one to which the KI6WX mod has been applied, the ALC won't noticeably degrade the linearity of the signal, so you can use the K2 power control to limit your transmitted output power. Set the power control to 5W, and the meter reading mode to ALC. Switch to transmit (both the K2 and the software) and increase the audio level until you're getting one bar of ALC showing. You should repeat this adjustment on each band, as you may need to use a lower audio level on the lower bands, or a higher one to get full output on ten meters. Once you've found the correct setting you can switch the meter back to RF mode. The ALC will keep the output constant as long as you don't stray too far from the center frequency of the SSB filter, so you don't need to worry about keeping a watchful eye on the RF output.
Checking your transmitted signal
If you have followed these instructions you should have a very clean signal, but if you can, monitor your own transmission on another receiver and another computer, or get a friend to do it for you. If only more people would do this, there would be fewer bad signals on the bands!
In PSK mode, hit the software's transmit button, but don't type any text. This sends an idle signal, which can be used to check your IMD (intermodulation distortion) level. If monitoring your own signal, transmit into a dummy load and adjust the coupling into the second receiver so that the signal strength is no stronger than normal received signals, as receiver or detector overload could affect the validity of the result.
The idle signal should appear on the waterfall display of the receiving computer as two separate lines. There should be no extra lines either side of these two lines, nor any other signals that come and go with your transmitted signal. Most PSK programs will read out the IMD level in dB when they receive an idle signal. As the screenshot above shows, the K2 transmitting at 5 watts and correctly adjusted as described, can produce an IMD of -34dB, which is an excellent result. Note: the faint lines visible in the trace shown are artefacts of the receiver used to do the test.
Making a contact
The dummy load is also a good tool to use while getting familiar with the software. Make some dummy contacts. This will also allow you to check that your macros send what you want them to send, so you avoid the embarrassment of sending things like "CQ CQ DE MYCALL MYCALL". Yes, we've all seen it!
Once the antenna is connected, making a contact is simply a matter of clicking on the station you want to call, or clicking in a quiet part of the band where you want to call CQ, and pressing the button that sends the appropriate macro. Don't forget to flick the transmit/receive toggle switch! The data modes software will generate tones that match the frequency of the other station, so you don't have to turn the K2 tuning knob to tune a received signal to an exact frequency. However, it's best to keep close to the 1200Hz center frequency, to be sure that you're receiving the best possible signal from your contact, and that other stations are attenuated a bit. Later on, you'll probably want to consider using narrow filters to pick out the weakest signals in conditions of heavy QRM.
Now you're ready to go on the air with a good, clean signal that will enhance the high reputation that the K2 already has. Enjoy your first data contacts with your Elecraft K2. I hope to work you on the air soon!