Zero Beating a CW Signal

Zero beating a CW signal is easy for some, art for others, and very difficult for many. Just listen on the bands, particularly during a contest and notice how may QSOs there are  where the two stations are not on the same frequency. It is also important to be able to zero beat two signals using the the sidetone offset available on many of the QRP kits on the market. The instructions for one kit read, "... adjust until you hear a nice mellow 800 Hz tone." Well my kids don't let me sing in church so the there is very little hope that I will recognize such a tone.

The two most important things you have to get right if you are going to play around at the power levels I do (not just QRP, but with mWatts) is to zero beat the other fellow and to be sure the offset on you new homebrew rig is where it should be. Without these the best antenna in the world will not help. You will be transmitting where he is not listening.

But there is hope for us who can't quite identify the correct tone. Spectrogram is available on the web and it significantly facilitates the setting of the sidetone offset of a rig without having to rely on your hearing to get the tone "just right," or even make a judgment call with regard to whether the two tones you're trying to match really are the same frequency.

The most obvious use of Spectrogram is to ensure that you have managed to accurately zero beat your signal with that of the other station. Many operators will purchase a new rig which will have its default sidetone preset to 800 Hz. If that person is accustomed to listening to a 700 Hz signal then when he thinks he has properly zero beat the other fellow, he will be 100 Hz off. Not a good thing.

Let's first look at a signal on the air. To do this yourself, download and install Spectrogram. Connect the speaker (or headphone output of your rig to the MIC INPUT of the sound card in your PC. Start Spectrogram running. Set the display type for SCROLL. When the screen comes up you want to set a FREQUENCY MARK (under the POINTERS menu item) to match the frequency of the sidetone you use when operating our rig. You can set two marks. In the screen captures below I have set one mark at 700 Hz and a second at 800 Hz. The rig I was using is set to use a 700 Hz sidetone. That is the lower of the two horizontal red lines. The second (upper) mark is set at 800 Hz and shows up as the second horizontal red line. So a zero beat CW signal will show up as a line of code, centered right on the 700 Hz line.

Take a look at the screen capture above. The was a glimpse of two stations in a QSO. Notice that the signal for the first station (left half of the screen) is on the 800 Hz line and the signal for the second is on the 700 Hz line. These two stations had not properly zero beat their signals. Had one of them been running QRP, or even less power, the other station might have never heard them. They would also not have had a successful QSO in poor propagation conditions.

The same procedure used here can be used to ensure that you are zero beat with the other station. Simply bring up Spectrogram. Feed the audio output from the rig to the PC, so  Spectrogram can 'hear' the signals. I do this the lazy way, by simply holding the PC's mike near the speaker of the rig. You can get fancier and run a cable between the two but that is more effort than it is worth to me. Then simply tune the station until his signal is centered on the line that is set to the sidetone you use. Then if the sidetone offset is  properly set in your rig you will have him zero beat.

This is another a good example of the kinds of signals you can find on the bands at times. This one I actually recorded from the CW county hunters net. It shows the signals of three stations. The station on the right was a mobile running a county on the net. I had him zero beat with an offset of 700 Hz. The station who signal is on the left of the screen was a fixed station working the mobile. He was off by 200 Hz. Finally notice the signal in the center. This was the net control who sent a "k." His signal was off by 100 Hz in the opposite direction. In this case the net control was facilitating the QSO between the fixed station and the mobile. This help was likely necessary because the fixed station had not zero beat the mobile as he should have. He is fortunate that the net control could help him. Most likely the net control and the fixed station were better placed for favorable propagation so the difference in frequency between the two was not as critical as it was between the fixed station and the mobile.

Setting the Offset on a New Rig

These same basic ideas can be used to help set the sidetone offset of a new rig (QRP or otherwise). You will need a second rig on which the offset has been properly set. Assume RIG #1 is the one for which you want to set the offset. RIG #2 is the rig that already has a properly set offset. Start Spectrogram on your PC. I usually hook a mike up to the PC and feed Spectrogram from the speakers of the two rigs. That is a lot easier than switching cables back and forth.

Note you don't have to transmit a steady tone from either rig to listen in on the second rig. A few simple DITs should be enough to help you get the VFO set at the zero beat. You don't want to overload the second receiver.

What can be done when you get it all set up right? Head back to my home page and take a look at the link to WAS QRPp. Take a look at my DXCC QRPp effort. Lastly go to the QRP page and click on the link to the 2 mWatts QSOs.

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