SSB Interference to IBP Beacons

This page addresses the problems of SSB interference to IBP Beacons and also the often misunderstood relationship between SSB frequencies and CW frequencies based on dial readout.

For more information about the International Beacon Project see the IBP Page.

This page here is intended primarily to explain to the SSB operator, why they need to set their transmit frequency as much as 2kHz above the beacons, to avoid harmful interference.

Firstly it is important to note that the IARU has mandated a 1kHz exclusion zone around the beacons, and that the beacons transmit down to QRPP levels of 100mW.

It is also important to note that the beacons have been transmitting 24 hours a day all year round since the 1980s, from 18 locations around the world, on 14100, 18110, 21150, 24930, 28200 kHz.

The above are explained for those interested on the IBP page, while here, we will focus on explaining how SSB transmissions interfere with CW reception also BELOW the dial frequency.

To do so, let us take a specific example of a beacon on 24930.0 kHz. Of course, the exact same example applies to those on other bands at 14100, 18110, 21150 and 28200.

So, from the point of view of the SSB operator, if he tunes his rig to 24930, and asks if the frequency is clear, and hears nothing, he is not going to cause interference to the IBP, right?

Wrong, and here is why. Firstly in the USB position, the SSB Operator will indeed not hear the CW transmissions on 24930, because 24930 is a CW transmission and so it takes place on what would be the SSB carrier frequency (if it was a DSB or AM transmission), namely exactly 24930.0.

When the SSB operator speaks using USB on 24930.0, his voice is going to be transmitted in a range of approximately 24930.3 to 24932.7 kHz. And he will also only hear any CW transmission that is taking place within that passband of his receiver, typically 24930 to 24932 or 24933.

HOWEVER the reverse is completely different, when you now switch to CW and want to listen to a CW transmission, in this case the IBP Beacons, on 24930.0, and here is why!

Now, we are the CW Operator and we have turned our receiver to the CW mode and we are listening on 24930.0 kHz, even in a very narrow filter of only 40Hz, given that the beacons are pretty much exactly on 24930 kHz.

But, what do we hear in the CW position, and why?

For a start, so that we CAN hear anything on CW, the receiver is actually TUNED AWAY from the transmit frequency by an offset of anything from 0.5 to 1kHz. If that was not so, we’d hear nothing! Just as the SSB operator on 24930.0 cannot hear CW on 24930.0 neither would we be able!

So, instead of manually having to move your VFO up (if in LSB) or down (if in USB) so that the inaudible CW now has a rising tone that we can hear with our ears (typically between 500 and 1000 Hz), the transceiver does this automatically.

But this means of course, that the effect is the same as if you were listening to 24930 in USB on 24929.5 in which case you hear it at a comfortable 500Hz tone. And this is exactly what happens, in CW-U, the receiver is doing exactly that, and in CW-L it will be sitting on 24930.5 kHz.

Most rigs do not display the CW RECEIVE frequency but by convention only the TRANSMIT frequency. Some do shift every time you press the key, but for convenience most rigs do not.

Now, what does this mean again in practical terms, to the person trying to listen to a beacon transmitting in CW (carrier) mode on 24930 kHz? What will that receiver here from QRM?

There are two possibilities. One, the CW is receiving upper side band (CW-U) in which case, it will hear everything from around 24929 to 24932. In CW-L, which is the case for most but not all transceivers by default, it will hear everything from around 24928 to 24931 or more.

OK so what about if we switch the narrowest CW filter in, will we still hear the SSB on 24930 USB? Yes indeed. Because the SSB transmission is taking place across the range of 24930.2 or so up to 24932.7 or so. So even in narrow filter, the CW is listening to some of that range, as above.

Another way to verify this if you skipped the radio amateur exam or were not tested on these important facts, as these days they have made the exams simple indeed, and if you do not believe us, is to test this for yourself. Nothing is as good as “seeing (in this case hearing) is believing”!

Find a nice clear frequency, let’s say you are now on 18150.0 kHz USB. Find a Kiwi SDR online receiver (search the web they are easy to find) that is a reasonable distance from you so that you can be heard clearly in it with a good signal. Tune the remote SDR to also 18150.0 USB.

Say your callsign testing into the microphone and verify that you can hear yourself. Now, switch the Kiwi online web SDR receiver to the CW position even in a narrow filter. And retune it so that the CW frequency is again exactly 18150.0 kHz. Now speak into the microphone again.

You won’t be able to hear WHAT you are saying, but you will clearly hear your muffled voice within the narrow CW filter passband. And keep in mind that this would now be wiping out any beacons especially the distant ones and even the nearest ones down to the QRP levels.

Now you should clearly understand why these facts were taught in radio amateur exams long ago when the world was a more sane place. And even if you missed out on that, you now know!

So in summary and brief, stay 2 kHz ABOVE those frequencies so as to avoid all interference even from the edge of your signal, and avoid using any frequency lower. This means your minimum SSB frequency should be 14102, 18112, 21152 or 24932 (10m not lower than 28302 kHz)!

Remember again, it is NOT the DISPLAY frequency on your radio that matters: that is merely the frequency where the carrier WOULD be if it were not SSB but DSB or AM. What matters only is where your transmission is occurring and how to avoid QRM to CW due to the CW-RX offset.

Thank you for your cooperation and enjoy the hobby, wishing you also QRM free SSB DXing!