Radio Frequency Noise Cancelling

By this time we are all familiar with the technology of noise cancellation.  The concept is that the noise is sampled along with the desired signal.  The noise is then turned 180 degrees and mixed with the received data to cancel out the noise, leaving only the intended received signal.

There are several products on the market that do this.  Here is just one device in particular that I have built from a kit.  This is not meant as an advertisement, but only a testimonial to the technology.  The QRM-180 kit by AG6YJ is a compact and clever kit.  The packaging is superb, documentation is good, and assembly is easy.

The noise cancelling device accepts two inputs.  The TX/RX antenna and a Noise Antenna input.  There is also a transceiver connection that goes to your radio.  In Transmit mode the signal from the transceiver goes straight out to the TX/RX antenna and can handle the power of a 100watt radio on HF.  Naturally a PTT is required to switch the device from receive to transmit mode.  This re-routes the RF from the radio straight out to the main antenna terminal.  A well designed device will also have RF detection to switch to transmit if a small amount of RF is detected on the radio output.  This prevents blowing out the circuitry of the device.  The QRM-180 uses tiny 6 volt incandescent lamps as fuses.  If something goes wrong, the fuse blows before the circuitry is damaged.

The receive function is what we are really interested in.  Noise cancellation will work only on local noise that you can receive with a sense antenna.  It won't be able to remove atmospheric noise, and it won't improve poor transmission quality.  If you can't eliminate a noise in your area by going around the home turning off things looking for the source, or if you have an essential device in the house that cannot be turned off, or the noise is somewhere in the neighborhood that you have no control over, then noise cancellation may work for you.  Removing noise allows weaker signals that are below the noise level to be heard by the receiver.  It may not be as effective as moving out into the country into your own private "radio-quiet zone" to escape the RF pollution in your neighborhood, but it will help.

The noise antenna can be anything that picks up the noise.  It can be resonant on the noise frequency, a length of wire, or a small telescopic mag mount.  It should not be resonant on the band that you are trying to receive on.  I small telescopic mag mount works for me on most bands.  A longwire strung outside works for some of the lower bands like 160m.  You have to find out what works for your troublesome noise.

The noise will be brought in and amplified and available for phase adjustment.  It will then be flipped in a 180° phase inverter.  The noise now becomes "anti-noise" which is the exact opposite amplitude of the noise with the same waveform.  The Main antenna will receive both the intended signal and some of the noise.  The signal is amplified and available for phase adjustment.  Tune off any strong transmissions and just listen to the background noise for a moment.  If you have an SDR you will be able to see the waterfall and the noise floor.  When the gain is turned down on both the QRM-180 and the RF gain of the radio, the phase adjustments (marked as ø in the diagram) can be made so that a null is seen on the S-meter of the radio.  In some cases the null may be very sharp and pronounced.  On an SDR you will see the noise floor across the band drop.  As the phase adjustment knobs are turned a null may even be visible moving up and down the band as the knobs are turned.  When both phase knobs are in the right position the noise floor will indicate the lowest S-meter reading.  This is an iterative process, and will also mean adjustment of the gain knob.  When the "anti-noise" amplitude matches the noise amplitude contained in the received signal the re-combination of those two signals (marked as x in the diagram) will pass the intended signal to the radio.  The removal of the unwanted noise signal is done by destructive interference.  There is a gain adjustment for the noise sense amplifier in the back of the unit, but unless the noise is unusually strong, the manual says to leave this gain up the whole way.

Now tune the radio receiver VFO to the desired signal.  Again adjust the gain knob to get the best signal with a maximum amount signal and minimal noise.  Turning the gain up too high will just make it worse again.  This seems complicated at first, but after using the QRM-180 for a few days the process becomes natural.

Reduce the gain --> find the null with the phase adjustment knobs --> adjust the gain for best signal to noise ratio.

Let's look at some real results on the QRM-180.  There is something in my surrounding neighborhood that creates a fairly high noise floor.  There are a lot of houses around and who knows what noise-producing products people have nowadays!  Adjustments within the band should only be minimal if at all once the noise is nulled out in one section of the band.  Here we see the different when I turn on the QRM-180, adjust the gain down and find the null.  With the gain down the whole way both signal and noise are attenuated which doesn't help.

I see the most benefit on 17, 20, and 40 meter bands using the telescopic mag-mount antenna as a noise sense.  Here we see the signal on the Flex waterfall with the QRM-180 turned off.  The input signal from the Main Antenna goes straight to the receiver, noise signal and all.  The S-meter reading is fairly high at S5 and we see only a few signals peaking up above the ugly noise floor.

Some gain is needed to find the null.  Once the null is found the amount of gain is a judgment call for the operator.  Using the QRM-180 I have been able to hear stations that were previously buried in the noise floor.  That's the beauty of it.  As the adage is told from the days of old, "You can't work them if you can't hear them."

This device works so well, I'm not sure why modern transceivers don't come equipped with this technology built in!  Digital Signal Processing and Software Defined Filters can only do so much to enhanced the readability of a signal or reject QRM from adjacent stations.  Notch Filters are great if the QRM is very narrow.  For total broadband noise the QRM-180 has allowed me to hear stations that I would have never heard without it.
.