Tuesday, May 26, 2009

Noise cancelling

When I moved to this QTH in 2001 the electrical noise on the HF bands was quite low considering this is a high density residential area. But noise levels have risen over the years.

About two years ago I was disturbed to hear a constant "arcing" type noise at a strength 7 to 8 affecting most of the HF bands 15m and below. I first noticed it when switching on the radio after a few months of inactivity, so I did not know exactly when it started. A neighbour had taken delivery of a new TV during that period, and the interference seemed to be coming from his house. However, when I tried to raise the subject the neighbour denied having anything that could be causing interference, so I was unable to pursue it. I resigned myself to having to live with the interference.

A week or so I ago I observed that what sounded like the same interference had increased in strength to S9+10dB on 20 metres, making the band unusable. Again, I wasn't sure exactly when it happened as my interests had been diverted to the higher bands 10, 6 and 2 metres for a couple of weeks. It seems possible that changes I made to the antenna system at the time - such as adding 10m and 6m elements to my multi-band dipole - may have increased the pick-up of the noise. 40m and 80m were similarly afflicted.


I was unenthusiastic about trying to raise the matter with my neighbour again, since even if I was able to show that his TV was causing the interference, it seemed unlikely that he would agree to buy a new one just to keep me happy. Someone recommended trying an MFJ-1026 noise cancelling unit, and reviews on eHam.net suggested that these can be very effective at eliminating noise from a single point source, so I decided to order one. It was delivered on Friday and I spent much of the holiday weekend trying it out.

The principle of the noise canceller is that you have a second antenna which ideally should receive just the noise you want to eliminate. You then balance the signal strength of the two antennas so that the strength of the noise is equal, and adjust the phase of the two signals so that the noise received on the noise antenna cancels out the noise received on the main antenna.

The MFJ-1026 is sometimes advertised as an "active antenna with noise cancelling", and comes with an internal antenna and preamp. However, this is really pathetic at picking up anything on the lower frequencies. Although it did pick up enough noise that I could cancel it out on 20m, this could only be achieved by reducing the gain of the main antenna to a low level to match the noise level received by the whip. It was completely ineffective on 80m.

The manual advises erecting a second "sense" antenna. However, even a random length of wire proved to be a poor antenna on the lower frequencies, just as it would be for normal receive use. What's needed, if you are going to use a non-resonant antenna, is an ATU to get the maximum signal out of it.

It occurred to me that I could use my Wonder Wand L-Match antenna (similar to a Miracle Whip) as my noise antenna. This picked up a considerably stronger noise signal on 20m, allowing a higher main antenna gain level to be used. It also allowed the MFJ to cancel noise more effectively on 80m.

I have made an MP3 sound clip that demonstrates the difference that the MFJ-1026 noise cancelling unit makes on 20 metres. The first five seconds of the recording are with the noise canceller switched out so you can hear the interference at full strength. Then I switch the noise canceller in, the noise vanishes, and you can hear a conversation that you could barely tell was there, never mind copy, before. I switch the noise canceller out and in again a couple more times during the 30 second clip so you can hear the contrast. Click here to listen to the sound clip.

I should be overjoyed to have eliminated this awful interference from 20m, my favourite band. Actually, I am a bit depressed. It all seems like a messy kludge. I had to rearrange my desk and make up new cables to locate the MFJ-1026 to one side, because it was rather an eyesore sitting above the K3 right in front of me with the huge Wonder Wand telescopic practically touching the ceiling.

Changing bands is no longer just a button press. I have to tune the Wonder Wand and then adjust the MFJ's level and phase controls as the settings change from band to band, even from one end of the band to another. And the MFJ-1026 is no use with my little QRP rigs, which don't generate a strong enough signal to activate the RF-sensed TX changeover in the noise cancelling unit.

But when all is said and done, the MFJ-1026 does a pretty good job of removing this particular noise source. It's better than going QRT, which may well have been the only alternative.
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