Combating HF Noise: what works?

Posted by Richard Newstead on 12th Sep 2018

First published in the SOTABEAMS Newsletter 1 September 2018. If you like this article, why not sign up to our newsletter? It comes out about monthly.

Combating HF Noise: what works?

Last year I bought a motorhome. It's been a great way to get about and has provided welcome accommodation at rallies. One thing that struck me when we started using it was just how much rubbish we generate in our day to day life. Everything we buy seems to have multiple layers of packaging and it's all the more obvious in a motorhome. Clearly this has an environmental impact and, the mantra of reduce, reuse, recycle neatly summarises a hierarchy for environmental impact reduction. As we will see, a similar hierarchy applies for noise reduction.

For effective combating of noise, your first action should be to eliminate the noise source or sources wherever possible. It's often surprising how much noise can come from your own property. When our children moved out earlier this year, the noise floor on the HF bands dropped significantly which I suspect was due to the removal of various low quality electronics devices that were left permanently plugged in and switched on. When you face noise problems, a good test is to rig up a battery powered radio receiver and then cut off all power to your own house at the main breaker to see if the noise level is reduced. If the noise level drops you can unplug everything, switch on the mains supply at the breaker and start plugging things in to see what is causing the noise. It is worth mentioning that home solar power systems have often been associated with high levels of HF noise. This leads us to the first of our "three Rs"; attempting to REMOVE the noise source should always be your first course of action.

If removing the noise source is not practical there are various techniques that can be used to improve matters. The use of balanced antenna systems (with baluns and common-mode chokeswhere required) can often be helpful to stop noise getting into your radio system. Common-mode chokes such as ferrite rings can also be used on the power leads of electrically noisy devices to reduce their tendency to radiate noise.

A co-ax fed dipole with a balun should have a deep null off the ends which can be oriented to reduce noise pick-up. Use of a directional antenna can be helpful if the noise source comes from one direction. In this respect, properly made, "Moxon rectangle" antennas are particularly effective. Although they have modest gain they can have an excellent deep null off the back - which in a noisy environment makes them sound much "gainier" than they actually are. It can also be helpful to move your antenna as far from a local noise source as possible.

A less-used, but potentially very effective technique for noise reduction, is "noise cancelling". There are various commercial products that do this. They rely of having two antennas; one designed to pick up as much local noise as possible and the other being your normal HF antenna. The noise canceller allows adjustment of the phase and amplitude of the noise and adds it to the main antenna. If done carefully, the noise cancels out leaving a nice clear band! Noise cancellers can work really well but there are a few things to be aware of:

* they only work over a small band of frequencies and will need adjusting frequently;

* actually getting the adjustment correct can be very challenging - expect to spend some hours getting it working correctly;

* they work best for single sources of noise - they can give poor results with distributed noise sources.

These techniques give rise to our second "R"; if we cannot remove the noise we should aim to REDUCE it.

If all these techniques have failed there are other methods that can be tried but they are seldom as effective. The use of digital signal processing in modern radios usually means that some sort of noise reduction algorithm is available. These have improved over the decades and can make a modest improvement to the signal to noise ratio. If used on SSB they can lead to a rather metallic sounding received audio at high levels of processing. For narrowband modes such as CW, reducing the bandwidth of the radio can help but be aware that this is not required for modes such as FT8 where the computer does all the audio processing. These techniques fall into a category that I have called reprocessing - giving us our final "R"REPROCESS.

So our noise hierarchy is complete. We should aim to: REMOVE, REDUCE, REPROCESS (in that order).

Useful resources:

...note that a listing here is not a product endorsement - just for info!

73 Richard G3CWI

First published in the SOTABEAMS Newsletter 1 September 2018. If you like this article, why not sign up to our newsletter? It comes out about monthly.