Radio User Technical Review: The Cross Country Wireless Mains Filter

Mike Richards reviews the new mains filter from Cross Country Wireless, after providing a brief introduction to the origins and types of noise we will all encounter at some point.

As featured in December 2017, Radio User
Author: Mike Richards

With the spread of switch-mode power supplies, internet data over mains and similar technologies, noise has become a major headache for many listeners. The new mains filter from Cross Country Wireless aims to improve life for the listener by cleaning up the mains supply to your receiver.

Origins of Noise 
Noise in electrical circuits is everywhere and a certain amount of noise is unavoidable. The most fundamental noise is the thermal noise produced in any conductor. This noise is very small and is the result of the random movement of electrons. This amount of noise is determined by heat and the hotter the conductor or component, the louder the noise. The only way to eliminate it is to cool the conductor or component down to absolute zero! Therefore, radio telescopes use cooled preamps close to the antenna.

However, the noise we are talking about here is on a much higher level. It consists, essentially, of spurious emissions from active equipment that is connected to the mains wiring. The two main offenders in a domestic mains environment are poorly-designed switch-mode power supplies and power line adaptors. PLAs are those plug-in data extenders that use the mains wiring to carry high-speed data. From a consumer’s viewpoint, they are great, as you can extend high-speed Ethernet connectivity anywhere in the house without having to run any cables.

If this sounds too good to be true, it is because the penalty is the emission of carriers throughout the HF bands. While the principle sounds fine, mains wiring is a long way from being a proper transmission line. It acts as an antenna that broadcasts these carriers. If you are using a PLA in the home, tune to the HF bands and try unplugging the PLAs (all of them) to see the difference it makes.

Avoiding the radiated components is difficult and the best way is to locate your antenna well away from any mains wiring. However, you will have some PLA data signals on the mains feed to your receiver and, in this case, a mains filter will help.
Similarly, switch-mode power supplies often use square waves (for the switching) that are rich in harmonics and have the potential to get into the mains supply. Good-quality power supplies will have plenty of internal filtering but there are many poor-quality products on the market. These frequently cut costs by leaving out the electromagnetic compliance filtering. They can be extremely noisy.

In addition to these sources of continuous noise, a typical home environment uses many motors and switches, all of which add to the cacophony of mains-borne noise.

Types of Mains Noise
There are essentially two main ways in which noise can be carried by the mains wiring. These are common mode noise and differential mode noise, so let us begin by explaining these.

Common mode noise is noise that is carried equally in both the live and neutral conductor and can be measured against ground (Fig. 1).

By contrast, differential mode noise exists between the live and neutral conductors (Fig. 2).

How do we stop both noise types from entering our receiver? The most common technique is to use a combination of capacitors and chokes. The chokes are used to provide a high impedance in series with the mains wiring and the capacitors are used as a low-impedance path to direct higher frequency signals and spikes down to earth. A typical differential and common mode mains filter schematic is shown in Fig. 3.

Cross Country Wireless Mains Filter
Fig. 4 shows an inside view of the Cross Country Wireless mains filter. You can see that there is a standard IEC EMC filter socket on one side, which is supplemented by a trifilar wound ferrite toroid choke. The unit is housed in a well-sealed ABS enclosure to create a very compact mains filter.

Without EMC test rigs, measuring the performance of a filter is not easy and I had to use my imagination. Rather than measure the frequency responses of the receiver, I decided to measure the improvement in receive performance when the filter was attached to the mains input of my Yaesu FT-897 transceiver.

In an attempt to eliminate radiated noise, I terminated the antenna socket with a 50R dummy load. I then connected the audio output to a soundcard input on the PC and measured the no-signal background noise, with and without the filter at various frequencies across the LF to HF bands.

I have shown the results in Table 1.

Here you can see that the filter provided a small improvement in background noise on most of the test frequencies. However, the mains in my shack is relatively quiet and you may well see a more noticeable improvement if you have a particularly noisy mains supply.

The Cross Country Wireless Mains filter is a useful add-on for the shack that might give you a small improvement in the noise floor for very low-level signals. The filter also provides useful protection against transient spikes that could damage sensitive components in your receiver.

The Cross Country Wireless Mains Filter costs £60 inclusive of VAT and is available directly at the CCW website.

My thanks to CCW for the loan of the review model.

Table 1: Signal-Noise Improvement Measurements (CCW Mains Filter) 
Frequency    Noise improvement
1.38MHz        1.9dB
5MHz        2.8dB
7MHz        0dB
14MHz        0.7dB
21MHz        1.8dB
28MHz        3dB