Poetic justice?

Following my last post it is perhaps ironic that for the last few evenings I have been plagued by an increase in QRM.

Trying some JT65 on 40m I was being plagued by S8-S9 of local noise, it is noise I have had regularly (even before the purchase of my PLT devices) so I reconnected up the WiMo QRM Eliminator, which has been collecting dust, to see if I could improve matters.

Using just a short piece of wire as the auxillary ‘noise’ antenna managed to null out most of it to greatly improve reception. No commentary on the video below but hopefully you can see it working.


Interestingly the noise seems to abate around 11pm when people are off to bed.

OFCOM undue interference consultation

Happy New Year!

2015 has started with some important developments. Ofcom have launched a consultation on draft regulations for new wireless telegraphy legislation. The proposals are intended to strengthen regulatory power and keep pace with technological advances with respect to interference of radio communications from electronic devices.

Current Electromagnetic Compatibility (EMC) regulations should prevent electrical and electronic apparatus emitting electromagnetic energy that would cause interference to radio communications but as most licensed operators have experienced this is not the case. In particular the growth of “power line” networking equipment (PLT) in recent years has caused much distress to amateur radio particularly in populated areas. I myself have suffered interference from what I believe are PLT devices.

It is certainly true that most newer devices are much better and are notched to prevent emissions in certain bands but there are still many older devices in use and imported non-compliant devices are still readily available. It is not just PLT devices that cause issues but solar panels with RF noisy inverters, unfiltered switch mode power supplies, plasma televisions and other equipment that pollute the spectrum.

Unfortunately at present Ofcom has limited enforcement powers so are proposing the law is amended to make it a criminal offence to continue using equipment identified as a source of undue interference.

Several agencies such as GCHQ, CAA and emergency services have voiced concerns in the past over the threat of RF pollution (Telegraph newspaper article) so not surprisingly this weeks announcement has been given the ‘spooks’ spin with some sensationalist headlines “You could be prosecuted over your broadband thanks to GCHQ” as the Telegraph reported.

I feel it is important that UK licensed amateurs respond to these proposals and the consultation is only open till next month it is very easy to respond on line at the Ofcom website

It is quite a timely announcement from Ofcom since I have been forced to use the QRM eliminator that I purchased back in August due to increasing interference. I have not been able to use it in line with the FT857D as it requires the PTT/TX-GND signal from the CAT/Linear socket from the transceiver to activate a bypass when transmitting. The issue being I wished to use the CAT functionality at the same time and this involved making up a harness with the appropriate 8-pin mini-DIN plug/socket. I purchased some plugs and sockets but kept putting it off due to the fiddly nature of the small connectors and I didn’t wish to cause any damage to the FT857D by shorting the pins.

By chance I spotted this cable on eBay which is the same as my current CAT cable but with the addition of the PTT/TX-GND signal on a short pigtail with a phono (RCA) socket making it a breeze to complete the installation.

I am hoping to improve the noise performance by putting up a new HF antenna in the future, to this end I have had a nice Christmas present. A Feature Tech AW07A HF-VHF-UHF Antenna Analyzer

Once I have used it in anger I will post a proper report, but it seems a great piece of kit with some encouraging reviews, currently available of £168 on eBay it is a massive £200 less than the identical MFJ-266

Noises Off – WiMo QRM Eliminator

As I posted at the end of June I was almost on the point of giving up on HF due to the high levels of QRN/M suffered at home.

In desperation I had been looking at some of the noise cancellers available from MFJ and others. I had heard conflicting options on their effectiveness but was willing to try one if I could obtain one cheaply. The MFJ units are in my opinion expensive to purchase new so I bid on several that came up second hand on eBay but they invariably also went for silly money. I was even contemplating building an home-brew one from the numerous designs available.

Then I discovered the WiMo QRM Eliminator, made by the German company WiMo Antennas and Electronics. Several online reviews and numerous YouTube videos seemed to indicate its effectiveness and taking up the then summer discount offer I ordered one for the princely sum of €147 including postage, around £116.

Due to heavy demand and low stock levels following the International Ham Radio Exhibition at Friedrichshafen I was told the unit wouldn’t be available till the middle of August, so I was pleasantly surprised when the unit turned up last Friday morning.

New toy in time for the weekend 😉 Thank you @wimo_de pic.twitter.com/4Xfuv5ccHB
— Andrew Garratt (@nerdsville) August 1, 2014

I have connected it up and briefly tried it out and am impressed, as this video illustrates.

Even using one for a short time I would agree that they are something of a black art to set up and use and can understand why people might consider them useless, but this may be due to lack of understanding of how they work.

I have created a diagram showing a typical scenario.

The operators main antenna is designed to pick up distant signals, not necessarily DX but signals not emanating from the immediate vicinity, however they will also pick up locally produced QRM as shown, perhaps generated by a neighbours TV or PLT device. 

The QRM eliminator has a second antenna which isn’t as efficient as the main antenna and ideally just receives the local QRM at a similar level to the main antenna. The device then takes this second signal and inverts the phase so when it is mixed with the main antenna signal the QRM is cancelled out.

The principal is quite simple, if you take two in phase signals and combine them you will end up with a signal with a larger amplitude. However if the signal is 180 degrees out of phase, the positive and negatives of the waveform cancel each other, producing a null signal. 

The tricky part is making sure the noise antenna just picks up the noise, if it picks up the main signal then that will also be cancelled out. The WiMo unit has three controls a gain and two phase controls it is a case of altering all three to maximise the cancelling effect without losing the main signal.

At present I just have several meters of wire running as the noise antenna along the side of the shack and this seems to fairly effective.

The unit is powered from 12V and can be left in line, but requires the PTT/TX-GND signal from the CAT/Linear socket from the transceiver to activate a bypass when transmitting and I don’t have the appropriate 8-pin plug at present. If the unit is powered off the bypass is automatically engaged.

All in all, first impressions are good and looks like a worthwhile purchase.