Category Archives: interference

My WRT54G has become noisy!

I haven’t done much ‘radio stuff’ over the last few weeks due to work/family commitments and holidays but when I have found time I have been exasperated by huge levels of interference all across the HF/VHF bands which has suddenly appeared. Manifesting itself as huge regular bands of hash across most of the spectrum (as pictured below)

I assumed that a neighbour had purchased some new electronic device that was responsible. However last night I decided to go around the house again just in case, and discovered my ancient Linksys WRT54G router was the source.

I have had issues with QRM before and I was aware that this router was responsible for some noise in the VHF bands, but I had mitigated this by replacing all the network cables with brand new properly shielded network cables (screened shielded twisted pair) and ferrite clamps. This noise is something new and appears even with the network cables unplugged.

I naturally assumed it was the PSU as poor quality switch-mode power supplies used by most peripherals can be sources of interference, but interestingly the PSU for this router is actually a huge linear type judging by its size, weight and the temperature it runs at. I did try another compatible supply with the same results.  

Doing a Google search shows these routers seem to have a reputation for being RF noisy, but until now I haven’t had any real issue on HF.

Due to my ISP upgrading the network I got a Virginmedia SuperHub last year but was forced to put it into ‘modem’ mode and use my existing WRT54G after struggling to configure it. I couldn’t find anyway to configure the LAN to use the correct subnet (it kept defaulting to never mind configuring the port forwarding and wireless access! The GUI and firmware was atrocious and I refused to go around reconfiguring all my devices. Maybe I might have to swallow my pride…

Away from the noise

High up and a hillside in Cumbria at the moment, nine metre fiber glass fishing pole and 10 meters of wire, a ground spike and my FUNCube Dongle PRO+ and the laptop and what a revelation!
The entire HF spectrum spread out before me with little or no noise anywhere, signals booming in! and this was the view last night
As they say Location, Location, Location!

QRM – It is a very noisy Friday

My last post was about the joys of SDR, this one is about the bane of not only SDR but radio in general namely the amount of electromagnetic rubbish that is thrown into the ether by shoddy equipment, with SDR you just see just how much s**t there is.

These are some waterfalls captured today, first exhibit showing lovely evenly space spikes, and a mass of hash and splatter.

A little further up the spectrum, another splatter, this I am convinced is some form of Power Line Network Adapter. You know the ones someone thought was a clever and expensive solution to that problem “How do you get network data from one room to another without the hassle of running a cheap proper shielded network cable or using wifi?” Their answer was to use unshielded mains wiring to transmit data at radio frequencies turning them into efficient antennas!

Mind you even using shielded network cabling isn’t always the answer especially if you have cheap probably Chinese made network equipment which some one is using nearby. It generates this pretty pattern right across the VHF 2 meter band, especially bad on 144MHz.

Hold on VHF is immune to PLA interference you say? Yeah right!

I have posted before about problems I have had before including some videos including the one below. Thanks to my direction finding skills I know which direction this annoying interference was coming from, but I haven’t seen it for a while, someone got themselves a new Plasma/LCD TV?

I am aware some of my equipment generates some noise, but even powering down the entire house and running on laptop batteries I see just as much rubbish, rubbish that disappears as soon as you remove the antenna.

RTLSDR – Part 4 – Cutting out the noise using a simple coax filter

One major limitation of both the FUNCube Dongle and the RTL-SDR based receivers is the poor front end filtering.

Connecting a wide band antenna such as a discone to the input often results in a spectrum full of noise which swamps the target signal.

In most cases the noise is actually a result of out of band high-power transmissions such as FM and TV broadcast stations and pagers which overload the receiver and get mixed in to the tuned signal. The effects can be mitigated to a degree by turning down the gain of the receiver but at the expense of sensitivity to the signal you are wanting to receive.

The FUNCube Dongle in my experience seems especially susceptible to the effects of FM broadcast signals when trying to monitor the 2-Meter amateur band. I’m not sure if the current weather conditions were the cause but yesterday evening it seemed the breakthrough was especially bad. So I decided to experiment with a simple coax ‘notch’ filter.

There are two approaches to filtering, one is to use a band-pass filter which passes frequencies within a certain range and rejects (attenuates) frequencies outside that range. The other is to use a band-stop filter that passes most frequencies unaltered, but attenuates those in a specific range to very low levels. A notch filter is a band-stop filter with a narrow stop band.

As it happens you can make an effective filter using ordinary coaxial cable which will generate a sharp, deep notch which can be used to attenuate the unwanted signal.

A coax stub notch filter can be made if you connect the antenna to the receiver using a T-connector then attach a piece of coax of the appropriate length (the stub) to the remaining connector.

I happen to have a fair bit of 10Base2 Thin-Ethernet cabling (RG-58U) and connectors which were repatriated from a previous employer over a decade ago. The network cards they used came supplied with lovely long high quality 10m patch leads with moulded BNC connectors and a T-Connector and terminator! Even back then the 10Base2 was completely redundant as the network infrastructure used Cat-5 cabling.

So I dug out one of the T-Connectors and a BNC connector and set to work, connecting the antenna and receiver was simple, all I needed was to construct the coax stub.

To calculate the length of the stub you must first calculate the wave length of the desired ‘notch frequency’ for the FM band a good central notch frequency is around 100MHz.

The wave length is found by dividing the speed of light (which is around 300,000km/sec in free space) by frequency

So for a 100MHz signal the wave length is 300/100 = 3 meters.

However the speed of the radio wave in a coax is affected by the material it is made of, this is called the velocity factor. For most solid polyethylene coax (e.g RG-58) this is 66% (a handy table can be found here)

So multiplying 3 x 0.66 = 1.98 meters

The coax stub is 1/4 of this wavelength = 1.98 x 0.25 = 0.495 (49.5 cm)

So cutting a piece of coax of that length, connecting it to the T-Connector and leaving the other end open completes the filter. The filter will have maximum attenuation at 100MHz (assuming the coax is cut accurately) and the attenuation will fall off away from this ‘notch frequency’ (Note the filter will also notch at 300MHz, 500MHz and so on)

Interestingly it seems my RTL based SDR receiver is more affected by pager breakthrough, so I plan to make an alternative stub, adjusting the calculation to use 153MHz as the notch frequency to reduce this. It is possible to add more than one filter at a time using multiple T-Connectors and alternative higher performance designs for coax notch and band-pass filters can be found with a quick Google search.

I made a quick video showing the effectiveness of this simple filter which took about 10 minutes to make!

Station Report

It has been a few weeks since I dug out my neglected scanners and despite not being able to spend hours with them I have still had an enjoyable time reacquainting myself with them and experimenting.

Despite the growing use of digital transmission systems there is still a lot to listen to on the airwaves. I should note at this point that using a scanner to monitor anything that is not intended for ‘General Reception’ is illegal.

The Amateur bands are fairly quite around my location, however I was pleasantly surprised to be able to here to receive a number of Amateur repeaters both on the 2 meter and 70cm bands despite appearing to be outside the predicated coverage.

The CB bands are also very quiet but I suspect that is more to do with the high levels of interference I seem to be suffering. I have been struck by the apparent increase in interference (QRM) on a lot of the bands since I last used the receivers. I can hardly hear anything on the short wave HF bands except the more powerful commercial transmitters.

I suspect this interference is down to the proliferation of computers and associated peripherals, wired and wireless networking. Energy saving fluorescent lamps and microprocessor system in all manner of consumer equipment such as TVs, PVRs, DVD players for example means the airwaves are full of noise.

The Radio Society of Great Britain (RSGB) who represent the thousands of Amateur radio operators have begun to campaign to save the radio spectrum from such interference, specifically they are currently campaigning against the threat poised by the proliferation of PLA/PLT devices (such as HomePlug) these devices use the house mains wiring to transmit network data. However what happens is the wiring then acts an effective radio antenna transmitting the traffic at such levels blocking out other legitimate radio transmissions. This sounds(sic) to be in direct contravention of the EMC Directive of the EU that prohibits the manufacturer of any device that interfers with radio and telecommunications equipment.

However despite representations by the RSGB, the BBC and others OFCOM and the last Government have dragged their heals and refused to act. Hopefully the new Collation Government will be different, but I won’t hold my breath. More information at UKQRM
However given all this noise there are still things to listen to, of course I haven’t since it is illegal, but if I were so inclined there are plenty of analogue VHF/UHF Private Mobile Radio (PMR) systems still being used by commerical organisations. Such as taxi firms, councils, security patrols or by large business to facilitate communication across sites.

In the past it would be difficult to know the source of the transmission often listening for clues to try help identify them such as names of locations, buildings, streets and people for example. However nowadays it has been made a little easier by OFCOM who have allowed on-line access to the Wireless Telegraphy Register database. The use and allocation of radio frequencies is strictly regulated so now if get a hit on a frequency it is possible to use the WRT website to help identify them.

The air and marine bands are still as active as ever, given my location near to a number of RAF bases it could prove interesting if I were also so inclined.