Wireless Waffle - A whole spectrum of radio related rubbish
RSGB Spectrum Defence Fundsignal strength
Sunday 31 January, 2010, 09:39 - Amateur Radio
Posted by Administrator
Sometimes Wireless Waffle can be rather frivolous and irreverent but today is not one of those days. On many previous occasions, Wireless Waffle has gone on about the threat to short-wave radio caused by power line telecoms (PLT), power line adaptors (PLA), broadband over power-line (BPL) and blaufunk zür electrische Breitband über bezahlen (BLZEBUB as it is known in the Germany) yet they continue to proliferate. But in the UK at least, it seems that there is going to be a properly monunted challenge to the blatant flouting of the law that these devices do on a daily basis.

Having thought that the UK spectrum regulator, Ofcom, would respond to the concerns of UK radio users, the Radio Society of Great Britain (RSGB), the body which represents radio amateurs, has played a very professional deck of cards to date; demonstrating that the equipment concerned does not conform to the necessary standards and using existing legislation and regulation to request that Ofcom take action. Ofcom's response, however, has been very disappointing, and seems to be acting on the basis that few complaints equals few problems. Presumably very few people complain when a vehicle knocks them down and kills them, so perhaps we should cease to be concerned about enforcing speed limits?

Ofcom seems to be following a circular argument that, on one hand, radio regulations don't apply because the devices which cause the problem are not intentionally generating radio transmissions; and on the other, that electromagnetic compatibility (EMC) regulations don't apply to the radiation they do cause, as EMC is not about controlling radiation from devices - as any such radiations are meant to be covered by radio regulations. It even turned down a sensible proposal from the German regulator, the Bundes Netz Argentur (BNetzA) for a purpose-designed specification for these types of device to ensure they didn't fall down the tiny pothole that might just exist between the two sets of regulations.

Several petitions have even been raised with Number 10 Downing Street (the Prime Minister's office as opposed to the Farnham Healing Centre) to which Ofcom's response has been equally lacklustre. 'No biggy...' seems to paraphrase their attitude.

rsgbspectrumfundSo kudos then to the RSGB for putting down their playing cards and taking up arms against the problem (whether you class the problem as the devices or as Ofcom) and establishing a Spectrum Defence Fund (SDF). The RSGB claim that to challenge Ofcom through the relevant legal channels will cost in the region of £75,000. The RSGB are asking anyone and everyone who has an interest in short-wave listening, either for pleasure or professionally, to put their hands in their pockets or break open their piggy banks and donate some money to the cause. They point out that if every radio amateur in the UK donated just £5, they would soon reach their target. On the other hand, if only a couple of thousand people donate the same amount, they will miss the target by a large margin.

rsgb in actionSome may question whether or not it is right for the RSGB to become so overtly political in its outlook given the fact that it has only really used its deck of cards to play snap in the past and has never ventured into more risky territory (such as whist for example), let alone expose itself to the dangers of games such as poker. For the past century, or thereabouts, it has been a forum for sharing news and information for the amateur community in the UK and although it has responded to many Ofcom consultations, it has done this in the role of a semi-passive observer: a (card playing) grandfather who passes down occasional nuggets of experience to the go-getting upstart grandchildren who now inhabit the spectrum. And rightly so. The RSGB should be a centre of expertise; a wise old sage to whom its members, and the wider UK radio community can turn to answer those difficult questions. But the threat from PLT/PLA/BPL/BLZEBUB is very real and it's time for granddad to stand up and batter the kids around the ears with a solid oak walking stick until they sit down and stop acting like spoilt brats.

The threat to the radio frequencies used by radio amateurs, however, comes from more than just BLZEBUB. Radio spectrum is becoming a very valuable commodity and amateurs sit on real-estate which in the UK alone would be valued at several million pounds if it were open to the markets to purchase. There is no evidence of an immediate threat in this regard, and no queues have formed to offer granddad an especially large pension in return for his deck, but if the amateur community does not keep its eyes open, someone is likely to creep up on granddad whilst he is out for a walk and take his wallet and keys, and his solid oak walking stick.

So the time has come for radio amateurs and short-wave listeners of all kinds across the United Kingdom to unite and support granddad. Why not have a night listening to the radio and remind yourself of what it's all about. Not Danny DJ spinning the same old tunes on Radio Local FM, but tune around the short-wave bands and see what you can hear. With sunspots beginning to acne the face of the sun again, you might just be surprised at what you can hear. And when you have rekindled your love of radio, pop along to the RSGB web-site and donate a few quid. Imagine what life would be like without your other favourite pass-times, whether beer, wine, fags or whatever. In fact, go without for a week. Then think how it would feel if you couldn't use your short-wave radio again and give the money you saved that week to the RSGB spectrum fund. In return, Wireless Waffle intends to donate all the money made from the various advertisements which are scattered around the site to the RSGB Spectrum Defence Fund. We'll let you know how much this is in a couple of months time...
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Noisy Neighbourssignal strength
Thursday 30 October, 2008, 22:23 - Amateur Radio
Posted by Administrator
It may appear to have been quiet here at Wireless Waffle over the past couple of months, but that's because a number of things have piqued the interest and we've been doing a bit of experimentation and investigation. The first of them is the issue of Power Line Telecommunications (PLT) also known as Broadband over Power Line (BPL) and in particular the problems being experienced by many UK short-wave radio listeners with it. One of these devices could recently be heard, albeit quietly, across the HF spectrum (oddly it has since disappeared) at Waffle HQ but it raised the question as to how many more there were in the area. To find out, I fitted and HF antenna to my car and connected it to an HF receiver and then drove around a nearby housing estate to see what could be heard.

The antenna used resonated in the 18 MHz (17 metre) amateur band but received perfectly well in the 17 MHz (16 metre) broadcast band. Tuned to a clear channel around 17460 kHz, the car was driven around the area under test. Over the area covered by a small local estate, three devices were detected. Two were almost certainly the 'Comtrend' device, sounding just as the ones that UKQRM has demonstrated on YouTube. The third (shown in red on the map) emitted a more continuous tone, interrupted by occasional blips; the range of this device was somewhat less than the other two. Their range at other frequencies was not tested but anecdotal evidence driving round the area using other antennas and the same receiver suggests that the coverage on frequencies around 27 MHz is similar.

plt bpl interference rangeI've plotted the approximate location of the three devices identified together with the area over which they were clearly receiveable on the map on the right (the map covers approximately a 500 metre by 500 metre area). The blue device had the largest interference range and within the areas marked it is unlikely that any short-wave reception would be possible.

The density of these devices means that over the whole estate, short-wave reception would be virtually impossible. A cursory glance around the area indicated no amateur radio antennas so it is likely the devices are going undetected (or just unreported!) Not a positive result and unless the powers that be do something to halt the spread of these devices, it would be easy to foresee a situation where HF reception could be pretty much impossible over whole towns and cities, in residential areas for certain.

One piece of positive news is that Ofcom have set up a special team to deal with PLT interference and appear to have begun taking the problem seriously. Let's hope that this is more than just paying lip service to the problem before the whole HF spectrum is lost to the laziness of those who can't be bothered to use WiFi or put a piece of wire between various bits of equipment in their home. These devices are a disgrace and a menace and before they wipe out all short-wave reception and neighbour-on-neighbour war breaks out, serious action by the authorities is absolutely necessary.
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A Noisy Noise Annoys (Part 3)signal strength
Sunday 27 July, 2008, 09:12 - Amateur Radio
Posted by Administrator
The European Telecommunications Standard Institute (ETSI) is currently in the process of finalising a standard for in-home Power Line Telecommunications (PLT). This is equipment that allows internal electrical wiring in a house to be used as a local area network, to carry computer and other data signals around without having to re-wire your house to do so. Whilst wireless networks do much the same, the advantage of PLT is that it could be built into, for example, a television, to allow it to interconnect with a video server or even a set top box, without the need for additional leads and connectors.

comtrend pltThese devices (for example those sold by Homeplug) work by sending signals around the mains wiring at frequencies between around 3 and 30 MHz. Now the wiring in a typical house is designed to carry signals at 50 or 60 Hz, depending which country you are in, and although it is possible, with sufficient brute force and ignorance, to get signals in the short-wave (high frequency or HF) frequency range from one socket to another, the network is very (very) leaky. Much of the signal leaves the wiring in the house with the potential to cause interference to any HF user in the vicinity (say, within 100 metres or so).

Much of the work being done by ETSI is to try and find ways of stopping these PLT devices from causing interference. To start with, the specification requires that there be notches in the transmitted spectrum in all the amateur radio bands to try and reduce interferece to radio amateurs. This is a good start but does not solve the problem for short wave listeners (e.g. those who like to listen to international short wave broadcasts).

short waveThe most recent studies that have been done have been to address specifically this issue and the result is a specification which attempts to monitor the various short-wave bands and if it senses activity in a band, will not use frequencies in that range. So if the device detects a signal on, say, 5.9 MHz, it will not use frequencies in the 49 metre band. There is a little more intelligence than this included, such that if only signals on low frequency ranges can be detected, the device will assume it is night-time (when few high frequencies propagate) and vice versa, and from this information will make further informed decisions about which frequencies might be clear to use.

This is fine, as long as: (a) the detection algorithms work properly and (b) the frequencies allocted to the short-wave broadcast bands remain as is and don't get expanded or moved around. Of course such a system will not protect out-of-band broadcasters or other services which use HF frequencies such as the military, maritime and aeronautical communications. Nonetheless, it at least shows a willingness to take account of domestic short-wave radio usage where the impact of the devices will be greatest.

Step up to the podium, then, BT Vision who have begun using PLT technology to connect their set top boxes into televisions (the two usually being in different rooms in the house) through the electrical wiring. However the devices they are using were developed prior to the completion of the ETSI standards and do not have the sensing mechanisms in place. One device in particular, made by Comtrend (Model DH-10 PFUK to be specific). It provides a 200 Mbps connection between sockets and can be bought independent of BT from various suppliers.

noisy noiseThis device (and some others like it) have started to cause enormous headaches to domestic HF reception, both amateur and broadcast. You will find various videos on YouTube demonstrating the problem and a Yahoo! group (UKQRM) has been set up for users to share experiences with these devices. What is encouraging is that Ofcom is taking the situation seriously. One radio amateur reports that after having reported the problems to Ofcom, action was taken and the offending devices replaced with alternative ones which seemed to cure the problem.

Driving around the town where I live, I have noted three of these devices, each blocking out reception right the way across the HF frequency bands over a range of around 100 metres. Thankfully none of them are in the immediate vicinity of my own station but the threat exists. If you are suffering this kind of interference, I would urge to you take a look at the UK QRM site or contact Ofcom.

Apparently BT are aware of this problem, with the specific units in question, and will replace them if contacted by their subscriber. This, however, requires anyone who is affected to determine where the interfering signal is coming from and then speak to the person whose house the equipment is installed it. If this is a friendly neighbour, you are fine. If it is not, then your only recourse is through Ofcom. Bon chance!
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The World's Simplest Harmonic Filtersignal strength
Saturday 31 May, 2008, 08:23 - Amateur Radio
Posted by Administrator
One problem that radio amateurs (and professionals for that matter) regularly stumble across, is the problem of adequately surpressing the harmonics that are produced in their transmitter. Harmonics are frequencies which are on multiples of the actual signal being produced (e.g. a transmitter at 100 MHz will produce harmonics on 200, 300, 400, 500 MHz and so on...) and are a natural and largely unavoidable bi-product of the transmitter.

Normally a low pass filter is used to try and stop harmonics from reaching the aerial. Such a filter will be designed to allow the wanted frequency through with little or no attenuation, whilst attenuating harmonics by 30 dB or more (the level of filtering required depends on how bad the harmonics being produced by the transmitter are). However, there are some instances where the surpression offered by a low pass filter might not quite be enough to reduce the harmonics to a level where they are no longer a problem. The diagram below illustrates the typical response of various low pass filters.

low pass filter response

Imagine an amateur radio station transmitting on 50 MHz. The second harmonic of this transmission is on 100 MHz, right in the middle of the FM broadcast band. If this happens to be the frequency of a local FM station, then even tiny amounts of harmonic radiation may upset neighbours trying to tune in.

Years ago I stumbled across a device which is one of the simplest (and cheapest) harmonic filters around. It comprises of... a piece of coax. Yes, it's that simple. Or almost that simple. It's called a stub and works like this:

A quarter wave transmission line (i.e. a quarter wavelength of coax, twin feeder or similar) has the weird property that the impedance at one end of it will be the reciprocal of the impedance at the opposite end of it, at the frequency where it is a quarter wavelength long (this has to take account of the velocity factor of the line - more of this later). So, if one end of the quarter wave is a short circuit, the other end will be an open circuit.

A half wave transmission line exhibits the property that the impedance at one end, will be exactly the same at the other end (which is why some folk recommend making all patch leads a half wavelength long).

So how can these facts be used to make a harmonic filter. Simple! Using the previous example, if we cut a piece of coax to be a quarter wavelength long at 50 MHz and then short circuit one end of it, the other end will show an open circuit at 50 MHz so we can place it across the output of the transmitter with no effect (if you aren't sure how to connect it across your transmitter output you shouldn't be playing with transmitters in the first place). At 100 MHz, however, it will be a half-wave long and the short circuit at one end will appear at the other end. It will therefore allow the 50 MHz signal to pass and block the 100 MHz signal - the perfect harmonic filter.

However, this pattern repeats such that the same 'stub' will also pass signals at 150, 250, 350 MHz and so on, and will block signals at 200, 300, 400 MHz and so on. Whilst this means that some harmonics are still passed unchanged, it does block half of them and in our particular example means that the problem signal that interferes with our neighbours' reception of the local FM station is addressed. Alternatively, it would simplify the design of the necessary low pass filter, potentially reducing the component count and hence cost. The frequency response of the stub (as connected across the output of a transmitter) is shown in the diagram below.

stub filter response

Making a stub couldn't be easier. Let's assume we are using standard RG-58 (URM-76) coax cable. This typically has a velocity factor of about 0.66 (meaning, freakily, that radio signals travel at only 0.66 times the speed of light inside the cable, compared to the speed in free space). Velocity factors vary between about 0.66 and 1 (foam filled coax has a factor of about 0.8).

stub filter diagramA quarter wavelength at 50 MHz is 1.5 metres long (300 divided by 50 MHz divided by 4). Multiplying this by the velocity factor gives a resulting stub length of exactly 1 metre. So, if we get 1 metre of coax, and short circuit one end of it, whilst connecting the other end across the output of our transmitter, it should have no effect whatsoever on the 50 MHz signal, but will short out the 100 MHz signal to the best of its ability.

Typically stubs of this nature attenuate the second harmonic by 30 dB. Careful tweaking to ensure that the second harmonic 'notch' is right on the second harmonic frequency can increase this to maybe 50 dB. Multiple stubs, separated by further quarter wave lengths of coax can be used to make the notch deeper.

The downside of such a filter is that it is frequency specific, so if we re-tune our transmitter to 52 MHz, with the harmonic falling now at 104 MHz, the second harmonic attenuation will be less (again multiple stubs, each on a slightly different frequency to the next can help here). Also, stubs of this type would be very large for low frequency operation (the same stub would require 14 metres of coax if the wanted frequency were in the 80 metre, 3.5 MHz band).

Nonetheless, the tuned stub harmonic filter has to be one of the simplest and cheapest ways to reduce harmonic emissions that anyone could make.
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