Wireless Waffle - A whole spectrum of radio related rubbish
http://www.mf2fm.com/blog/index.php
Pennies from Heaven?
http://www.mf2fm.com/blog/index.php?entry=entry080409-154036
Wireless Waffle that takes an active interest in matters pertaining to spectrum management. No less than the fourth highest authority in the land (after God, the Queen and Chris Moyles), the Rt. Hon. Alastair Darling MP, chancellor of the Exchequer has obviously been taking note of what we've been saying. Section 5.16 of the his Budget 2008 entitled, innocuously enough 'Spectrum Management', says:
As broadcast media and mobile technologies continue to grow in importance and diversity, efficient use of the electromagnetic spectrum to deliver the services that society demands remains an important issue for both the public and private sectors. In line with the Government's Forward Look on public sector spectrum, the Ministry of Defence will by May 2008 publish an implementation plan setting out its plans for the release of spectrum to the market. Other departments are adopting similar processes. To ensure best use of spectrum by the private sector, Ofcom has also confirmed that in contrast to some previous spectrum releases which were available for specific uses only (notably mobile telephone services) the spectrum released by digital switchover will be available for all technologies. The Government fully supports this decision.
Does this tell us anything? Are the Government about to tax spectrum? Will spectrum be blamed for the economic downturn that we now seem to be staring into the barrels of? Will spectrum be the reason for the 2 point increase in income tax at the next budget? Or much worse, will poor spectrum management cause an extra 1 pence on the price of a pint of beer (except 'lager' which isn't really beer in the true British definition of the term)? Probably not, other than the fact that the estimated bill that Ofcom is going to charge the MoD to 'recognise its access' to the spectrum is £300 million, and that decisions on MoD ('and other departments' meaning the Civil Aviation Authority mainly) spectrum use are taken at cabinet level and so are visible on the radar.
One might go so far as to suggest that the chancellor at the time of the £22 billion 3G licence windfall, none other than the now Prime Minister, Gordon Brown, is keeping an eye on matters to make sure that the Government doesn't lose out should the MoD find a way to profit from the sudden interest in one of their previously lesser valued assets. One might do that. One might not be far wrong!]]>Frequency Sharpener
http://www.mf2fm.com/blog/index.php?entry=entry080401-102400
You've heard of pencil sharpeners and knife sharpeners, both designed to ensure the maximum of performance from the devices on which they operate. Well, on April 1 this year, researchers at the University of Al Tayr, in Tripoli, Libya released details of a 5 year study that they have been conducting where they believe they have discovered the world's first 'frequency sharpener'. Details are somewhat scant but if the device can deliver the kind of results that have been quoted, the frequency sharpener could be being included in many new digital radios of the future.
In their press release, Professor Ali P'Frool says of the frequency sharpener:
With weak signals, it is like a thousand camels standing through your receiver to blunt the sound. The frequency sharpener acts between these blunt and dull edges using a special digital encoded Saif Al Nisan algorithm that swipes away and cuts off the noise to leave only clear oasis of surrounding signal.
The release is rather thin on technical details of how the sharpener functions but goes on to claim:
Tests in laboratory of testing have shown, masha'allah, that 12 times improvement in frequency sharpness can be made to receive signal with almost no hump blockage in background or foreground. Works of programme suggest that remaining disturbance no worse than size of golden sand rabbits of similar proportion. By development, frequency sharpener will deliver 20 to 25 times decrease in annoyance of radio camel noise for new design of radio receiver in Arabian region. Better results with Saif Al Nisan algorithm than traditional threshold extension or synchronous detection methods employed before this discovery.
We've sent an e-mail to the professor to see whether he could provide more information on how the frequency sharpener works, however from reading between the lines of the press release, we have been able to piece together how we think the device works:
* The sharpener first stores ambient noise received when the wanted signal is gone, or from areas around the wanted signal. * When required, this noise can be metabolised to act as a source of 'anti-noise' (similar to noise cancelling headphones) which cancels out the incoming 'live' received noise. * This yields both a reduction in noise and, as a bi-product, generates spare energy that can be used to boost the wanted signal. * The process of noise metabolisation generates a net reduction in both the stored and received noise, as well as amplifying the wanted signal. * The system though, if it works as we suspect, requires the noise store to be 'topped up' from time-to-time. Once the noise store is empty (for example, if there is no longer noise on the received signal), the device returns to the normal state with no noise reduction taking place.
This fascinating development is something we, at Wireless Waffle are keen to keep an eye on and will bring you updates as we get them.]]>Duel-Casting
http://www.mf2fm.com/blog/index.php?entry=entry080313-074109
Finding a frequency must be one of the most fraught tasks for any prospective London pirate radio operators. The band is now so crammed with stations that there are next to no gaps anywhere. The problem is not made any easier by the BBC using lots of frequencies to infill coverage of its local stations; nor by the new wave of community stations taking to the air. Now don't get me wrong, BBC, commercial and community radio stations have a licence and are authorised and legitimate users of the FM band and as such should be respected, and nothing hereinafter should be in any way taken as non-recognition of that important fact. But the fact also remains that pirate radio stations exist and are likely to continue to exist until technology renders them irrelevant and that finding a frequency that doesn't cause interference to these legitimate users, which is a goal to be aimed for if both legal and illegal stations are to co-exist, is nigh-on impossible.
Since 2000 Ofcom (and its predecessor the Radiocommunications Agency) have been aware (through an oft overlooked study that examined the re-planning of the FM band) that there are small pockets of the FM band that could be used for low-power, limited geographical coverage transmitters in and around London, and it is the results of this study that are, to a large extent, enabling the licensing of the community stations. It's also interesting to note that many of these community stations are using frequencies that were esrtwhile used by pirates. Question: If they can now be used legally for such services without causing interference, can it be completely true that when being used illegally by pirates that the interference they cause was really so bad? Well the power of the community stations is typically less than 100 Watts and they are specifically located in areas where the frequencies they use will not cause interference, whereas the pirates on the same frequencies were often using 250 Watts or more in an attempt to cover a much wider, or a different, area. So it is quite likely that the pirates did cause interference in some areas, but clearly not in others.
One of the interesting side-effects of this use of previous 'pirate' frequencies by the new community stations is that the pirates have been forced to take action to try and maintain their coverage and listenership without causing (too many) problems to the new stations. Blasting several hundred Watts over the top of a new community stations is the perfect recipe to get busted. Hats off, therefore, to Passion FM who, having been forced off their long-time frequency of 91.8 MHz by community station Hayes FM in West London, have taken to using two different frequencies, with directional antennas, to protect Hayes FM yet maintain their service area. Passion can now be found in East London on 91.8 MHz and in West London on 97.9 MHz, thereby making an effort not to interfere with Hayes FM at the expense of having two lots of transmitters to replace each time they are taken off-air.
West Londoners Point Blank FM also deserve a mention. They are broadcasting to South West London on 103.6 MHz (and thus avoiding Life FM in Harlesden, North London and TGR Sound on 103.7 MHz in South East London) and to Central London on 90.2 MHz, having moved off 108.0 MHz where they used to cause undue interference to Radio Jackie on 107.8 MHz. 108.0 is now used by Unknown FM whose service area, being further East causes fewer problems to Jackie. Both Passion FM and Point Blank FM use the correct RDS Alternative Frequency ('AF') flag so that listeners driving around London will automatically be re-tuned to the clearest frequency - smart! Freeze FM are also 'dual-casting' on 92.7 and 99.5 MHz - it's not clear why but possibly one of the community stations yet to come on-air (Radio Ummah and Irish FM) may use a frequency near 92.7.
Pirates are often accused of not caring about interference to other stations, but the actions of these stations would tend to suggest that they do take some care - not least, perhaps, to protect themselves from an excessive number of raids from the authorities.
For the record, other pirate/community frequency clashes that will no doubt resolve themselves in the end are Westside FM (Southhall, West London) and Select-UK (Rotherhithe, South London), both on 89.6 MHz, Nu-Sound (Forest Gate, East London) and Powerjam (Battersea, South London) on 92.0 MHz, and Voice of Africa (Newham, East London) and Tempo on 94.3 and 94.4 MHz respectively.]]>Farnborough Airport Frequencies
http://www.mf2fm.com/blog/index.php?entry=entry080311-080832
To add to the previous list of frequencies for London Heathrow, Gatwick and City that went before, here is a set of active frequencies for Farnborough (ICAO code: EGLF) which might come in handy for the 2008 airshow. All frequencies are in MHz and use AM. Those shown in italics are unconfirmed but are listed widely enough to indicate that they are highly likely to remain current.
One additional frequency of note is 444.3375 MHz (FM not AM), which is a local repeat of the tower and is interestingly in the 443.500 to 445.500 MHz Ministry of Defence (MoD) sub-band. Presumably this is assignment a hang-over from when the airport undertook more military operations than it does now.]]>A Noisy Noise Annoys (Part 2)
http://www.mf2fm.com/blog/index.php?entry=entry080310-172543
<rant> Whilst visiting Oxford this week, a quick scan around the 70cm band yielded a number of repeaters that aren't normally receiveable from my regular location. Amongst these was GB3WO, near Witney in Oxfordshire. Nothing unusual there. Except that upon setting the right CTCSS tone and firing up the repeater, it was distressing to find that it remained open and was emitting a horrid buzzing/whining noise like someone was attacking it with a chainsaw. The culprit for the noise was clearly one of the many low-power licence-exempt data links used for devices such as weather stations, doorbells and the like.
I've already discussed the issue of allowing these devices into the UK 70cm repeater band and poor GB3WO was suffering the consequences (of course, the addition of a requirement for all transmissions to have CTCSS as some repeater operators have done would help the situation but would not stop the repeater input being jammed by such a device, only stop it being relayed). I've also recently highlighted the problems of being a secondary user in the 70cm band and the associated difficulties in getting interference dealt with.
But it seems that the problem is getting even worse. Repeater GB3NS in Banstead, Surrey had to be taken off-air because of these low power devices. It used to operate on RB10 (output frequency of 433.250 MHz) but was forced to cease operation when it was shown that it was causing blocking of car key-fob receivers in a nearby car-park which was in turn causing the car rescue organisations to be repeatedly called out to cars that wouldn't open or start. It was eventually shown, following practical on-air tests, that a change to a 70cm wide-space (7.6 MHz split) channel assignment circumvented the problem and eventually 'NS was re-licensed and radio hams can now use this repeater again (output is on 430.925 MHz and input on 438.525 MHz which also keeps the input frequency thankfully free of these annoyances). So this is a real example of a secondary user of a band being forced to cease transmissions to protect a tertiary or non-interference basis 'NIB' user - clearly in this band, radio amateurs are not just second-class citizens but have no rights at all.
Worst still, through an almighty lack of foresight, many of the new, digital D-Star repeaters have been given frequency assignments with an input frequency of 433.9125 MHz - just 7.5 kHz down from the 'centre' of the low-power licence-exempt band. Interference at this frequency is so bad that one repeater has already had to be taken off-air until a new assignment can be allocated, as the following press-release tells:
A newly operational D-Star repeater in the United Kingdom has been forced off the air due to interference on its input from unlicensed devices. The Radio Society of Great Britain's Emerging Technology Co-ordination Committee website reports that the GB7YD-C, 70cm D-Star system has been removed from service until an alternative frequency can be found. According to the coordinating committee, problems have been experienced at other United Kingdom 70 cm D-Star repeaters with an input on 433.9125 MHz.
The most annoying aspect of this situation is that the rules under which these low-power licence-exempt devices operate require them to accept any interference that they might experience, and not to cause any interference to licenced users of the frequencies on which they operate. Yet (in the UK at least) they are being given privileges that are greater than that of secondary users whose status is legally higher. They are certainly becoming more than just a noisy annoyance, but can anything be done to rectify the situation?
A review conducted by the European Radiocommunications Office, known as the Detailed Spectrum Investigation Phase II (DSI 2) and published on 13 March 1995 (i.e. 13 years ago) specifically recommended:
… an allocation be agreed for a general low power band at 403-404.5 MHz intended for new applications and to avoid placing new equipment at 433 MHz unless absolutely essential, the 433 MHz band to be subject to a general review at an appropriate time.
It made this recommendation because:
Amateurs in CEPT countries, particularly suffer from ISM interference in the 433.92 MHz ISM band. Similarly manufacturers of low power systems using this band are concerned at the interference potential of amateur emissions.
So this problem was identified, and a solution proposed, so long ago that it could now be something of the past. Obviously something has changed at the ERO, who now seem intent upon converting radio amateurs (who have been responsible for much of the propagational and technical research and innovation that drives today's wireless industry) into operators who can do little more than clean up the sweepings on the spectrum floor and be content with any titbits they might be thrown. Might this change of heart be something to do with the fact that at the time of the DSI, the head of the ERO was a radio amateur (OZ3SDL) but it is now headed by Mark Thomas, ex-Ofcom, a man who abolished the minimum bit-rate for UK DAB radio services and of whom Google brings up the thoughts (check for yourself if you don't believe me):
Gone to ERO in Denmark, and good bloody riddance …
So, it seems, radio amateurs are right royally stuffed! Having realised this, one thought springs to mind - why not take away lots of the amateur bands that are lightly used (30m, 12m, 23cm, 13cm, 9cm and so on) and instead allow amateurs to 'roam free', and use any frequency that is otherwise available. This is the concept being argued for by proponents of cognitive radio, whereby sensitive receivers sweep available frequencies to identify unused ones and then communicate on those - if a transmission on that frequency is later detected the cognitive radio senses this and moves elsewhere. Now amateur radio systems are almost all set up with the intention of being able to detect weak signals, and thus if any frequency appears unused at a given location, there's a fair chance that it is unused in that neighbourhood. And instead of being controlled by software, an 'amateur cognitive' transmitter would be controlled by a living, thinking person with a respect for the radio spectrum and its users. Indeed amateurs could once again be at the forefront of technology by conducting real-life assessments of the potential for cognitive radios to cause interference and thus informing the wider discussion on the use of such devices.
Maybe, just maybe, taking some action now might return amateur radio and its users to a position of innovation and respect amongst the wider radio community instead of just being viewed as bolshy CB operators? </rant>]]>Intermodulation: What's All This Then
http://www.mf2fm.com/blog/index.php?entry=entry080229-081046
It works a bit like this: Whistle or sing two notes at the same time (OK, this bit is rather difficult, but run with it for now...) In their natural form, each note will be 'pure' or 'clean' and both notes will be distinct from each other. Now whistle or sing the same two notes through a kazoo. If you've ever heard a kazoo played, you will realise that it works by distorting the sound going through it by moving a membrane (often paper) to its extremities, in essence limiting the audio and producing a square wave output from the sine wave input. The effect of any such non-linear distortion on the two notes will be to mix them together and the resulting output will be rich in all sorts of notes and sounds that weren't there in the first place.
The same can happen with radio transmissions. Any two signals passing through a non-linear device produce outputs that were not there to begin with. Though I could run through the maths and prove that such signals actually do exist, it's a little easier just to tell you what the result is.
Let's assume that the two frequencies that we are interested in are f¹ and f². The non-linearity will produce harmonics of these frequencies at 2f¹, 2f², 3f¹, 3f², 4f¹, 4f² and so on... In addition to this, it will 'mix' these harmonics together with themselves and with the original signals to produce frequencies like f¹+f², f¹-f² (these are the outputs we would want if we were using the process for mixing). Frequencies of 2f¹-f² and 2f²-f¹ are known as the 'third order intermodulation products', third because they are composed of three lots of the input signals (two of one and one of the other) and are usually the most problematic because they are closest in frequency to the original signals. Fifth order intermodulation products 3f¹-2f² and 3f²-2f¹ are the next nearest; then seventh (and every odd number thereafter). The problem gets even more complex when there are more than two signals getting mixed together. The even order intermodulation products are usually far removed (in frequency terms) from the original signals and thus cause fewer problems.
If we use real frequencies as an example, let's say we have transmitters on 80 and 85 MHz, the third order products will be at 75 and 90 MHz, the fifth order at 70 and 95 MHz. So we can end up with signals in the FM broadcast band from transmitters that were originally well outside it (and vice versa).
Intermodulation commonly occurs at the receiver (due to distortion in the sensitive amplifiers) but can sometimes occur at a transmitter, though this is more often caused by dodgy connections than by the transmitters themselves. There are stories of 'rusty bolts' on metal structures such as cranes acting as crude diodes (which are highly non-linear) producing intermodulation products if they are in strong radio fields. Because of this problem with receivers, it's not at all uncommon to receive a signal on a frequency where no signal is actually present, a 'ghost signal' as some have called it. Normally, putting an attenuator in line with the antenna will make the ghost signal completely disappear, proving that it is an intermodulation product and not a real signal (for every dB that a real signal decreases, the third order intermodulation products will usually drop by 3dB making them easy to detect).
Those responsible for choosing frequencies for transmitters in a given area, usually try to avoid putting transmissions on frequencies where the intermods would fall on the frequency of a nearby receiver, especially if the victim receiver is on the same site. Taking the example above, at a site where a receiver operates at 75 MHz, planners would usually avoid a combination of frequencies (e.g. 80 and 85 MHz) that might result in intermodulation causing ghost signals to cause interference. This is normally one of the rules employed when planning the FM broadcast band (though oddly, the frequencies for BBC's national networks are totally counter to this logic and seem to work fine), and within PMR, cellular and microwave bands great care us taken to try and avoid a ghost signal appearing in any particularly scary locations!
With radio transmitters proliferating rapidly, the problem of intermodulation is growing, especially in dense radio environments and on busy sites. Improvements in receiver electronics are able to tackle some of the problems, but as pressure increases to make efficient and effective use of the spectrum, the problem of intermodulation isn't going away and in the end may prove one of the major limiting factors in maximising the density with which radio services can be packed together.]]>Roof Raiders II - The Council of Life
http://www.mf2fm.com/blog/index.php?entry=entry080227-094915
It seems as if Ofcom has been up to its tricks again. On February 19th they announced that they had conducted another large-scale raid on London's pirate radio stations similar to one it conducted in 2005, however this time it was in conjunction with the local council authorities whose buildings are often the home of the pirates' transmitters. 22 transmitters were seized and 3 people were arrested.
I won't repeat the musings I made last time this happened concerning the probability of pirates causing the kind of wide-spread radio interference they are accused of. The latest news release from Ofcom actually plays down the interference aspects compared to their previous press release, and now plays up the difficulties and dangers that councils face due to the damage caused to property when pirates break into buildings to install their equipment. There can be no doubt that such damage does get caused when pirates break into lift-shafts and onto the rooves of blocks of flats which, if anything, only serves to highlight the desperation that such stations face and their determination to bring the kind of music they play to their audiences. Clearly mainstream radio is not catering for a whole swathe of society.
Whether or not they are working in cahoots or not, it seems that the BBC is intent on reducing the number of frequencies which might otherwise be useful to community (or pirate!) radio broadcasting. It has recently expanded coverage of 'Three Counties Radio (3CR) by adding relay stations in East Herts, South Herts and High Wycombe. However unlike the majority of other BBC local radio stations, these relays use frequencies in the bands usually used by the BBC for relays of national radio stations (90.4, 92.1 and 98.0 MHz respectively). Now the BBC are quite at liberty to use their own spectrum as they see fit, but this move away from a logically planned system to something more flexible surely suggests that a similarly flexible approach might be taken on a wider basis to allow for frequencies for more, new, innovative radio stations rather than just repeating existing ones. These three frequencies could have been used for new commercial or community stations in these areas. It's not that BBC coverage did not exist there before, it's just that it wasn't quite as good as the BBC had hoped.
I still maintain that it's quite possible that pirate stations can (and do) cause interference to legitimate users of the radio spectrum. It's clear, though, that Ofcom is only making small inroads into solving the problem and that other parties are doing nothing to assist. There has to be a long-term solution to the problem if it is ever to be solved and one commentator has suggested that when radio broadcasters have moved over to digital broadcasting (whether DAB, DRM, or something else), some of the 'digital dividend' that results should be given to low power, lightly licensed, radio broadcasting. Sounds like an eminently good idea to me.]]>Frequency Asked Questions
http://www.mf2fm.com/blog/index.php?entry=entry080131-202234
The first of these were foreign voices being transmitted on what was clearly some kind of repeater on a frequency of 430.0375 MHz. This turned out to be an Echolink node near Heathrow Airport in London that was being accessed by some Maltese radio amateurs. OK, so I should have known about this and it's certainly no mystery. But then I heard something similar on 430.0875 MHz and as far as I can make out, there are no Echolink nodes on this frequency in the UK.
The next signal was encountered when listening to the GB3FN repeater on 433.375 MHz. I kept receiving bad adjacent channel interference. In the end this turned out to be a strong voice transmission on 433.3875 MHz (12.5 kHz up from the repeater output). From my memory of the 433 MHz band in the UK, the Ministry of Defence (MoD)'s MOULD repeater network has outputs that are interleaved between the UK repeater network outputs (a bit of a silly arrangement if you ask me - just asking to be overheard), so this is likely to have been one of these - but could equally be an illegal user making use of amateur equipment with the hope of going unnoticed (in which case it was a bad choice of frequency).
Finally, I have noticed a data carrier on 437.575 MHz which seems to be on-air continuously (at least it's always been there when I've listened). At first I thought this might be a spurious response from some other frequency but checking on a second receiver the signal was there too which pretty much confirms it. This part of the 70cm band is (according to the UK bandplan) assigned for amateur television and satellites. The signal is too fluttery when mobile, and too steady when not, to be a satellite (and anyway the frequency is not listed for any amateur satellites I could find) and is certainly not television. All this means that the things I've heard could be:
* Perfectly legitimate transmissions from the primary user of the band (the MoD) * Radio amateurs operating outside the bandplan * (In the case of the 437 MHz signal) An illegal link set up in the band hoping no-one will notice
The difficulty is that, because radio amateurs are secondary users of 70cm in the UK, it is to be expected that other signals will be found in the band. However, as it would be against the terms of the operating licence to listen to non-amateur transmissions, it is effectively illegal to listen to these signals to try and identify their source - we cannot police our own frequencies. Unless the 'powers that be' (or the primary user) take an active interest in monitoring the band, it is therefore quite possible for illegal usage to go unnoticed and unreported. It is quite likely that if anyone is monitoring they put any anomolies down to radio amateurs.
Now the use of frequencies (430.0875, 433.3875 and 437.5750 MHz) by some other user does not necessarily represent a mass invasion of the band by unwanted types, but it does serve to highlight a problem. The only (legal) recourse to action is to report this usage to Ofcom and let them investigate (though by doing so you would have admitted to having heard them and thus breached your licence already). Which offers up a second problem - if the transmission is found to be legitimate, you have to pay Ofcom for the privelege of them telling you so (at least that seems to be the case - their web-site does not specifically touch the matter as amateur radio is not a business radio user nor is it domestic television or radio reception). Whether Ofcom have a database of MoD usage and thus would be able to dismiss any enquiry rapidly is unclear.
As secondary users, radio amateurs must accept any interference caused by the primary user, and this is an accepted symptom of sharing the band. But does it also mean that we are to suffer interference from potentially unlicensed and illegal users, without any clear method of redress? Or worse, be blamed for such intrusions? It seems this may be the case. For now, I thought the best thing I could do was post the frequencies here and see if it raises any interest from the UK radio amateur community.]]>
Does this tell us anything? Are the Government about to tax spectrum? Will spectrum be blamed for the economic downturn that we now seem to be staring into the barrels of? Will spectrum be the reason for the 2 point increase in income tax at the next budget? Or much worse, will poor spectrum management cause an extra 1 pence on the price of a pint of beer (except 'lager' which isn't really beer in the true British definition of the term)? Probably not, other than the fact that the estimated bill that Ofcom is going to charge the MoD to 'recognise its access' to the spectrum is £300 million, and that decisions on MoD ('and other departments' meaning the Civil Aviation Authority mainly) spectrum use are taken at cabinet level and so are visible on the radar.
We've sent an e-mail to the professor to see whether he could provide more information on how the frequency sharpener works, however from reading between the lines of the press release, we have been able to piece together how we think the device works:
Since 2000 Ofcom (and its predecessor the Radiocommunications Agency) have been aware (through an oft overlooked study that examined the re-planning of the FM band) that there are small pockets of the FM band that could be used for low-power, limited geographical coverage transmitters in and around London, and it is the results of this study that are, to a large extent, enabling the licensing of the community stations. It's also interesting to note that many of these community stations are using frequencies that were esrtwhile used by pirates. Question: If they can now be used legally for such services without causing interference, can it be completely true that when being used illegally by pirates that the interference they cause was really so bad? Well the power of the community stations is typically less than 100 Watts and they are specifically located in areas where the frequencies they use will not cause interference, whereas the pirates on the same frequencies were often using 250 Watts or more in an attempt to cover a much wider, or a different, area. So it is quite likely that the pirates did cause interference in some areas, but clearly not in others.
One of the interesting side-effects of this use of previous 'pirate' frequencies by the new community stations is that the pirates have been forced to take action to try and maintain their coverage and listenership without causing (too many) problems to the new stations. Blasting several hundred Watts over the top of a new community stations is the perfect recipe to get busted. Hats off, therefore, to Passion FM who, having been forced off their long-time frequency of 91.8 MHz by community station Hayes FM in West London, have taken to using two different frequencies, with directional antennas, to protect Hayes FM yet maintain their service area. Passion can now be found in East London on 91.8 MHz and in West London on 97.9 MHz, thereby making an effort not to interfere with Hayes FM at the expense of having two lots of transmitters to replace each time they are taken off-air.
West Londoners Point Blank FM also deserve a mention. They are broadcasting to South West London on 103.6 MHz (and thus avoiding Life FM in Harlesden, North London and TGR Sound on 103.7 MHz in South East London) and to Central London on 90.2 MHz, having moved off 108.0 MHz where they used to cause undue interference to Radio Jackie on 107.8 MHz. 108.0 is now used by Unknown FM whose service area, being further East causes fewer problems to Jackie. Both Passion FM and Point Blank FM use the correct RDS Alternative Frequency ('AF') flag so that listeners driving around London will automatically be re-tuned to the clearest frequency - smart! Freeze FM are also 'dual-casting' on 92.7 and 99.5 MHz - it's not clear why but possibly one of the community stations yet to come on-air (Radio Ummah and Irish FM) may use a frequency near 92.7.
I've already discussed the issue of allowing these devices into the UK 70cm repeater band and poor GB3WO was suffering the consequences (of course, the addition of a requirement for all transmissions to have CTCSS as some repeater operators have done would help the situation but would not stop the repeater input being jammed by such a device, only stop it being relayed). I've also recently highlighted the problems of being a secondary user in the 70cm band and the associated difficulties in getting interference dealt with.
But it seems that the problem is getting even worse. Repeater GB3NS in Banstead, Surrey had to be taken off-air because of these low power devices. It used to operate on RB10 (output frequency of 433.250 MHz) but was forced to cease operation when it was shown that it was causing blocking of car key-fob receivers in a nearby car-park which was in turn causing the car rescue organisations to be repeatedly called out to cars that wouldn't open or start. It was eventually shown, following practical on-air tests, that a change to a 70cm wide-space (7.6 MHz split) channel assignment circumvented the problem and eventually 'NS was re-licensed and radio hams can now use this repeater again (output is on 430.925 MHz and input on 438.525 MHz which also keeps the input frequency thankfully free of these annoyances). So this is a real example of a secondary user of a band being forced to cease transmissions to protect a tertiary or non-interference basis 'NIB' user - clearly in this band, radio amateurs are not just second-class citizens but have no rights at all. 
So, it seems, radio amateurs are right royally stuffed! Having realised this, one thought springs to mind - why not take away lots of the amateur bands that are lightly used (30m, 12m, 23cm, 13cm, 9cm and so on) and instead allow amateurs to 'roam free', and use any frequency that is otherwise available. This is the concept being argued for by proponents of cognitive radio, whereby sensitive receivers sweep available frequencies to identify unused ones and then communicate on those - if a transmission on that frequency is later detected the cognitive radio senses this and moves elsewhere. Now amateur radio systems are almost all set up with the intention of being able to detect weak signals, and thus if any frequency appears unused at a given location, there's a fair chance that it is unused in that neighbourhood. And instead of being controlled by software, an 'amateur cognitive' transmitter would be controlled by a living, thinking person with a respect for the radio spectrum and its users. Indeed amateurs could once again be at the forefront of technology by conducting real-life assessments of the potential for cognitive radios to cause interference and thus informing the wider discussion on the use of such devices.
Let's assume that the two frequencies that we are interested in are f¹ and f². The non-linearity will produce harmonics of these frequencies at 2f¹, 2f², 3f¹, 3f², 4f¹, 4f² and so on... In addition to this, it will 'mix' these harmonics together with themselves and with the original signals to produce frequencies like f¹+f², f¹-f² (these are the outputs we would want if we were using the process for mixing). Frequencies of 2f¹-f² and 2f²-f¹ are known as the 'third order intermodulation products', third because they are composed of three lots of the input signals (two of one and one of the other) and are usually the most problematic because they are closest in frequency to the original signals. Fifth order intermodulation products 3f¹-2f² and 3f²-2f¹ are the next nearest; then seventh (and every odd number thereafter). The problem gets even more complex when there are more than two signals getting mixed together. The even order intermodulation products are usually far removed (in frequency terms) from the original signals and thus cause fewer problems.
Those responsible for choosing frequencies for transmitters in a given area, usually try to avoid putting transmissions on frequencies where the intermods would fall on the frequency of a nearby receiver, especially if the victim receiver is on the same site. Taking the example above, at a site where a receiver operates at 75 MHz, planners would usually avoid a combination of frequencies (e.g. 80 and 85 MHz) that might result in intermodulation causing ghost signals to cause interference. This is normally one of the rules employed when planning the FM broadcast band (though oddly, the frequencies for BBC's national networks are totally counter to this logic and seem to work fine), and within PMR, cellular and microwave bands great care us taken to try and avoid a ghost signal appearing in any particularly scary locations!
The first of these were foreign voices being transmitted on what was clearly some kind of repeater on a frequency of 430.0375 MHz. This turned out to be an Echolink node near Heathrow Airport in London that was being accessed by some Maltese radio amateurs. OK, so I should have known about this and it's certainly no mystery. But then I heard something similar on 430.0875 MHz and as far as I can make out, there are no Echolink nodes on this frequency in the UK.
Finally, I have noticed a data carrier on 437.575 MHz which seems to be on-air continuously (at least it's always been there when I've listened). At first I thought this might be a spurious response from some other frequency but checking on a second receiver the signal was there too which pretty much confirms it. This part of the 70cm band is (according to the UK bandplan) assigned for amateur television and satellites. The signal is too fluttery when mobile, and too steady when not, to be a satellite (and anyway the frequency is not listed for any amateur satellites I could find) and is certainly not television. All this means that the things I've heard could be:
Unless the 'powers that be' (or the primary user) take an active interest in monitoring the band, it is therefore quite possible for illegal usage to go unnoticed and unreported. It is quite likely that if anyone is monitoring they put any anomolies down to radio amateurs.