Thursday 13 March, 2008, 08:41 - Pirate/ClandestineFinding 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.
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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.
Tuesday 11 March, 2008, 09:08 - Spectrum ManagementTo 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.
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London Farnborough Airport
Radar Standby 130.050
Operations 130.375 (TAG Handling)
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.
Monday 10 March, 2008, 18:25 - Amateur Radio<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.
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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>
Friday 29 February, 2008, 08:10 - Spectrum ManagementIntermodulation (intermod for short) is a common problem besetting virtually every radio system in existence. This particular problem occurs when two (or more) signals mix together in a non-linear device creating emissions on frequencies which are directly related to the signals being mixed together. It is the same process that is used in the mixers of most superheterodyne (don't worry if this means nothing to you) receivers, where it is a wanted outcome. Intermodulation is therefore mixing which produces unwanted outcomes.
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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.