Wireless Waffle - A whole spectrum of radio related rubbish

Intermodulation: What's All This Thensignal strength
Friday 29 February, 2008, 08:10 - Spectrum Management
Posted by Administrator
Intermodulation (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.

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.

intermodulation diagramLet's assume that the two frequencies that we are interested in are and . 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).

inter mod ulationThose 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.
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Roof Raiders II - The Council of Lifesignal strength
Wednesday 27 February, 2008, 09:49 - Pirate/Clandestine
Posted by Administrator
lara croft roof raiderIt 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.

spondwick by ormWhether 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.
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Stereo-Phonicssignal strength
Friday 8 February, 2008, 19:10 - Licensed
Posted by Administrator
radio bognor logoThe Wireless Waffle team used to supply VHF FM radio transmitters for Restricted Service Licence (RSL) stations in the UK. The licences allowed a maximum transmitter power of 25 Watts and a maximum antenna height of 10 metres above the ground. As such, the range of such stations was normally very limited compared to regular FM stations who used powers typically in excess of 400 Watts and with much higher antennas. To maximise the range of the RSLs, it was best to identify and use transmitter sites which were as high above the surrounding land as possible but in some areas the land was so flat that no such sites existed.

One question which was constantly asked by the operators of such stations was, "Should I broadcast in stereo?". On the face of it, stereo is the norm for FM broadcasts and most stations believed that if they weren't in stereo they would be seen somehow as inferior. However, what most failed to take into account was the fact that in order to receive a good quality stereo signal, the signal strength has to be 10 times (20dB) higher than that required to receive a good quality mono signal. This translates into a reduction in coverage area of 100 times, i.e. the coverage in stereo is only a hundredth of the coverage area achieved by the same transmitter in mono (ignoring topographical issues such as terrain and buildings).

Why is there so much difference? The answer lies in the bandwidth which a stereo signal occupies compared to a mono signal. The audio bandwidth of an FM transmission lies in the range of 30 Hz to 15000 Hz (15 kHz). However the way that a stereo signal is generated expands this bandwidth to 53000 Hz (53 kHz). (Don't get this confused with the RF bandwidth of the signal which is 180 kHz for mono and 256 kHz for stereo).

fmspectrumHow does the audio bandwidth extend to 53 kHz? Well the mono signal which is made by adding the left (L) and right (R) channels together - expressed as L+R - is transmitted as usual so that the resulting signal is compatible with mono receivers. The difference between the left and right channel (L-R) is amplitude modulated onto a carrier at 38000 Hz (38kHz). This produces a signal which occupies the audio frequencies from 23 to 53 kHz - above the standard audio range and thus inaudible on a mono receiver.

A 'pilot' tone which is a low-level tone at 19000 Hz (19kHz) is also added to this signal and then the whole lot is sent to the FM transmitter. In a stereo receiver the presence of the pilot tone triggers the stereo decoder to recover the original signals. The left channel is reproduced by adding the mono to the stereo difference signal (L+R+L-R=2L) and the right channel is produced by subtracting the difference signal from the mono signal (L+R-(L-R)=L+R-L+R=2R).

The noise received by an FM receiver increases as the square of the bandwidth of the modulated/demodulated signal and as such the increase in noise (i.e. the decrease in signal to noise) for a stereo signal is (53/15)² or 12.5 times. Some of this increase is counterbalanced by 'pre-emphasis' where higher audio frequencies are enhanced before transmission and then reduced at the receiver, reducing the effect of some of the noise. The resulting improvement leaves a difference of the factor of 10 mentioned above.

hisssThe question of whether to broadcast in stereo for a low-power RSL (or indeed a pirate!) FM station is therefore a question of quality and coverage. If you have a high site and can expect that most of the listeners you wish to target will receive a good strong signal, stereo is great. If not (which is usually the case), using mono ensures your coverage is maximised. Of course, in fringe stereo areas where the signal becomes 'hissy' the listeners could always switch to mono, but how many people actually know that this solves the problem, let alone know where the mono/stereo switch on their receiver is?!

There are some pirate stations who transmit only the pilot tone so that the stereo 'light' on receivers comes on, which looks nice, but don't actually transmit in stereo. This is the worst possible case, as all it will serve to do is reduce the coverage, without giving any additional benefit to the listeners!
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Frequency Asked Questionssignal strength
Thursday 31 January, 2008, 20:22 - Amateur Radio
Posted by Administrator
A quick scan of the 430-440 MHz (70cm) amateur band in the UK usually reveals very little, other than the occasional repeater and one or two low-power licence exempt data transmitters (for weather stations and so on). But the other day I stumbled across a number of transmissions in the band which puzzled me.

transmitter1The 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).

transmitter2Finally, 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. police radioUnless 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.
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