Wireless Waffle - A whole spectrum of radio related rubbish
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.
add comment ( 1865 views )   |  0 trackbacks   |  permalink   |   ( 3.1 / 4667 )

Pennies from Heaven?signal strength
Wednesday 9 April, 2008, 15:40 - Spectrum Management
Posted by Administrator
It seems it's not just 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.

spectrum pricingDoes 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!
add comment ( 1066 views )   |  0 trackbacks   |  permalink   |   ( 3 / 4783 )

Frequency Sharpenersignal strength
Tuesday 1 April, 2008, 10:24 - Radio Randomness
Posted by Administrator
frequency sharpenerYou've no doubt 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 the results of a 5 year study that they have been conducting in which 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 in the research, the frequency sharpener could be being included in many new digital radios of the future as soon April of next year.

In the University's press release, Prof. Ali Lo 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.

desert radioWe'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.
add comment ( 1553 views )   |  0 trackbacks   |  permalink   |   ( 3.1 / 40 )

Duel-Castingsignal strength
Thursday 13 March, 2008, 08:41 - Pirate/Clandestine
Posted by Administrator
100 watt pirate transmitterFinding 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.

some pirates betterSince 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.

walk the plankOne 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.

rubbish pirateWest 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.
2 comments ( 2135 views )   |  0 trackbacks   |  permalink   |   ( 3 / 3435 )

<<First <Back | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | Next> Last>>