Wireless Waffle - A whole spectrum of radio related rubbish
Height versus Powersignal strength
Tuesday 10 August, 2010, 05:17 - Broadcasting
Posted by Administrator
One of the most common questions that the Wireless Waffle team are asked by those setting up radio transmitters is, "How much power do I need to cover an area X miles wide?". Such a question is virtually unanswerable as there are so many factors to take account of including the frequency of operation, the topography of the area, the kind of structures (buildings, trees) which are in the required coverage area, what kind of receivers people are using and much more. The observant will note that these factors are not ones which can necessarily be changed by the person operating the transmitter - unless they fancied chopping down a forest for example. What can be changed at the transmitting site are two relatively simple factors: the height of the antenna, and the power of the transmitter.

Such discussions therefore end up focussing on how high the antenna needs to be and what power the transmitter should be. But which is most effective in increasing coverage: height or power?

Let's tackle height first. Assuming we are trying to provide a signal over the earth and that there are no obstacles at all and that the earth has no undulations (hills and so on), then the range of a transmitter can easily be calculated from a simple line-of-sight rule. This tells us that for a particular height above the ground, the horizon (and thus the edge of the coverage area) will be a specific distance away. One oddity in this is that radio signals tend to get defracted a little by the Earth's atmosphere which has the effect of making the planet appear slightly less curved and thus extends the radio horizon about a third beyond the optical horizon. The chart below shows the optical and radio horizons for a transmitting antenna mounted at a certain height.

optical radio horizon

With an antenna about 10 metres above the ground, the radio horizon is about 10km away. If the height of the antenna is increased to 50 metres, the radio horizon increases to about 24 km - a very healthy improvement. It's perhaps worth noting that 'height above ground' could be generated by raising the height of the antenna, or by mounting it on top of heigh point (eg a hill).

Increasing the transmitter power also increases coverage, but not in quite the same way. Getting signals much beyond the radio horizon relies on various odd propagation techniques including refraction, defraction and scatter. In free space, increasing the power by a factor of 2 will increase the distance at which the signal is of equal strength by the square root of 2. So, if the signal is 30 dB at a distance of 10km, increasing the power by a factor of 2 will move the point at which the signal is 30 dB to a point approximately 14km away from the transmitter. Sadly, the Earth is not generally a 'free space' environment and signals fall away much quicker than this, even before the horizon is reached. The chart below shows a simulation of coverage for different transmitter powers, assuming an antenna height of 20 metres.

distance versus power

The distance to the radio horizon for a 20 metre heigh aerial is 15 km and in 'free space', in this example, this is reached by a power of 10 Watts. For the 'real life' example, 10 Watts only achieves a distance of around 10 km because of the fact that the Earth is not a free space environment. To achieve 15 km in 'real life' requires a power of nearer 50 Watts. What is immediately clear is that enormous increases in power are required to extend coverage. Even with 100 Watts, in our theoretical example, the distance acheived is still less than 20 km.

Increasing the height of the transmitting antenna is therefore, theoretically, a much more effective way of increasing coverage than turning up the power. Of course, it's not always possible to put up a high antenna, and in this situation more power is clearly better, but in general height wins every time. To show the difference, the map below (made using Radiomobile) shows the coverage for a transmitter nominally located in the centre of Oxford. It's animated (oo-err!) and cycles through the coverage which would be acheived for:

* A 10 Watt transmitter with an antenna height of 10 metres
* A 40 Watt transmitter with an antenna height of 10 metres
* A 10 Watt transmitter with an antenna height of 20 metres

coverage map example

high receptionThe coverage achieved in the latter two cases is very similar, however in the map with the higher antenna, the coverage is more 'solid' than that with higher power. If this were a radio station, the higher antenna would provide a more reliable signal, especially for people on the move, than the lower antenna with higher power. The extent of the advantage of height over power means that it is generally more beneficial to identify an elevated transmitter site towards the edge of an area where coverage is required, rather than settle for one which is nearer the centre but lower. A transmitter on a hill overlooking a town will provide more solid coverage in the town for the same transmitter power than a site in a town centre. Hopefully, those now considering how best to maximise their coverage will think beyond Watts and consider that factor well understood by estate agents, location, location, location.
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Super Signal Holiday HF Antenna Apparel (Part II)signal strength
Tuesday 27 July, 2010, 17:20 - Radio Randomness
Posted by Administrator
bikini hf antennaLast summer, here at Wireless Waffle, we came up with a design for an increadible piece of beach-wear for the short-wave listener which we cristened the 'Wireless Waffle Super Signal Holiday HF Antenna Apparel'. Not only has this become the must have item for improving reception whilst soaking up the sun, devotees have coined the nickname, 'SuSi' and it's an idea that has clearly caught on. At the end of our revelation of this unbelievable breakthrough in summer attire last year, we asked you to submit your own photos of the 'SuSi' which we would then share with other Wireless Waffle readers. And submit them you did! Here we present the 2010 SuSi Snapshot Selection. Together with the original device, it is enthralling to see so many variants in use, however we have our doubts about how effective some of the modified versions might be - so together with your photos, we have also included our view on how well the device pictured might perform.

wireless beach wearThis first picture was sent in by Jim Thrisby of Humberside and was taken in his laboratory with the device under test conditions, rather than out in the field. The unit in question has been modified to include a double, side-mounted dipole which will imbue it with some directionality. The use of an interconnecting cross-bar, however, will act as a short-circuit at higher frequencies and may limit the broadband functionality of the device. The inclusion of three distinct connection pads does, however, offer the wearer some flexibility in adopting the best position for reception whilst allowing the interconnecting wires not to get in the way.

hf antenna bikiniThis next image came from Tyrone Mulligan from South Carolina and shows a similar variant to that sent in by Jim. In this case, however, the horizontal interconnecting cross-bar is missing and its omission should ensure a wider frequency response. The three interconnection pads are also present, however the triangular (instead of square) shape and blue paint used to make them 'look pretty' may introduce a high impedance into the connection which is undesirable. The use of a conductive gel or paint will ensure a more solid contact. The modified SuSi is shown in use under good conditions and is clearly being well received by the wearer.

short wave bikiniTuning of a SuSi should not require any user intervention, however some experimenters such as Dave Brookes of Sydney, Australia, have suggested that some manual adjustment to the position of the various connecting wires can improve the clarity of reception. His photo shows a relatively standard SuSi but in which the support structure has been angled so as to increase the capture of incoming waves. Dave tells us that by using the device in this way, it is possible to receive many more short-waves and even some medium and long waves, but that effectiveness was reduced as it was too easy to get 'Chile' (or that's what we think Dave said).

cornish witch bikiniThis photo, which arrived by e-mail from Harman Tallow of Newquay, Cornwall, is apparently an attempt to use some Cornish witchcraft to improve the device's effectiveness. Whilst there addition of the two 'tuning coils' may improve the reception in some directions they also act to obscure significant amounts of the underlying support structure and may even weigh it down to produce a highly undesirable 'sag' in performance at some wavelengths. According to Harman, the field trials were relatively successful in that it was easier to mount the device on the support structure compared to the original design, but that overall, the reception was disappointing.

bikini phased arrayWe were particularly impressed by the efforts of Damian Hextonwick. Damian doesn't tell us where he's from but does indicate that using multiple devices in the form of a 'phased array' produces one of the largest increases in the achievable range of the device that he has witnessed (though we once again note the erroneous use of the bandwidth restricting horizontal cross-strut). His attempt to join four such devices together as shown in his picture, produced sufficient voltage to excite receivers which were at a significant distance from the array. Whereas it is necessary to connect directly to a single device to get its benefits, standing in close proximity to multiple interconnected devices has impressive results.

mesh satellite dish bikiniFinally, Heinz Wiedemann of North Germany has attempted to take the concept of the SuSi one step further and produce a device which can be used for satellites. By expanding upon the concept of a mesh dish, he has produced this mesh SuSi which has an integral 'low navel block' (LNB) at its lowest point which focuses incoming signals. Unfortunately, the alignment proves very critical and without careful hands-on positining of the LNB, the supporting structure becomes badly distorted leading to highly unsatisfactory reception. Heinz does add, however, that the hands-on nature of this variant of the SuSi has many distinct benefits which he then refuses to scientifically quantify, rendering the results of his experiments somewhat suspect.

Overall, we have been highly impressed by the look, feel and ingenuity of the various models of SuSi that have been submitted to us. Please keep up the experiments and remember to send pictures your results to us. We can't wait to see what the next year of experimentation may bring and to share the joyous fruits of your labours and to enjoy the summer sunshine wherever you may be.
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Speakin' Beaconssignal strength
Sunday 13 June, 2010, 11:43 - Radio Randomness
Posted by Administrator
Most short-wave listeners would probably love to know whether reception conditions are good or not at any given time on any particular band. One way to do this might be to use an on-line tool, such as the one shown on the right, which tries to interpret solar conditions (eg sunspot numbers) to provide an indication of whether specific frequencies are likely to perform well. This is a good start but gives no idea of from which areas signal are being received. Knowing that frequencies below 10 MHz are suject to 'fair' propagation tells you little about whether this is to the East, West, North or some other direction.

Another way, therefore, might be to try and tune in to radio broadcasts from specific areas to see what can actually be received. This works pretty well and tools like that provided at short-wave.info can help you find where signals are being transmitted from and thus what else you might be able to hear.

But radio broadcasts are not the only short-wave transmissions which can be regularly received. There are many other signals which can give indications of propagation. Radio amateurs have networks of worldwide beacons. These can be found on frequencies of 14100, 18110, 21150, 24930 and 28200 kHz and each beacon sends its callsign in morse code at a relatively high power (100 Watts) and then decreases the power down to a few milliWatts. Once it is done, another beacon uses the frequency, with the frequencies time shared between them. These beacons (and other amateur beacons which operate) provide an alternative means of testing propagation. Of course if you can't read morse code (CW) then they are of little use. Also, frequencies below 14 MHz are not well served and though 100 Watts is a relatively strong signal for radio amateurs, it is not in the kiloWatt region which radio broadcasters use and thus may be more difficult to hear for the average short wave listener.

At Wireless Waffle it was realised that it would be useful if there were other means of checking propagation which didn't rely on knowing morse code, covered more frequencies, was easy to receive, and would give an idea of in which direction signals were coming from. Step up the the challenge volmets! met report londonA volmet is a radio broadcast of weather information (meteo in French) for aircraft (vol is French for flight). Volmets exist in many countries around the world and there are several on short-wave which use relatively high power transmissions (normally between 1 and 10 kW) on various frequencies ranging from 2.8 to around 15 MHz. Most frequencies are shared between multiple volmets who take it in turn to broadcast local weather conditions for 5 minutes and then pass the frequency on to the next station (sound familiar?)

There is a relatively up-to-date list of active, inactive and planned volmets available on-line. One evening recently a receiver was set on 6676 kHz. This is the frequency used by a series of Asian volmets in Australia, India, Thailand, Pakistan and Singapore, each of which uses the frequency for 5 minutes at a time, suprisingly at least three of these were clearly heard (though there was some interference from the Echo Charlie band radio pirates who also use these frequencies!) The following evening on the same frequency, nada, nix, nothing. None of them were audible. An almost perfect example of the use of these stations as propagation beacons.

newyork to hatyaiAn even odder one but of startling usefulness is the frequency of 13270 kHz. This is used by two volmets, one in the USA (New York) and one in Canada (Gander). It is also used for a digital HF radio system for aicraft to report their position when outside of radar coverage called ACARS by a station in Hat Yai in Thailand. Tuning to this frequency recently, both the volmet and the ACARS service could be heard! The New York volmet weather man was churning out temperatures and dew points and the like, whilst over the top came the occasional 'beep fluff' noise of ACARS. It seems a little odd that two aeronautical services would be put on the same frequency but when you consider that Hat Yai is 9190 miles from New York (as the crow flies - though he would get pretty cold whilst going over the north pole) then the probability of interference is probably too small to worry about under normal circumstances. It makes a very handy propagation beacon though - almost around the whole world on one frequency!
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GiffGaff RiffRaffsignal strength
Sunday 30 May, 2010, 14:23 - Much Ado About Nothing
Posted by Administrator
fgaffTravelling on the London Underground last week, Wireless Waffle was intrigued by an advertisement for GiffGaff which marketed itself as 'the mobile network run by you'. Visions of self installed cell sites connected back to the network infrastructure by home (or business) broadband connections flashed in front of the eyes. Notions of a new style of organic mobile network where coverage is provided by the users themselves caused a twitch of our technology antennae.

Sadly, upon visiting the GiffGaff web-site it turns out to be yet another Mobile Virtual Network Operator (MVNO) like Virgin Mobile, Tesco Mobile and others - and in this case, one which uses O2's mobile network. The 'network run by you' is simply an incentive to get you to get your friends to use GiffGaff SIM cards, a marketing ploy and not a new kind of network at all.

But would it be possible to construct a 'network truly run by you'? Believe it or not, the technology to do this already exists:

Firstly, there are phone handsets which use a WiFi connection to connect to the Internet and then using a suitable service provider, act as extended range cordless phones. They are extended in that if you take them with you to another place where there is a suitable WiFi connection, they log back on to the network and enable you to make and receive calls. Unlike a true mobile service, however, there is no hand-over between WiFi connections meaning that if you move in or out of coverage the call you are making will drop out. But with a WiFi phone you can sit in a coffee shop and make and receive calls as if you were at home - whether or not there is really any value in doing this when you could use an everyday mobile phone is a moot point.

In Japan and some other Asian countries, there is a mobile phone technology known as the Personal Handyphone System (PHS for short) which is technologically similar to the DECT digital cordless phones used in Europe. PHS (and it's 3G variant known as XGP) use short range, cordless phone like base stations which connect back to a central newtork control centre via a standard telecommunications connection (historically an ISDN circuit). The resulting network is similar to that of WiFi phones in that you have a home cordless phone, but which you can take with you to other houses where coverage exists (eg other PHS subscribers). The main difference between PHS and WiFi phones is that there is proper hand-over between cells such that you can walk down a street where there is PHS coverage and continue to make your call. Indeed, by adding a few cell sites at key locations and relying on users to extend coverage by their home connections, reasonably large coverage has been achieved without the operator needing to put expensive cell sites everywhere. One of the main problems of PHS is that cell sizes are relatively small due to the low power nature of the technology, but no smaller than that of a WiFi connection. It is now also rather antiquated technology and it is not clear whether operators will upgrade to XGP or some other technology - PHS subscriber numbers are on the decline.

femto cellMore recently there has been a development typified by services such as Vodafone's Sure Signal. These are miniature cell sites known as 'femto cells' which plug into your broadband connection and provide (in this case) 3G coverage in the immediate area. Having a 3G cell site in your house (or office) gets around the problem of coverage blackspots in a big way. Femto cells are fully working parts of the mobile network to which they are connected and therefore you can seamlessly roam in and out of coverage as you leave home (as indeed you can with PHS). 3G femto cells allow you to use your 3G phone or data card at home and in nearby areas.

Now... imagine a situation where a whole street had installed femto cells such that there was unbroken coverage as you walked or drove along it. There would be no need for coverage provided by any official network operator. In this case all the network coverage would be provided by the users. All that would be required would be some organisation to provide the central functions such as allocating phone numbers and managing mobility (handing over calls between cells) and hey presto! - truly a 'network run by you'. The only thing stopping anyone from launching such a service is lack of (harmonised) radio spectrum. In theory, someone could launch a 'network run by you' in the same, unlicensed spectrum, used by WiFi devices but the handsets they would need would be proprietary and thus expensive. If they could roll-out 3G femto cells instead, the handsets would be available off the shelf. But all of the 3G frequencies have already been assigned to licensed operators who are, in no way, going to allow some upstart to usurp some of their valuable spectrum.

http   fgaff com banner 06 250x52Which is kind of where organisations such as Google and Microsoft step up to the lectern with their desire to operate 'WiFi 2.0' in spectrum known as whitespace. Whitespace spectrum is that which lives inbetween terrestrial television transmitters but which cannot be used for more television without causing interferenece. It could be used for lower power short-range services though - such as femto cells for example. So combine the idea of femto cells, widely available, licence-free spectrum with big money backers, and perhaps the concept of a network 'run by you' is closer than might have first been apparent.
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