Wednesday 19 July, 2006, 15:56 - Amateur RadioTuning around the amateur bands during recent sporadic-E openings, I regularly stumbled across voice traffic in a slavic language on 29612.0 kHz USB (upper side band). The format of the traffic was clearly not amateur, and given this, and the fact that it is right in the middle of the relatively active 10 metre FM section of the band but was not FM made me wonder what it might be. Enjoying a mystery I managed to make a short recording of some of the traffic [304kB windows media file] at around 14:15 GMT on 17 July 2006.
Not speaking whatever language it is I posted a message on the Utility DXers Forum message board. What a great set of individuals! Within half a day I'd received a response that the most likely culprit of the signal I heard was the Russian Air Defence (the Protivovozdushnoy Oborony or PVO) who track all aircraft through their airspace giving a continuous read-out (quite literally!) of the sector quadrant and the corresponding coordinates of each aircraft.
It turns out that the PVO also run a morse code based variant of this service which has been given the designator 'M21' by Enigma 2000, the group who monitor 'number stations' such as the Lincolshire Poacher. Looking back through their records shows that a station called 'V21' was active on 30220 kHz in the past. I don't know whether V21 is the voice equivalent of M21 or not, but if it is, presumably they have now moved to 29612 kHz for the time being in a random attempt not to be intercepted. Bit of a silly exercise really given where they've plopped themselves. It makes you wonder whether military radio operators have any idea what goes on around them. To change frequency in order to maintain some kind of covertness is vaguely clever, but to have changed into the middle of a fairly active section of an amateur band is just a bit daft.
I also dropped a note to the UK representative of the IARU Region 1 Monitoring Service, who record and monitor all 'intruders' into the amateur bands. Don't know whether this will make their offender list or not, but it's nice to be on the discovery end of something rather than just following the lead of others!
Thursday 13 July, 2006, 08:36 - LicensedDriving along the M4 motorway yesterday evening, I was trying to listen to the news on BBC Radio 4. As usual, the RDS Alternative Frequency (AF) service was doing a good job at re-tuning my radio to a new transmitter once I disappeared out of the coverage of the one I was tuned to. Not long into my journey I noticed that my radio was struggling to find a clear frequency and that even the best it could find was swamped by interference. Having experienced this before, I wondered whether there might be Sporadic-E propagation around. A quick tune to the low end of the FM band confirmed there was.
Sporadic-E propagation exists when the sun's radiation ionises layers of gas in the 'E'-layers of the ionosphere. These ionised layers refract radio signals, often up to high VHF frequencies, enabling signals from far afield to be easily received. Usually such ionisation forms in relatively small 'clouds' such that the signals which are refracted in any one area might differ significantly from those received in another. Often from any given point the signals received are from one specific area.
It can be fun tuning around the FM band at times when Sprodic-E is active as stations from typically 1,000 or more miles away can swamp local stations. With the advent of RDS it's also relatively easy to identify the location of the transmitter you are hearing. A good place to listen (in the UK) is the bottom end of the FM band (87.5 - 88.0 MHz) where, unless there is a local RSL station active, there tends to be nothing but static. This is also the frequency range that is first to respond when the Sporadic-E clouds are around. Yesterday, for example, the highest frequency on which I could hear foreign stations was around 90 MHz.
On my journey yesterday I logged the following stations (the location has been added in after a brief web-search). The map on the right shows the path between me and these stations - you can pretty much see that there must have been 2 'clouds' (one over the Bay of Biscay and the other somewhere near Austria) and how the areas where signals were received from are similar:
87.6 Radio Speranza (Pescara, Italy)
87.7 Antena 1 (Mendro, Portugal)
87.7 HRT HR-1 (Pljesvica, Croatia)
87.8 RNE-3 (Baza, Spain)
87.9 Canal Extremadura (Merida, Spain)
88.0 RNE-5 (probably Huelva, definitely Spain!)
There was also a very strong Spanish station on 88.5 MHz but it never quite registered its RDS and as such remains a mystery! There were also other stations fading in and out, many Spanish, some Italian and Portuguese and a few French, however none were receivable long enough to register their name or RDS.
None of this is particularly amazing. Sproadic-E propagation is common, especially during the summer months. However, it is most common at the height of the sun's 11 year solar cycle. At the moment, though, the sun is at the lowest point of its 11 year cycle. And radio amateurs monitoring the 6 metre amateur band have also regularly experienced enhanced propagation over the past 2 months. This shouldn't really occur. It might be that the sun has been a bit more lively than it should have been - I'm useless at interpreting the complex solar data - but those that do can tell me that the recent conditions are not normal. So what is going on? My own theory, for what it's worth, is that propagation conditions are not just a function of the sun's activity but are also connected to weather conditions here on Earth. This year in the UK we've had at least 3 weeks of weather where temperatures were 10C above the seasonal norm. Could global warming also be playing havoc with radio propagation? If anyone has a few hundred thousand pounds to spare, I would be happy to investigate further!
Tuesday 27 June, 2006, 08:13 - Radio RandomnessLooking back at the analysis of WiFi antenna performance I conducted recently, it struck me that to maximise the performance of a Wireless LAN there are two factors at play. One is the strength of the signal, clearly enhanced by the higher gain antenna. The other is the amount of background noise. It is not for nothing that the quality of the link from the wireless access point to the computers is measured in terms of 'signal to noise'.
Wireless LANs (at least variants 802.11b and 802.11g) operate in the frequency range 2400 - 2483.5 MHz (this extends to 2495 MHz in Japan only, and not all of the band is available in all countries, noteably France which does not have access to frequencies below 2450 MHz), known as the 2.4 GHz band. This band is not exclusively set aside for use by WiFi equipment, but is in fact shared with many different users. The transmitters of each of these users will increase the overall background noise in the band and if strong enough, will cause direct interference.
The 2.4 GHz band is classified as an 'Industrial, Scientific and Medical' or ISM band, meaning that it can be used by a range of non-communicating radio transmitters such as microwave ovens, industrial paint and biscuit drying machines and medical thermal heating devices. These ISM devices typically use very high powered transmitters (900 Watts plus even for a domestic microwave oven) and thus have the potential to cause enormous amounts of interference in their immediate proximity. However the band is also shared with a number of other users including the military, aeronautical radars, wireless video cameras (both professional and domestic), radio amateurs and a whole plethora of low-powered devices in particular Bluetooth. And, of course, other WiFi systems.
If we want to maximise the range of our Wireless LAN installation, it is therefore important to try and select a frequency (channel) which contains the lowest possible number of other users and thus is likely to have the lowest possible amount of interference or noise present.
Wireless LANs, based on the 802.11b/g standard offer us the option of 13 different channels depending on which country we're in (14 in Japan). However these channels are not independent of each other, they overlap very significantly. The 13 European channels have centre frequencies from 2412 to 2472 MHz inclusive, spaced at 5 MHz intervals. However the bandwidth occupied by a transmitter is 22 MHz. This means that transmissions on channel 1 actually extend from 2401 to 2423 MHz. This overlaps with transmissions on channels 2, 3, 4 and 5, meaning the next 'clear' channel is number 6. Thus if our neighbour is using channel 1, we should only use channels from 6 upwards if we are to avoid direct interference. Likewise channel 6 overlaps with all channels from 2 to 10! Thus if we are to avoid interference we can only really use 3 channels, either 1, 6 and 11, or if we have the option, 1, 7 and 13 which will give a bit more separation and hence lower interference. The diagram below attempts to illustrate the situation.
But is there any reason to select any of these channels over another? Which ones might have the lowest inherent level of interference and noise from the other sources in the band? Well of the other users, Bluetooth uses the whole band, so there's no particularly better place to go to be to be protected from this. The military and radio amateurs typically use frequencies from 2400 to 2450 MHz, affecting channels 1 to 10 inclusive, however activity is rare. Certain short-range applications (in particular equipment for the detection of movement as well as high power 'RFID' tags) use 2445 - 2455 MHz affecting channels 6 to 11 inclusive (though the effect is most profound on channels 7 to 10). Emissions from microwave ovens (and most other ISM equipment) centre around 2450 MHz, and as such would also affect channels 6 to 11. So from this simple analysis, it would seem like channels 12 and 13 are the most likely to be clearest of interference.
However, by far the most likely source of interference is... other Wireless LAN users. So which channels are most commonly used by other WiFi bods? On a recent train journey from Manchester to London, I let my laptop and trusty 'Netstumbler' software scan the band for LANs, to see which channel was most commonly in use. The results are shown below:
The most commonly used channel was 11 - given the earlier analysis about the channels least likely to be interfered with, it is perhaps not surprising that this is the default channel on which most equipment operates when initially purchased. Equally, it seems that most people give no further thought to the channel number and leave it on 11. Channels 1 and 6 were also relatively well used - again not surprising given that 1, 6 and 11 are the most common interference free line-up.
So, after all this, which channel is most likely to present the lowest overall noise and interference? If there are unlikely to be any other WiFi users in the neighbourhood, I'd choose channel 12 or 13. If there is likely to be lots of other WiFi use nearby, channel 1 is the best bet.
Tuesday 20 June, 2006, 12:11 - Radio RandomnessHaving gone on about how to extend the range of a wireless LAN using a high gain antenna, the need suddenly arose for the range of my own WiFi connection to be extended so I though I would purchase a 9dBi antenna to see what happened. Being a hardcore engineer, I wanted to try and see whether this antenna really delivered the gain over the original 2dBi antenna which it promised.
Antennas and laptop in hand, I used a programme called 'Netstumbler' to record the signal to noise of the reception of my WiFi connection over about a 1 hour period, changing between the standard 2dBi and the higher-gain 9dBi antenna about half way through. The first graph (below) shows the received signal strength over the period. The orange line is the rolling average over a 2 minute period. Without any further analysis, it is immediately apparent that the signal strength received when the 9dBi antenna is installed is higher than with the 2dBi antenna, indicating that it did have some gain as promised.
A quick calculation of the average over the 2 periods showed that the average signal to noise with the 2dBi antenna was 28.8dB, whereas the average signal to noise with the 9dBi antenna was 34.6dB, an increase of 5.8dB - not quite the 7dB increase that should in theory materialise, but not bad nonetheless. For the statisticians amongst you, the standard deviation in both cases was remarkably close at 3.96 and 3.92 dB for the 2 and 9 dBi antennas respectively, indicating that the signal was equally stable (despite the obvious variations) in both instances.
A further analysis of the results (above) shows the distribution of signal strengths produced by the 2 antenns. In this case it is easy to see that the signal produced by the 9dBi antenna (in orange) is consistantly and significantly better than that of the 2dBi antenna (in dark blue).
Whilst the experiment was less than scientific, taken at face value, it does suggest that the signal produced by the 9dBi antenna is a worthwhile improvement over the signal produced by the standard 2dBi antenna as supplied with most WiFi routers. The claims of a '3 to 4 times' increase in the range of coverage have not been tested - maybe I'll do that one day soon.