Tuesday 11 September, 2007, 14:07 - Radio RandomnessOK, so it's not as snappy as 'spot the ball' but the idea is the same. Look at the picture below and see if you can spot the HF antenna. Clue: Look for the 'x's.
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Posted by Administrator
A bit silly perhaps, but the thing of interest is that this building is the Admiralty Building as seen from Horseguards Parade in Central London and that those really are HF antennas (see the expanded picture below). Whilst it's easy to dismiss good old fashioned short wave communications as outdated, especially in the age of satellites and mobile phones, it's very satisfying to see that those who need some assuredness of communication think it worthwhile to defile historic buildings in the centre of an area of tranquility and beauty with whopping great big, ugly HF aerials.
It doesn't necessarily follow, but it might be fair to presume that the people who did this (let's call them the 'military' for want of a better word) will be keen to ensure continued, low-interference access, to HF spectrum for some time to come. Which has to be good news for short-wave listeners and radio amateurs alike.
Friday 31 August, 2007, 10:48 - Pirate/ClandestineSince the late 1990's rumours have abounded that it was possible to hack into American military satellites and use them for wide area communication. The satellites, originally the 'FleetSatCom' newtork (often abbreviated to FLTSATCOM) use basic FM modulation and have uplinks in the area of 300 MHz and downlinks in the area of 260 MHz. Stories went that tuning in to the downlinks it was possible to hear illegal pirates, from Brazil in particular, who were usurping these US military satellites to use for wide-area communications. It was also said that 'Smile 93.9 FM' (rumoured to be from Manila) was using one of the channels as a studio to transmitter link and could often be heard on the downlink frequency of 269.950 MHz.
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Posted by Administrator
This seemed a little far fetched and unbelievable: How could one of the world's most super-sophisticated armed forces allow their multi-million dollar military hardware be taken control of by such an unsophisticated enemy armed with nothing more than a simple UHF FM transmitter? Using a simple VHF/UHF receiver and a bog standard roof mounted VHF/UHF antenna, I set out to try and debunk the myth.
Within seconds I was listening to a conversation between two likely sounding chaps on a frequency of 255.550 MHz. Next I stumbled across more voice traffic (definitely in Portuguese, the language spoken in Brazil) on 258.650 MHz. And before long I found more voice traffic on 253.850 MHz. Intrigued that this long reported phenomena was still in evidence I did a bit of digging on the internet to find out more.
The original FleetSatCom satellites which were launched in the late 1970's and early 1980's are no longer operational. They were initially replaced by satellites known as Leased Satellites (Leasat) which have also since been replaced by the UHF Follow-On series of satellites, ironically acronymised as UFO. The UFO satellites continue to provide the same communications capabilities as the earlier ones but with somewhat higher transmitter powers, making reception of them fairly straightforward.
A bit more digging uncovered military standard MIL-STD-188-181A which describes the interface specification for the satellites (i.e. the technical requirements for equipment used to access them) and in it we find a list of the uplink and downlink frequencies used. All the frequencies I could hear are in group 'Charlie', now known as group 'Quebec' (Q) on the UFO satellites. Group Q comprises the following 25 kHz wide downlink frequencies (uplink frequencies are 41 MHz higher):
Q1 250.650 MHz (Fleet Broadcast)
Q2 252.150 MHz (Navy Channels)
Q3 253.850 MHz
Q4 255.550 MHz
Q5 257.150 MHz
Q6 258.650 MHz
Q7 265.550 MHz
Q8 267.050 MHz
Q9 269.450 MHz
Q10 269.950 MHz
Q11 260.625 MHz (DoD Channels)
Q12 260.725 MHz
Q13 262.125 MHz
Q14 262.225 MHz
Q15 262.325 MHz
Q16 262.425 MHz
Q17 263.825 MHz
Q18 263.925 MHz
So far, I have heard sporadic voice traffic on channels Q2, Q3, Q4, Q5 and Q6 and something, albeit rather weak on Q7. It seems as if the satellite I am hearing is UFO-7 which is situated over the Atlantic. But is this traffic really pirates using the satellites on purpose, or is it something else? Surely there is no longer the need, in Brazil or other countries, to use US military satellites for communications, especially now that mobile phones and mobile coverage are virtually ubiquitous?
A quick look at the Brazilian frequency allocation table, the Plano de Destinação de Faixas de Freqüência, shows us that the frequency range 270 - 326.8 MHz is assigned to the fixed and mobile service, and in particular to public correspondence. So the frequencies are quite legally in use for various communication services; could it be that they are being relayed by the satellite is incidental and a result of the fact that the uplink frequencies are used differently in different parts of the world? So maybe there are no Brazilian pirate radio mafia trying to jam US military satellites after all then? What a shame, it seemed like such a good story.
Tuesday 31 July, 2007, 15:26 - Radio RandomnessAnyone who really knows their stuff when it comes to radio propagation and stuff like that will tell you that any aerial (or an antenna for that matter) has a certain capture area.
Its capture area defines, in effect, how much of the signal that is in the ether it can capture and thus present to an attached receiver. The bigger the aerial, the greater its capture area and the stronger the received signal. Obvious really.
For some antennas, such as satellite dishes, the capture area is relatively obvious. A satellite dish which covers an area of 1 square metre has a capture area of roughly 1 square metre (ignoring edge effects). For linear aerials, however, the capture area is less obvious. Which has the greatest gain - a 4 element yagi or a 4 wavelength long colinear? Not so easy is it. However, it seemed to me that there ought to be a simple rule-of-thumb which allowed simple comparison between antennas. I therefore hypothesise that:
* The gain of a linear aerial ought to be somehow related to the amount (length) of metal it represents. Thus if you add up the length of the elements on a yagi and stretch them out into a colinear, the two should have the same gain.
* A doubling of the amount (length) of metal should double the overall gain (i.e. increase it by 3dB).
For now, I'll leave my first hypothesis and concentrate on trying to test the second one. Taking the quoted gain figures for over 20 amateur radio colinear antennas I've plotted their length in multiples of half a wavelength against their gain (in dBi). The length is done logarithmically, so '1' represents a length of half a wavelength, '2' represents one wavelength, '3' represents 2 wavelengths, '4' represents 4 wavelengths, '5' represents 8 wavelengths and so on, doubling in length for each increase in the index. Some of the antennas are multi-band (and thus might represent a compromise of gain balanced across more than one band), some are single-band.
The dotted line represents my hypothesis - i.e. that the gain rises by 3dB for each doubling of length. The solid line represents a 'best fit' line. As the antenna gets long (in terms of wavelengths) there is a noticable drop in gain with respect to my hypothesis. It is probable that as the antennas become long, the losses in the 'metal' itself cause a fall away from the my theoretical figure. Alternatively my hypothesis is incorrect. Either way, the original premise isn't that far off!
Next job is to see whether my hypothesis about the total length of elements of a yagi and the length of 'wire' in a a colinear holds any water.
Monday 18 June, 2007, 15:31 - Radio RandomnessIn a previous entry, I discussed the massive security hole presented to their neighbours like a baboon's bottom, by those still using analogue cordless telephones, as they can be easily received with cheap radio scanners, over quite large distances. However this is as nothing compared to the relatively common practise of bugging one's own house; otherwise known as 'installing a baby monitor'. The majority of these low power (10 mW) transmitters operate using basic analogue FM modulation on frequencies between 49.820 and 49.980 MHz (a low-power, short-range, licence-exempt band in the UK). These devices, like their cordless phone counterparts, can be picked up over several hundred metres, if not further. And whilst the owners often switch off the receivers when their child isn't in range of the monitor, they rarely switch off the transmitter meaning that it's often possible to tune-in to your neighbours going ons all day long (though such activities are strictly illegal in the UK and should not be entered into).
Around the wireless waffle HQ, there are several such baby monitors clearly audible on frequencies of 49.830, 49.840, 49.890, 49.930, 49.940, 49.950 and 49.962 MHz (the latter possibly intending to be on 49.960 MHz but is off-tune). There are also carriers on several other frequencies in this range but which are too lost in noise and interference (caused by other transmitters on the same frequency) to clearly make out. Some devices just produce a steady carrier, modulated with audio, others transmit data too, either as a 'warble' every second or so, or as a continous 'chuff-chuff-chuff-chuff' type noise. The latter types typically revert to being audio transmitters once the microphones detect any sound.
As well as allowing anyone with a cheap receiver to tune in to your private moments, many of these devices are poorly designed or built and have the capacity to cause significant amounts of interference to nearby radio frequencies, in particular the 6 metre (50 MHz) amateur band. It was as a result of such interference that my attention was drawn to the use of these 49 MHz frequencies in the first place, as reception from around 50.000 to 50.200 MHz suffers from out-of-band emissions from these devices (especially the warble and chuff-chuff-chuff-chuff models). What effect a 200 Watt SSB transmission on 50.150 MHz has on reception on neighbouring devices, I have no idea but it's to be hoped the receivers are as bad as the transmitters and that mummy and daddy are startled to find their 6 month old baby calling 'CQ'. It seems that the power supplies used for these transmitters are often badly regulated or smoothed meaning that there's broadband 50 Hz powerline noise, or worse, switch-mode noise emitted along with the intended transmissions.
There are some newer digital baby monitors available which operate either in the 900 MHz range (US models only, not licenseable in the UK), the 2.4 GHz and 5.8 GHz range. These models are virtually impossible to eavesdrop upon as they use digital modulation and are usually spread-spectrum. That being said, they're not encrypted so I guess it would be possible for some enterprising brainbox to figure out how to listen in, but that's hardly a Sunday afternoon activity (an activity which we do not condone, as it demonstrates utter contempt for people's privacy and is, at least conceptually, even more illegal than listening to the analogue ones). If it were me with a young child, I'd opt for a digital system safe in the knowledge that (a) it offers better features, (b) it's much more difficult to intercept and (c) it causes less noxious emissions that have the capacity to cause severe damage a young humans brain and body tissue. This latter point is exceptionally important, or completely made up, I'm not telling: you decide!