Wireless Waffle - A whole spectrum of radio related rubbish
A noisy noise annoys a noisy oystersignal strength
Wednesday 8 November, 2006, 15:29 - Amateur Radio
anoisynoiseWhen I'm at home, my receiver is usually switched on monitoring one frequency or another, commonly 145.500 or the local 70 cm repeater. Yesterday I was doing the latter and it soon became apparent that there was some active tropogation around. In addition to my local repeater, other amateur repeaters from typically 300km distant were fading in and out with regularity. What I noticed, however, was that the audio for all these distant repeaters (and, mysteriously, the local ones too) was accompanied by a rather annoying crackle.

My first thought was that the coax cable feeding the antenna must have a dry joint in it somewhere and the gentle wind blowing it back and forth was causing an intermittent connection. But I checked reception of repeaters on 2 metres and these were clear of the crackling noise which indicated that the effect was real, and was being received right across the 430-440 MHz band.

The next port of call was the various items of household equipment that could cause such relatively focussed wide-band noise; thermostats, fluorescent lights, computers, vacuum cleaners and similar devices being the typical culprits. But wandering around the house with a hand-held UHF receiver, it soon became apparent that none of these were the problem, nor could the crackle now be heard at all on the local repeater, though it was still there on more distant stations.

Then I remembered the BMEWS radar at Fylingdales near Scarborough and Whitby on the North Yorkshire Moors. This operates from 420 to 450 MHz and is renowned for producing interference resembling a crackling sound right across this frequency range. Obviously the tropogation was carrying the BMEWS signal the 350 km across the UK to my receiver.

Having now heard the kind of noise that this early warning radar produces, it amazes me that radio amateurs anywhere in Yorkshire or Durham can use the 70 cm band at all, but apparently they do! But this isn't the only noisy noise that annoys users of this band.

On several channels (including repeater input frequency 434.600, 433.900, 433.525 and repeater output 433.050 MHz) in my area is the familiar buzzing of low-power, licence exempt data links. Such links are commonly used for cheap wireless devices such as weather stations, doorbells, key-fobs and more. According to Recommendation 70-03 of the European Radiocommunications Office (ERO), which details which frequencies must be made available on a licence-exempt basis in European countries, such links are free to use the frequency range 433.050 to 434.790 MHz (centre frequency 433.920 MHz) with powers of up to 10 mW. What a stupid range of frequencies to choose as this coincides with many 70 cm repeater output and input frequencies as well as common FM working channels SU20, SU21 and so on. One can only assume that someone at the ERO (an agency of CEPT) has it in for radio amateurs. Why not limit the range to, say, 434.000 to 434.500 MHz, frequencies which are rarely used.

keyfob womanRecommendation 70-03 largely prohibits the use of voice or audio signals in this sub-band, but that doesn't stop (cheap) equipment being produced that is capable of doing just that. And if cheap equipment is available, someone will buy it. Monitoring 433.925 MHz in my area yields what sounds like a wirless audio link being used to relay television sound from one room to another, as well as occasional walkie-talkie like conversations. But who is going to stop this equipment being used illegally like this? Ofcom? They can't even keep obvious spectrum miscreants such as pirate radio operators down, so they're unlikely to be bothered about people who are using voice in a non-voice band.

So can anything be done to recover the band for radio amateurs? Well the old adage 'use it or lose it' has never been more applicable. If the band were full of transmissions, the ERO would never have considered it for such low-power devices. But it's not too late. A good 20 Watt transmitter on 433.925 MHz should wipe-out key-fobs, freezer failure detectors and other mindless devices over a good few hundred or more metre radius. So next time you want to chat with a local amateur, why not go simplex on 70 cm on, say, 433.925 MHz just for the hell of it?! Antisocial? Me? It's legal. It's simple and it's worthwhile. See you on 70 cm then...

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