Wireless Waffle - A whole spectrum of radio related rubbish
The Digital Dividesignal strength
Thursday 27 October, 2005, 09:45 - Licensed
Posted by Administrator
After many years in a sleepy backwater, Digital Audio Broadcasting (DAB) in Europe is finally becoming more mainstream (with one or two exceptions). This is certainly in part due to the fact that the prices for DAB receivers have now dropped to the level where, when buying a new radio for the kitchen, a DAB radio is in the running; and with the extra stations and 'improved quality' they offer, DAB radios are now becoming a must have item.

However, the gradual move towards DAB brings with it a problem for local FM broadcasters whose coverage is limited to a relatively small geographic area. For you see, DAB works by means of a multiplex (mux), whereby a number (typically 8 or more) stations are combined together and transmitted from one or more transmitter sites. The content of the mux must remain identical on all the sites such that all the stations on the mux achieve exactly the same coverage area. This is fine for national stations and for those who cover a relatively large geographic area, or the same city, such that there are likely to be other broadcasters who wish to achieve the same coverage and could share a multiplex; but who would wish to share a multiplex and duplicate the coverage of a neighbourhood or a small rural enclave? The straighforward answer is no-one! So these small scale stations are left with no option other than to continue using FM and hope that DAB does not become the de facto broadcasting standard.

Salvation, however, may come in another form of digital radio, otherwise known as Digital Radio Mondiale (DRM). DRM is designed to offer a replacement for outmoded AM radio on the long, medium and short wave bands, but it has recently had its frequency range extended to 120 MHz so as to offer a possible replacement for FM broadcasters too. For existing AM broadcasters, a move to DRM offers the chance to replicate their existing coverage but with much improved quality ('near-FM quality' is often quoted as being achieveable in an AM channel using DRM).

For AM broadcasters, this is great news: Using the same spectrum they can improve their transmission quality and many broadcasters, especially in mainland Europe, have already done so.

Using DRM in the FM band is a largely untried quantity and in theory there is no reason why it could not only work but at the same time expand the number of stations that could be broadcast in the FM band, as a DRM transmission takes up only about one tenth the bandwidth of an analogue FM transmission. It may not, though, be an ideal solution for a local FM broadcaster, particularly as at present, the very few receivers that exist for DRM do not cover the FM band (the manufacturers assumed that the standard would only be used on the HF bands below 30 MHz). Also (and I'm sure there are cleverer people than me working on this), the carrier spacing for the OFDM signal of a DRM transmisison varies between 41.66 and 107.5 Hz. At a frequency of 108 MHz, a vehicle travelling at 250 km/h (a train for example, or a BMW on a German motorway) experiences a Doppler shift of 25 Hz. Compared with the 41.66 Hz carrier spacing this is significant, and even at 107.5 Hz carrier spacing it represents a significant difference for the receiver to have to track, and must introduce lots of errors in reception.

There is another cat in a bag which offers a potential solution to the question of a digital replacement for local FM broadcasting in the shape of a little used HF broadcast band. The 11 metre band (25670 - 26100 kHz) is virtually unused by broadcasters as, other than at the peak of the 11 year sunspot cycle, propagation is virtually non-existent except for local ground-wave coverage. During the day at the peak of the cycle, signals travel thousands of miles, especially trans-equatorially (i.e. north to south and vice versa). Experiments have been conducted in Germany which showed that using DRM in the 11 metre band offers good local coverage similar to that which FM enjoys, however there is the potential for occasional long-distance interference during the day. This is, if anything, an improvement on the situation which traditional AM broadcasters enjoy in which there is a virtual guarantee of interference from distant stations every night!

Experiments using the 11 metre band are continuing in Germany and France and in London by WRN (1kW on 26070 kHz from Crystal Palace) and if all goes well, I think we could see a raft of local stations opening up DRM transmissions in this band, complementing their FM coverage and giving them a digital outlet to compete with the growing number of DAB stations. For the sake of small local radio stations, let's hope so.
1 comment ( 1589 views )   |  0 trackbacks   |  permalink   |   ( 2.5 / 7009 )

Cluster Beaconssignal strength
Thursday 20 October, 2005, 09:07 - Radio Randomness
You often read about 'strange signals' being heard on various frequencies; there are lots of web-sites which provide listings of the frequencies on which these oddities can be heard. But I was surprised to discover that there are some weird goings on in the amateur bands. Not, that is, because they are in any way sacrosanct, but because these things often occur in the 'twighlight zone', i.e. those frequencies which don't seem to belong to anyone in particular because they sit between Fixed, Mobile, Amateur, Broadcasting and other allocations.

My attention was drawn to 'Cluster Beacons': a series of CW (Morse) transmitters which are clustered very closely together in frequency and do nothing other than transmit the same letter over and over (and over) again. Such beacons have apparently been heard in the 40m amateur band occupying frequencies on or just below 7039 kHz. Now usually such things can only be heard by people with mile-long multi-element beam antennas and ultra sensitive receivers so I tuned my receiver to the frequency in question expecting to hear absolutely nothing (always the optimist). To my surprise, not only were these cluster beacons audible but (especially during the hours of darkness) they put quite a good signal into the UK. In particular the 'C', 'D', 'O' and 'S' beacons are audible for most of the day.

clusterspectrogramNot much is officially written about what the purpose of such beacons might be, however the received consensus is that they are either for monitoring propagation or are for navigation (the spectrogram on the right shows a waterfall of one of the clusters in action). What is known is that they come from various cities in the Former Soviet Union; each city is represented by a unique letter. This is a list I found elsewhere, I have no idea how accurate or up to date it is:

'A' Unid.
'B' Archangelsk
'C' Moscow
'D' Odessa
'F' Vladivostok
'K' Peteropavlovsk Kamchatskiy
'L' St. Petersburg
'M' Magadan
'O' Moscow
'P' Kaliningrad
'R' Izhevsk
'S' Arkhangelsk
'V' Khiva

As well as being clustered around 7039 kHz in the 40m amateur band, other cluster hot-spots are noted as being 5154, 7039, 8495, 10872, 13258, 16332 and 20048 kHz. I checked these and found that as well as those around 7039, beacons around 10872, 16332 and 20048 kHz were audible during the day (as of October 2005).

Whatever purpose these beacons officially serve, they are useful as a propagation check, if you happen to want to use HF to be communicating with Russia. Alternatively, if you can find a nice clear frequency where only the beacons are audible, the gentle tonal pulsating of the beacons is strangely hypnotic!
add comment ( 1231 views )   |  0 trackbacks   |  permalink   |   ( 2.5 / 10491 )

Over-the-Hill Radarsignal strength
Wednesday 19 October, 2005, 08:19 - Amateur Radio
Listening to the 17 and 15 metre amateur bands recently, I was puzzled by a buzzing that occasionally appeared. Centred around 21300 and 18130 kHz, the odd 50 Hz buzz spread over about 30 kHz (which almost wipes out the whole SSB section of the 17 metre band, which is only 57 kHz wide in total!)

A quick web-search and I discovered the IARU Region 1 Monitoring System (the IARU is the International Amateur Radio Union in case you were wondering) who monitor intruders in the amateur bands. To my surprise (I thought these things had died in the late 1980's) the signal is none other than an over-the-horizon (OTH) radar.

But to make matters worse, this particular one emanates from Cyprus and is brought to us courtesy of the British Military base at Akrotiri (see the picture on the right) - my own side! - where there have recently been riots due to the installation of new antennas...

And it seems that Cyprus is not the only one. The Russian Woodpecker is still drilling holes in the amateur bands, and has been joined by Iran who have updated their OTH radar which can apparently be heard on 14000 and 21000 kHz from time to time. There are also some civil radars that occasionally splash against the amateur bands which use HF frequencies to monitor the state of the sea (cold, wet, salty... what more do you need to know?)

The real surprise in all this is that with modern day spy satellites and other surveillance mechanisms, it's amazing that such primitive technology is still useful. Apparently the radars are only used in times of heightened security risk but what with the atrocities of 9/11 in the US (and the UK's much smaller 7/11 bombings in London) I guess risk is high again. Shame. And shame on the British Military for causing all that nasty interference!

Postscript: I was (cough, ahem) checking the frequencies used by the RAF for 'Architect', their HF communication network and lo and behold up cropped the Cyprus radar centred on about 9040 kHz, so they are 'crapping in their own back yard' too!
add comment ( 1425 views )   |  0 trackbacks   |  permalink   |   ( 3 / 16633 )


<<First <Back | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 |