Wednesday 8 February, 2006, 12:10 - LicensedI used to supply VHF FM radio transmitters for Restricted Service Licence (RSL) stations in the UK. The licences allowed a maximum transmitter power of 25 Watts and a maximum antenna height of 10 metres above the ground. As such, the range of such stations was normally very limited compared to regular FM stations who used powers typically in excess of 400 Watts and with much higher antennas. To maximise the range of the RSLs, it was best to identify and use transmitter sites which were as high above the surrounding land as possible but in some areas the land was so flat that no such sites existed.
One question which was constantly asked by the operators of such stations was, "Should I broadcast in stereo?". On the face of it, stereo is the norm for FM broadcasts and most stations believed that if they weren't in stereo they would be seen somehow as inferior. However, what most failed to take into account was the fact that in order to receive a good quality stereo signal, the signal strength has to be 10 times (20dB) higher than that required to receive a good quality mono signal. This translates into a reduction in coverage area of 100 times, i.e. the coverage in stereo is only a hundredth of the coverage area achieved by the same transmitter in mono (ignoring topographical issues such as terrain and buildings).
Why is there so much difference? The answer lies in the bandwidth which a stereo signal occupies compared to a mono signal. The audio bandwidth of an FM transmission lies in the range of 30 Hz to 15000 Hz (15 kHz). However the way that a stereo signal is generated expands this bandwidth to 53000 Hz (53 kHz). (Don't get this confused with the RF bandwidth of the signal which is 180 kHz for mono and 256 kHz for stereo).
How does the audio bandwidth extend to 53 kHz? Well the mono signal which is made by adding the left (L) and right (R) channels together - expressed as L+R - is transmitted as usual so that the resulting signal is compatible with mono receivers. The difference between the left and right channel (L-R) is amplitude modulated onto a carrier at 38000 Hz (38kHz). This produces a signal which occupies the audio frequencies from 23 to 53 kHz - above the standard audio range and thus inaudible on a mono receiver.
A 'pilot' tone which is a low-level tone at 19000 Hz (19kHz) is also added to this signal and then the whole lot is sent to the FM transmitter. In a stereo receiver the presence of the pilot tone triggers the stereo decoder to recover the original signals. The left channel is reproduced by adding the mono to the stereo difference signal (L+R+L-R=2L) and the right channel is produced by subtracting the difference signal from the mono signal (L+R-(L-R)=L+R-L+R=2R).
The noise received by an FM receiver increases as the square of the bandwidth of the modulated/demodulated signal and as such the increase in noise (i.e. the decrease in signal to noise) for a stereo signal is (53/15)˛ or 12.5 times. Some of this increase is counterbalanced by 'pre-emphasis' where higher audio frequencies are enhanced before transmission and then reduced at the receiver, reducing the effect of some of the noise. The resulting improvement leaves a difference of the factor of 10 mentioned above.
The question of whether to broadcast in stereo for a low-power RSL (or indeed a pirate!) FM station is therefore a question of quality and coverage. If you have a high site and can expect that most of the listeners you wish to target will receive a good strong signal, stereo is great. If not (which is usually the case), using mono ensures your coverage is maximised. Of course, in fringe stereo areas where the signal becomes 'hissy' the listeners could always switch to mono, but how many people actually know that this solves the problem, let alone know where the mono/stereo switch on their receiver is?!
There are some pirate stations I have heard who transmit only the pilot tone so that the stereo 'light' on receivers comes on, which looks nice, but don't actually transmit in stereo. This is the worst possible case, as all it will serve to do is reduce the coverage, without giving any additional benefit to the listeners!
Friday 3 February, 2006, 18:32 - Much Ado About NothingMy attention was recently drawn to a product called the 'Power Strip Antenna Booster' (though I believe these devices first came to fame in the USA in about 2001). It was being offered for only 1 pence by a mobile retailer (I won't embarrass them by naming First Phone Shop, oops...) if bought in conjunction with other cellular products worth more than GBP12.
The claims being made for this amazing device are quite impressive, "It dramatically reduces static and increases reception by up to 50%", and "It's like having a FIVE foot antenna on your phone". But what I particularly enjoyed were the descriptions of how the device functioned...
"It is a passive device designed to capture the stray radiation inside the body of the phone and re-direct the signal to improve the phone's performance."
"The Signal Booster captures stray static electricity around your phone and focuses it back to the signal, dramatically improving reception."
"It creates a megnetic [sic] field around the handset and draws in more signal also giving better clarity."
To most people these explanations might seem perfectly plausible. As someone who has handled radio for many years, its utter bunkum. 'Focusing stray static electricity back into the signal' makes about as much sense as 'Gathering stray exhaust fumes and channelling them back into the steering wheel'. Surely the fact that the device can be stuck anywhere on the phone, particularly inside the battery compartment, must at least have raised doubts in some people's minds. How can signals get out better if they are trapped behind the battery?
There are devices which can 'focus radio signals, increasing reception quality'. These magical devices are known as aerials (or antennas to our North American cousins). However, there is already an antenna inside every cellular phone, otherwise it wouldn't receive any signal at all!
There are a number of devices around which genuinely try to improve upon the performance of these in-built antennas, however the resulting product looks remarkably like... an aerial. A 'patch' that focuses stray radiation, what will they think of next?
Incidentally, having said all this, I do have a supply of Radio Atomising Crystals. Sprinkling these crystals around your phone, particularly under the key-pad and in that hole where the SIM-card fits has the effect of absorbing nearby interference channels, dispersing weak signals and leaving stronger signals to be more easily filtered by the phone's in-built diplexing separator. These crystals are guaranteed to deliver a massive increase in my wealth and are available for only GBP19.99 for a 5 gram sachet from all bad retailers and some rather gullible good ones who got taken in…
Thursday 2 February, 2006, 21:28 - Radio RandomnessNo doubt a lot of people received cordless phones in their Christmas stocking, not least because with cellular phones all the fashion, it seems rather restrictive to have to use a fixed phone at home. Cordless phones come in many flavours, but in particular come in analogue and digital varieties. It's not always that simple to identify, however, which phone is which. In the radio sense, an analogue phone is one which uses analogue modulation (usually FM) to carry the audio to and from the phone. Conversely, a digital phone is one which uses digital modulation. However some unscrupulous retailers who believe that the 'digital' tag is something which sells, often call phones with digital answerphones (i.e. that record messages digitally) as 'digital cordless phones' - they aren't!
Why does all this matter? Well, only true digital phones offer any level of security over eavesdropping on your calls. Analogue phones are as very easy to tune into (more of which later...) I know of people who have bought 'digital cordless phones' in the belief that their calls were secure. A brief demonstration using a portable receiver as to how easy it is to overhear their calls sent them rushing back to the retailer from who they purchased the phone. Would the retailer swap it for a proper digital phone - not without them paying an additional fee (It's true to say that true digital phones still command a slight margin over analogue phones).
So how do you know whether the cordless phone you have is digital or not? Well, firstly if it is clearly marked as a 'DECT' phone (usually shown as the letters 'DECT', standing for Digital Enhanced Cordless Telephony, inside an oval shaped blob) then you're fine. As well as being digital, DECT, a European standard, normally operates at frequencies between 1880 and 1900 MHz which means that signals don't tend to travel far outside the house. There is an equivalent American DECT standard known as DCT-U and alternative American digital standards, all operating around 900 MHz.
For a practical test, try holding your phone next to an electrically noisy device such as a food blender, electric drill or lift/elevator motor. If you can hear crackles or whooshes over the top of your call, chances are you are using an analogue phone.
In the UK, there are three bands in which analogue cordless telephones, termed CT0 - cordless telephony 0 - to represent the '0th' generation of technology - can legally operate (phones imported from other European countries or outside Europe may operate in different bands).
The first of these bands, specified in UK standard MPT 1322 has a base transmitter on frequencies between 1.642 and 1.782 MHz, just off the end of the medium-wave broadcast band, paired with handset transmitters on frequencies between 47.44375 and 47.54375 MHz with a maximum radiated power of 10mW (erp). They can be identified as the base unit has a wire, up to 3 metres long, which trails out of the back. Using such a low frequency, the base transmitters can radiate over quite some distance. These phones are being phased out in preference to the band identified below, but a tune over the lower frequency band will usually yield a number of units still in operation. The handsets, being low power and with small antennas, tend not to radiate over such a wide area.
In 1996 the Radiocommunications Agency, then responsible for managing the radio spectrum in the UK, realised the need for additional channels for analogue cordless telephony to relieve the congestion that was occuring in the existing bands. A further 8 channels were released (specification MPT 1384) with base units transmitting in the range 31.0375 to 31.2125 MHz, and handsets in the range 39.9375 to 40.1125 MHz again at a power of 10mW (erp). Ranges for the base transmitters vary but can extend to over a km.
One final range of frequencies is used for long range cordless phones (complying with UK standard MPT 1371) with only two channels available and a maximum radiated power of 100mW (erp). Base units transmit on either 47.43125 or 47.41875 MHz and handsets on 77.5125 or 77.5500 MHz.
If you have a receiver that tunes across the range of frequencies used by these analogue phones, particularly those used by the base units, have a listen (though obviously I haven't told you to do this as doing so would force you to break the law and I can't possibly condone that kind of thing). It's amazing what range 10mW can give at some of these frequencies. Far be it for me to suggest that you could make a game out of guessing which of your neighbours regularly visits a 'medium', or which one's son has just come out as or which has insurmountable debts or ...! And remember, if you buy a cordless phone for yourself, make sure it's digital!
Tuesday 3 January, 2006, 16:50 - Radio RandomnessCitizens' Band (CB) Radio began in the UK in the late 1970's when truckers and a few other enthusiasts (illegally) imported CB radios from the USA where they were widely available and relatively low cost.
Prices for these illegal AM sets in the UK were typically around GBP50 which wasn't totally out of reach of even a few school kids. I fondly remember communicating via CB with many of a my school friends whose parents would surely have been worried if they'd known the penalties for operating the equipment!
Under immense populist pressure, the UK government finally relented and on 2 November 1981 the Radiocommunication Agency licensed CB radio of two flavours. CB in America (and in most other countries which had licensed it) operated on 40 channels spread out between 26.965 and 27.405 MHz. The UK decided that, as CB radio was meant to be a local communications service, it would license a different set of channels to stop UK 'breakers' from chatting to overseas CBers when conditions permitted - thus 40 channels from 27.60125 to 27.99125 were licenced with a maximum power of 4 watts and a maximum antenna size of 1.5 metres and height of 10 metres above the ground (these antenna restrictions have since been relaxed). Equipment (marked 'CB 27/81') for the band was readily available from day one at prices similar to the illigaly available US equipment.
In addition to the 27 MHz channels, an additional 20 channels were licensed in the frequency range 934.0125 - 934.9625 MHz. The 934 MHz band was mainly for enthusiasts as equipment was relatively cutting edge and hence expensive and operation required a great deal of skill. A licence fee of GBP10 per year (now GBP15) was payable to use either (or both) bands, though many users, having been used to operating illegally, never bothered with this formailty.
In 1988 the Radiocommunications Agency succumbed to pressure from Europe and licensed the US 27 MHz channels (which had been formally adopted by CEPT for use across Europe). At the same time the withdrawal of the 934 MHz band (at the end of 1998) was announced to make way for GSM phones, and the use of the original UK-only 27 MHz channels was frozen with no new equipment allowed to be manufactured in favour of the CEPT channels.
With the ready availability of equipment and the boosted awareness of CB amongst the general population through films such as 'Convoy' and 'Smokey and the Bandit' and TV shows such as 'The Dukes of Hazzard', the CB channels quickly became the playground for every kind of nutter you could imagine. Many channels were filled with the likes of frustrated DJ's who would spend hours playing music from their bedrooms and people who thought that swearing incessantly on the calling channel represented sport. Towards the end of the 1980's CB usage began to wane and today, even in a highly populated area such as London a tune across the 40 UK and 40 CEPT CB channels yields very little usage. Also, 27 MHz is not an ideal frequency for local communications. At certain times of the year, especially in the summer, propagation allows communication over 1000's of km and channels become clogged as it becomes easier to talk to breakers from faraway places such as Italy and Russia than to talk to people only a few km away.
So is CB radio dead? Well in its original format, possibly. The new UK regulator, Ofcom, has allowed an experiment called the 'Community Audio Distribution System (CADS)' in the 27 MHz band to try and make use of these otherwise empty channels. Meanwhile, in April 1999, Ofcom de-regulated a set of frequencies in the range 446.00625 to 446.09375 for a service called 'PMR446'. PMR446 was a replacement for the previous 'Short Range Business Radio (SRBR)' service which allowed the use of 3 channels at 461 MHz at low power for just GBP30 per annum and was intended to be used by businesses whose radio requirements did not merit a wider-area licence (such as between shop assistants in department stores). However, being licence-exempt, the PMR446 service soon found favour amongst a variety of radio users who might otherwise have used CB. Eight channels are available and equipment is restricted to 500 mW of power and integral antennas, however reliable ranges of around 1km are common.
Thus those who want to use radio for non-commercial use such as families and event organisers have access to channels which are largely, though not exclusively, idiot-free and that do not suffer from frequent continental interference. PMR446 is an analogue service, however having regocnised its potential, a digital version of PMR446 which should supposedly offer greater range and better audio quality is already on the cards.
Where does this leave CB then? With 27 MHz mostly dead and most 'community' use of radio now using the short-range PMR446 service, there is still a need for a wider area CB type service to replicate the service offered by CB for businesses such as taxi's and logistics companies. Help may be at hand in the form of the little recognised 'UK General' licence. This allows the use of a number of VHF and UHF frequencies with 5 Watts of power (10 times more than PMR 446), however a licence fee of GBP60 every 3 years is payable. The frequencies which can be used are:
A quick scan of this odd collection of channels in West London shows that many are relatively unoccupied and would offer great potential for a supplemental service to PMR446. A more sensible pricing regime for the use of these frequencies (say GBP15 per annum...) or indeed the inclusion of these channels in the UK CB licence might just tip the balance back in favour of CB radio as a sensible solution to the radio needs of small businesses.