[cma-l] Extending Community Radio on FM

Tue Sep 30 17:24:44 BST 2014

The possible problem with a housing estate is not that the 54dBuV/m figure is wrong, but is the difficulty of accurately predicting the fine detail of field strength - although top flight packages with the up-to-date databases are incredibly good at taking these things into account. (Actually I would have thought modern housing estates might well be less of a problem than the traditional streets of terraced houses prevalent in the 1950s when the figures were first set.)

You seem to be implying that because it is hard to calculate urban clutter we just should chuck a bit more power at it - ie aim for a higher rooftop field strength overall. Ironically, an unintended consequence of that would inevitably be that there would be fewer available slots in the spectrum overall, since for any given protection ratio the more you increase transmitter powers the fewer there can be (in non-terrain limited parts of the country).

Yes, it was doubts over whether existing  field strengths for DAB indoors were sufficient in the light of experience that was a key aspect of the 2011 review - but that was for DAB, not for FM. The FM study done in preparation for the DAB review  concluded that it was still the case that "...the current assumed level of rooftop field strength (54 dBμV/m) provides a good way of predicting good, indoor mono FM reception on modern portable receivers. However, many receivers will produce what some listeners might regard as an acceptable service at a field strength of 48 dBμV/m, and so this could be used to define variable, indoor portable FM reception." So, far from saying 54dB might be inadequate, the study concluded that it might be more than is needed with modern radios and listening habits.

Alex

PS
The point about the screening of modern houses is very true. I think I mentioned it in an earlier post as it is a bit of a hobby-horse of mine! Not only do builders almost universally use foil-backed plasterboard for walls and ceilings nowadays rather than only where a vapour barrier is needed, but if you add to that energy saving low-e glass (which is metal plated), then you really are making life tough for radiowaves. But it doesn't affect most people with a typical kitchen window radio, and it's still only a partial effect at Band II frequencies. It does affects DAB considerably worse, and that could partly account for why practical results  for DAB were proving worse than anticipated when using a similar rooftop proxy (it had been 58 dBμV/m but was raised to 69 dBμV/m as a result of review).

----- Original Message ----- 

  From: Alan Coote 
  To: 'The Community Media Association Discussion List' 
  Sent: Tuesday, September 30, 2014 4:37 PM
  Subject: Re: [cma-l] Extending Community Radio on FM

  I take your point on 54dBuV/m being a proxy, but this coupled with terrain data accuracy means the plots are a rough estimate. 

  That’s not a problem on a large scale. But at the granularity of the average community station, it means if coverage of a typical housing estate is calculated to be passable and in reality it’s not, it could be the difference between a viable station or not.   

  Another factor overlooked is that modern construction techniques dictate buildings are better insulated. The knock of effect is some houses and offices are very good Faraday cages. 

  Wasn’t poor indoor reception the reason DABs coverage was reviewed? I know out of town retailers put a signal booster in the roof so that their DAB radios work on the shelf. 

  Kind Regards

  Alan

  Hear Alan Every Week on Let’s Talk Business The UK’s Premier Radio Programme For Current and Future Entrepreneurs - Now Broadcast To 4.3 Million People  

  Email - alan.coote at 5digital.co.uk

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  From: cma-l-bounces at mailman.commedia.org.uk [mailto:cma-l-bounces at mailman.commedia.org.uk] On Behalf Of Two Lochs Radio
  Sent: 30 September 2014 12:38
  To: 'The Community Media Association Discussion List'
  Subject: Re: [cma-l] Extending Community Radio on FM

  I find it very hard to get worked up about that 54dBuV/m contour at 10m. It seems to work very well as a practical guideline to the required field strength for satisfactory reception in typical cases. 

  Ofcom has looked at the issue several times, including a careful review in 2011 as part of planning DAB coverage, and every time has come to the conclusion that 54dBuV/m VHF FM at 10m height makes a perfectly good proxy for what actual field strength is required at the windowsill for typical portable radio reception.

  It fits well with what I've found in practice - a rooftop level of 54dBuV/m is fine for transistor portables on windowsills, 60dBuV/m better for general indoor reception. Cars seem very happy with 42-48dBuV/m. Your mileage may vary!

  Of course it's true that more transmitter power may be needed in towns to achieve these field strengths, and I'm not arguing for one second that a "25W for all" approach would be realistic, but  the 10m target field strength levels are not the issue - you still need 54-60dBuV/m for reliable reception, even in a densely built up area. 

  What may change for an urban or hilly setting is the power needed to achieve that level at a given position. But the answer to getting that right is not to say you need a greater field strength at the receiver - that's still 54dBuV/m at 10m - it's down to using good quality propagation prediction software that can take urban clutter etc into account and tell you what transmission power and location is likely to provide that field strength. At least in a town there is more chance of finding a block of flats or other relatively high structure that can help.

  Ofcom clearly does to some extent take into account that 25W does not always delivered the desired signal at 5km - that's shown by Ian Hickling's recent analysis showing that 34 CR stations have more than 25W+25W in use. It's also noticeable that three-quarters of existing licencees could improve their effective strength by up to 3dB by using their horizonatal 25W clearances, but aren't doing so. Yes, it's a signifcant extra cost for quite a small gain, but every little helps, and it's hard to argue with Ofcom for more power if you aren't already using everything allocated.

  It's also the case that many stations could with a bit of effort/cost find more advantageous transmitter locations to make the most of their 25+25 allocation. Just for example, 25W at the top of a tower block in Glasgow or Leeds should give a comfortable 5km 54dBuV/m range in most locations across the cities. Again, there's usually a cost to a remote TX site, but if it makes a huge difference to the station's likely coverage and/or income you have to do the sums and decide.

  The effects of TX height are really very important to coverage in hilly terrain. I noticed the other day that Speysound CR broadcasting from Cairngorm with 68W power comes through at excellent car radio strength in the hilly suburbs of Dingwall, some 80km away. But they do have a rather favourable TX site at 1100m ASL!

  I'm not familiar with Bollington and its neighbouring 'hamlet' to the south-west, except from what I see on maps. If the present antenna is on the tower of Clarence Mill you'd think it would provide reasonable coverage to the wider area, but it seems to me that, again just for example, 25W+25W on a 10m elevation in the Kerridge / Hollin area would provide excellent coverage of Bollington as well much stronger coverage of its, ahem, neighbouring settlement to the south west. Those options and others should have been apparent right from the start.

  Don't get me wrong, I'm not arguing that Ofcom couldn't do a lot more to relax some of the power/spectrum restrictions, and maybe doesn't put as much effort or innovation into it as it could. However, arguing that the 54dBuV/m at 10m guiideline is inadequate doesn't seem to me to be a productive way forward - it's a complete diversion.

  Alex

    ----- Original Message ----- 

    From: Alan Coote 

    To: 'The Community Media Association Discussion List' ; 'ROBERT TYLER' ; ravensound at pilgrimsound.co.uk 

    Sent: Tuesday, September 30, 2014 10:06 AM

    Subject: Re: [cma-l] Extending Community Radio on FM

    It’s amazing that 54dBuV/m at 10m has been maintained for so long when it has little resemblance to the vast majority of setups. When projected to antennas at a more normal 1.5 – 2.5 metres above ground level, the field strength needs to be significantly greater.

    The other very significant practical issue is that the field strength required to cover dense urban, urban and  open country is different. Greater power helps in towns and cities due to the losses (refractions, reflections and absorptions) which affect the wanted signal.   

    This is very apparent in lower power stations (like community radio) as the fringes of coverage are often in areas which the station wishes to reach.     

    If we accepted that 25W does not always = 5km, I wonder therefore if Ofcom should take this more into account when licensing stations? An additional benefit would be that spectrum is better managed and (ref’ to Ian’s original post) more groups would end up getting licensed.     

    Kind Regards

    Alan

    Hear Alan Every Week on Let’s Talk Business The UK’s Premier Radio Programme For Current and Future Entrepreneurs - Now Broadcast To 4.3 Million People  

    Email - alan.coote at 5digital.co.uk

    Phone - 0800 949 6655

    Mobile - 07801 518858

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    From: cma-l-bounces at mailman.commedia.org.uk [mailto:cma-l-bounces at mailman.commedia.org.uk] On Behalf Of Two Lochs Radio
    Sent: 29 September 2014 18:17
    To: ROBERT TYLER; The Community Media Association Discussion List; ravensound at pilgrimsound.co.uk
    Subject: Re: [cma-l] Extending Community Radio on FM

    Bob

    You're right about the original signal specs being based on the assumption of outdoor aerials, but receivers were much less sensitive at the time, and by happy chance the improvements in receiver sensitivity have approximately counteracted the move to indoor whip aerials, so the planning based on the same 54dBuv/m signal level at 10m height still gives a useful guide as to the required signal for robust coverage.

    When the BBC VHF service first began they worked to a definition of 60dBuv/m needed for good service in mono (later 66dBuV/m for stereo), and  48dBuV/m (later 54dBuV/m for stereo) as the 'nominal limit of satisfactory reception'. Ofcom still uses this, summarizing it like this:

    I would be cautious about assuming much better selectivity or sensitivity on modern FM receivers - there are plenty of shower, bedside clock-radio and other cheapy FM radios that tell a different tale!

    I was interested in your reference to "the Home Service Bedford transmitter". What was that? I understood the VHF Home Service for the southeast began in 1955 on much the same site as today -  Wrotham in Kent (there was 120kW at Wrotham, Norwich and Sutton Coldfield to start with). Bow Brickhill which now serves Bedford on VHF wasn't set up until the 1980s as I recall, and was mainly as a result of the need to cover the burgeoning Milton Keynes.

    It's not correct that in 1955 it was thought there would be only two services on VHF - the Third programme was already 9 years old and went onto VHF simultaneously with the Home Service and the Light Programme. 

    When the BBC first planned it all for three services the band was one-third its present size (88.0-94.6 MHz), and as for us having more space between stations than any other country in Europe - a quick look at a transmitter map of Northern Europe both historically and today would show that not to be true. Southern Europe (okay, Italy really) is another story of course!

    Alex

      ----- Original Message ----- 

      From: ROBERT TYLER 

      To: The Community Media Association Discussion List ; ravensound at pilgrimsound.co.uk 

      Sent: Sunday, September 28, 2014 8:09 PM

      Subject: Re: [cma-l] Extending Community Radio on FM

      Tony

      It is my understanding that the British specification for the FM (then VHF) band was established before the Second World War. At the time it was considered that only the two BBC services would be on VHF. I believe that the specifications laid down are still current and are based on roof top aerials, as it was considered that to receive VHF such an aerial would be required. 

      Also for some reason, we in the UK also decided to employ  extra separation/guarding, in order to prevent ANY possibility of  the Home Service Bedford transmitter interfering with the Home Service Dover transmitter or Crewe Relay. In other words, we have more space between services than any other country in Europe. In many respects we have the perfect VHF/FM transmitter network.

      Of course then, it was not expected that the reception of radio would extend to telescopic aerials or tiny coil antenna, thereby making the roof top aerial redundant. Nor even then, was the prospect of commercial or community radio in the future mix. As a side issue, a few commercial stations and many community radio station are in allocated BBC spectrum at the behest of Dear Old Aunty. 

      The mystery of so called �expertise� in planning the VHF/FM spectrum has remained in the hand of the regulators. I do remember asking my MP sometime in the 70�s  to lodge a written question about an available frequency to The Home Secretary, only to receive the reply that 101 point something was �used to track otters� . 

      I did have it confirmed that 102.2 was assigned wrongly and should have been able to accommodate several UK allocations but apparently the spectrum had been badly assigned at a too high a power in the East of England, (flat terrain)  thereby rendering the space sterile for a large part of the country. 

      In summary there are many issues (and many question) surrounding spectrum allocation and the regulator of the day always hold the mystery ticket. The �beauty contest� of selecting future licence holders compounds the decision process. The reality is that the only policy is to switch to DAB so any FM debate is a waste of time. 

      Hope this helps

      Bob

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