[cma-l] Extending Community Radio on FM

Wed Oct 1 09:31:52 BST 2014

There's another aspect here which we might be missing - well two really.We as broadcasters - controlled by our eminent Regulator - can only provide a finite signal at a strength measured by whichever method we choose to employ - and there are points there which need to be clarified.What we have no control over as Alex says is "clutter" - whatever that may constitute and which in reality is legion and unpredictable - and the efficiency or indeed the format of receiver function.If we assume the concept of planar rather than particle propagation then the antenna should correspond in orientation and resonant length with the incoming signal for an optimal result.We all know I think that this simply doesn't happen with a portable radio because the consumer has no concept of the requirement - and indeed why should he?But with an external antenna those two requirements are (hopefully) correctly fulfilled.So we have three major attenuation factor areas - Specific site clutter (impossible to predict)Antenna/signal alignment (small element of control)Antenna efficiency (indeterminate for portable equipment)The fourth of course is receiver selectivity which is probably poor and equally indeterminate.We're on a lose/lose status here and we can do nothing about the risk level.The only significant thing we can do within the power/protection/launch height parameters we are given and of course our financial constraints is to very carefully plan the format we use for launching our signal.To quote PMQs - "I refer my honourable friends to the answer I gave a few moments ago":FM stations using 25W vertical only = 152 (72%)FM stations using balanced mixed polarity = 37 (18%)FM stations licensed for but not using mixed polarity = 157 (75%)The words "horse" and "hobby" come to mind - sorry!
From: tlr at gairloch.co.uk
To: cma-l at mailman.commedia.org.uk
Date: Tue, 30 Sep 2014 17:24:44 +0100
Subject: Re: [cma-l] Extending Community Radio on FM

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
  Phone 
  - 0800 949 6655
  Mobile 
  - 07801 518858
  Twitter 
  - @TheAlanCoote
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  - http://www.5digital.co.uk

  The 
  Media Production, Broadcasting and Training 
Company

  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
    Twitter 
    - @TheAlanCoote
    Web 
    - http://www.5digital.co.uk

    The 
    Media Production, Broadcasting and Training 
    Company

    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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