[cma-l] Extending Community Radio on FM

[Alan Coote]

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

 

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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 <mailto:bobtyler at btinternet.com>  

To: The Community Media Association Discussion List <mailto:cma-l at mailman.commedia.org.uk>  ; 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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