[cma-l] Extending Community Radio on FM
Tony Bailey
ravensound at pilgrimsound.co.uk
Tue Sep 30 11:21:01 BST 2014
Don't forget that on VHF the effective antenna height is a factor, for
example, I once logged a 1 watt RSL on a car radio around ten miles away
(from their Wimbledon Hill site) (on about 87 meg)! And similarly, four
miles to a walkman from 1 watt in Orpington (I was listening on the roof
of a tower block). Unfortunately, this also means hollows = signal
loss, something difficult to deal with if on limited power/antenna height.
Which leads on to the subject which has had most research in recent
times: protection ratios. For all practical purposes, service areas are
interference limited. Protection ratios are effected by all sorts of
variables, see Rec. ITU-R BS.412-9. (There's an interesting section on
synchronised motor way services, as per Channel Travel Radio). There is
a school of thought that we simply jack up all the RF levels and just
keep the protection ratios intact, I believe this has been done in some
metro areas already?
We no longer have the option of 9 x 2.2 MHz slots everywhere, but if you
want to avoid those annoying neighbours you will still need your own
ball park. If the DAB move is off, there is a case for making room for
more CR in the National sub bands, which are populated by less
contentious relays.
Regards, Tony Bailey
On 30/09/14 10:06, Alan Coote wrote:
>
> 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 <mailto: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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