[cma-l] Ofcom announces trials to help small stations join digitalradio - 100w limit
Tony Bailey
ravensound at pilgrimsound.co.uk
Sun Mar 8 15:58:21 GMT 2015
Ofcom states that the coverage limit is set at 40% of the "corresponding
local DAB multiplex area" and that a practical limit of 100 W ERP "may
achieve a service area of approximately 10 km radius". A synchronised
two tx (not repeater) system would be spaced at no more than 15 km
apart. As pointed out below, this has to based on a practical antenna
situation to have any relevance.
Tony Bailey
On 08/03/15 14:01, Ian Hickling wrote:
> It seem there's a lot of second-guessing going on here from people who
> may know a lot about administration and encoding but possibly not so
> about the black magic that is RF propagation.
> There's no point in trying to relate 100W ERP to 5km for Band III DAB
> - just as it's equally irrelevant to relate 25W with FM to 5km - sorry.
> Topography, geology, refraction, refraction, foliation, antenna
> efficiency and launch conditions have far too large an influence.
> In terms of propagated signal transit, there's not a huge difference
> in practical terms between FM at say 100 MHz and DAB at 200 MHz when
> you take into account antenna size, efficiency, reflection and refraction.
> Because of the difference between demodulation formats, a receiver
> can tolerate a much lower signal level on DAB than on FM to resolve an
> acceptable audio service.
> This was originally proposed at 20dB from the point of view of
> transmitted power but then revised to 10dB - meaning that a DAB
> transmitter in Band III would need one tenth of the ERP of an FM
> transmitter in Band II to achieve the same audience.
> Hence it is puzzling why Ofcom has set so high a required signal level
> for a DAB service area of the order of 72dBuV/m as opposed to 54
> dBuV/m for FM.
> Beware - there is a distinct difference between a Power Decibel in
> transmission and a Voltage Decibel in reception!
>
> Let's not invoke DAB+ and DRM - Ofcom specifically rules them out in
> 2.30 and 2.32
>
> Yes, Block 5A would be ideal as it's relatively clear, allocated and
> accessible to modern receivers - but Ofcom apparently doesn't accept
> that as it hasn't headed straight for it.
>
> As I've protested many times, there is technically nothing at all to
> prevent a standalone transmitter radiating a single programme stream
> to serve a discrete area either on DAB, DAB+ or DRM as far as I'm
> aware. If I'm wrong I'd appreciate the exact reasons why.
>
> Looking at only the RF component in the transmission chain, several UK
> manufacturers could offer a 2U Band III 300W unit at around £2000 if
> the demand were high enough - no real cost differences from today's
> Band II units.
>
> Let's not get distracted - the encoding is software-defined - the
> actual RF transmitter is not!
>
> Ian
>
> ------------------------------------------------------------------------
> Date: Sun, 8 Mar 2015 11:13:25 +0000
> Subject: Re: [cma-l] Ofcom announces trials to help small stations
> join digitalradio - 100w limit
> From: alan.coote at 5digital.co.uk
> To: tlr at gairloch.co.uk; transplanfm at hotmail.com; info at a-bc.co.uk
> CC: cma-l at mailman.commedia.org.uk
>
> I can't help thinking that someone at Ofcom ran the simulations and
> came up with 100W = 5km radius.
>
> Therefore if small scale DAB became a reality it wouldn't annoy Radio
> Centre too much (they'd still complain as that's their mentality) and
> at worst secondary legislation could make it happen.
>
> 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 Over
> 5 Million People <http://www.letstalkbusinessonline.com/>
>
>
> From: "tlr at gairloch.co.uk <mailto:tlr at gairloch.co.uk>"
> <tlr at gairloch.co.uk <mailto:tlr at gairloch.co.uk>>
> Reply-To: "tlr at gairloch.co.uk <mailto:tlr at gairloch.co.uk>"
> <tlr at gairloch.co.uk <mailto:tlr at gairloch.co.uk>>
> Date: Sunday, 8 March 2015 00:45
> To: "transplanfm at hotmail.com <mailto:transplanfm at hotmail.com>"
> <transplanfm at hotmail.com <mailto:transplanfm at hotmail.com>>, Associated
> Consultants <info at a-bc.co.uk <mailto:info at a-bc.co.uk>>
> Cc: "cma-l at mailman.commedia.org.uk
> <mailto:cma-l at mailman.commedia.org.uk>" <cma-l at mailman.commedia.org.uk
> <mailto:cma-l at mailman.commedia.org.uk>>
> Subject: Re: [cma-l] Ofcom announces trials to help small stations
> join digitalradio - 100w limit
>
> I simplistically presumed they settled on the 100W suggested limit on
> the basis that at the Band III frequencies of DAB it would give
> roughly the same coverage area (at 58dBuV/99%) as 25W on Band II (at
> 54dBuV/90%).
> NB the average *local* DAB multiplex power is 1.3kW, not 2kW, but of
> course they tend to be from sites with much higher antennas than
> economically available to community stations, so the chances are the
> 100W represents an even tinier coverage area in comparison to current
> local multiplexes than might appear at first sight from a simple
> comparison of powers. But I can see it is much easier for Ofcom to
> control the allowed power than to get into arguments over exact
> percentages of area covered. Maybe 500W would have been more realistic
> if they wanted to take that simplistic approach, with a lower limit
> applied in the few cases where 500W coud cause difficulties.
> (I guess there is also the question that Ofcom is paying for the
> transmitters in the trial, and a band III amplifier running at , say,
> 250W is a lot more expensive than a 50W one, especially if one uses
> the technique of greatly underrunning a much higher power design to
> help achieve the necessary linearity.).
> Seems to me that block 5A, (currently unused, but allocated for local
> DAB) could be used as a UK-wide frequency block for terrain limited
> single station services up to 500W to deal with all the areas where
> there is a low density of local stations (ie only one within the
> interference range of a 500W TX) and it could be done tomorrow,
> without any fancy trials or risk of interference, clearing out one
> whole tier of demand without any fuss, leaving trials and more
> complicated sharing and co-channel planning issues to be threshed out
> over time in the other seven frequency blocks allocated to local
> ensembles in areas of more dense demand. It's also much lower in
> frequency than the other blocks, which reduces the demands on the
> low-cost software defined transmitter.
> Alex
>
> On 25 February 2015 at 13:04 Associated Broadcast Consultants
> <info at a-bc.co.uk <mailto:info at a-bc.co.uk>> wrote:
>
> We challenged the 100w limit in the consultation - suggesting that
> the "no greater than 40% of the local commercial Mux area" was an
> adequate limit. 100w is roughly 5% of the average existing DAB
> transmitter power, so presuming community stations don't deploy
> their DAB transmitters using tethered balloons or satellites etc
> they unlikely ever to get near 40% unless they deploy multiple
> numbers of transmitters (thus undermining the low-cost aim).
> The standard consultation deflection response was invoked (ie:
> address a different question) - stating that "it is not
> necessarily the case that allowing a higher power will in all
> cases reduce the number of transmitters needed". We never said it
> would in all cases, but were suggesting that by removing the 100w
> cap you retain some flexibility when it /would/ make a difference
> in some cases! Unfortunately though, consultations are single shot
> - no possibility to clarify the point or challenge the response.
> I think we can all imagine the real (unstated) reason why they are
> limiting it to 100 watts ;-)
> Don't get me wrong - 100w at 200MHz can still provide useful
> coverage if planned correctly (other DAB coverage planning
> services are available!), but in some cases more may be required.
> Otherwise we risk repeating the same problem that analogue CR has
> - the paltry standard 25w power is often inadequate and quite
> literally blasted off the dial by much stronger commercial and BBC
> signals. And this problem is even worse with DAB (for technical
> reasons that I will not go into here).
> Glyn
> --
> Glyn Roylance - Principal Consultant
> Associated Broadcast Consultants <http://www.a-bc.co.uk/>
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Local Reports at http://www.ravensound.pilgrimsound.co.uk
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