[cma-l] Ofcom announces trials to help small stations join digitalradio - 100w limit

[tlr at gairloch.co.uk]

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> 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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