All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
Mark Barden: My belief is that the CAI-recommended levels for digital TV were set mainly to ensure that the analogue signals didn't drive amplifiers into non-linear regions, causing intermodulation. It wasn't because 60 dBuV on digital alone would cause intermod. If your signals are currently in spec, an additional 10 dB on each is unlikely to be a problem.
You are only 5 km away from Bluebell Hill and appear to have clear line-of-sight. Plugging those figures into Field Strength Calculator suggests that even with no aerial gain and 10m of downlead it could be over 80 dBuV on C39 (ArqA mux). I'd say that you might need attenuation - it'll depend on how much the signal is attenuated by neighbouring buildings and trees.
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Dave Lindsay: Digital UK's database has no way to represent 'change to antenna radiation pattern' so they list it as 'PSB Power Up'. The antenna was originally built with more beam tilt - how far the main beam is angled down from horizontal - than ultimately intended. This was to protect some transmitters in the Meridian region that used the same channels before switchover. The changes on 27 June, once those transmitters no longer need those channels, are to insert pieces of rigid feeder line to change the electrical lengths of the feed to each tier of panels, which will reduce the beam tilt and therefore increase the distance covered.
I believe the parts have to be inserted into the antenna itself. If so, it will cause some interruptions as they switch to and from the reserve antenna, possibly some complete interruptions as the workers climb past the antennas. The changes may be delayed if the weather is too severe (typically too windy) to climb the mast safely.
People in the far south of Sandy Heath's coverage area may notice a small improvement. Anyone to the north, west or east of the transmitter should ignore this; it won't have any effect on their reception.
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Mr Edwards: This is a SELF HELP website with volunteers who put their own time in to help people. The broadcasters do not routinely monitor this website (as far as we know). If you are currently having a problem and you think it's down to transmission (though usually it isn't), contact the broadcasters. There's a list of contact numbers etc at Broadcaster complaints details | ukfree.tv - independent free digital TV advice .
The BBC track fault reports, but mostly they and the other broadcasters rely on telemetry from the transmitters. If the fault occurs anywhere before the transmitting antenna, on a major station, they know about it and, if it affects the BBC, it should be recorded at BBC - Reception problems . This site scrapes that BBC site periodically and puts the result here as a 'Transmitter Engineering' post.
Signal levels do vary with weather conditions - just as you get red skies with dust in the atmosphere and 'heat haze', limited visibility due to fog and rain, and indeed even mirages. These are all changes to the way electromagnetic waves travel - radio and TV have much longer wavelengths than light but many of the principles are the same. Digital UK's predictor takes account of the expected variation and still reckons that better than 90% of locations in your grid square should have reliable reception for more than 99% of the time (for some services, the prediction is ALL locations in the square). That prediction is based on a good quality high-gain aerial and a fairly short downlead (cabling).
Given that, I think it's likely that it's been raining and that water has got into your aerial system. That makes it much harder for the signals to travel down the cables (coaxial cables at high frequencies are waveguides with the signal travelling on the surface of the conductors, it's very different from power cables). The improvement is likely because it's dried out. Different frequencies are affected by different amounts - usually the highest frequencies are worst affected.
You should check that any connection boxes are mounted so that the lid is facing downward, to avoid water getting in and ensure that any water that does get in can drain out. You should also check that all cables are properly secured, so they cannot rub against roof tiles or brickwork, and check that the insulation is fully intact. Also check that the cable insulation is still flexible: the insulation is damaged by UV light and eventually becomes brittle and porous, again allowing water to get in.
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bernard: a group C/D aerial will not provide useful gain down on C29. You really do need to replace it with a wideband, I'm afraid.
The Midlands is very crowded with signals and it just wasn't possible to fit everything into Group C/D, particularly with half that group, C63-C68, being released (C61 and C62 are also to be released, BBC A will move to C49 some time next year, expected to be October).
Anyone who had Freeview before switchover needed a wideband already. You would have had best results on Channel 5 analogue by changing to a Group E or wideband. That was probably taken into account, along with the expected population coverage, when deciding which transmitters had to have services outside the old analogue group.
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Yvonne: The prediction for the postcode you've provided suggests an excellent chance of getting reliable reception from the Whitby transmitter. This will provide the PSB multiplexes only after switchover in September - see references to 'Freeview Lite' or 'Freeview Light' elsewhere on this site, or the Whitby transmitter page, to see what channels will be available.
It's very unlikely that a full service from Bilsdale will be possible at that postcode, even after switchover completes. The chances of even getting the PSBs reliably are very low indeed. Currently, Whitby only provides BBC One, BBC Two, ITV1 and Channel 4 on analogue - Freeview is only currently available from Bilsdale in your area.
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Jay: Access to the channels is based on the serial number of the viewing card in the box. If you ask them for a new box they'll send out a new card with it, and the subscription will be associated with that card. The recording features, including playback, will go along with that card as well.
So, if you put the new card in the old box, you should be able to play back the old recordings, but you wouldn't be able to record on the new box. You probably want to keep the new card with the new box, and put that wherever you intend to watch the pay channels.
To get all features on all boxes, you need a multiroom subscription. Sky probably still have the serial number of the old card on record and could probably reactivate it; if not, they'll send out a new card for the old box.
I'm not sure if Sky's offers would extend to adding a second set of cables to the dish. Their help page on multiroom setup says:
"A standard installation assumes that no additional cabling, bracketry or other equipment is required or any works which are beyond the scope usual in a normal installation. A standard installation further assumes that if you already have satellite equipment, you will not continue to use it and your existing cabling will be used in the connection of the Digital Satellite Equipment. If the installation of your Digital Satellite Equipment is not standard you will be charged a reasonable sum for any additional equipment or works that are required.
"We will advise you about these charges before we install the Digital Satellite Equipment, which may need to be re-arranged for another time. If you already have satellite equipment and you wish to continue using it and/or have it relocated within your home, you must tell us at least 24 hours prior to the installation, and in that case a separate installation charge may be payable by you. If so, we will advise you of the amount of this charge, which must be paid before the day of installation of your Digital Satellite Equipment."
Source: Sky TV | What equipment do I need with Sky Multiroom ? , under "I already have satellite equipment in my home, can I link it to my Sky box with Multiroom?"
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Jean Moore: If you stop subscribing to Sky, the box simply becomes a 'Freesat-from-Sky' receiver. See the "Compare TV" section at Compare Freeview and Freesat TV | ukfree.tv - independent free digital TV advice to see what's available on that service, and how it differs from the BBC/ITV Freesat service and from what's available through an aerial.
If you have a Sky+ box, be aware that Sky will disable the recording and time-shifting ('pause live TV') features. Also, you lose access to any previously-recorded programmes.
If you want to get recording and time-shifting features back without a subscription, you can buy a Freesat+ recorder, which just plugs in in place of your current Sky box.
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Steve: Signals from plain dipoles are diffused in a ring-doughnut shape. With more tiers of panels connected in-phase with each other, you get more gain in one plane and a number of additional smaller lobes. There are some angles that would indeed get no or nearly-no signal, referred to as nulls. The antenna designer therefore adds small differences in the lengths of the connections to each panel, which has the effect of reducing the nulls, at the cost of some efficiency. This is called 'null filling'.
Also, the main lobe of the transmitted signal would normally be exactly horizontal, which - due to the curve of the earth - actually misses the ground completely. Again, the changes to the relative phases allow this to be adjusted down a bit so that the peak signal lands at a reasonable point just short of the horizon - this is called 'beam tilt'.
Both null-filling and beam-tilt adjustment are performed to ensure that even those living very close will get sufficient signal. It will be less than the full 200kW, but people living this close do not need anything like that amount of power.
We don't have the design for the current transmitting aerial, but the BBC report for the old analogue aerial, with graphs showing its performance, is available at http://downloads.bbc.co.u….pdf . The vertical radiation patterns are on p13-p15 (figures 10-12). One-tenth or one-twentieth of the field strength is still very high indeed, there are many local relays intended to serve areas 1-2km away from the mast that have less than 100W. There are transmitters with less than 2W output, indeed there are some 'Link' sites with less than 1W, although these are really to get the signal to another relay . People that close to CP are more likely to have too much signal than too little.
As for the leaves: I believe they're more effective at blocking radio signals at UHF than your walls are, because they're mainly water (even worse when wet leaves). They also move in and out of the path when the wind blows, making for a lot of additional variation.
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Switchover at Dover and Bluebell Hill also *starts* early tomorrow, with BBC Two closing down to be replaced by high-power BBC A.
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Thursday 31 May 2012 2:45PM
Information on the NI mini-mux is now on Digital UK's postcode checker. The selected mode is DVB-T2, QPSK modulation, FEC 2/3, 32K carriers.
Frequencies:
Carnmoney Hill on C48 at 16 watts
Black Mountain on C39+ at 1kW/2kW (directional)
Brougher Mountain on C30- at 1kW
No information on the Guard Interval, but since the obvious pairing of Carnmoney Hill and Black Mountain are on different frequencies, I expect it'll be 1/128. Similarly it will probably use Pilot Pattern 7 to match the HD mode. The recommended configuration in the DVB-T2 Implementation Guide gives 9.954 Mbit/s (the frame lengths can also be altered in T2). Using this mode requires 2.6 - 2.8 dB less signal, compared to the noise level, than the 16QAM 1/2 mode previously suggested, and 14.7 - 15.8 dB less than the HD mode. That's equivalent to broadcasting at 29.5x - 38x the power of the HD mux.
The change in mode could be down to the power level permitted at Brougher Mountain, which is half that previously indicated (i.e. 3 dB less).