All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
steven wallace: Postcode?
A 'new digital aerial' is often over-specified for the job, leading to excessive signal levels.
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Jack Cannon: Most likely you have too much signal. If you have a booster or amplifier, you should remove it. If that doesn't help, add an attenuator.
Since you were able to receive the BBC channels before last night, your box must be able to handle 8K mode.
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Jack Cannon: though at that postcode, Digital UK reckon Tay Bridge, Angus or Craigkelly are better options than the Kinross relay (which the predictor doesn't even list). Can you confirm which transmitter you're using?
Angus is your best bet for a six-multiplex service. Craigkelly will provide a service from late 2012 (possibly after Tyne Tees switches over?) but it's expected to be poor on two of the commercial multiplexes and variable on the other. Black Hill is expected to provide a good PSB service from next week, but no prediction is shown for the commercial multiplexes.
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Gaz: If the flat uses a communal aerial, the system may need adjusting for the new channels and power levels. Check with your landlord or agent - see PARAS - Professional Aerial Riggers Against The Sharks for some more advice.
Looking at the prediction for that postcode - which shows 100% probability of reliable reception across the board - it's probable that you have too much signal. If the landlord won't sort it out, try fitting an attenuator, although if the distortion is occurring at the master distribution amplifier, it won't help you.
Check if your neighbours are affected too.
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Gaz: Sorry, I've just noticed that you posted at 2am, when all services were off. They should have been restored by 6am, so if you haven't retuned after that, try doing that first.
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Dave: The Rosehearty relay only transmits the three public service multiplexes. The commercial multiplex operators - which carry the other channels - don't want to spend the money on having equipment installed and maintained at all the small relay sites. They were quoted about three times as much as their current running costs, about £12m more, for all the relays.
You would likely need a new aerial for Rumster Forest, as the existing aerial is probably Group B, while Rumster Forest transmits in Group A and C/D (i.e. below and above the range that your existing aerial is designed to receive).
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GCD: See PARAS - Professional Aerial Riggers Against The Sharks for some thoughts about communal aerials. It's likely the council will have to have it recalibrated for digital next week.
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Gary: Make sure you're doing a full reset? New TVs should come with a leaflet specifically telling you how to do a retune on the switchover dates.
Apart from that, my next best hypothesis is that you now have too much signal, for which the solution is to reduce signal levels. If you have a booster or amplifier, remove it or turn it down. If not, or that doesn't help, try adding an attenuator (a device that reduces signal levels).
If you previously had interference on BBC Two analogue, it could still be there and interfering with the BBC digital multiplex that replaced BBC Two last week. See Single Frequency Interference for suggestions.
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Nick Battle: The predictions for Mux 2 are better than for many of the other muxes, so see Single Frequency Interference for possible sources of problems. It could be a case of impulse interference caused by electrical equipment switching on or off.
If signal levels are noticeably lower than the other multiplexes, it could be Short Reflector Syndrome, that the aerial's reflector is too short to do the job properly. http://www.wrightsaerials.tv/articles/whatsat-short-reflector-syndrome.pdf
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Wednesday 15 June 2011 6:17PM
Stuart O.: If a set-top aerial works but an outdoor aerial doesn't, it's most likely to be down to getting too much signal from the outdoor type.
Go for a fairly small aerial, I'd try a 25 or even 18-element log periodic, and fit an attenuator to reduce the signal levels. Obviously don't use the amplifier!
Amplifiers - including the automatic gain control and the mixer in the receiver - can only handle so much signal before distorting. This distortion essentially creates frequency-shifted copies of the signal (intermodulation products) which interfere with the original signals. It's the overall signal level, the level from each multiplex added together, from all transmitters received, that is important. (Filtering is usually done after mixing, to put the selected channel on a specific intermediate frequency, as it's easy to tune the mixer but hard to tune the filter.)
The individual carriers making up one multiplex are 'orthogonal', deliberately spaced so this problem doesn't happen within one multiplex. Unfortunately the multiplexes are not orthogonal to one another!
The 'outdoor' aerial should still have better directional properties than the set-top, so you're still likely to get better results overall.
Theoretical signal levels can be calculated with Field Strength Calculator . For an aerial with 7 dB of gain, I get 70.4 dBuV of output from Winter Hill (100kW on C61 at a distance of 71.6km), which is more than the CAI's recommended level of 65 dBuV. The Wrekin (20kW on C23, 55.8km away) gives 69.7 dBuV. You lose some signal through tiles and rafters, of course.
Sutton Coldfield is 97.3km away but you do have line of sight, power levels are currently low at 8kW on C41 giving a theoretical 58.6 dBuV - that's still plenty for the box to decode. Post-switchover level will be 200kW on C43, giving 72.4 dBuV.