All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
Nick: Brian's claim about 'directly in line' is wrong. He persists in reporting, incorrectly, that aerials receive as much signal from behind as they do in front. In fact the level received from behind is significantly less than that from in front - although it is not zero, so a very powerful or close transmission from behind can still be picked up by the box.
Also, aerials have cross-polar rejection. Transmissions are polarized, with the electrons vibrating side-to-side (horizontal polarization) or up-and-down (vertical). For best results the aerial has to be oriented the same way as the transmission. Again, they pick up far less signal from the opposite polarization. Aldeburgh transmits on vertical polarization while Sudbury and Bluebell Hill use horizontal polarization.
It's not clear how cross-polar rejection and the front/back ratio interact. Digital UK's prediction model only allows a maximum of 16 dB of rejection from both sources, the same as the maximum from either source. (Most aerials perform better than this.)
I think Digital UK's predictor has all sources of foreign interference down as occurring in 2013, rather than actually following the foreign switchover dates. In practice many of the foreign interferers have already switched over.
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Andy Moss: You didn't previously mention it was an apartment block. It may be that the communal aerial system is a 'channelized' system, in which case it probably needs retuning.
Channelized systems filter and amplify each frequency separately. If a transmission changes channels, the filter needs to be adjusted to the new frequency.
The landlord or building manager needs to ensure that the system is retuned and the levels set correctly at each switchover step. See http://www.digitaluk.co.uk/transmitternetwork/tools__and__resources/almanac/installer_newsletters_transmitter_groups2009_pdfs/Special_edition-installers_Waltham_retuneFINAL_Aug11.pdf for the remaining steps.
It might be necessary for the system to be adjusted again when the SDN multiplex reaches its final power level on 12 October. You will need to retune on this day because SDN also changes mode.
Waltham is also currently allocated a frequency for the BBC A multiplex, C61, that is going to be released for 800MHz mobile phone services.
Generally allocations of C61 move to C49, which Waltham already uses for low-power Mux 1, but it may not be able to go high-power on that frequency immediately because The Wrekin is still using it for SDN. The final allocation for that service at The Wrekin is being used by Sutton Coldfield until 21 September. If they're sensible, they'll move Waltham BBC A on the 12th of October as well.
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Tudor: It is possible to have too much signal as well as too little. Too much signal overloads the booster (if any) or the amplifier in the receiver. This is known in RF circles as intermodulation or cross-modulation, and the resulting unwanted frequencies are known as intermodulation products. Most often the strongest intermodulation products land in adjacent channels and knock them out.
To see whether your problems might be down to too much signal, please provide a full postcode.
If professional installers are using smaller aerials, it's a good sign that yours is over the top.
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David: By channel 43, I assume you mean Multiplex 2 rather than Gems TV!
The problem here is that Hannington's digital signals are broadcast at seriously reduced signal levels to the east, to protect Guildford's coverage area. Channel 43 is also used by Guildford for analogue ITV1, at quite high power.
For this reason, Digital UK predicts no coverage for you from Hannington. The predictor is often pessimistic - aerials often reject more signal from behind and from the other polarization than the model used for the predictor - but a zero prediction means there will be frequent problems. The prediction is a probability of finding a location for the aerial that gives a probability of reliable results, so you can get a frequently-failing service where the predictor says zero.
The restriction will be removed at switchover, when the digital transmissions move to the main antenna at the top of the mast (which currently provides the analogue services).
It's likely that other changes at other transmitters have caused a change in the amount of interference you're getting, though I can't pin-point a specific change.
If you can't wait until next February, you will have to get an aerial pointed at Crystal Palace, but the post-switchover service from Hannington is expected to be easier to obtain. Alternatively, look into Freesat.
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Reg Burns: Very few Freeview devices have an RF modulator output and none understand Magic Eye, as far as I know. You can get plug-in RF modulators that connect to the box's SCART socket, and other infra-red remote control extenders, but I don't know of any generic remote control extenders that piggy-back on the TV wiring.
If you're not bothered about watching recorded programmes in the bedroom, you could just get a regular Freeview or Freeview HD set-top box.
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nat: Have you checked that all the cables are plugged in, and that they don't seem to be broken? That, or a dislodged aerial, are the most likely causes.
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Derek: The current prediction is non-existent, so you're lucky to get the BBC channels at all. The prediction after switchover is variable to poor, but it should work some of the time.
It should be easy to get a reliable service from Sudbury.
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karen: You should use the right size aerial for the job. Bigger is not necessarily better - you can have too much signal, which can be as bad as not enough, and the aerial will be subject to more force from the wind. This force can damage the mast or even the building itself if the mast is too small or not properly secured for the load.
In most cases, a new aerial does not have to be any larger than the existing one, though it depends on whether the transmitter provides, or will provide, all services within the same group as before.
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Ian Brown: A longer run of cable will drop more signal than a shorter one. One TV might be more or less sensitive than the other one.
If you're going to split an aerial feed to more than one location, you should put the split as close to the aerial as possible. Don't run the cable to the back of one TV and split it there - you've already lost a significant amount of signal before it gets to the first point, and the cable run is likely to be longer than taking a second cable directly from the aerial.
Standard 'low-loss' coax cable is anything but. Use satellite-grade cable (dense copper braid over copper foil screening) to reduce the amount of signal lost in the cable.
Use proper splitters - don't just wire multiple cables into the aerial, because it will no longer be properly matched. If the cables aren't properly matched, you get reflections bouncing up and down the cable. There is some insertion loss just from using the splitter - if this is losing too much signal for one TV, you might need to add a small amplifier to offset this loss.
If it was working fine before switchover, and the non-working TV is on the shorter cable run, you could have too much signal. If you have an amplified splitter, try turning the amplifier down or removing the amp entirely.
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Thursday 5 May 2011 3:53PM
Michael: If your analogue signal is now also poor, that would suggest that the aerial is damaged or become misaligned, or that a cable is damaged.
The prediction for digital service from Bilsdale, at that postcode, is good for Mux 1, but poor for Mux A and C and no prediction given for Mux 2, B and D.
It's possible that the switchovers of Sandy Heath, Ridge Hill, The Wrekin, Lark Stoke and Bromsgrove, plus changes at Waltham and Fenton - all of which now transmit digital signals in Group A - may have had an effect on interference, though this should only have an impact at night (when signals travel further) and in high-pressure weather conditions.
Digital UK's prediction suggests that Emley Moor could be a better alternative in the long run anyway. It switches over at the end of September.