All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
Steve: My understanding is that the length of the reflector elements is more important than the number and type.
The stamped plates are usually indicative of 'contract' aerials which have relatively poor gain and usually uneven gain across the band.
Justin Smith of AerialsAndTV.com performed some tests with the reflector missing and fitted incorrectly at Aerial Positioning Tests . Bill Wright has written about short reflectors at http://www.wrightsaerials.tv/articles/whatsat-short-reflector-syndrome.pdf . I'm sure I've seen one of them comment on folding the reflector plate to reduce its height, with no noticeable impact on the performance, though that probably matters more for vertical polarisation acceptance angle than for the horizontal example.
Briantist: Please look at some polar diagrams at Aerial Polar Reponse Diagrams . The claim that aerials work nearly as well behind as in front is completely unsupportable. If the reflector is missing, this is possible, but if all elements are intact, it's simply untrue. The diagrams show -20 to -24 dB front to back in the examples.
Digital UK's predictor only allows for -16 dB of attenuation, based on ITU-R recommendation P.419-3, so it can indicate co-channel problems that turn out not to be a problem.
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Steve: The problem is simply the very slow roll-out across the country. By deciding to increase power at some locations before others, it causes problems for digital reception at the transmitters that are yet to switch over. (They're trying harder to avoid interference to analogue transmissions, which generally isn't hard because they're reusing the analogue frequencies where possible, which were *designed* not to overlap.)
They have also decided to increase power at all sites by more than is really required, which is causing more interference from digital signals than from the analogue signals that previously occupied those channels.
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Richard: You need to find out if your TV or box can do DiSEqC signals. If so, you can buy a DiSEqC switch that automatically switches between the dishes when instructed by the box.
There are also manual switches available, but - like any device inserted in-line - they do reduce the signal level a bit.
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Robin Smithett: You're very close to the transmitter. It's probable that you now have too much signal, which causes amplifiers and tuners to be over-driven. This causes intermodulation - signals bleeding out of their assigned channels and causing errors in, particularly, adjacent channels. If you have an amplifier or booster, remove it. If not, try adding an attenuator to bring the signal levels down.
If you have a PVR-9200T, aim for less than 60% on the signal strength meter. The calibration of this meter might be fine for a clean-room environment with only one digital multiplex, but poor for six muxes plus fringe pick-up of other transmitters.
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AlvinF: You're very close to a very high-power transmitter. If you have a roof aerial, it's possible that you have too much signal. If you have any amplifiers or boosters, try removing them.
My expectation is that they're moving services from one antenna to another (though I thought they'd finished this already). The antennas and transmitters are constructed in halves for redundancy. It's likely that they're powering down one half of the first antenna, moving that connection to the first half of the second, then powering down the second half of the first, turning on the first half of the second, then repeating with the other half.
A reduction to half power is not actually all that significant, and would only cause problems if your reception was already marginal - but it doesn't seem likely at such a short distance from such a major transmitter.
You've mentioned a loft aerial before - for tips on siting a loft aerial see http://www.wrightsaerials.tv/articles/whatsat-201007.pdf . I would strongly recommend a roof-top aerial, particularly if, as it appears from Google StreetView, the aerial might have to point through neighbouring properties.
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chris pugh: My guess is that your box is one of those that tunes in the first version of the signals that it finds, rather than the strongest/best-quality version. It's probably tuned in the weak (because off-beam) signals from The Wrekin on channels 26 and 23 rather than the strong ones from Brierley Hill.
The Wrekin switched over starting on the 6th and completing on the 20th, so now transmits much stronger signals than it did a few weeks ago.
Try deleting all the channels, then manually tuning using the frequencies at the top of the page.
Note that there was work on the Brierley Hill transmitter for a few days before the 15th, so it's possible that just retuning the TV will sort out the problem. It's possible that a new transmitting aerial was fitted, which could have changed the received signal levels. At less than two miles from the transmitter, it's more likely that you have too much signal than too little - remove any amplifiers or boosters you have. If none, or it doesn't help, try fitting an attenuator.
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Mick: Digital UK's predictor gives you a slightly better chance of receiving all six multiplexes reliably from Bluebell Hill rather than from Crystal Palace, both now and after switchover.
DUK's predictor is based on better information - that the broadcasters won't release, or which costs a big chunk of money - than this site.
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mike: Then the property developers have provided a communal system. Hopefully they were well-advised on selecting the proper transmitter to point the communal antenna at.
At that postcode, Digital UK advises that the best transmitter is Sandy Heath, which has just switched over. You should get good results from this transmitter, as long as they have had the communal system adjusted for the new frequencies. There will also be retunes on 31 August, 14 September, 23 November, and the middle of next year, as the commercial multiplexes straggle in to their final channels - again, each one may need adjustments if it's a 'channelised' system.
Sudbury and Tacolneston are closer, but it looks like there are terrain problems.
There is a slight possibility that they haven't provided Freeview at all, and the wiring is designed to take the RF output of a Sky box and distribute it around the house. This is most common where there is no possibility of Freeview, though.
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NickA: The *commercial* muxes - SDN, ArqA and ArqB - remain on pre-switchover channels at pre-switchover power levels. The HD multiplex has launched at full power and on its final channel.
If BBC One is working reliably, if your equipment is Freeview HD-capable, you should get reliable HD channels (but do see Single Frequency Interference).
"HD Ready" does not mean it can receive Freeview HD signals. It means that it can handle HD-format pictures supplied by another box. Freeview HD uses a very new transmission standard only completed in mid-2009. You must look for the Freeview HD logo.
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Tuesday 26 April 2011 5:20PM
Gerry Lowe: The Wenvoe transmitter, near Cardiff, only transmits the Welsh variants. It has only ever transmitted the Welsh variants. If you want English variants, you need to point your aerial at the Mendip transmitter. However, there are parts of the Bristol Channel coast that are shadowed - Digital UK suggests that your reception from Mendip will be variable and will get worse as the switchover programme completes.
If the aerial is already pointing at Mendip, see Digital Region Overlap for suggestions.