All posts by Michael Perry
Below are all of Michael Perry's postings, with the most recent are at the bottom of the page.I agree with jb38, on site diagnosis is always best - just not always possible.
Those thinking there is any difference between a group C/D aerial for analogue TV signals and for digital TV signals has been misinformed, the signals are all UHF RF and the only difference is the modulation format and that doesn't alter the design requirements for the aerial. The key criteria are primarily about frequency range, gain and directivity as well as ability to reject any 'back/side lobe' reception. It is a sad fact that some of the 'less expensive' aerials have poor performance towards the extremities of the designed reception band, so some group C/D aerials may not perform as well in the Ch48/9 part as they do in the Ch61 part. Hence it may be worth having the aerial changed to a Group E so ensuring you get good coverage for the reasonable future - unless the 'powers that be' decide to make even more changes as has been rumoured.
Further information at http://www.bbc.co.uk/rece….pdf It also gives the colour codes for the different groups, E=brown, C/D=green (it gives all the others as well).
The aerial described by Richard Tonkin is a typical X-fire pattern commonly used for the modified Yagi pattern aerial needed.
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Julian
Take a look at the BBC factsheet, http://www.bbc.co.uk/rece…pdf.
A group C/D aerial will have a green colour code somewhere and a good example covers channels 48 to 68, cheap ones may not do so well though at the ends, so Ch48/9 might not be so good. As the change has reduced the signal quality of the pictures, you may benefit from changing the aerial. A Group E aerial will cover all the current channels easily, they are identified with a brown colour code. Note that there is no difference between an aerial for UHF digital TV reception or any other form of UHF TV signals.
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Julian
Glad to be of some help. A loft mounted aerial tends to receive slightly less signal than an externally mounted one due to absorption by the tiles, especially when they are wet. It being 1m lower is of little consequence except in critically difficult locations. If you have a good chimney stack in good condition and accessible by ladders then a good aerial contractor should be able to mount a new Group E aerial externally. Without actually knowing the local reception conditions I couldn't guarantee anything but technical experience suggests it should at least help and be suitable for the future unless 'they' (Ofcom, et al) decide to change their plans.
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Aerial height discussion.
The actual height an aerial needs to be depends entirely on local signal reception conditions. Higher is not always better and lower is not always worse. Example, in 1969 a road in South Croydon was found to have very poor reception of UHF ITV and several houses had aerials fitted on very long, guyed, poles but still had problems. Investigations showed that in many cases a lower aerial height cured the problem. It was caused by the curved copper roof of the Fairfield Halls about half a mile away, the curve acting as a director for some UHF signals and 'bending' the ITV signal to leave a blank spot. So higher is not always the cure and such instances should always be carefully investigated and analysed.
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Aiming dishes at satellites, getting the correct aiming angles.
Though the 'orbital' position may be at 13 degrees East, that is not the correct azimuth (the East or West angle away from the Greenwich Meridian) for the dish to see the satellite! You need to calculate both azimuth and elevation for the exact location of the dish and receiving site. You could use Calculate Azimuth And Elevation Angles and enter the latitude and longitude as well as the height above sea level, it does matter, and then add the orbital longitude position of the wanted satellite, e.g. +18 for Hotbird with a latitude of 0 (it's nominally geostationary above the Equator). The calculator will show the correct aiming angles for the dish. Remember that an 'offset' dish, where the LNB is on a boom and is below the dead centre of the dish, actually receives the signals from the equivalent angle above the dish, it actually 'looks over' the the LNB. So, take a measurement of the angle that the LNB is below a perpendicular drawn from the dish face top and bottom edges, then the elevation setting (using the same front edges of the dish as the reference) should be the calculated angle minus the offset angle. Yes, it looks at a point higher in the sky than it appears to.
The further north the receiving location the closer the azimuth is to the orbital position, but it's not the same. Elevation is lower further north too. Further south, the azimuth is rather greater than the orbital position suggests.
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Whilst it is often not advisable to diplex two aerials into a single feed there is another possible solution, one that I used when living in Swindon and could receive from Mendip and Oxford main transmitters. Both aerials were mounted on a good strong pole, thicker than is normally used, and each had a separate good quality downlead into the lounge by the TV set. They connected to a coaxial switch that had two inputs and one output, so only one aerial was connected at a time. Simply moving the manual switch allowed selection between sources. The switch has to be a good quality coaxial one and fully screened to avoid picking up interference and to maintain the integrity of the coaxial connections. It must be designed for 75 Ohm coaxial UHF systems, can be either IEC coax or F-connector but not BNC connectors. Works with all UHF TV signals with no problems if cables and connections are made properly. You'll need to search for one but avoid any 'automatic' ones.
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Looking at the Freeview Channel Guide, at http://www.freeview.co.uk…els, shows that Eurosport is not available on Freeview! Channel 33 on Freeview is used for ITV+1. Any TV guides that show it as available must be in error.
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Tim
It is rare for an aerial to give 'too much' signal without a booster/amplifier unless you are very close to a main transmitter.
In analogue days you would see strong cross modulation patterns on the screen but with digital you can only go by the signal strength and quality guides. If the signal is too strong then the Strength will be 100% but the quality will be very low. You can check it by inserting a coaxial attenuator in the aerial lead, preferably at the TV UHF input for this test purpose. They are available with different attenuation, typically 6dB, 12dB or even 18dB. If by trying one the strength stays high but the quality improves significantly then there may be too strong a signal.
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James
There is no difference between a UHF aerial for digital or analogue reception in Bands 4 or 5, see what the BBC has to say on the matter at http://www.bbc.co.uk/rece….pdf
This shows that you do NOT need to have any 'special' aerial at all, but you do need the correct one for the signals to be received (i.e. correct group and suitable for the location) and of good quality properly mounted with good coax cable feeding the TV.
Many Group C/D aerials have poor performance around Channels 48 & 49 so reception of the mux's on those channels can be poor compared to higher channel numbers.
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Sunday 7 April 2013 11:55AM
Winchester
To KMJ, Derby (and Chloe). That's what I suspect as well and hence contacting BBC Reception for them to be aware of it and hopefully investigate. Arqiva only give a postal facility (Arqiva, Crawley Court, Winchester, SO21 2QA, or phone their main switchboard +44 (0)1962 823434) and no means for a viewer to contact them by email, webmail, etc!
To jb38. Aerial alignment used to be easier in analogue days as you could see the effects of reflected signals, as 'ghost' images, and align for enough signal with minimum 'ghosting' (by minimising back/side lobe reception). But that is not possible with a digital system even though the reflections may be a significant cause of poor signal quality. Hence the need first to ensure sufficient signal strength before looking at the quality of that signal, either on a professional test instrument (expensive) or the receiver's own software systems. Most TV viewers only want to watch the programmes and not be troubled by the technicalities, so one should start by following the KISS principle. Check the simplest/most obvious first then if that doesn't reveal the cause investigate further. Remember that many who post here are/were not in the business but concerned viewers who have lost the ability to watch favourite programmes due to the changes made at Mendip (and the potential for problems glossed over by the regulators) and are seeking help and guidance.