What is the Inversion Effect and why does it effect my Freeview TV reception?
Under normal circumstances, the signals from each television transmitter can only be received by those homes and businesses that have aerials that have a direct line-of-sight to the transmitter.
The part of the signal from the transmitter that is directed upwards simply escapes into space and are lost.
When there is high atmospheric pressure (BBC Weather - UK Pressure chart) as the sun heats up the ground the warm air gets trapped underneath the colder air higher up. At the point where the warm air gets trapped under the cold air this creates a layer that is, in effect, a mirror for the television signals.
This means that signals that would otherwise be impossible to receive can suddenly effect your television reception.
The digital television signal uses a format called COFDM (coded orthogonal frequency division multiplexing) and this incorporates the ability to ignore reflected digital signals.
However, analogue television signals do not co-operate with each other and the picture quality is reduced. The very weak digital signals that were used before digital switchover for Freeview were badly effected by interference from reflected signals from adjacent analogue transmissions.
I recommend you look at the Tropospheric Ducting Forecast for VHF and UHF Radio and TV page for a current forecast - yellow, orange, red, and pink indicate that conditions are perfect for the effects listed above.
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alan mcintyre: multiplexes 2 and A are more prone to problems with unusual weather conditions due to their use of 64QAM mode. Philip JesseMonday 2 July 2007 6:05PM
Caterham Dgital is going to be a disaster what with extra costs to be incurred by viewers and, apparently - according to BBC Information" rain falling on nearby trees will disrupt the signal.
Whenever it rains all my BBC channels go haywire, so much for progressPhilip Jesse: It's true that "rain falling on trees" can have that effect. However, I should point out that the current digital signals are at very low power levels because they use the "padding frequencies" that were left between analogue transmitters to prevent interference. For reliable digital reception you must have a roof-mounted aerial that has a line-of-sight to the transmitter. This is also true for analogue, but results in snow rather than the loss of signal. You happen to live somewhere with complex reception options. Simply using a larger aerial will help right now, and you can also use other transmitters, but you will need a pole long enough to get up high enough to avoid your trees! Also the Crystal Palace transmitter will have 10 times stronger digital signals after switchover. Gerald HillSaturday 4 August 2007 8:10PM
Potters Bar Although I am supposed to be in a good reception area (EN6 2BU), reception, especially in sunny weather is pixilated.
Today 3/8/07 at 1700 approx all digital channels became useless and yet very soon, digital TV through my existing arial will be my only choice.
How do I overcome this technical stupidity without spending even more money than the cost of two set top boxes?Gerald HillSaturday 4 August 2007 8:14PM
Useless response. I do not want excuses, I want improved reception.Laura JThursday 9 August 2007 7:58PM
Liss I am having similar problems. For months, all signals have been fine. now, for the last month, I have found that the ITV channels and most of Channel4's channels are unwatchable, due to poor reception. I thought this was initially because of the terrible weather we were having, but now it has cleared up there is no change. My partner and I thought if we upgraded our box (which is a very old ondigital box!) it would help - is this the case? Or is it a more permanent problem. I'm not waiting for the switchon to be able to watch television!(location - Liss, Hants)Laura J: Upgrading your box may work, but the problem suggests that your aerial is too small, or not located high up. You should probably check your cables and connectors first too, as this is a free fix. You might find a Freeview Playback box a good upgrade! Laura JWednesday 22 August 2007 8:58PM
Liss Briantist: had checked all the cables and connectors beforehand using previous advice on these pages, rescanned and replugged etc., to no avail.
I don't understand why my aerial would suddenly be too small and low when for six months we were perfectly able to get every digital channel going, unless the winds have knocked it in the wrong direction (which we're having checked). Thanks for the advice on the box. However,I'm still cross as I can only receive 3 digital channels overall now! Laura J: You are in a location where reception should be straightforward, so I'm sure that once the aerial is checked you will be back to full service. BenWednesday 17 October 2007 6:02PM
At least when analogue is effected by bad weather you only get a degraded picture. Digital splutters, coughs and freezes to the point where you cannot actually watch a program.
We live in a strong reception area, BA3, and the rain last night meant that BBC3 was completely unwatchable. And our aerial and setup is confirmed as perfect.
The digital switchover is going to have its problems despite what the advocates say.Mr. A.J. PointerFriday 14 December 2007 3:29PM
I have just been told (14th December 2007) that Sky will no longer sell cards for 'free to view' channels.Is this true please?Mr. A.J. Pointer: Call 0870-606 1111 and they are £20+VAT, pay by debit card. S J ArbiterWednesday 26 December 2007 4:22PM
I have a Panasonic TV with built-in Freeview. Until about a month ago I could get good reception on all the Freeview stations except on odd days when (I assume) meterological conditions caused a drop in quality. Now Freeview (and ITV1 and Ch4) is poor every day with extreme pixellation and frequently 'No Signal' captions. What are my options to improve reception? I'm in IP31 and I should say my high-gain aerial is pointed to Sudbury tx rather than Tacolneston tx as there are huge trees in the way (in my neighbour's garden).roman haraburdaSaturday 15 March 2008 10:23PM
I live in coventry. I have noticed that when it rains I lose Film 4. I have a bog standard DC18 aerial pointed at Sutton Coldfield. My Dad who also lives in Coventry sees that same thing even though he has a proper digital aerial.
Why does this happen?
ThanksBrian HallThursday 1 May 2008 7:47PM
Stoke-on-trent I live in Stoke - ST6 6XL - and about four weeks ago my Freeview signal from Sutton Coldfield became very poor, and has remained so. It has been fine for at least three years previously. My mother, 10 minutes away, has also suffered the same problems. Retuning doesn't help. Any ideas about this sudden poor reception ? Richard MerrittThursday 1 May 2008 10:26PM
Ivybridge Here in Devon at PL21 9SD we don't yet get a Freeview digital signal but we do receive analogue signals for BBC 1 and BBC 2 as well as ITV and Channel 4, but not Channel 5. This part of Devon has always been a TV backwater. However, I discover from the www.digitaluk.co.uk website that after digital switchover next year we will have 11 digital TV channels and this will not improve afterwards. These channels are:
ITV1
Channel 4
ITV 2
ITV3
Channel 4+1
More Four
ITV4
E4
Teletext
Teletext Cars
Teletext on 4
Note that this does not include any BBC channels. So after switchover next year our digital ready TV will not enable us to watch core BBC programming. Is this progress? I certainly find it totally unacceptable and question why we should continue to pay a licence fee for a service that will not be provided to us.
James Muir: What 'Engineering enhancements' are you on about? James MuirFriday 2 May 2008 10:06AM
I can't get any specifics from NGW as its the equivalent of trying to get blood from a stone. Engineering enhancements made to the Sutton Coldfield transmitter may affect viewers using older set-top boxes in the Sutton Coldfield transmitter region including Birmingham and parts of the West Midlands.
These enhancements are required as part of the digital switchover programme. Some older set-top boxes are not compliant with the standard required for digital switchover and will no longer work. These boxes were designed before the full standard for digital switchover had been worked out. It is not possible to upgrade these boxes and a new replacement set-top box will be needed.
James Muir: What makes you think there are 'Engineering enhancements' then? Sorry, but there can be no changes to the broadcast format without Ofcom approval. James MuirFriday 2 May 2008 2:31PM
I'am under the impression that the changes at Sutton are to do with the format of the metadata. A change from 2K to 8K FFT. Its the next step in prep work and certain older boxes will not meet this need requirement. Ofcom has with prior approval allowed specified sites to adopt this method and Sutton Coldfield is included. This is quote from one of their own documents. The parameters have been revised to permit the use of the 8K DVB-T transmission mode at specified transmitter sites, with prior Ofcom approval. This change will help facilitate digital switchover at those transmitter sites where, for technical reasons, 8K mode will need to be adopted.
James Muir: They won't change the transmission mode until switchover, it's part of the agreed process. Denys EdwardsTuesday 13 May 2008 1:32PM
Leatherhead We have a major problem with the hannngton transmitter -Our customers are regularly loosing all digital channels-retuning brings them back -our biggest sales are of Sharp tvs and as most of our customers are over 60!! we end up going out to the customers f.o.c-we are also amazed at the government allowing a monopoly of multiples who,s staff would have difficulty recognizing one end of an F connector from the other-to be the only ones "capable of selling" Freesat.-oh really?-I admit we have a few idiot net dealers in our group-"Euronics" who would just add 2 % to the buying price in order to get the best volume-but that is called compettion and I though our goverment was for that
Richard MerrittThursday 22 May 2008 1:24AM
Briantist: You replied to my posting on May 2nd stating that everyone will get the three public service multiplexes. My transmitter will carry them but the prediction for reception gives me only one, Mux2. If you don't believe me look at 
Postcode Checker - Trade View Currently I get perfectly good analogue BBC signals but the one multiplex that I am predicted to receive does not include BBC channels. I guess I have to go Freesat.
Richard Merritt: I don't know what you are looking at, but the link you provide does indeed show that you "Mux1/ BBC A, Mux 2/ D3&4 , Mux B/ BBC B" after switchover... Colin LawrenceSaturday 24 May 2008 8:06PM
Runcorn Since the middle of last week i've beeb experiencing piture break up on the itv 1 2 3 4, qvc & sky sports news channels via my freeview.All bbc based channels are fine.signal strength is fluctuating between 0 & 100%.
This used to happen some 12 months ago, but stopped.
I recieve my signal from winter hill, any ideas? Richard MerrittThursday 29 May 2008 9:21PM
Ivybridge Briantist: Re follow up 4 days ago. The link I provided is for the aerial trade and shows that Mux 1 and Mux B will not meet the planning standard for acceptable reception at my location. The consumer information provided on the same www.digitaluk.co.uk website states that after digital switchover next year we will have 11 digital TV channels and this will not improve afterwards. These channels I assume will be on Mux 2 and do not include any BBC channels. Most of the people I know in this part of Devon outside of Plymouth currently have no freeview transmitter and have similar or even worse predictions, yet we are to be one of the first regions to have our analogue signal switched off. Most of us have decided that we have to go freesat. How typical is this likely to be for rural areas?Richard Merritt: I've looked at it again, I guess you are taking the 'Marginal – may be usable but subject to periodic interference' value to mean 'no reception', rather than 'occasional interference'.. To answer your question, 'how typical is this for rural areas?' - it depends. You are right to note that Freeview coverage is being predicted for 'percentage of population' and therefore there is a bias against rural areas. However, as most TV transmitters are located in rural areas, there are many rural places where reception will be better than those in Urban areas (where tall buildings often spoil reception). However, the problem tends to be hilly areas, such as your location to the south of Dartmoor. So, basically, if you have problems with TV reception now OR your are using a low-power relay transmitter (such as you are from Ivybridge) then get Freesat now, because it will give you more than the Freeview Lite you will get after switchover. So, in summary, it is hilly areas, not rural ones, that will have problems and Freeview Lite. Jos JoslynSunday 8 June 2008 3:45PM
This discussion about inversion affected reception is not always going to be accurate. My 'argument' could be quite lengthy, so forgive me...
I live in postcode RG28 - about 6 miles 'line of sight' from Hannington mast(Hampshire), with about 3 trees between my roof-mounted high-gain 40+ element aerial. Reception, generally, is excellent but occasionally there is a "dip" in the signal quality & strength. One such incident happened in early May when we were experiencing a good spell of weather. BBC channels using MUX1 16QAM were severely affected, whereas ALL other channels - i.e. those NOT using MUX1 were totally unaffected. I have two issue here :
1. The topic of inversion affected reception... the May incident was NOT affected by inversions, as there was no such inversion. Yet reception for all MUX1 channels were severely degraded. As an example the 'sounding' from Herstmonceaux for early May had only two very minor inversions - and describing them as inversions is stretching it somewhat, at barely a 1deg C increase for just a few metres before correcting and returning to the normal adiabatic curve. In fact today, where we have similar conditions - a southeasterly airflow, high pressure - we have a significantly larger inversion of nearly 5deg C. The reception is unaffected, signal strength and quality are 100% for both. In May it could barely register 5%
2. The use of 16QAM over 64QAM due to robustness. The fact that the MUX1 channels which use 16QAM were so easily degraded, beggars the question 'is 16QAM really that more robust' or 'is there an issue with the implementation of 16QAM'??
So, if it isn't inversion, and everyone insists that 16QAM would make signals more resilient than 64QAM, why were these channels unaffected, whereas the more 'robust' channels were useless. Is there a third reason?Jos Joslyn: Inversion is usually only a problem when there is static high pressure. If the areas move around or appear and disappear there effects are tranistory, less of a problem. I would have thought that Inversion would not be the problem at your location, but I can't be sure without a full postcode. Even if Inversion is a problem, the effect has little to do with the transmission mode and all to do with what signals the inversion is bouncing your way. If you have most Mux 1 from a transmitter, the problem will be a distant analogue transmitter using the same frequency. However, the problems you have is much more likely to be a local source, not a distant one. 
Single frequency interference | ukfree.tv - independent digital television and switchover advice, since 2002 There is considerable research into the effects of 16QAM vs 64QAM mode, for example: 
BBC - R&D - Publications - WHP056 Jos JoslynSunday 8 June 2008 4:59PM
Basingstoke Briantist : My postcode RG28 7NF. If loss of MUX1 - 16QAM signal was due to "a distant analogue transmitter using the same frequency", Then it does bring into question the "apparent" 'robustness' of 16QAM, given that it was so easily disrupted. And therefore the assertion that 16QAM is the better option is questionable, by any measure of scientific reasoning. Clearly inversion is as I have stated all along not an issue, but was the one offered up by the broadcast communities. Again, I state the meteorological evidence never supported a day-time inversion during that periodJordySunday 8 June 2008 8:34PM
Jos Loselyn - I agree with Briantist the research into the changing of the transmission modes was not only well known but endlessly researched by myself among many others. The entire broadcast network is in a state of flux as 2 formats are being broadcast from single transmission sites as part of a large network, it seems to me now that everyone thinks they're a broadcast expert but don't grasp how complex this project is. The decision to use the 16qam mode over 64qam is a sound one long term which is what is planned for. It has nothing do do with inversion, tropo or even science. You may well find that the BBC channels are slightly more robust because they use a different method of transmission, 16 QAM rather than 64 QAM. This is at the cost of fewer channels per MUX but I think it was a wise decision to use it. Having said that, the most likely cause of superior BBC channels is because they are often broadcast within the original transmission group. A Digital TV picture is not superior to a good analogue picture. In fact it is actually worse because the broadcasters want as many programme channels as possible they have sacrificed picture quality and a robust signal to that end. The issue you raise about distant transmitters interfering with one another will continue, there are not enough spare frequencies to broadcast 2 formats across this country as recently demonstrated in the south east of england. The robustness of 16 qam will in years to come prove its worth and people need patience as spectrum is at breaking point. Jordy: It is so strange watching BBC HD and ITV HD on an SD TV as the picture looks like an analogue one again, without the awful compression artifacts.
Jos JoslynSunday 8 June 2008 11:58PM
Jordy : If you had read my thread you would have seen that far from stating that the BBC channels are more robust I am stating that they seem more 'prone' to reception degredation, and therefore are NOT more robust than the channels still opting to use 64QAM. Rather than repeat myself I would suggest that you refer back to my input into the thread. And my name is Jos Joslyn .... no idea who this fella Jos Loselyn isJordyMonday 9 June 2008 1:28AM
Jos Joslyn - It nonsense to suggest that BBC channels are more prone to degredation than any other uhf channel or frequency. The major differences between digital and analogue is the more stringent reception conditions that will be required, 16qam is a much more reliable carrier signal. The compression rates with this bandwidth are not as good as 64qam but its allows the conditions that have been layed down for broadcast to be met. You need to under stand the technical parametres required to operate a dtt network with the spectrum available. The current network is under constant change and people like yourself don't seem to realise the technical specifics involved . Explain to me what reception degredation is experienced on 16qam channels. Digital television pictures can be affected even more than analogue pictures because the broadband carrier broadcasts when affected by interference or cci its causes a pixalted or frozen picture to form on screen, the more stringent requirements with 16qam transmission lessen this impact but this will be always be a fact of life with dtt. I've read your input to this thread but it is simply incorrect. I suggest you research this topic and i'd be happy to answer any questions.Jos Joslyn: Please note that "robustness" has a very specific meaning. It is a measure of how, in a given area of coverage there will be less places where the signal is rendered below the reception threshold by interference. It does not mean, and should not be taken to imply that ALL 16QAM signals at ALL locations will be easier to receive than ALL 64QAM signals. That would, given the mixture of modes and signal levels and the intermix with the existing analogue network, be impossible. Please, please just read this -> BBC - R&D - Publications - WHP056 Paul NaishThursday 26 June 2008 11:29PM
Question for Jos Joslyn. Our Postcode is RG40 (Hannington/Guildford notch), we still cannot receive MUX1 and its been at least a month now! Ignoring 16/64 QAM arguments for now; Why can't we receive MUX1 after all this time?, and is the loss going to be permanent? Is there a stronger distant transmitter come on line that is swamping our high gain setup?
Mux 1 used to give the best reception, all gone now, only the other muxes ;-(
PNJordyFriday 27 June 2008 3:34AM
Paul Naish - Your issue is Rowridge. Your answer is this - There were some co-channel issues/ conflicts involving Midhurst and Rowridge since dtt started. From memory there was no reliable reception on muxes 60 & 64 but a slight null in the tx aerial due east of midhurst had to be adopted to stop co channel from Reigate and Heathfield.
Paul NaishFriday 27 June 2008 12:26PM
Rowridge is close enough but doesn't broadcast MUX1 on ch50. I'm not clear what the rest of the posting means. What happened at the beginning of May?
PN JordyFriday 27 June 2008 1:52PM
Paul - When dtt was first explored it was fairly well known that there would be issues in the south east of england? I assume this is what you meant by the beginning of may? Certain protections had to be included such as protection for transmitters in holland and france. The amount of transmitters in the area and hence it being the most populated area in the country left co channel issues. Co channel refers to two stations broadcasting on the same frequency. In turn you get (cci) co channel interference. During high lift conditions these stations would interfere with one another causing poor reception. Nulls were adopted at most of the sites (nulls, voids or gaps in service) Midhurst does not transmit to the east of its location so it does not interfere with Reigate or heathfield, the same can be said of hannington which does not transmit to the north and east so as not to interfere with guildford and so forth. Recent changes have been made at certain sites unfortunately as i'm no longer contracted by arqiva i'm not privy to new information and getting radiation patterns from arqiva is impossible, i have no idea why i don't know what a terrorist would do with a map of the hannington's erp? Paul NaishFriday 27 June 2008 3:17PM
Hi Jordy, I'm not sure that there's been any change in Hannington or the null to the east. I firmly believe there's a new DTT service somewhere, probably northern France, which is now interfering on ch50. We have an extra high gain wideband (a rough guess at about 50 elements) on a 3 metre pole at the apex of our gable-end, and a mast mounted 3dB NF LNA + about 12dB gain in our downstairs DA/PSU. We are probably right on the knee of process gain so any additional interferer, even distant, will take us over that knee. I'd like to know what has come on air in the past month so I can decide whether or not to go over to freesat. If its simply UHF lift, I'll put it off. I wish they'd stop procrastinating about DSO and just do it. Thnx 4 your 2p-eth PNPaul NaishFriday 27 June 2008 4:16PM
Wokingham Just received from the man at the Beeb; "I understand you're experiencing problems receiving some of our channels.
"There have been a number of faults as well as planned maintenance at the Hannington transmitter since the beginning of May, so there would have been periods of reduced power or complete shutdown. However we generally tend to limit low power periods to non peak viewing hours, usually during mid-morning and early afternoon."
MUX1 is giving between 0% & 5%, MUX2 is effortlessly cruising at 100% without dipping at all (at 4.00pm).
I just hope it'll all be back to normal before too long. Nice of them to publish it. PN
JordyFriday 27 June 2008 6:01PM
Paul - Hannington has a 'Horizontal Radiation Pattern' that significantly reduces the amount of signal radiating towards the east. The reason for this is because it happens to be co-channel on some multiplexes with the transmitter at Guildford, hence increasing Hannington would end up reducing the service area of Guildford so ultimately be pretty pointless presumably since more people are served by analogue Guildford than live to the east of Hannington who aren't served by other digital transmitters. The plan was to re-channel some multiplexes to remove this co-channel problem and then allow the radiating pattern to be changed to allow Hannington to broadcast more signal to the east.
Following the implementation of the CP5 package, Hannington is operating at 10kW on all channels. This proposal requires only that the restriction on the HRP of Hannington antenna protecting Guildford analogue viewers on channels 40, 43, 36, 50 is removed, increasing the service to the east. The removal of restrictions results in a large gain in Hannington DTT coverage at the cost of significant re-tuning in the gross Guildford analogue service area. The Guilford analogue relay was designed o provide a higher level of field-strength than is now considered necessary and as a consequence it has a gross service area which has unnecessary overlaps with adjacent stations. It is mainly in these areas that interference will be evident and as there are a choice of alternative services, in the large majority of cases re-tunes should be problem free. However a proportion of these will be changed to a different region.Paul NaishFriday 27 June 2008 6:19PM
Wokingham So what's changed? Did they decided in May that the null to the east of Hannington should be deeper? And why the lack of information? MUX1 on ch50 has been the best mux until the beginning of May. Now we cannot receive it at all. And what's the CP5 package? PN 
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