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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PedantWednesday 25 July 2012 6:26PM
That should be "why does it affect..." (not "effect"). "To effect" is to cause, to bring into being - it does not mean "to influence". MeganThursday 26 July 2012 7:58AM
I had the digital switch over at the end of May but I'm still being affected during the hot weather! I loose channels, get picture break up and just general annoyance. Our aerial is fine, and we keep retuning the boxes but nothing seems to help. Seems to me that the so called 'boosted' signal was all talk!MikeFriday 27 July 2012 8:32PM
Chatham Hi,
My Mother is situtated about halfway down the valley in Walderslade. Has a roof-moun ted aerial and experiences picture break-up on a Samsung HDD/DVD recorder. When recording, it not only breaks up but locks up, stopping the recording. The Samsung TV is not affected so badly. The BBC FAQs suggest that the issue is co-channel interference for which there is no cure. However, posts on here suggest that an attenuator might help. Is that the case? Where would I find one? Is manual tuning the way to go first? Many thanks!Trevor goldsmithThursday 9 August 2012 5:58PM
We have high pressure in Westbrook margate Kent time 17:30 and have lost all picture quality on all bbc tv stations with no picture.Mike,Chatham.You can purchase attenuators from most TV & Aerial retailers as well as DIY stores,the likes of B&Q,Wickes,etc.I would look up the ATV Aerials and Television of Sheffield website as they stock good quality attenuators and i personally only recommend them as such.
As for your mother's Samsung HDD/DVD recorder i would consider resetting that appliance as such as the current memory could be contributing to the pixellations and freezing up during recording.
As for manual tuning if the appliance allows it,is recommended as such,automatic scanning from lowest to highest frequencies often picks up and stores unrequired multiplexes such as weak pixellated ones lower down the frequency band while a stronger one higher up often fails to pick up and store.So yes manual scanning/tuning is the best bet. MikeFriday 10 August 2012 12:07PM
Chatham Mark Fletcher, thank you for your help - much appreciated. I'll try and manually tune the samsung 'box first and try an attenuator next.Jay.Friday 10 August 2012 9:25PM
Often picture quality is great during the day but usually ends up rubbish at around the same time each evening ( 9pm ). I always blame atmospherics, but why is it always around 9pm, its like the margate transmitter is switched off at this point., and why are only some channels affected, ie BBC channels not ITV or vice versa, I live in central margate which is not a massive distance from the transmitter site. Currently have no ITV channels, 5, 4, quest, dave ect.JayFriday 10 August 2012 10:17PM
@Dave Lindsay.
Thanks Dave, looks like i might have to invest in a more 'up-to-date' aerial as the one i have is about 5 years old and was bought for freeview reception as apposed to digital reception.Thanks for your help.Jay: Freeview is digital! In any case, there is no such thing as a "digital" aerial — aerials work based on the frequency they are receiving.
If your aerial is vertical and pointing towards Margate transmitter, then you have been lucky to receive Quest, Dave and other COM services. This may indicate that the COM channels are strong (as they can be picked up by a vertical aerial probably facing the wrong way). With this in mind, you might be able to re-orientate to Dover, switching it for horizontal polarisation. Gordon MowattSaturday 18 August 2012 12:17PM
Bo'ness Hi,i have two l.e.d.televisions in the upstairs bedrooms of my house both have built in freeview and recieve there reception from an aerial in my loft,i am currently having periodic reception problems from one of the televisions several times during the day,i have checked the connections and do not know if i need to install an outside aerial or a booster please advise me on how i can solve this problem, Regards Gordon.Stephen PSaturday 18 August 2012 2:44PM
Gordon - swap them round to see if the problem is with the TV or the aerial feed.
Is it a single aerial with a splitter? Powered or not? See if by-passing the splitter helps?
Does it feed anywhere else?
jb38Saturday 18 August 2012 3:22PM
Gordon Mowatt: Obviously try what has been suggested by Stephen P, but as far as only one of your TV's having problems is concerned, excluding any possibility of it having a defective aerial connection then the usual cause of this is simply because that the tuner in the set with the problem is less sensitive than the one fitted in the other TV as no two tuners are the same unless the TV's are of identical model numbers, and without knowledge of the method you are using to feed the aerial to both TV's it should really be done by using a two way powered splitter, as that guarantees the same level of signal to each TV plus gives the signal a slight boost.
Also, you should verify that both TV's are receiving from the same transmitter as they might not be as Blackhill and Craigkelly are both possible in your area (having previous knowledge of) this can be checked by carrying out a signal test starting on BBC1 because the mux (transmitter) channel number being received will (or should) be shown along with the strength / quality and this can be cross checked with the mux channel numbers used by both stations as listed below.
You should carry out this test on both TV's using "EPG programme" numbers 1 - 3 - 50 (if HD used) - 10 - 11 - 12, as that checks all six multiplexes used by each station, making a note of the strength / quality seen on each mux channel test and comparing the results between the TV's, as one of is liable to be lower than the other, giving an update on results, and when if necessary, further advice can be given.
Blackhills channels: 46 (BBC1) - 43 (ITV) - 40 (HD) - 41 (ITV3 etc) - 44 (Pick TV) - 47 (Yesterday)
Craigkelly (in same order) : 27 - 24 - 21 - 42 - 45 - 39Carol HallThursday 7 February 2013 2:29PM
Hi, the reception on my tv is a joke,for days now every channel has been to bad to watch.Does anyone know why this is happening?
I dont think its the aerial because occassionally I have received good reception.KMJ,DerbyThursday 7 February 2013 3:54PM
Carol Hall: The William Hepburn Tropospheric ducting forecast shows that conditions are not currently those that cause interference from distant transmitters to occur through the inversion effect. As you are having problems on all channels it suggests that you could either be experiencing impulse interference from nearby electrical equipment or a faulty connection or there could be a problem with your aerial/cables. Try to observe the signal strength and quality on affected channels when reception is poor. Is the frequency being received the correct one for the transmitter to which the aerial is pointing? Try repositioning the aerial cable (and any HDMI cable) relative to the receiver in case there is any interaction. Also check the condition of cable and connectors and do a visual check that the aerial is still mounted in its correct position.Mrs. E BenatarFriday 15 March 2013 4:34PM
Richmond In the last few days I have been unable to get reception on channels ITV 3, 5 USA, 5` I have never had any problems with these channels before (even when others were erratic). Now I just get "No Signal". Why? i would be grateful for advice. Simple please as I do not understand most of the advice I have been reading addressed to other questioners,
Thank youMrs. E BenatarFriday 15 March 2013 4:38PM
Richmond Channels ITV3, 5USA, 5% have suddenly started to give me "No Signal" messages after several years of perfect reception. Why should this happen and what can I do? I have tried going back to Installation but this has not helped.
I am not at all technically minded so a simple answer would be lovely! Thank youupdates
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