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What is the Inversion Effect and why does it effect my Freeview TV reception?

What is the Inversion Effect and why does it effect my Freeview TV reception?

What is the Inversion Effect and why does it effect my Freeview
published on UK Free TV

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 Inversion Effect is also known as "Tropospheric Ducting")

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 (Met Office - Surface pressure charts) 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 did 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.

Links to current pressure data

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Wednesday, 4 May 2016
alexei romanov

4:53 PM

Anje Yelf: I have the exact same problem with my Samsung.

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alexei romanov's 48 posts GB flag
Thursday, 5 May 2016

2:53 PM

Anje and Alexei:

One possibility could be that you are too close to the transmitter and have too much signal. Try searching this website for 'too much of a good thing'. You should also give yopur full post code (or that of a very nearby shop of post office) so that the relative locations of your equipment and the transmitters can be determioned, plus allowing a look at the local terrain which can sometimes cause such issues.

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MikeP's 3,056 posts GB flag
MikeP's: mapM's Freeview map terrainM's terrain plot wavesM's frequency data M's Freeview Detailed Coverage
Friday, 13 May 2016
Brian Passingham
12:27 PM

Recently the TV channel 8(Solent) I had very poor reception and Signal. It has now gone onto CH7, it is a little better, but even now I am only getting 47%strength, 50/100signal. But all other channels are fine, 80% on an indoor aerial, and 100% signal. With a Wolsey booster on the Aerial (2/in and 4/out)
I have installed a new Aerial, and rewired with WB100 cable.

Otherwise keep up the good work.


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Brian Passingham's 1 post GB flag

9:32 PM

Brian Passingham: Although the EPG number used by "That's Solent" was changed to 7, the multiplex transmitter carrying the programme is exactly the same, and so any variation in strength is more likely down to atmospheric or weather related conditions rather than anything else.

On the subject of signal strength, it has to be appreciated that the LSO transmitter the programme is carried on is only rated at 10Kw (H polarity) compared to that of 200Kw used by the main PSB channels, this power applying to vertical as well as horizontal transmissions , COM 4/5/6 also radiating at 200Kw but only on vertical polarity, 50Kw on horizontal. Taking this into consideration its not terribly unusual for the LSO mux to be received at a lower level, although you might be able to increase the strength by moving your aerial slightly to the right or left of its present position.

However, another factor exists about reception from the LSO mux, insomuch that the signal is beamed out in a fan shape from the mast to cover Portsmouth and parts of Southampton, the centre line of the fan shape being Fareham / Hambleton, and so although you are receiving the LSO mux, the strength / quality its being received at all depends on where you are located within the coverage area.

A post code or one from nearby (e.g: shop inc Post office) enabling access to the reception predictor as well as the terrain indicator. Could you also confirm (or otherwise) that the indoor aerial you refer to is of the loft mounted variety?

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jb38's 7,179 posts GB flag
Sunday, 30 April 2017
11:53 PM

Please help me, every so often I lose all BBC programs from my TV and when I retune it only brings up programs from 307 upwards. I cannot have my Laptop on at the same time as the TV as I lose all programs then.
This has only started in the last 2 weeks,I have had an engineer from BT and he could not find the answer!!

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Joan's 1 post GB flag
Monday, 1 May 2017

9:20 AM


Please provide a full post code so that we contributors can examine the reception conditions where you live.

Retuning when you lose a channel is not going to help but may cause other channels to be lost as well.

Please tell us the signal strengths of the BBC channels you are losing.

If your laptop computer causes loss of all channels then you have a problem with it. The WiFi should not affect TV reception.

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MikeP's 3,056 posts GB flag
Saturday, 10 March 2018
10:09 AM

We have just had an aerial fitted and can get several channels but all BBC channels appear well then disappear completely after 5 seconds. The aerial people say it is the TV but I don't understand why we see the channel then it disappears, only on BBC channels. Any help appreciated. Thankyou

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joanne's 1 post GB flag

11:25 AM


Please provide a full post code so that we can see what the reception conditions are like at your location.

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MikeP's 3,056 posts GB flag
Thursday, 7 June 2018
Jon Dooley
2:03 PM

In Margate facing the sea. Rarely perfect. For the past two weeks or so, all signals have come from East Anglia and occasionally Nottingham! Especially from about 8 pm onwards, break up and loss of channels is dreadful until after breakfast. Manual Channel Tuning, of every channel, shows signal variations between 0 and 100% on the same channels. Retuning is successful randomly but soon breaks up, including Autotuning. Overnight we are reduced to virtually no channels from anywhere. Highly specialist aerial guy has been in from time to time but during the day when he produces a good result. Are you cutting the power overnight, as was said to be the case of reduced water pressure overnight?! I see you have been working on the Dover transmitter but stated only briefly. For us it doesn't seem to be working at all. Driverless cars?! CT9 5BJ.

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Jon Dooley's 1 post GB flag
Jon's: mapJ's Freeview map terrainJ's terrain plot wavesJ's frequency data J's Freeview Detailed Coverage

4:39 PM

Jon Dooley: Firstly, no one on this website has anything to do with any broadcaster or transmitter company. The transmitters do not reduce their power level overnight. If the problem is only happening overnight, the most likely explanation is that you are suffering from electrical interference being emitted by something in your home (or a near neighbour) which is operating on a timer and probably has an electric motor.

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StevensOnln1's 3,622 posts GB flag
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