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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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Tuesday, 12 June 2018
11:39 AM

Lost itv1 no signal , bbc 1 OK, Bbc news hd channel poor signal pixelating. Area pr3 1tb

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Jh's 1 post GB flag
Jh's: ...

2:17 PM

Jh: Have you checked for any loose or damaged cables or connections behind your TV? Do not attempt to retune.

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StevensOnln1's 3,601 posts GB flag
Saturday, 3 October 2020
martin grimes
2:16 PM

bbc radio and tv transmission has been lost. itv channels are ok. 3/10/20

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martin grimes's 1 post GB flag

6:35 PM

martin grimes:

There is very little "Tropospheric Ducting" around causing interference at this time, see Effect of tropospheric ducting on Freeview | RTIS for a simplistic explanation.
A lot could depend on when you last retuned.
You are not advised to retune when you have lost signals or they are badly pixellated as this usually just clears correct tuning and you have to repeat a retune when signals are stable.

Equally, your transmitter might be listed for Planned Engineering and as we haven't a clue about your location or which transmitter(s) you might receive - a full postcode is needed - then it's impossible to make any further constructive comment.

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Chris.SE's 4,079 posts GB flag
Monday, 9 October 2023
Gary Grace
4:00 PM

Does having the aerial in my loft as opposed to on my roof contribute to the bad signal I'm getting from the transmitter?

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Gary Grace's 1 post GB flag

10:20 PM

Gary Grace:

Not necessarily. It depends on which transmitter you receive and how good the signals are.
As you haven't given a full postcode, we're not able to constructively comment on this.
If you provide a full postcode we can check your predicted reception.

BUT also note current weather conditions have been causing Tropospheric Ducting affecting much of the south coast and southern parts of the country on Saturday and more of the south of the country through Sunday. This causes interfering signals from distant transmitters in Europe or the UK to affect reception of your wanted signals. This can periodically last, seconds, minutes and sometimes much longer - Do NOT Retune.
There is nothing you can do about this apart from wait for conditions to change, or use online streaming if available.

The BBC and Freeview have issued warnings -
High pressure weather conditions impacting TV & Radio services - from 07 October | Help receiving TV and radio
High pressure could affect reception across parts of the UK this week | Freeview

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Chris.SE's 4,079 posts GB flag
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