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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 3,645 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 4,232 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 4,232 posts GB flag
Tuesday, 16 April 2024
Jan Knights
8:16 PM
High Wycombe

What channel numbers do I need when manual tuning my small tv. Post code HP13 5GL. I am very close to the transmitter at Downley. Is this a problem. Automatic tuning gives a broken picture. I live in a flat with a communal aerial but my main tv works fine through a freeview box. Many thanks

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Jan Knights's 13 posts GB flag
Jan's: mapJ's Freeview map terrainJ's terrain plot wavesJ's frequency data J's Freeview Detailed Coverage

11:24 PM

Jan Knights: If you're receiving the High Wycombe transmitter (approx 1km away on a bearing of 294 degrees) it uses UHF channels 31, 32 and 37. You're also predicted to receive a good signal from Chepping Wycombe (approx 3km away on a bearing of 153 degrees) which uses UHF channels 40, 43 and 46. It also predicted to be possible to receive Crystal Palace at your postcode (approx 53km away on a bearing of 115 degrees) but at a lower signal strength, which uses UHF channels 22, 23, 25, 26, 28, 30 and 35.

If your getting a broken up picture it's very possible that the TV has auto tuned to the wrong transmitter, rather than the one the aerial is pointed at, so take a look at the direction the aerial is pointing if you can get a clear enough view of it.

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StevensOnln1's 3,645 posts GB flag

11:41 PM

Jan Knights:

Is your small TV connected to the communal aerial as well?
Roughly how many TV channels do you get on your main TV, is it below 50 or over a 100? (This gives us an idea which transmitter the communal aerial is pointing at) - unless you know which way it points!

If it points to the High Wycombe Relay transmitter (Downley - the closest) roughly compass direction WNW it's UHF channels 37, 31 and 32.
If it points roughly SSE at the Chepping Wycombe Relay transmitter, it's UHF channels 40,43, and 46.
If your small TV doesn't have HD capability for the HD TV channels, ignore UHF C32 or C46.

If the communal aerial points at the main Crystal Palace transmitter roughly ESE and you get over 100 channels on your main TV then it's UHF channels 23, 26, 30, 25, 22, 28, and 35. If the set isn't HD capable ignore C30.
The UHF channels numbers are in the multiplex order BBCA/PSB!, D3&4/PSB2, BBCB HD/PSB3, SDN/COM4, ArqA/COM5, ArqB/COM6 and the Local multiplex. Relays only have the PSB multiplexes.

See Channel listings for Industry Professionals | Freeview for which TV channels are carried on which multiplex.

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Chris.SE's 4,232 posts GB flag

11:42 PM

Ha, looks like two of us were compiling responses at the same time :)

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Chris.SE's 4,232 posts GB flag
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