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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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Sunday, 5 June 2011
1:55 PM

Rick Whitehead:
We're the same (approx 5 miles South of Ely)....the Sandy Heath signal has been perfect since the switchover but has been getting progressively worse for the last couple of weeks.... seems to have gone back to the same dire state as it was when they were fiddling with it for months last year leading up to the switchover. Or it could just be another inversion but if the weather is going to ruin our viewing for more than 10% of the year as it is now then maybe someone ought to come up with a solution.

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John's 7 posts GB flag
John's: mapJ's Freeview map terrainJ's terrain plot wavesJ's frequency data J's Freeview Detailed Coverage

4:34 PM

John: Inversion is very unlikely with the current weather conditions. I would also check Freeview reception has changed? | - independent free digital TV advice - trees coming into leaf near to your aerial can often be the problem.

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Briantist's 38,915 posts GB flag

6:52 PM

Brian - is there a NOT omitted from the above/below? What iIS "inversion weather"?

Thursday 26 May 2011 8:05AMLiam Guest: Loft aerials are recommended for reliable Freeview reception,>>

Rick W - Do you have neighbours with or without the same problem? 45dB signal should be plenty

UK digital TV reception predictor

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Steve's 1,173 posts GB flag

6:56 PM

Steve: Yes, they are NOT recommend. I can't think how many times I have said that, must be in the tens of thousand by now...

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Briantist's 38,915 posts GB flag
Monday, 6 June 2011
Rick Whitehead
9:33 AM

Briantist: Thank you kindly for your reply , your comments are very much appreciated, but sadly it does not answer my question at all.
I'm on a hill on the west of MK, my aerial sits 12m off the ground with nothing at all blocking the view of Sandy Heath which itself should be fairly tall??? So it's definitely not trees but thank you for the suggestion.
Is it possible to find out the actual power of signal now being transmitted [from Sandy Heath] instead of relying on what we're being told? Is Sandy heath due to have power increases to some muxes in the future? Is this a ploy to get everybody over to Satellite TV?
My log40 aerial is classed as directional but not high gain and this was fine until recently - I have spare wideband aerial of a poorer physical quality so I suppose I should test this next to see if this helps (I tried amplification but this had absolutely no effect).
I find it difficult to believe that what would appear to be a perfect aerial setup on paper which was working perfectly can now suddenly need attention despite no obvious local environmental changes without this being a change at the transmitter?
kind regards

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Rick Whitehead's 2 posts GB flag

10:49 AM

- Freeview on Sandy Heath TV transmitter | - independent free digital TV advice -
RW - have a look at the Sandy H page. In fact your questions might get responses from locals there.
NB the note about temporary low power.
Did you ask neighbours?

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Steve's 1,173 posts GB flag
Monday, 4 July 2011

10:01 AM

Brian - I think a little banning needed!

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Steve's 1,173 posts GB flag
Wednesday, 6 July 2011
H demou
9:24 PM

Aerial on roof clear line of site good signal till summer started , now very week bbc1 and signal strengh as shown on our freeview box is down

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H demou's 1 post GB flag
H's: mapH's Freeview map terrainH's terrain plot wavesH's frequency data H's Freeview Detailed Coverage
Steve P

11:44 PM

... UK digital TV reception predictor

Hi H Demou - from the above, you are on a fairly weak signal so might notice a seasonal change - leaves on trees? Or an aerial that has moved, or a cable problem.

Have you talked to others nearby?

Otr checked the page here for whichever transmitter your aerial points to? Just search it.

PS Brian, if you see this

You remember the old ads about the best friend who told you you had dandruff?

Every time I see this page it hurts me that you say in the title "effect"(which means "cause") not "affect" (which means "influence")

Please edit it for my peace of mind!


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Steve P's 1,173 posts GB flag
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