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Single frequency interference

What to do when several channels are missing, but they are all on the same 'multiplex'

What to do when several channels are missing, but they are all
published on UK Free TV

There are several different type of interference that cause problems. This page deals with the situation where:

There are two main causes of this type of intermitted interference: The current Freeview mulitplexes are shown in this diagram. If all of your missing channels are on a single row, you have "single frequency interference":

Local analogue interference

Local analogue interference is caused by household video devices that you can attach to a television such as:
  • Sky Digibox;
  • Video cassette recorder (VCR);
  • Personal digital video recorder (PVR);
  • Games console;
If you have any of these devices, you will need to work out which of these is blocking the reception of a Freeview multiplex by turning off the suspect device and then rescanning for channels on the Freeview box.

If powering off the device restores the Freeview channels from the blocked multiplex, you will need to either stop using the problem device's RF output by disconnecting the RF output cable permanently, or by changing the RF channel number that that device uses. See, for example, How do I change the RF output channel on a Sky Digibox?

You can find a list of the frequencies that are not in use at your location by putting your postcode into the My Freeview reception and looking under the "Free RF channels" section.

Non-wideband aerial in some areas

In some places you will need a wideband aerial to receive all the Freeview multiplexes. For example in Central Birmingham, the Sutton Coldfield has multiplex D on an 'out of group', so you cannot view Film4 or ITV2+1 without an aerial upgrade.

To find out if you need an aerial upgrade, see My Freeview reception.

Help with Freeview, aerials?
How do I get a test card with Freeview1
I would like to know if it is possible to receive UK terrestrial Freeview servic2
I have been told I would receive too much singal from my Freeview tansmitter as 3
Can my Freeview box receive more than one BBC and ITV region?4
Is it true that my 87 year old mother is entitled to a FREE upgrade when the ana5
In this section
Official aerial installers guide to the TV spectrum future1
Which free digital TV system will give me the most reliable reception?2
High pressure causing channel loss through "Inversion"3
Digital Region Overlap4
Freeview reception has changed?5
Two frequency interference 6

Thursday, 26 April 2012
8:20 PM

Robert Dallenger: Good News on my sysytem. All now appears to be working ok. I have recently decorated in the room I was having problems with and must have disturbed the connection at the splitter. I have remade all connections and ALL Channels appear to be coming through OK again. Will have to keep an eye on it though.

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Robert's 4 posts GB
Rob Godwin
10:07 PM

HI, we have recently lost Dave & the channels on that group, I forget the name. Ok 'til about a week ago. We have a Sony Bravia TV & we are postcode PL19 0JE.
I have re tuned a couple of times & those channels no longer show up on the list.
Any ideas what we can do?
Thanks, Rob.

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Rob Godwin's 1 post GB
10:15 PM

Not good news on my system - exactly as it was when I posted four days ago. Anyone have any ideas at all?

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Dominic's 3 posts GB
Dave Lindsay

11:04 PM

Rob Godwin: If your aerial is pointing south and the elements are vertical (up/down), then it is facing Tavistock transmitter which does not carry these services. You have therefore been receiving those channels from another transmitter (probably Caradon Hill) and therefore the solution to reliable reception is to replace your current aerial with on on Caradon which is due west. For Caradon, aerials are horizontal (elements flat).

Have you been along this road and had an aerial on Caradon fitted?

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Dave Lindsay's 5,724 posts GB
Dave Lindsay

11:27 PM

Dominic: At such close proximity to the high power transmitter, I think that your receiver may be being overloaded:

Freeview signals: too much of a good thing is bad for you | - independent free digital TV advice

Try something that will pick up less signal and see if the problem goes away or at least subsides. A set-top aerial maybe. An aerial lead on its own may pick up enough signal; you may have to move it about to best effect.

Or if the plug on your aerial lead is a screwed together one, remove the outer part and put the inner pin into socket.

You may need quite a bit of attenuation. A 20dB variable attenuator may not be enough. Perhaps a high value fixed, e.g. 24dB in conjunction with a 20dB variable will give you enough attenuation whilst allowing you to fine tune.

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Dave Lindsay's 5,724 posts GB
Friday, 27 April 2012
Terry Ward
9:34 AM

My postcode is BT30 9QS and I am considering installing 3 aerials using a splitter onto one downlead to receive
1. Clermont Cairn,Saorview Group C/D
2. Divis, Freeview Group A
3. Caldbeck Freeview Group A
Can you advise if the offset frequencies utilised by Caldbeck are compatible with reception from Divis after it becomes exclusively digital on full power in October 2012.

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Terry Ward's 2 posts GB
Dave Lindsay

9:50 AM

Terry Ward: Combine Divis and Calbeck signals. You ain't gonna do it!

Why do you want Caldbeck anyway, when Divis is a full service transmitter?

You need a diplexer such as these:

Online TV Splitters, Amps & Diplexers sales

You need one which splits the two at a workable channel.

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Dave Lindsay's 5,724 posts GB
Terry Ward
1:14 PM

Hi Dave,

thanks for your reply. I appreciate that I will lose channels that share the EXACT same channels i.e 23-, 26- and 33- but I should get most of this programming from freesat. Can you advise that the Caldbeck offset frequencies on others will be sufficent to avoid signal cancellation
My main reason for trying this is to avail of a recordable STV HD service which differs from the rest of the ITV network.

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Terry Ward's 2 posts GB
Dave Lindsay

2:26 PM

Terry Ward: I'm not sure of how much spacing there needs to be between digital channels.

There are a number of issues you really need to be aware of before you start:

1. You really need filters or a diplexer to combine the aerial feeds which you can't use because of Divis and Caldbeck both being Group A.

2. Calbeck broadcasts two variants of the Public Service Channels (English and Scottish), both of which are ITV Border and not STV. Scottish broadcasts reportedly use directional antennas, unsurprisingly in the direction of Scotland.

3. All of Calbeck's nine channels have negative offsets. I have read where all those transmitters that use C30 have a negative offset. So I assume that reason for them all needing negative offsets is because all neighbouring channels from 22 through to 30 are used by Caldbeck. Perhaps one of the experts could confirm that digital broadcasts can be on adjacent channels. Also, that a positive offset on C22, for example, means that if C23 is to be used, then it must also have a positive offset.

I like a challenge too, but would it not be easier to receive ITV1 HD from Isle of Man?!

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Dave Lindsay's 5,724 posts GB
Dave Lindsay

2:39 PM

Terry Ward: Having looked at this page:

TV regions overview map | - independent free digital TV advice

I suggest that Port St Mary, which is Group B may be the best. At 2kW it is the most powerful transmitter on the island.

The only caution I would give is that the powers that be won't release the radition pattern so the projection by this website of its coverage assumes an omnidirectional antenna has been used.

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Dave Lindsay's 5,724 posts GB
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