Single frequency interference
Brian Butterworth published on UK Free TV There are several different type of interference that cause problems. This page deals with the situation where:
- Freeview reception is perfect or near-perfect most of the time
- Several channels are missing, but they are all on the same 'multiplex'
Local analogue interference
Local analogue interference is caused by household video devices that you can attach to a television such as: 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?
Thursday, 3 February 2011
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rob10:31 PM
Sheffield
Brian: I dont know to what you are referring. i ve tried changing the RF outputs but as the aerial extension isn't connected to the sky box can it still be affecting the signal? I ve changed the RF channel and tried retuning the tv, as described at the top of this page, but it still doesn't work.
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rob's: mapR's Freeview map terrainR's terrain plot wavesR's frequency data R's Freeview Detailed Coverage
Friday, 4 February 2011
rob: I mean check for other devices that might be causing the interference.
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Tony10:08 AM
Leicester
I bypassed the amp last night and picture no different. So amp OK. Also I did a continuity check on cable up to the aerial, centre core back to the shielding. On the 200M (presumably milliamps) range, I got a reading of 12. I would expect zero. Suggests damp in aerial connector box?
I'm going for a loft aerial. Not ideal I know but it's lack of pennies.
What type should I go for? Ideas would be welome
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Tony's: mapT's Freeview map terrainT's terrain plot wavesT's frequency data T's Freeview Detailed Coverage
Tony: As I said, loft aerial are unsuitable for Freeview reception, so I am not in a position to suggest one.
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Tony4:41 PM
Leicester
Brian: I know they are of limited success but have decided against one due to lack of headroom and too so much clutter up there.
The existing aerial's on the roof. I shall have to go up to check the connections and maybe replace the cable to the amp. What is the latest standard?
jb38: Thanks for your suggestions. I'm satisfied the amp is working as I bypassed it and there was no change. All TVs have picture and sound breakup on same Freeview channels. Dawn lives on a village estate and there's no new buildings or factories nearby. Mains interference. That's a possibility. She could try a filter.
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Tony's: mapT's Freeview map terrainT's terrain plot wavesT's frequency data T's Freeview Detailed Coverage
Tony: I'm not stopping you if you want to, but I can't recommend something that I know will not work.
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jb388:54 PM
Tony: Just a little point about the measurements you made on the cable, I think the 12 you mention will have been ohms and which should just be about the resistance expected up a co-ax centre, round the aerial dipole, then down the outer screen (or vice-versa!)and to me doesn't indicate any fault.
Of course if you had disconnected the lead where it joins the aerial and you had still got a reading then that could indicate a problem as you shouldn't get any reading, provided that is you didn't set your meter on a range that would indicate anything if you held the end of a test lead in each hand, as if you did the test would not be valid as the meter was set too high.
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jb389:34 PM
Tony: The second point I wanted to make was that as far as you by-passing the amp is concerned only the method I described can prove if its OK, or not as the case may be! and as also mentioned this having to be done whilst viewing an analogue signal, but as aforementioned in my previous posting aerial amplifiers are amazingly reliable devices that either work or they don't, and any that does fail are usually always outdoor types that have suffered high static rain damage during thunderstorms.
Regarding a loft aerial, I realise Brian will always state the utopian mounting position, that being the roof, but fully realising that this is just not possible in very many circumstances loft aerials can be quite successful if carefully positioned, that being not sited next to the water tank, and likewise the tank not being in front of the aerial.
Loft aerials only really lose efficiency when the roof tiles are sodden wet, or worse snow on the roof, but in most tests I have carried out over the years if a signal strength reading is taken from a loft mounted aerial and that same aerial is then passed through the skylight onto the open roof, "if" any difference at all are seen (as mostly not!) then its only of a most fractional nature.
The only occasional down side that loft mounted aerials can all suffer from is that they are more vulnerable to picking up mains interference from the inevitable cabling that's always found running across loft floors.
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Saturday, 5 February 2011
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jb389:06 AM
Tony: Should you see this before carrying out any further tests, another good way to test a suspect outdoor aerial is on a breezy day measure the resistance (ohms) reading across the cable end and KEEP observing this for a few minutes, if you see any fluctuations taking place in the reading then that indicates a bad joint somewhere.
Please note that excluding the intermittent aspects of the readings mentioned above, my comment on the readings expected is referring to the most commonly used types of aerials that use a folded dipole element, (elongated loop seen in front of reflector) or use a balun inside the connection cap, however some other types of aerials exist that will always indicate an open circuit on an ohmmeter and which therefore cannot then be assessed using this method, as it does have its limitations.
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jb389:02 PM
rob: I am rather intrigued by the strange fault you mention and so should you still be fault finding I would like if you could clarify a little point I wondered about, that being, you say the TV upstairs is fed by a cable from a booster (downstairs?) and whose input is from a wall socket, what I wondered was is the wall socket fed directly from your receiving aerial? and is that same wall socket connected in any way with either your problem TV or the Sky HD box's RF sockets? (which the problem TV is running from)
The other point I wondered about is you stating that there is still a signal with the booster off, are you referring to the upstairs TV viewing an analogue picture?
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