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?
Wednesday, 31 December 2025
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Steve Donaldson3:36 PM
Mike Sanders: The PSB have the better channels, where there is generally more distance to the next transmitter on the same channel. The COMs fit in where they can, with channels that may have incoming interference or be not good enough to be received in parts of the PSB coverage area of the transmitter.
With respect to Rowridge COMs, they are co-channel to the COMs of Crystal Palace to the north east and Stockland Hill to the west. They are also co-channel with Caen (Mont Pincon). All three of these transmitters do not use the Rowridge PSB channels. The R3 multiplex in France closed earlier this year, and it was on C22 from Caen. Should this be reassigned then expect C22 to recommence from Caen.
At your location you are 47 miles from Crystal Palace, and being high up there is line-of-sight, barring local obstructions such as trees. Crystal Palace is on a bearing of 61degrees and Rowridge 205degrees. Thus, you may have to contend with incoming interference from Crystal Palace.
Since digital switchover, Rowridge transmits with vertical polarity as well as horizontal polarity. The PSBs are 200kW HP and the COMs 50kW HP. The PSBs and COMs are both 200kW VP. Thus, VP is generally better as it has the higher powered COMs and is opposite polarity to the other three aforementioned transmitters which are HP-only and are co-channel with Rowridge's COMs.
The potential issue with taking advantage of VP with Rowridge - the fly in the ointment - is that the local mux on C37 is HP-only. The local mux is not available to all viewers of Rowridge and is directional, with the strongest part of its beam approximately north north east. The transmission mode used for the local mux means it has better coverage for a lower power than the other muxes.
Thus, assuming you have set your aerial for HP (which I imagine you have as you are referring to the COMs as being 50kW) you might try VP in order to improve the COM reception (and increase rejection of unwanted, interfering signals from co-channel transmitters which are HP-only). It might be a case of seeing how good the local mux is with the aerial set at opposite polarity.
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Chris.SE10:01 PM
Mike Sanders:
Steve has addressed most of what I'd already been thinking before I read his post.
I'm not certain from what you've posted whether your reception conditions are still the same now as they were in the past week, as we've not long had a period of tropospheric conditions and there is still some variable residual such conditions still about. As Steve points out you could be getting co-channel issues from other transmitters Crystal Palace being just one. I hadn't/still haven't looked at Stockland Hill, but from the many other surrounding transmitters, Oxford is the only one where you don't have a direct line-of-sight and so might receive signals from especially under tropo conditions.
Your antenna will have rear and side lobes that would receive signals (especially with amplification) so none of the other co-channel possibilities should be ruled out in these conditions even from other transmitters in the UK further afield.
As Steve has suggested, changing your aerial to VP could solve the co-channel issues but maybe at the sacrifice of the Local mux on C37, it would be a case of suck it and see. If you didn't get a good enough signal with VP from the local mux and you really wanted it, you could considered a separate HP antenna with a C37 filter connected into your system, but that will of course come at a cost.
BUT One thing that seems to have been ignored, is the possibility of interference from new/upgraded mobile phone masts using the 700MHz band. There are at least 4 masts within a 1.5km distance from you, and I'm not sure how up-to-date that information is, there could well be more!
Your postcode should have received a postcard from Restore TV, see - https://restoretv.uk/postcards-not-sure/
We have seen a number of cases where channels like C25 and C28 have had such interference where others don't appear to have, and the Free Filter from Restore TV has solved the problem.
If your first amp is a masthead, and you personally don't have easy access to it (to even consider changing to VP as well) then if you advise Restore TV they should send an engineer Free of charge to fit an External Weatherproof Filter, and from reports we've seen in the past they have generally done other things that could be needed to resolve the reception problems.
Any filtering has always to go BEFORE any amplification. If your amp is internal and you have easy access then make sure the filter you get has the relevant connectors, coax or f-connector.
Try the filtering first, then consider the change to VP if the problems aren't resolved.
HTH.
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