Why do I get poor signal on some channels, but not others?
Question submitted by Alasdair Bampton
“I have a new digital television connected to my old aerial and I’m currently receiving Freeview. This appears to work perfectly for every channel except the BBC ones which are all prone to breaking up or total loss of signal. Why is this?
If my aerial is faulty, why do all the other channels work perfectly? Is it that the BBC signal is weaker and if so why is that? Will a new aerial make a difference? I have been quoted £150.00 for a new aerial but will it make a difference?”
Our Answer
It’s not uncommon for people to experience this problem, and there’s not one easy answer that applies to everyone. The problem is likely to be one of two things, which we’ll handle separately: Lack of signal, or Interference.
Lack of signal
If your Freeview receiver can get some Freeview channels perfectly, but not others, there are a few possible reasons:
Aerial Type:
The Broadcast TV band goes from 21UHF to 68UHF. In the early decades of analogue TV, TV aerial installers would install an aerial that performed best at certain parts of the band. There were 5 groups of aerial – A, B, C, D and E. Type A performed best from 21 to 27, Type B, from 35 to 53, etc. London’s main TV transmitter broadcast at the lower part of the band, so best performance could come from a Type A aerial.
Now that we have Freeview, with more channels, some people will find that some of the channels are outside the part of the band that their aerial is optimised for. Accordingly, channels outside of the aerial’s optimal band will be worse than others. The normal fix for this is to get a “wideband aerial”, which will cover the entire 21 to 68UHF band.
Signal from transmitter:
Freeview signals are broadcast in groups, known as Multiplexes. It’s not uncommon for one or two multiplexes to be broadcast on reduced power. This is particularly common during the 2008-2012 Digital Switchover. You can find out if your local transmitter is operating on low power, or if there is transmitter work ongoing using the BBC Transmitter Information Tool
Your TV set or Freeview box may have an on-screen signal strength meter – You can use this to check whether the signal for the problem channels is lower than for the other channels. The best way to check though, is to use the special signal strength measuring equipment used by TV aerial installers.
The other thing you can do, is to check with neighbours. If they are getting the problem channels without an issue, that helps to confirm whether it’s a problem with the transmission power, or your aerial
Interference
The other possible reason why some of your Freeview channels are suffering from break-up while others aren’t, is interference. Here are some possible causes:
Use of a modulator
If you are using a TV modulator somewhere in this house, this could be causing a problem. Typically, you’ll find a modulator in a video recorder and a Sky box. They are used to convert the output of a piece of equipment so that it can be sent over TV aerial cable. In the case of Sky, these are used to send signals from a Sky box to a second TV set. In the case of a video recorder, these were used to get a signal to a TV without using a SCART cable. A modulator “broadcasts” a signal on one of the UHF channels between 21 and 68, and your TV would tune in to this channel to get a picture.
If you have a modulator in use, it could be that this is on, or close to, the frequency of the channels you’re having a problem with. You may need to turn the modulator off (if not in use), or tune it to a different channel – one that’s not in use.
Local ‘Noise’:
Admittedly, this is less likely, but it’s still a possible cause. Digital TV equipment can be quite sensitive to various forms of interference – from your home’s main electrical wiring, cordless phones, radio signals (CB, taxis, etc), weather and atmospherics, domestic appliances and heating thermostats.
Some types of interference affect different frequencies. What’s known as RF (radio interference) will be worse at some frequencies than others, and there’s a chance that if the interference is worse for one group of channels, it could be something like your Wi-fi transmitter, cordless phone, baby monitor, or a local taxi company, that’s generating radio frequency interference that’s upsetting some, but not all, of your channels.
If you are getting interference, there are various things to try, such as using high quality aerial cable and connectors, keeping aerial leads as short as possible, and making sure your cables are kept away from mains cables and radio transmitters.
Again, finding out whether other people in your area are similarly affected, is a great way of working out whether the problem is a general one, or something specific to your aerial or home setup.
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