Just a quick recap
- You live somewhere in the Anglia TV region
- Your main transmitter is Tacolneston
- You have a loft aerial
- There's a multi-way splitter fitted in the loft too
- The house had aerial cable installed in-wall by the house builder, but you've had a new piece run by an aerial guy which did temporarily fix the interference issue [this could be coincidental]
- The issue affects only the lounge TV
- The issue affects only Mux c59 (PSB2 D3+4)
- Signal strength and signal quality were both at 10 on the Panasonic TV's meter
- You've done the postcode checker at 4G AT800 and it predicts no issue with mobile phone mast interference
- Following advice here, you've bought and fitted a variable attenuator. It is set to max. Signal strength 8, Quality 10. This improved things for a while but now you've got some interference creeping in again
- The issue is worse at night
My gut reaction is you've still got too much signal coming off the aerial. Solution: Add a 10dB attenuator and turn down the variable until the problem stops. i.e you might need 22dB of attenuation, so the variable alone (if it's a 20dB attenuator) isn't quite enough. If you look at the sensitivity of a typical wideband high-gain aerial they have a rising hump (see the graph in post #6). That means they put out most signal strength at the upper-end of the channel range.
Look at how that tallies with your experience: Your issue is with c59, but not 55 or any of the other lower channel numbers.
But that's not the whole story.
Adding a 10dB in-line attenuator is easy. However, you run the risk of fixing one problem and causing another elsewhere.
Referring back to that sensitivity graph in post #6, you can see that the wideband high-gain aerial is poor at lower frequencies. Your family's main TV is already picking up on c45 and c42, but you seemed to be missing out on c39
before the attenuator was fitted. Throwing in yet more attenuation could put c42 & 45 at risk as well.
With this in mind, and factoring in that over the next couple of year all the channel space from c49 to c60 will be cleared ready to be sold off to mobile phone companies, then the outlook for your current aerial isn't great. All the TV transmitters will eventually go to frequencies that are c48 and lower. It's ironic that it'll fix your c59 reception issue, and probably move those channels to somewhere you can't get a signal.
The bottom line is that sooner or later, just like tens of thousands of other homes in the UK, a wideband high-gain aerial of the type being sold as a "universal aerial" for the last 20 years is going to become junk. A Log Periodic aerial is good for now and for the future when the channels all move to c48 or lower. Fitting a Log now will also fix the over-saturation at c59.
There are two other areas I'd look at just to dot the I's and cross the T's.
1) The aerial position in the loft. Ideally, move the aerial to outside. But if it has to be in the loft because of some covenant on the building then the installer must really spend some time measuring and mapping out the signal levels across the loft space. It's not enough to assume that the middle of the loft is the best place.
2) House builders use cheap aerial gear. That goes for the wall socket plates as well as for cable. If you haven't already had it changed, swap out the wall plate for a shielded version.
Finally, the time of day (or night) could indicate that your site is experiencing a phenomenon called tropospheric propagation. ("What?" I hear you say
) It's radio signals (and yes, TV is part of the radio spectrum) bouncing off a boundary layer within the atmosphere. Folk who listen to short-wave radio take advantage of this phenomenon all the time. Due to the curvature of the earth there's no direct line of sight to distance places such as Russia, and since radio signals only travel in straight lines then it shouldn't be possible to pick up from- or transmit to- places beyond the horizon. The surprising thing is that the horizon is much closer than you think. For a 6ft tall person at ground level it's only about 3 miles away. This is why transmitters are very tall and why getting the receiving aerial up high makes a difference.
TV signals work at much longer wavelengths than SW radio, so this bouncing effect is far less pronounced. A lot of the signal just passes straight through the atmosphere on on eventually in to space. However, in the right conditions then the signal from distant transmitters can be detected, and the signal from local transmitters can become a little bit stronger. Night time is typically when this happens. The result; if your signal was already borderline 'too much' then this small boost could be enough to tip the balance.
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