Channel 61 erratic, others OK

voltaire

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We're having trouble getting channel 61 from Mendip reliably. Signal is usually around 60-70%, but intermittently drops to 'no signal', sometimes briefly, sometimes for extended periods.

Other channels are OK, often 100%.

We're about 40 miles away, but very high up, so signal path is excellent.

I don't really understand what sort of problem I'm trying to fix, any ideas?
 
We're having trouble getting channel 61 from Mendip reliably. Signal is usually around 60-70%, but intermittently drops to 'no signal', sometimes briefly, sometimes for extended periods.

Other channels are OK, often 100%.

We're about 40 miles away, but very high up, so signal path is excellent.

I don't really understand what sort of problem I'm trying to fix, any ideas?

Welcome to the forum.

Can you clarify, when you refer to percentages, are you referring to signal strength or signal quality?

Also, it may assist those who may be able to help you to describe your setup and what device(s) you are receiving on, and if more than one, how they are interconnected.
 
Welcome to the forum.

Can you clarify, when you refer to percentages, are you referring to signal strength or signal quality?

Also, it may assist those who may be able to help you to describe you setup and what device(s) you are receiving on, and if more than one, how they are interconnected.

Thanks.

Setup is simple, rooftop aerial feeding Techwood FV2A digibox feeding old Philips TV. No aerial splitting or amplifiers etc - in fact no other TV in the house.

Percentages are guestimates of signal strength from bar type readout.

At the moment, Channel 61 is flicking between 75 and 100% strength and between good and very good signal quality.

Other channels:

56: 75%, good

54, 52, 48: 100%, very good

I've checked the signal path on Megalithia and it looks very good, and I'm pretty confident about aerial alignment.
 
Try removing any hdmi cables you have, these can radiate rf in the uhf band. Strangely cheaper cables appear less problematical in this respect. Use double screened WF100 to make up the coax interconnects.

You haven't got any other source of RF close to the TV like a wifi router or cordless phone have you ?
 
Try removing any hdmi cables you have, these can radiate rf in the uhf band. Strangely cheaper cables appear less problematical in this respect. Use double screened WF100 to make up the coax interconnects.

You haven't got any other source of RF close to the TV like a wifi router or cordless phone have you ?

Nothing modern like HDMI here, and no cordless phone.

The wifi router is in the loft, just under the ridge, probably about 5 meters from the aerial, but at 90 degrees to the bearing to the transmitter. Could this be the source of the problem? I'll experiment later. (Router needs to be high up to get a good signal for the 3G mobile broadband we use, which is much faster than landline broadband here.)

There are also 2 mobile phone masts at 200 and 300 metres at about 45 degrees to the bearing to the transmitter.
 
The wifi router is in the loft, just under the ridge, probably about 5 meters from the aerial, but at 90 degrees to the bearing to the transmitter. Could this be the source of the problem? I'll experiment later.

Doubtfull but no harm in turning it off to make sure. Your symptoms do sound like a local source of interference around 794MHz. They can be the devil to find the source. In the past even a faulty street light was found to be the culprit. It may not even be in your property.

This may help

http://www.stevelarkins.freeuk.com/tv_interference.htm
 
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Doubtfull but no harm in turning it off to make sure. Your symptoms do sound like a local source of interference around 794MHz. They can be the devil to find the source. In the past even a faulty street light was found to be the culprit. It may not even be in your property.

This may help

TV Interference Advice, Cure TVI Problems with TVI Filters

Thanks for the suggestion - I'd assumed interference was unlikely because of our rural location and the fact that we're on the side of a valley, so there's not much close to us in the direction of the transmitter. I've tried turning off the router and my laptop (which completely wrecks the DAB signal) with no luck, but there's a few other things I can try.

Left the signal monitoring screen on this afternoon while doing other things and of course channel 61 has been a rock steady 75% level, quality good, while other channels have been varying. We don't watch much TV, and more channel 61 than anything else, so its not surprising we haven't noticed a problem on other channels.

If anyone else has any ideas I'd been interested, as I don't feel optimistic about finding the source.
 
Your location is important. Where (approx) are you?

Check your TV predictions here Digital UK - Postcode checker using the 'detailed view' option. This may show that your frequency ch61 reception is liable to interference from other sources more than the other muxes?

Note that on 27 March next year the BBC mux moves from 61 to 49 and the predictions will change, as will your reception (to some extent).

Reception at 40 mile distant can be affected by all sorts of things between you and the transmitter... (e.g. Tidal variations caused problems to the Countisbury analogue transmitter's rebroadcast link from Mendip. Trees and foliage wet/dry moving/still can affect some frequencies more than others.) as well as interfering signals. Water on the receive aerial can sometimes cause variations (as can a dodgy connection be influenced by wind movements of cables and aerials). Hopefully, though, you'd have spotted a 'pattern' relating to such weather conditions?

If no amplifiers are in use then it's unlikely that the mobile masts are having any influence.
 
Your location is important. Where (approx) are you?

Check your TV predictions here Digital UK - Postcode checker using the 'detailed view' option. This may show that your frequency ch61 reception is liable to interference from other sources more than the other muxes?

Note that on 27 March next year the BBC mux moves from 61 to 49 and the predictions will change, as will your reception (to some extent).

Reception at 40 mile distant can be affected by all sorts of things between you and the transmitter... (e.g. Tidal variations caused problems to the Countisbury analogue transmitter's rebroadcast link from Mendip. Trees and foliage wet/dry moving/still can affect some frequencies more than others.) as well as interfering signals. Water on the receive aerial can sometimes cause variations (as can a dodgy connection be influenced by wind movements of cables and aerials). Hopefully, though, you'd have spotted a 'pattern' relating to such weather conditions?

If no amplifiers are in use then it's unlikely that the mobile masts are having any influence.

In the Cotswolds, a few miles east of Stroud, about 600 feet up. Bearing to Mendip is 210 degrees from memory.

Thanks for that checker, I hadn't seen that version. For Mendip it shows 99/100 on all channels. Rather surprised to find one alternative is Rowridge, 129km away.

I think our aerial is wideband, would that make it more susceptible to interference? There's a cable junction box a few metres below the aerial which I want to get up and check when the weather's better, but I would have thought a problem there would be all or nothing rather than frequency dependent.

Having spent more time looking at signal strength I've realised the problem can affect all the channels, but never at the same time, there's always at least one that's close to 100%.

Currently waiting for a quiet time in the house when I can turn off everything to eliminate internal interference.

Thanks for your interest.
 
I think our aerial is wideband, would that make it more susceptible to interference? There's a cable junction box a few metres below the aerial which I want to get up and check when the weather's better, but I would have thought a problem there would be all or nothing rather than frequency dependent.
Wideband and group C/D aerials have fairly similar gain curves - BUT aerial design style can have radically different polar responses and pick-up from the rear may be part of your problem? Aerial Polar Reponse Diagrams (and much of the rest of the site makes good reading)

The junction box is a concern... why is one there? (definitely not a masthead amp - one may be needed). Slightly poor connections can cause internal reflections giving problems at specific frequencies - but these frequencies shouldn't change as you describe. However it's always worth going over all the ones inside with a fine tooth comb... it's surprising how often that can and has fixed similar problems on here!! :thumbsup:

Check out UK digital TV reception predictor {although I take the prediction numbers and aerial recommendations with a large pinch of salt} the site also links to a path profile plotter...

One I did for a nearby postcode (GL6 7NQ) showed that a ridge a few miles toward Mendip that partially obstructed signal and even with a much higher receive antenna (than 10 metres agl) would cause 'diffraction' effects (these may change as any vegetation on the ridge altered with the weather and seasons - frequency by frequency being slightly different). Also there was another ridge behind which could - if aerial pick-up from the rear was significant - give similar issues. YOUR path profile will be somewhat different but may not be too dissimilar?

A picture or good description of the aerial may help us?
 
Wideband and group C/D aerials have fairly similar gain curves - BUT aerial design style can have radically different polar responses and pick-up from the rear may be part of your problem? Aerial Polar Reponse Diagrams (and much of the rest of the site makes good reading)

The junction box is a concern... why is one there? (definitely not a masthead amp - one may be needed). Slightly poor connections can cause internal reflections giving problems at specific frequencies - but these frequencies shouldn't change as you describe. However it's always worth going over all the ones inside with a fine tooth comb... it's surprising how often that can and has fixed similar problems on here!! :thumbsup:

Check out UK digital TV reception predictor {although I take the prediction numbers and aerial recommendations with a large pinch of salt} the site also links to a path profile plotter...

One I did for a nearby postcode (GL6 7NQ) showed that a ridge a few miles toward Mendip that partially obstructed signal and even with a much higher receive antenna (than 10 metres agl) would cause 'diffraction' effects (these may change as any vegetation on the ridge altered with the weather and seasons - frequency by frequency being slightly different). Also there was another ridge behind which could - if aerial pick-up from the rear was significant - give similar issues. YOUR path profile will be somewhat different but may not be too dissimilar?

A picture or good description of the aerial may help us?

The junction box was the work of the previous owner. I replaced the aerial, but didn't have enough coax to not use the junction box. If and when I've got time, I'll replace the cable with a single run.

You're approximately right about the signal path profile. We're on the north side of a steeply sloping east-west valley. The south side potentially interferes with the path, but exact elevations are critical, and I suspect the online plots aren't accurate enough - there's a 10 metre height difference between the two corners of my garden, and I should think at least 30 metres over the area of a postcode.

I've done my own survey which gives an aerial elevation of 197m +/- 4m. The highest point on the signal path on the south side of the valley is at 192m at 1.5 miles.

I did a plot on Megalithia with an accurate grid reference and my own elevation estimate, and that showed the first fresnel zone as 20 metres above the south side of the valley, so only grazing taller trees. Wolfbane gives a much more pessimistic projection, but I think puts me at about 180m, so significantly too low.

I can't really find any pattern to the interference. My impression is that it gets worse after dark. (Summer evenings were relatively OK, at least on channel 61.) At this moment (a sunny morning) it's the lower frequencies that are affected (48, 52, 54), which are fluctuating from poor to medium, but recently channel 61 has been often been 'No signal' in the evening (it's 75%/good, no fluctuation right now).

If the problem is trees blowing around in the signal path I'm wasting my time changing the cable. I'm confused.
 
Not sure if this is heresy here :rotfl:

A £20.00 dish and lnb (from e-bay) and a Freesat box will solve Voltaire's problem at a stroke. Given a suitable location for the dish a couple of hours work should solve the problem for ever :D
 
Of course it's heresy, but you're forgiven, as it's (nearly) the season of goodwill to all! :D

Actually your solution applies to virtually all Freeview reception problems, although I assume that if someone can't/won't install a proper aerial, then they can't/won't install a dish...:(
 
I'm tempted to suggest that this is one case where a masthead amplifier might be needed to provide a (more) stable higher level signal ... but before blundering in blindly with that what type/style of aerial (make, model, if known) did you install, voltaire? {I'm thinking now mostly of reflections into the back which a log periodic or grid antenna might be better suited to reject - most probably also requiring an amplifier}.
 
OK, so I've done the turning everything in the house off experiment, all circuits off at the switchboard apart from the one for the TV, didn't make the slightest difference, so it's not interference from anything in the house.

Didn't get chance to take a picture of the aerial before dark. I think it's a 14 (or 15?) element wideband Yagi. Most of my neighbours seem to have ones about half the length, and I'm at least 10 metres higher than them, so I think it's overkill. It is an analogue aerial, mounted on a standoff from the chimney, so slightly above the ridge of the roof. The ground drops away sharply from the house, so nothing close can block the path. I installed it over 10 years ago, and it worked fine. Then for quite long time we lived without television, and got it again for the Olympics, so this problem has been ongoing since July. The downlead is old fashioned low loss coax, so if the problem is interference getting in to the cable (as opposed to being picked up by the aerial), replacing it would go along way to keeping the interference out, if it is an interference problem.

Would a masthead amplifier help? I thought they amplified interference as well. The digibox often shows 100% signal level on some channels. For a while I thought I had an overload problem, because channel 61 dropped from 75% strength to nothing, while others showed a strong signal. But having realised the weak/poor signal keeps affecting different channels, interference seems more likely (but I'm not an expert).

We wouldn't be keen on a dish - it would have to go on the front of a relatively pretty cottage whereas the aerial is too high to matter much.

Access to the aerial is easily possible with stepladders because of the sloping ground and a shed, so I feel I ought to be able to fix this, if only I can work out what I'm trying to fix.
 
All aerials are analogue , the uhf signals they recieve is analogue, the data the uhf carrier is modulated with is digital for digital TV.

Masthead amplifiers are very low noise devices and as they are mounted at the aerial they don't amplify any noise picked up on the downlead.

My suggestion is fit a new log40 wired with wF100 grade coax. (Log periodics are very good at rejecting impulse noise). If you still have problems you can add a masthead amplifier later (you will need a power supply for it fitted by the TV). If you get a masthead get a variable gain one, I swear by Vision electronics masthead amps.

eg

Vision V20-1420 Variable Gain Masthead Amp + PSU - dastechnology.co.uk
 
Meters on receiving equipment are not often calibrated against a known input level figure. Some just give a guide the levels after the automatic gain control of the tuner, and thus may indicate much the same levels against a wide input level variation...

My Panasonic TV is like that and the levels hardly changed before and after DSO (when power increased by 10x or 20x). My Humax 9200T however did show an improvement from 50-60% strength to over 70%. This does fluctuate slightly over time... but then I'd expect it to as signals do vary a little over time.

If Wolfbane's calculations are to be believed your field strength figures are around 35dBuV/m. Assuming an extra high gain aerial gain of +17dB, allowing -3dB for cable loss another -6dB for termination losses and -18dB (due to the lambda/pi conversion factor for C/D frequencies) then you'll have a terminated signal strength of around 35 + 17 -3 -6 -18 = 25 dBuV... The recommended levels are between 45 and 65 dBuV (although some receivers will cope with more and less signal of course)...

Wolfbane's numbers would have to be significantly out as even a factor of 100x (20dB) would still leave you towards the low side with the biggest available non-amplified aerial (+17dB gain... a log periodic is only around 6-7dB, and a 'standard' +11dB).

Graham's antenna suggestion is a good one but - personally - I'd try the amplifier first with the existing aerial if of a decent quality... NB some of the Vision ones work with 5V supply that some set-top boxes and TVs can output (check for an antenna power option in the setup menu of your receiver) and won't need a power supply brick.
 
Decided to target the downlead first, as I knew that was substandard. Unfortunately didn't get home with the replacement (good quality double screened) until after dark, so spent an hour outside in the wind and rain with a head torch and got it fixed up.

Result! Signal level 100% on all channels. Signal quality 'Very Good' on all channels. I have some more work to do tomorrow on cable routing and with cable clips, but that's fairly trivial.

Thanks to Rodders53 and everyone else who has helped.
 
Decided to target the downlead first, as I knew that was substandard. Unfortunately didn't get home with the replacement (good quality double screened) until after dark, so spent an hour outside in the wind and rain with a head torch and got it fixed up.

Result! Signal level 100% on all channels. Signal quality 'Very Good' on all channels. I have some more work to do tomorrow on cable routing and with cable clips, but that's fairly trivial.

Thanks to Rodders53 and everyone else who has helped.

Excellent news... and well done on braving the elements.
 
Result! Signal level 100% on all channels. Signal quality 'Very Good' on all channels. I have some more work to do tomorrow on cable routing and with cable clips, but that's fairly trivial.
Brilliant news! (and you're much braver than I to climb ladders in the wet and dark!!)

Probably a slightly dodgy connection rectified by renewal and/or damage due to old age within the old cable. The sheath can perish/become brittle and start to let in water, for example.

(Many years ago my self-install telephone extension cable ran up an external wall and failed - a BT installer reckoned it was due to repeated expansion contraction in the sun of the solid core conductors causing an eventual break.)
 

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