HD signal strenght from CP

[Dave Plowman (News)]

A few weeks ago, there was an alteration to the frequencies used for FreeView from the main London transmitter, Crystal Palace. Since then, the HD signals read very low on the TVs, and indeed sometimes do have problems.

Aerial system is a Log Periodic from Blake (not sure of the model as don't have the receipt, but less than 10 years old). That feeds a 10 output DA.

Have several TVs on that DA, and all show much the same. Near 100% signal strength and quality on CH 22-35, but very low on 55-56.

Have bypassed the DA and looked at the signal display on a TV in that condition. Pretty well the same.

The log beam has a flying lead as supplied, and the F connectors between that and the downlead are good and clean.

Everything is pointing at the aerial. But thought log beams were all wideband?

Luckily, not that difficult to get at the aerial. Should I just replace it

- or am I missing something obvious?

[alan_m]

According to the digitaluk site Bluebell Hill (a neighbouring transmitter) is also transmitting on 55 and 56 (com7 and com 8) so I'm not sure how that's meant to work.

Put your postcode, house number AND TICK THE BOX that says "detailed view" and you should get the channel numbers from all applicable transmitters

[Andy Burns]

COM7 and 8 are now a national SFN, all transmitters are moving to CH55/56 for further spectrum clearance.

[Dave Plowman (News)]

Looked at that before and it just gives '100%' everywhere.

Not what I'm getting here. I can see the CP mast from the aerial position

- about 5 miles away.

[Robin]

I have a Blake log periodic (DML26WB) on a low pole 9 miles from CP and, while signal strength from Chs55-56 is lower than others, the quality as indicated by TV and PC is still 100%. Suggests to me an issue in the path from the aerial rather than in the aerial itself.

FU set to uk.tech.digital-tv

[Dave Plowman (News)]

It really is line of site. And zero problems with the aerial position before that retune. Signal strength is so low, the TV in this workshop won't even find anything on 55 and 56.

If I'd wanted to post to that group, I would have done.

[Bill Wright]

Difficult without an analyser. For a Blake LP just check that there's nothing between the screw heads (the screws that hold the elements to the boom) and the other boom.

There are various other weirdnesses that can affect the frequency response of logs, so I'd be inclined to try a different log of a different type.

If you're 5 miles from CP and you've been feeding the aerial output into a DA it suggests that there's some attenuation somewhere, otherwise the DA would likely be overloaded. That attenuation could be more severe on the higher frequencies. Another reason to try another aerial, and when you do have a 12dB attenuator handy.

The other possibility is that there are two signal paths to the aerial and they are in anti-phase as they enter the aerial. Ground bounce, bounce from the side of a building that is just to one side of the main signal path. Quick and easy to test. Move the aerial without changing its orientation about half a yard up, down, left and right. Observe the results. Moving the ae towards and away from the tx that sort of distance doesn't change the phase relationship enough to make a difference.

Bill

[Andy Burns]

I have a DML26WB pointing at Waltham 22 miles away, COM8/COM6/PSB3 muxes on CH56/57/58 get strength 97-100 and quality 100, the COM7 mux hasn't yet moved to CH55 here.

So there's no issue with the aerial type, in analogue days Waltham used to go up to CH64.

[The Natural Philosopher]

My immedaite though was a bad connection and nulls at that multiplex frequency OR actually not tuned into CP at all, but some other transmitter.

I used to get Sandy Heath, Sudbury and occasioanlly CP...on the right day!

[Bill Wright]

That's the group with RF experts in it. It would be the obvious place I would have thought.

Bill

[Adrian Caspersz]

Mine's also line of site, normally 100%, and 13 miles away here in Harrow. I can see the CP mast on a very good day.

Except my aerial is a group A - it needs to be wideband for 55/56.

I can't be bothered changing it, for the HD stations I'm lacking.

Makes more sense either installing a dish, or getting them via IP.

[Robin]

When I referred to "the path from the aerial" I assumed you were receiving rather than transmitting. Your response suggests I go that wrong.

Sorry, I forgot you were perfect.

[Fredxx]

Perhaps uk.tech.digital-tv is less conducive to posters of topics about Brexit?

It certainly is the logical group to ask the question!

[Robin]

A neighbour had a problem after CP moved to Chs 55-56 and I couldn't see any obvious fault. So I took my poor man's signal strength/quality meter - a 22 inch Samsung LED TV - onto the roof and connected it direct to the aerial. (Mind you, this was an aerial mounted on a short pole on a short chimney at the back of the house in the middle of 2 low pitch (c. 22 degree) rooves I could sit on!) The TV worked fine so I replaced her cable and all was sorted. ISTR Bill published a piece once on the performance of different cables at different frequencies but I couldn't see what the old cable was. I may well have just got lucky sorting a bad connection.

[Ian Jackson]

Can I also suggest that Brexit (and other political discussions) take place in uk.politics.misc - or even the little-used uk.politics? That's where the experts should be hanging out. It's just plain stupid to use uk.d-i-y.

[Dave Plowman (News)]

It seems to be cross posted here with OT posts for here.

Are you saying TV aerials out with the scope of DIY?

[Dave Plowman (News)]

I'll check that tomorrow, weather permitting.

I've a variable attenuator fitted - also from Blake. Removed it, but no difference. All the aerial feeders arrive direct into the airing cupboard, and are patched onwards from there. The DA is in the cellar. All about not having cables showing or running round the outside of the house. But all cable runs are well clear of mains.

I patched the aerial direct to one TV after checking each cable run for shorts or open circuit, etc, but just with a DVM. I'd doubt any of the cables could be damaged where they can't be seen. Made zero difference to the readings on the TV display.

Right - up and down the pole is easy. Side to side, more of a problem.

Incidentally, others in the area have reported problems with HD too since the change. But may well not had wide band aerials.

[Dave Plowman (News)]

I'd say I've got a clearer path than perhaps the majority round here. Of course RF being a black art doesn't follow it is better.

Why would I want to download another group to see any replies?

You could just have crossposted your post if you want to read replies to it elsewhere.

[Fredxx]

Are you so stupid to ask the question? When have I said it is out of scope to ask here in uk.d-i-y?

Anyone with half a brain would ask in uk.tech.digital-tv instead of, or as well as, here.

[charles]

When Crystal Palace uhf originally opened - in 1964 - there was a problem known as the "Hammersmith Effect". The transmitting aerial (which is a combination of bits) wasn't correctly installed, leaving a gap in coverage. I have met this on other sites at both uhf and vhf. That could be the case here. 5 miles away is well below the main beam and all sorts of things could be happening to the signal.