Interference between signals on dual output lnb?

ckrys1

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#1
I`ve recently installed a Syntec dual output lnb on my 1.2m prime focus motorised dish. The receivers to which the 2 outputs are attached(Humax 5300 and Manhattan Skyline 1000), although physically in different rooms, are piggy backed together with uhf leads, along with another digital receiver, 2 analogue receivers,2 Sky digiboxes, ITV digital receiver,and a video, and distributed round the house with the 5 terrestial channels to 13 tv's. (I know it seems a lot, I'm a bit of an addict), but it all works with no interference until now.
When the Humax and Manhattan are both on, there is an electrical type of interference on 6 of the tv's on all channels, the sort I seem to remeber you used to get when the hoover was on! It is not uhf interference, and changing uhf channel makes no difference. But, switching either of the 2 receivers off clears the problem.
The cables from the lnb are taped to the same arm of the dish, and then go their separate ways round the house into separate rooms (using CT100 cable). The only explananation I can come up with is that the signals are interfering at the lnb, or in the cables while they are near each other - is this likely? And is the lnb therefore faulty, and should I get a replacement? Or is it just a fact of life with dual output lnb's?
Thanks for any help (sorry to be so long!)

Ckrys1
 

2old4this

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#2
I can't imagine it's the LNB or proximity of the cables. Good quality cable is excellently isolated.

I've got three dishes, and cables from those plus one from a normal antenna and another from a high-gain antenna are all bundled together and fed some 20+ meters through piping and even a junction box into my house, where they continue to the respective receiving devices. I don't have any interference.
The only time I have had, it was related to the uhf channels I select on any device I'm looping through. Are you sure it's not that? I'm not sure I understand what exactly you mean with 'piggybacked together using uhf cable".

If you disconnect one of the LNB outputs, does the problem go away?

Do you have a normal uhf/vhf antenna connected to any of your boxes, looping through for terrestrial TV? If so, try disconnecting that.

Do you have any of your boxes connected to TV via uhf cable (rather than SCART otr video/audio plugs)? Try replacing that connection by a SCART/plugs.

2old
 

ckrys1

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#3
Thanks for your reply - delighted to say I have resolved the problem, tho` I`m not sure why or how!
When I say `piggybacked with uhf cable`, I should have said looped through with rf cable. Perhaps `piggybacked` is a Scottish word only?. Anyway the terrestial aerial cable goes into the ITV digital rf input, the output of that goes to sky digital input , and so on until the last receiver goes into a uhf distribution box, from which there are 6 outputs, some of which branch again in other rooms to supply the 13 tv`s.
Changing the uhf channel on the two interfering receivers made no difference, but disconnecting the lnb cable from either did, but obviously this was not a satisfactory solution.
I realised that the 2 interfering receivers were next to each other in the chain, ie the output of the humax was connected to the input of the manhattan, so I simply changed the order by putting a receiver `in between` the interfering 2. Amazed and pleased to see no interference and perfect pictures everywhere. Don`t know why - if anyone can explain I`d be interested.

Ckrys1
 

Channel Hopper

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#4
It could be that the signals used at RF for both recveivers have not followed the N+5 and N+9 rule where none of the broadcast frequencies should be set either 40 or 72MHz from each other.

However inside the more modern receivers are many operating frequencies.

Those from the PSU (switchmode), the tuner/demodulator, the main and sub processor circuits, each with their own crystal or oscillator settings.

Each creates both main and harmonic frequencies, and owing to the nature of the beast, these are rarely filtered out completely, leaving some to propogate to the rear panel connections.

CE approval does have specific requirements for output signals through the usual RF, SCART and accessory ports, but these are rarely if ever adhered to once full production has started on a product

I believe that whichever receiver was fitted in the RF chain first was the one that has a fault and the frequencies generated have created interference with the RF output

Sometimes however the individual boxes can interfere with each other as a matter of RF connections being made in a fashion that generates trouble on their own.
 
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