DECT Phone causing interference to satellite TV reception.

[Robbo]

If you are having reception problems with some satellite TV channels, one of the things to check is that it isn't your cordless DECT phone causing it.


DECT Frquencies are 1880 to 1900 MHz


Hi band problems:-

12480 to 12500 MHz (10600 MHz LO) IF is 1880 to 1900 MHz

Lo band problems:-

11630 to 11650 MHz (9750 MHz LO) IF is 1880 to 1900 MHz


For example if the interference occurs on a channel that is on 11642 MHz for example, the LNB will down-convert it to an IF frequency of 1892 MHz, which is right in the middle of the DECT band. Poor screening and/or DECT equipment too close, could cause problems.

[Analoguesat]

These Dect phones commonly cause problems with the free music channels on 11642V on Astra 2/Sky - Scuzz / Flava / NME etc.

[TheCoder]

It would have to be a pretty bad design or you'd have to be using the phone inches from the box for this to happen though. Once out of the tuner can the baseband should be at least several hundred times more powerful than any extraneous phone signals.

If your having interference problems, its much more likely they are entering the dish/lnb and the interference is being downconverted along with the signal. You can get all kinds of harmonics from all kinds of strange sources producing problems with sat signals. I've even seen a motorbike engine having an effect !

Actually, thinking about it, the IF never actually comes out of the tuner can and thats always screened. The output of the tuner can is always baseband.

[Robbo]

I would suspect that the interference is caused on the intermediate frequency coming from the LNB, either through the cabling or as it enters the tuner.

I would have thought it very unlikely to be occuring at the dish itself, at the microwave frequencies.

[Analoguesat]

It is the IF that causes the problems. It leaks into badly shielded equipment, mixes with the IF's downconverted by the lnb nails bands of transmissions.

[TheCoder]

Originally Posted by Analoguesat

It is the IF that causes the problems. It leaks into badly shielded equipment, mixes with the IF's downconverted by the lnb nails bands of transmissions.

Question is, whats the I.F leaking out of ?

Pretty much everything that the IF is present in should be fully shielded. Thats the lnb itself, the co-ax transfer cable and the tuner can within the stb. The I.F shouldn't exist on the stb PCB itself.

tbh, i'm still not sure how DECT phones or mobile phones could cause any interference at all to a sat signal as the modulation techniques are fundamentally different. Even if completely mixed, the demodulators on these devices should still be able to recover their own seperate signals.

[Robbo]

In a perfect world yes.

But in reality, nothing is fully shielded, just varying degrees of shielding.


The modulation type is irrelevant, if two carriers are on the same frequency, or at least within the channel pass band, and the interference is of sufficient amplitude, it will bugger up your wanted signal.

The 2 signals don't 'mix' as such, they add together.


The IF isn't leaking out, it's getting in, the interfering signal being transmitted deliberately by the DECT system.

[Thompson9100]

Hi.

I'm new to these forums, and when it comes to STB's, etc, I'm not that techy. However, I can definately say that DECT interferes with Sky. Specifically the True Movies channels. I live in a block of flats (very new) with a distribution system (recently checked out fully due to a faulty amplifier that was replaced).

The engineer told me the problem is likely caused by my DECT telephone. I tested the theory by turning the phone off at the mains, and guess what? No Interference!

True Movies actually run adverts asking you to move your DECT phone to over 2 meters from your STB, which is ok if it would work! My phone is over 3 meters from the STB and still interferes. It's really bugging my wife as she loves True Movies!

It is apararently due to the low frequency that they transmit the signal on. I'd love to know how to stop it without banning the use of DECT phones in my house. I personally hate corded phones so there's a bit of a stand-off in my house at the moment!

Anyways, any suggestions more than welcome.

[SAT-WORLD]

Have you made sure that the DECT phone base station,is actually as far a possible from the Sky digibox,not just the handset.

[Robbo]

Interesting..


I ve never had the problem myself, so have never had the chance to find a cure for it, but I do have a little understanding of the likely causes.

For one, good quality cable must be used between the outlet plate and the sat box. Ideally the outlet plate should be fully screened too. You could try triple screened cable if there is already CAI approved double screened cable already in place. If the cable is the crap stuff, try using cai aproved, eg wf100 or equivalent.

With well screen cable, providing an earth to the-f connector may help too, not sure.

I think it is unlikely that it gets in through the receiver, as the tuner should be canned, but as an experiment, I suppose you could wrap the whole box in tin-foil or something, LOL

Try the above, and if you get an any success at all let us know how you got on.


Unfortunately, if low quality cabling has been used inside the wall, there's not much you can do about that.