Analogue distribution of signal from Sky box

[JohnW2]

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If I needed it, I have potential access to the big brother of that (so wouldn't have to wait until Christmas!), but I don't think it would necessarily help in the situation I was describing. If the Sky RF were being interfered with (by 'brute force') by some very strong signal on a frequency totally divorced from that of the Sky RF, then I don't think that a spectrum analyser would necessarily help in identifying which one it was!

Kind Regards, John

 

[Lucid]

However, don't the filters filter both the inner and outer of the feed? If they did (which is what I would have expected) then, as I said before, I don't see why it should matter whether the unwanted signal was coming from the aerial or from the cable. Am I missing something?

The filter blocks the signal from upstream. But of you've got poorly shielded coax by modern standards for digital TV, and now the new information about some amateur radio transmissions then the coax downstream of the filter could well be the route in for RFI. Whether that RFI is as strong as the same RFI signal picked up by the aerial I couldn't say for certain.

 

[JohnW2]

I'd just put in the 4G filter and go from there.

As I've said, that is/was my next planned step, as soon as the filter arrives. However, patience is not one of my virtues, and I may now not need to wait!

You will recall that one of the other approaches I muttered about was to simply put an attenuator in the aerial feed to the Sky box, the idea being that it might reduce the strength of whatever it is interfering with the Sky RF, whilst leaving enough of my (very strong) DTV signal for the TVs to use.

Somewhere I have some attenuators, but I haven't yet found them! So, being impatient, I've just thrown together a crude 'signal reducer', simply by putting a tiny variable capacitor (I think 15-20 pF max) in the signal path (see piccie below) - and that seems to have 'done the trick'. The Sky RF signal is now (at least to my eyes) totally clean throughout the house, and the DTV signal also remains OK (i.e. adequate) throughout the house - and all that whilst still using ch48. Adjustment of the variable capacitor is in no way critical, so I could easily substitute a (very small) fixed capacitor and make the whole thing physically much smaller.

So, although I'll certainly try the filter when it comes, it seems that I probably have 'a solution' without it!

Kind Regards, John

 

[JohnW2]

The filter blocks the signal from upstream. But of you've got poorly shielded coax by modern standards for digital TV, and now the new information about some amateur radio transmissions then the coax downstream of the filter could well be the route in for RFI. Whether that RFI is as strong as the same RFI signal picked up by the aerial I couldn't say for certain.

See what I've just posted - which somewhat over-rides most else.

I may have confused you with mention of amateur radio transmissions, since you refer to it as 'new information'. The recent discussion on the sister forum was about the need (or not) for screening of (modern) signalling cables, and the talk about amateur radio transmissions related to anecdotes from some of us from many decades ago. There are certainly no amateur radio transmissions (of which I am aware!) in the current equation!

I was talking about installing the filter very close to the Sky box, so only a few centimetres of cable downstream of the filter before it got to the box. Are you now perhaps talking about the post-Sky box (and post-distribution box) feeds to the TVs picking up interference, and hence causing problems at the TVs' tuners?
[Edit: if so, then I suppose that filters at the downstream end of each distribution cable would be the answer. However, I very much doubt that is the problem, since the degradation of the Sky RF is essentially the same with the shortest and longest ('distribution') cable runs.]

Whatever, as you will have read by the time you see this, I seem to have found 'a solution'

Kind Regards, John

 

[winston1]

The RF converter/modulator in a sky box passes RF from around 48 to 860MHz not just UHF. (Because Ireland where sky is also marketed used VHF for analogue TV). This means that any local strong signal below the UHF band could get in and cause cross mod in the modulator. Clutching at straws but something to be aware of.

EDIT. Your capacitor would have more attenuation the lower the frequency.

 

[JohnW2]

The RF converter/modulator in a sky box passes RF from around 48 to 860MHz not just UHF.

Indeed.

(Because Ireland where sky is also marketed used VHF for analogue TV).

So did we (UK), way back ('Band I' TV), and that could be a real problem for radio amateurs - apart from anything else, even small amounts of 2nd./3rd harmonics from 15m/20m band transmissions could be on, or very close to, Band I TV channels

This means that any local strong signal below the UHF band could get in and cause cross mod in the modulator. Clutching at straws but something to be aware of.

Exactly - I was avoiding 'technical terms', but that cross-modulation is precisely what I was referring to when I talked of 'brute force' interference - and is the reason why a low-pass filter (such as a 4G one) or a UHF notch filter (on Sky RF channel) would not inevitably solve problems.

EDIT. Your capacitor would have more attenuation the lower the frequency.

Indeed so - but, pragmatically, the relative attenuations it is providing at (known) wanted frequencies and the (unknown) frequency of whatever is doing the interfering appear to be "doing the trick' satisfactorily (i.e. getting rid of the 'unwanted' whilst retaining enough of the ';wanted')

 

[RandomGrinch]

If I needed it, I have potential access to the big brother of that (so wouldn't have to wait until Christmas!),

I somehow guessed you would!
...and the 'signal reducer' is a work of art!

 

[JohnW2]

I somehow guessed you would!

Having 'friends ' can be useful at times!

...and the 'signal reducer' is a work of art!

Thanks, but I have to say that aesthetics were not high on my priority list! As for the description I had to call it something, and 'attenuator' did not really seem appropriate, hence 'signal reducer'! Anyway, whatever one calls it, it certainly does seem to do the job. I suppose that the filter (which still hasn't arrived) might be fractionally 'better', but I'm not sure there is enough scope for further improvement for it to be noticeable to my eyes!

Kind Regards, John

 

[JohnW2]

.... I had to call it something, and 'attenuator' did not really seem appropriate, hence 'signal reducer'! Anyway, whatever one calls it, it certainly does seem to do the job. I suppose that the filter (which still hasn't arrived) might be fractionally 'better', but I'm not sure there is enough scope for further improvement for it to be noticeable to my eyes!

This is interesting. The 4G/5G filter has arrived, but the outcome is not one which was amongst my mental list of the most likely ones!

There is, even to my eyes, a marked difference between the filter and my 'signal reducer' in terms of the picture quality of the Sky RF signal. However, although the quality is much better with the filter than with 'nothing', it is clearly inferior to the quality I'm seeing with my 'signal reducer'!

At first sight, this might suggest that whatever is interfering with the Sky RF signal is on a frequency corresponding to a channel below 60 (which I gather is the approximate cut-off the filter). However, unless the insertion loss of the filter at such frequencies is considerably greater than the claimed "<1dB", that would not explain why the filter is resulting in appreciable improvement (albeit not as much as my 'signal reducer').

The only other explanation I can think of is that the interfering signal is, indeed, above ch60 (quite possibly 4G/5G) but that my 'signal reducer' is resulting in appreciably more attenuation at the frequency concerned than the 40dB claimed for the filter.

Any other thoughts/ideas?

Kind Regards, John

 

[RandomGrinch]

Just as a case of dotting the i's and crossing the t's (and of no consequence whatsoever ), I wondered if you had substituted a portable antenna for your fixed one, during your experimentation?

 

[JohnW2]

Just as a case of dotting the i's and crossing the t's (and of no consequence whatsoever ), I wondered if you had substituted a portable antenna for your fixed one, during your experimentation?

I haven't - but, other than as 'an experiment', I'm not sure that I could. Probably because most of the walls of my house are extremely thick, few RF signals seem to be able to get in at all - there are certainly only a very few places in the house where i can get any mobile phone signal at all (so good luck to anyone who tries to install a 'smart' meter ).

In as much as I know that unplugging the aerial from the Sky box cures the problem (of the Sky RF signal),I imagine that substituting an indoor 'portable' aerial would probably do the same - but quite probably without getting adequate DTV signals.

For the above reason, I'm not even sure that I've still got any portable aerials but, if I can find one, I will do the experiment, " just to dot the i's "!

Kind Regards, John

 

[RandomGrinch]

I haven't - but, other than as 'an experiment', I'm not sure that I could

In my non-specialised mind, I was also wondering if some directionality for the interference could be ascertained - I can see that wouldn't be possible now!
...and only be of academic interest, as the problem is substantially solved!

 

[JohnW2]

In my non-specialised mind, I was also wondering if some directionality for the interference could be ascertained - I can see that wouldn't be possible now!

Well, as I said, it might be possible -and, if that proves to be the case, I'll certainly try (and report back!)

...and only be of academic interest, as the problem is substantially solved!

Indeed -but an interesting question I'm now pondering is how I would have reacted/behaved if I had 'been patient'. If I had simply waited for the filter to arrive, and then found that it substantially, but not completely, 'solved' the problem, I may well have concluded that such was probably the best that could be achieved, such that I would have to 'live with that' - without knowing that a crude 'signal reducer' could actually do better than the filter!

I'd be interested to hear what some of the experts have to say about what I have now experienced/achieved!

Kind Regards, John

 

[Lucid]

What would be interesting is taking the 4G filter out of the equation, and then comparing the capacitor to a standard wideband signal attenuator.

In fact, I'd go one step further and get hold of a 0-20dB variable attenuator such as the £6 ProCeption unit from Toolstation (lots of branches up and down the country, easy to check local branch to stock, click and collect ordering available to guarantee availability).

Try this before and after the Sky box adds its signal.

This would help point in the direction of whether it's tuner oversaturation or crossmodulation noise from outside the band.

 

[JohnW2]

What would be interesting is taling the 4G filter out of the equation, and then comparing the capacitor to a standard wideband signal attenuator. In fact, I'd go one step further and get hold of a 0-20dB variable attenuator such as the £6 ProCeption unit from Toolstation ...

Indeed. In fact, as I said before, I knew that I had some attenuators somewhere, but couldn't find them, but have just found a couple of them, one of which is a 0-20dB variable one such as you mention ...

Try this before and after the Sky box adds its signal. ... This would help point in the direction of whether it's tuner oversaturation or crossmodulation noise from outside the band.

I will. Watch this space.

However, I was about to write a note to confess that I may not have given the filter a fair trial. Looking at the spec of the (Labgear) filter, I see that it's cut-off frequency is about 700 MHz, so that it is meant to 'pass' up to ch48 and 'reject' everything from ch50 upwards. However, although I had intended to change back to ch68 before I tried the filter, I forgot to do that - so what I recently reported with the filter was whilst still using ch48. Hence, once 'er indoors has stopped watching Sky, I'll change it back to c68 and see how the filter then performs.

Kind Regards, John