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 ...
Try this before and after the Sky box adds its signal.
I've so far only tried it before (upstream of) the Sky box. ..... [using an adjustable attenuator] [with] 10dB-20dB...., the result is similar to the ('near-perfect') picture I see with the (reduced value) capacitor.
Do you think there is still something to be gained by trying the attenuator (or, indeed, my capacitor) downstream of the Sky box?
With no aerial connection in to the modulator (that's important), then testing the capacitor then the attenuator is about seeing whether the analogue RF output is too strong for the TVs' tuners.
I'm a bit confused there. I've already demonstrated that if I disconnect the aerial, then 'the problem' goes away (i.e. the distributed Sky RF seems to produce a near-perfect picture), so there isn't then really anything left for the capacitor (or an attenuator) to improve.
........ On the other had, I suppose there would be some academic (but really no practical) interest in trying capacitor and/or attenuator downstream of the Sky box,
with the aerial connected (hence some degradation of the analogue RF signal) - since that would presumably clarify whether the problem was arising at the level of the modulator and/or the tuners of the remote TVs. Maybe I'll try that 'for completeness'!
Just a quick update, for anyone interested.
Just to remind everyone, remote TVs were receiving poor quality (nosing/'snowstorm') signals distributed from the RF output of a Sky box. When the TV aerial (very strong DTV signals) was disconnected from the Sky box, the problem went away and the Sky RF distributed from the Sky Box became near perfect.
Inserting a very small capacitance, or 10-20dB attenuation using a variable wideband attenuator, between aerial and Sky box made this problem go away (distributed Sky RF 'near-perfect'), whist still passing enough of the DTV signals for the remote TVs to receive them satisfactorily. A 700 MHz low-pass filter inserted in the same place (upstream of the Sky box) did not produce as much improvement as did the capacitor or attenuator). That's where I left the story before.
I've done only two more things since then. The first was trivial, replacing the variable attenuator (between aerial and Sky box) with a fixed 15 dB one and, needless to say, that changed nothing (i.e. the distributed Sky RF signal remained 'near perfect'.
Secondly, I tried moving the (15 dB) attenuator (from pre-Sky box) to (a) between Sky box and the 6-way distribution box/amplifier and (b) between outputs of that 6-way distribution box and the remote TVs. In either location (a) or (b), the attenuator achieved nothing - if anything, the distributed Sky RF got even worse.
I still find this all a bit odd/confusing. Particularly given that I'm now back to using ch68 (>700 MHz), the fact that an attenuator is better at improving things than is a 700 MHz low-pass filter and the fact that either will only produce improvement if installed between aerial and Sky box seems to indicate that some strong signal(s) with a frequency below 700 MHz is/are upsetting the Sky box's modulator to the extent that the Sky RF signal deteriorates. Those "some strong signal(s)" could be DTV broadcasts or anything else - and without acquiring, and playing with, spectrum analysers (as previously mentioned!) and tuneable wide-range notch or band-pass filters I'm probably never going to know the answer.
Whilst I'm always frustrated by 'not knowing', I'm also very pragmatic, so since I have found 'a solution' which works to my satisfaction, I'm not going to lose too much sleep because I don't fully understand how/why the 'solution' is working
Thanks to everyone foe their interest in this exercise!
Kind Regards, John
