[mythtv-users] OT: Moved aerial & lost one mux (UK Freeview)

John Pilkington J.Pilk at tesco.net
Tue Oct 17 10:06:24 UTC 2017

On 17/10/17 04:02, Stephen Worthington wrote:
> On Mon, 16 Oct 2017 22:39:15 +0100, you wrote:
>> On 16/10/17 21:16, Simon Hobson wrote:
>>> Slightly OT but I suspect there's a few people here with more RF/antenna knowledge than I have ...
>>> We've just moved, and no aerial on the new house - prev owner used Sky and I don't have any sat equipment.
>>> Until I get round to having a decent one put up outside, I've put one in the loft for now. When I was testing with the aerial in the lounge (didn't want to miss Red Dwarf !) I did a full scan on the TV and got all 5 muxes I should. Now I've moved it to the loft, pointing in the same direction, but with a longer cable (WC100/WF100) - I can't get anything on one mux.
>>> The other 4 muxes are fine, plenty of signal, just the Com6/ArqB mux on C22+ (482.2MHz) is "missing". It's almost as though there's a notch filter in place - connected direct to the TV it gives "no signal" but just occasionally flickers a bit of pixellated picture and a "click" of sound.
>>> Anyone come across this effect before, got any suggestions as to the cause ?
>>> It's the same for the TV, and both my tuner cards (HVR1350 IIRC). Transmitter is Lancaster (Gp A, Vertical pol), Wolfbane says I have a clear LoS and should be OK with a Hi-Gain antenna (50dBµV/m), and the aerial I'm using at the moment is a Gp A XB10 http://aerialsandtv.com/gaincurves.html#Agroups. All connections are F type, except for the TV which is the old Belling Lee "UHF" plug. Roof is slate on timber rafters & battens.
>>> I'll be putting a Yagi 18A up when I get round to calling the installer in.
>> It does seem strange that one mux is received so differently;
>> frequencies are closely spaced, and the same listed power and
>> polarisation.  Indoor locations can be affected by reflections, even
>> with your roof materials, and quite small movements can have pronounced
>> effects, but the attic ought to be generally better than the lounge.  I
>> assume that your cable run doesn't have any spurs, which will cause
>> (possibly irrelevant to your circumstances) frequency dependent losses.
>> You don't say how long the run is.  Can you try a preamp, as near to the
>> aerial in the run as you can conveniently get it?
>> But the local knowledge of an installer might be your best bet.
>> Good luck,
>> John
> DVB-T transmissions are very resistant to reflections.  That is due to
> the way they were designed.  A reflection normally causes a small
> phase difference between the signals, and that is sufficient to allow
> the DVB-T receiver to lock to one or the other of the two signals on
> the same frequency.  It is common to have nearby DVB-T transmitters on
> the same frequency where people can receive from both, and it does not
> cause problems.

Yes; I haven't looked up any field tests, but I suspect this is a "can 
be, when locked, and with appropriate hardware" rather than "are."  And 
reflections can give dead spots and frequency dependence too: see eg 
"Lloyd's mirror."  All rather unlikely.

Anyway, if you really really want "Yesterday" and "Dave ja vu," and have 
nothing else to do after moving house, you can try moving the aerial a 
bit and rescanning.  You'll perhaps lose something else :-)

> As to the original problem, if the aerial works from downstairs
> without the cable, and the cable is a temporary one that is loose, I
> would suggest trying the aerial downstairs with the same cable in
> place, to see if the cable is the problem.
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