Terrestrial TV Reception in SE Reading, UK - C-H and TETRA Interference (LONG!)

Apologies if two copies arrive - I'm in the middle of swapping ISPs and
have not received the first one back from the news server after several
hours.

Regulars here may recall that I live in SE Reading, UK, and am trying to
improve terrestrial TV reception. I'm indebted to Bill Wright and other
professionals here for their advice, but should emphasise that any views
expressed and mistakes described below are my own, and that my area of
technical expertise lies elsewhere. I record problems and solutions
encountered hoping that I can save others the sometimes unnecessary expense,
time, and exasperation that I've undergone.

Useful general background on TV Interference can be found he

http://www.ofcom.org.uk/static/archi...a323/ra323.htm

Local symptoms a
1) Mains spikes &/or electromagnetic interference, which occur in winter
months to the extent that in practical terms it's impossible to make any
recording I'd want to keep. On analogue this appears as about 2 or 3
seconds of snow on picture and crackling on audio, and on digital at best
freezing on picture possibly with clicks on audio, at worst complete loss
and brief 'No signal' messages.

I believe this is interference from my own and neighbouring central-heating
systems. The area is mostly 80s housing estates, and presumably those
systems which have not since been replaced or at least well maintained are
wearing out. Useful background can be found he
http://www.ofcom.org.uk/static/archi...info/ra272.htm

2) Continuous faint patterning on analogue channels.

As I now watch almost exclusively digital, I didn't realise the
significance of this until I tried to cure the more important spikes
problem. It's a type of interference called cross-modulation - an
interaction between UHF TV frequencies and others outside the UHF TV band
emanating from TETRA transmitters, used by local emergency services, and
possibly also from some or all of 4 nearby mobile phone masts (note a).
Useful background can be found in the first link given, section 2.2 Masthead
Amplifiers, and he

http://www.ofcom.org.uk/ind_groups/i...tetra/?a=87101


FWTW My Conclusions/Advice:

Ensure that you have your aerial pointing at the optimum transmitter for
your location, but IME below don't just assume that even a reputable local
rigger will necessarily know which transmitter that is. Try and cross-check
for yourself. Remember that signal power and what lies along the line of
sight are important, and that the former is not always equi-directional from
the transmitter, and falls off sharply with distance (in another thread we
concluded that each doubling of distance quarters the signal power). Useful
sources are (note that clicking on the map in the first link seems to give
you analogue info, while the links at the end of the text give you digital
info):
http://www.bbc.co.uk/reception/tv_tr...rs/index.shtml
http://www.megalithia.com/elect/terrain.html

In areas of high interference such as built-up areas, choose a
highly-directional aerial (with a central mount if long).

As far as possible, avoid a mounting site pointing out over other nearby
houses, and get the aerial as high as practicable.

Use double-screened components throughout.

Amplification won't cure interference glitches, because the interference
will be amplified as well. It may help - the ratio between the signal and
the glitches may be improved if the glitches are weaker than the signal
beforehand - but amplify primarily for other reasons such as to improve
overall reception or to distribute the signal, not solely in the expectation
that it will help with this problem.

Choosing to amplify your aerial system will probably require filtering
against TETRA and other sources of cross-modulation, especially in built-up
areas. For reasons such as in the next paragraph, this might have to be
specific channel-pass filtering, rather than general UHF TV band pass
filtering.

Amplification may degrade UHF signals from equipment such as VCRs as their
signal will be superimposed on whatever the aerial receives, and it may be
more difficult to find for them channels totally free of aerial signal.
This will be a significant problem if your only means of connecting
equipment is chaining the UHF aerial signal through various boxes to your
TV.


What I did:

My previous system was two aerials, pointing at Hannington and Crystal
Palace, combined through a masthead Triax TMC 3335 STP UHF diplexer, and a
circular FM aerial, TV & FM being diplexed in turn by a LabGear MSA241
distribution amp in the attic, with single downleads to twin TV/FM sockets.
Digital signal levels were about 48-50%.

I first unplugged the CP aerial - the one pointing out over other houses.
This resulted in significantly fewer spikes, though still too many.

Next I replaced the system with a single, more sensitive wideband pointing
at Hannington, amplified by an SLx2M masthead amp powered from the original
distribution amp via the downlead, with double-screened components
throughout. I put spike protection between the distribution amp and the
mains. Digital signal levels were then about 75-80%, yet the result was
*worse* than before. Cross-modulation was bad on some analogue channels,
and digital muxes 2, A, & C were unwatchable.

The masthead amp amplifies *everything* it receives, even frequencies
outside the UHF TV band, and it was being driven into overload by
cross-modulation from strong unwanted signals such as TETRA. I cured this
by adding a passive UHF TV band pass filter between the aerial and the
masthead amp.

I now have as near perfect analogue reception as I'm likely to get in this
location, and digital signal levels around the 80% mark.

However, I'm still getting glitches - my own c-h has not been nullified by
the spike protector on the amp system, and I still get them even with my own
c-h turned off.

So, unless anyone has any better ideas, I suppose the only thing left to try
would be to call in a c-h engineer to do the thermostat, and then the
airwave cops to find all the other offending systems - that'd be popular
with the neighbours! Can anyone here suggest how much that'd cost each
problem household? I wonder just how many would be involved?

A mistake I made: I bought a cheap aerial advertised for weak signal areas.
It was long (ie: had more elements), my recollection being that longer
aerials had a more directional response. and would therefore pick up more
signal and less noise. However, when I showed Bill a picture after
installation, he described it as 'not very directional'!


Historical Notes

In 1997, when I first moved in, TV was unwatchable. I noticed a cable
outlet in the house and some neighbouring satellite dishes. Not wishing to
pay subscription, I replaced a T-junction in the attic with a cheap aerial
amp from B&Q, which at least made life watchable.

Later, I wanted better FM reception. A local firm replaced my previous
aerial with the rig described above. At the time, not knowing better, their
work seemed reasonable to me, but now, as it cost £300-400 (GBP) and was
done by CAI-approved riggers, I think it should have been better (note b).

After a while, I began to notice interference on FM radio and TV. I
suspected mains spikes from my own fridge, c-h, or similar. Accordingly I
tried switching off the rest of my house, and was puzzled when it failed to
cure the problem.

Around 1999, NTL offered a deal on cable broadband, and I had their TV as
well. *Even* this was somehow affected by this interference! Either mains
spikes were still getting through the spike protection which everything was
plugged into, or the cable STB, leads, &/or old TV must have been picking it
up directly via electromagnetic spikes. NTL were such a painful firm to do
business with that I dumped them after three years.

Around 2001/2 I bought a Thomson DTT STB, then a Pace Twin in early 2003.
Surprisingly, digital reception was adequate, but its extra clarity on new
LCD TVs and its greater vulnerability to interference meant that the latter
could no longer be ignored.

I found its intermittent nature, and that I'd even got it with cable,
extremely confusing. I only became convinced it was c-h after:
1) My electricity supplier checked my mains and gave it a clean bill of
health.
2) I consciously noted the seasonal change in severity throughout a year.

I wonder, if I were to add up all the money I've spent over the years trying
directly to solve this problem or indirectly to get round it with cable or
sat systems, perhaps it would have been cheaper just to move house?


Foot Notes

(a) For locals: Official info on the location of TETRA masts is not freely
available, but the new police station on Rushey Way near Asda is a very
likely source. There is a mobile phone mast on Rushey Way near the junction
with Beeston Way, two others just across the M4 from the newly rebuilt
roundabout system at the western end of Lower Earley Way, and a new,
temporary one next to this roundabout itself - perhaps a test for a
proposed permanent mast.


(b) The CAI-approved installers ...
i) Tried to steer me towards Crystal Palace. In retrospect, I have no idea
why. They should have known or noticed that:

Hannington ...
+ Is the correct regional transmitter.
+ Is half the distance.
+ Has a clear path from Lower Earley.
+ Lies beyond immediate open ground.

Crystal Palace ...
- Is the wrong regional transmitter.
- Is twice as far away.
- Has no clear path from Lower Earley.
- Lies beyond neighbouring houses on higher ground.

I insisted on having the correct regional news and weather. Also, unwisely
with 20-20 hindsight, I favoured being able to receive other areas (note c),
but IIRC they mentioned that this wasn't a good idea. Nevertheless, having
gained the impression that it was either CP alone, or CP & Hannington, I
chose the latter. Only recently, when I disconnected the CP aerial, did I
discover it was the one picking up most of the interference spikes!

ii) Misaligned the Hannington aerial by 11 degrees to the north.

iii) Mounted the FM and Hannington aerials too close together.

iv) I don't know if they reused an existing aerial entry hole, but it:
- Comes in behind a roof truss timber.
- Does not slope upwards and inwards (to prevent water ingress).
- Was without an exterior loop of cable hanging downwards (ditto).
- Was unsealed (ditto).

v) Used the giant rawlplug type of fixing for the mast mount. When I tried
to set the mast truly vertical, the screws just rode round in the rawlplugs.

vi) Over crimped the downleads with a staple gun.

vii) Used one double-screened outlet and one not, and not double-screened
cable.

viii) Didn't mention anything about interference or a need to guard against
it. It took me a long time to wise up, but I'm in IT and have no relevant
training or professional experience here. Had they really *never*
encountered it before, when the whole area seems to be *riddled* with it?


(c) It's totally unimportant now, and may already have been then, but
historically there were many more differences in ITV regional programming,
so that, for example, I remember being utterly exasperated at missing the
last episode of 'The Prisoner' through being in a neighbouring region the
night it was broadcast.

"Java Jive" wrote in
:

snip the story - you've prolly got it above!


viii) Didn't mention anything about interference or a need to guard
against it. It took me a long time to wise up, but I'm in IT and have
no relevant training or professional experience here. Had they really
*never* encountered it before, when the whole area seems to be
*riddled* with it?

Tell me about it- I've got the same, though not I think as bad, and I
can't identify the main culprits.

I like the filter before the masthead amp - I'll try that.

As regards the CH, I replaced my old room stat; it was really well
clapped anyhow, with a programmable digital.

This is well worth doing anyhow for the much superior control.

Have you tried dealing with mains spikes? Ferrite beads on the mains
leads.

You should try to eliminate interference at source, ie on the room stat,
on the fridge, freezer, etc, But spikes beyond your control can only be
dealt with on the lead to your apparatus. You may also have trouble with
these spikes being put on to the masthead amp by the power supply mains.

You can try Ferrite suppressors in all these spots.

I still have one source of interference causing sound dropout on my STB
that I can't identify sigh

mike

"mike ring" wrote in message
. 1.4...

You should try to eliminate interference at source, ie on the room stat,
on the fridge, freezer, etc,

Yes, I entirely agree with that. It's just that before, when I was picking
up so many more spikes from the CP aerial, to spend that much money on c-h
'repair' of a working system seemed a bit pointless! The next thing though,
definitely.

But spikes beyond your control can only be
dealt with on the lead to your apparatus. You may also have trouble with
these spikes being put on to the masthead amp by the power supply mains.
You can try Ferrite suppressors in all these spots.

I've got everything plugged into spike protected extension leads. Do you
think it would be worth trying ferrites as well (my midi system came with
one, and I'm not convinced as to how much good it's doing)?