TV on different aerial causing interference?

Strange one this ...

Parents got a new TV for their kitchen recently (Samsung LE19R8) and sure
enough their indoor aerial wasn't good enough as expected.

So, they had another aerial installed and a seperate downlead to the
kitchen. The aerials are both on the same chimney mast and the downleads
run along side each other until they reach a lower level and go their
seperate ways. (No, I don't know why they didn't get a booster and a
splitter).

2 TVs in the front room, one old analogue and an IDTV fed from one aerial
with a passive splitter (yes, I know). There's a VCR in there somewhere
also. Kitchen TV on it's own aerial.

Everything works fine ... Until the kitchen TV is put on ITV on digital,
and then Channel 4 in the front room is unwatchable on analogue - Snowy
vertical rolling. Fine on digital on the IDTV. Change channel on the
kitchen TV or go to ITV via analogue and everything is fine.

If I swap the kitchen TV and the analogue TV over there is no problem. It
sort of implies that there's something weird happening when the 2 digital
TVs are on different aerials.

I wanted to remove the VCR and the splitter and try some more combinations
but the parents are paranoid that it won't work again (nice to be
trusted!).

I can't believe that the kitchen TV is putting interference up the aerial
lead and that it's cross coupling with the other aerial. The coax looks to
be good quality foil shielded and both aerials seem to be giving good
signal strength.

I'm baffled

Al.

In article ,
Al wrote:
Strange one this ...

[Snip]

I can't believe that the kitchen TV is putting interference up the aerial
lead and that it's cross coupling with the other aerial. The coax looks
to be good quality foil shielded and both aerials seem to be giving good
signal strength.

not knowing where you live, and therefore which transmitter you receive,
make a proper diagnosis difficult. I would think that your kitchen tv is
indeed sending a signal back up the aerial lead and that is getting across
into the other aerial. This does happen, or certainly used to in the
analogue days. The solution is to move the two aerials further apart.

--
From KT24 - in "Leafy Surrey"

Using a RISC OS computer running v5.11

On Feb 23, 11:52 am, Al wrote:
I can't believe that the kitchen TV is putting interference up the aerial
lead and that it's cross coupling with the other aerial.


Well, I can.
From what you're describing, it sounds like the second aerial has been
mounted too close to the first one.
Just as Charles has said.
Cheers.

My mate was tuning his analogue TV some years ago, and as it was scanning
the stations it suddenly stopped and he could see someone typing in a Basic
program on his TV screen! A bit snowy and wobbly, but perfectly clear
enough to read.

This was in the days when people used their TV as a computer monitor.
Presumably a close neighbour was playing with his Spectrum or whatever, and
something was leaking enough for my mate's aerial to pick it up, and his
auto-tuning to lock on to it.

SteveT

"Al" wrote in message
. 4...
Strange one this ...

Parents got a new TV for their kitchen recently (Samsung LE19R8) and
sure
enough their indoor aerial wasn't good enough as expected.

So, they had another aerial installed and a seperate downlead to the
kitchen. The aerials are both on the same chimney mast and the
downleads
run along side each other until they reach a lower level and go their
seperate ways. (No, I don't know why they didn't get a booster and a
splitter).

2 TVs in the front room, one old analogue and an IDTV fed from one
aerial
with a passive splitter (yes, I know). There's a VCR in there
somewhere
also. Kitchen TV on it's own aerial.

Everything works fine ... Until the kitchen TV is put on ITV on
digital,
and then Channel 4 in the front room is unwatchable on analogue -
Snowy
vertical rolling. Fine on digital on the IDTV. Change channel on the
kitchen TV or go to ITV via analogue and everything is fine.

If I swap the kitchen TV and the analogue TV over there is no problem.
It
sort of implies that there's something weird happening when the 2
digital
TVs are on different aerials.

I wanted to remove the VCR and the splitter and try some more
combinations
but the parents are paranoid that it won't work again (nice to be
trusted!).

I can't believe that the kitchen TV is putting interference up the
aerial
lead and that it's cross coupling with the other aerial. The coax
looks to
be good quality foil shielded and both aerials seem to be giving good
signal strength.

I'm baffled

Al.



Likely that the local oscillator (or possibly the computer clock) is
getting back up the aerial and coupling across to the analogue aerial or
downlead.

If you swap the TVs you said yourself that there is a VCR in there and
if the TV is the last item in the chain then there is no path back
through the VCR to the aerial - many VCR local outputs are amplified.

As others have said relocate one of the aerials, or if you have enough
signal fit an attenuator in the DTTV cable at the TV end. You will then
maximise the interferring losses and possibly reduce it enough to remove
the Ch4 problem.


--
Woody

harrogate three at ntlworld dot com

If you swap the TVs you said yourself that there is a VCR in there and
if the TV is the last item in the chain then there is no path back
through the VCR to the aerial - many VCR local outputs are amplified.

Aha! That's one thing that I forgot. My Dad said something about the VCR
output being boosted, which made me suspicious and wanting to take it out
of the equation.

As others have said relocate one of the aerials

I'm not sure how easy that would be. Next time I visit I'll take a look.
Not sure how far away they should be spaced. This isn't a combined array,
so the half wavelength rule is (probably) irrelevant.

I'm still struggling to see how/why a TV would be pushing a signal *up* the
feed, but my hunch is that the tuner impedance changes enough to
effectively cause a load on the other aerial.

fit an attenuator in the DTTV cable at the TV end.

Good idea.

Thanks to all that have posted - I'm much now somewhat wiser

Al.

In message , Woody
writes
"Al" wrote in message
.4...
Strange one this ...

Parents got a new TV for their kitchen recently (Samsung LE19R8) and
sure
enough their indoor aerial wasn't good enough as expected.

So, they had another aerial installed and a seperate downlead to the
kitchen. The aerials are both on the same chimney mast and the
downleads
run along side each other until they reach a lower level and go their
seperate ways. (No, I don't know why they didn't get a booster and a
splitter).

2 TVs in the front room, one old analogue and an IDTV fed from one
aerial
with a passive splitter (yes, I know). There's a VCR in there
somewhere
also. Kitchen TV on it's own aerial.

Everything works fine ... Until the kitchen TV is put on ITV on
digital,
and then Channel 4 in the front room is unwatchable on analogue -
Snowy
vertical rolling. Fine on digital on the IDTV. Change channel on the
kitchen TV or go to ITV via analogue and everything is fine.

If I swap the kitchen TV and the analogue TV over there is no problem.
It
sort of implies that there's something weird happening when the 2
digital
TVs are on different aerials.

I wanted to remove the VCR and the splitter and try some more
combinations
but the parents are paranoid that it won't work again (nice to be
trusted!).

I can't believe that the kitchen TV is putting interference up the
aerial
lead and that it's cross coupling with the other aerial. The coax
looks to
be good quality foil shielded and both aerials seem to be giving good
signal strength.

I'm baffled

Al.



Likely that the local oscillator (or possibly the computer clock) is
getting back up the aerial and coupling across to the analogue aerial or
downlead.

If you swap the TVs you said yourself that there is a VCR in there and
if the TV is the last item in the chain then there is no path back
through the VCR to the aerial - many VCR local outputs are amplified.

As others have said relocate one of the aerials, or if you have enough
signal fit an attenuator in the DTTV cable at the TV end. You will then
maximise the interferring losses and possibly reduce it enough to remove
the Ch4 problem.

For analogue channels, the 'N+5' and 'N=5' allocations were strictly
been abandoned? If so, I can see some muxes getting clobbered by an
analogue TV tuned to a channel five channels down.

But maybe they have still avoided this relationship? For example, I see
that, with the Crystal Palace allocation, no digital mux is N+5 wrt an
analogue channel. However, there are three allocations where there is
something N+5 wrt a digital mux (25 on 30 analogue, 28 on 30 analogue,
and 29 wrt 34 digital).

But do the digital STBs and TVs have the same local oscillator and IF as
analogue TVs - or do they use something different? If they are the same,
do the digital tuners simply have much less radiation of the local
oscillator?
--
Ian

In message , Ian Jackson
writes
28 on 30 analogue,

Oops! Should be 28 on 33 analogue.
--
Ian

Al wrote:

I'm not sure how easy that would be. Next time I visit I'll take a look.
Not sure how far away they should be spaced. This isn't a combined array,
so the half wavelength rule is (probably) irrelevant.

What half-wavelength rule would that be?

IME multiple aerial installations like yours invariably have the aerials
mounted too close.

I'm still struggling to see how/why a TV would be pushing a signal *up* the
feed,

There's always some LO leakage, hence the N±5 rule for analogue. The
problem here though doesn't sound like LO interference; that would show
as herring-bone patterning and what you've described seems more like a
noise-like signal coming out of the kitchen TV's tuner - an
intermodulation product involving the LO and one or more DTT signals,
perhaps?

Are you in a high-signal strength area? As Charles said, the most
important piece of information needed is the name of the transmitter
site that the aerials are looking at.

Things you could try: (i) attenuate the input to the kitchen TV: a 6 dB
pad could make a big difference - it will reduce the level of any
intermod products in the tuner's front-end as well as improving the
effective isolation between the aerials; (ii) buffer the i/p to the
kitchen TV using a set-back booster type amplifier, followed by a 10 dB
pad (you don't need the gain, but the buffering effect of the amplifier
should certainly help).

--
Andy

"Al" wrote in message
. 4...
fit an attenuator in the DTTV cable at the TV end.

When this sort of problem occurs, a small one-in one-out amplifier, followed
by an attenuator if necessary, prevents anything getting back up the
downlead.

Bill