I have a very peculiar problem

Doug · #1 24-03-2010, 08:21 PM

Periodically we have been experiencing severe interference to our TV reception. The neighbors are experiencing TVI as well.
We have a high gain digital antenna etc etc, but now and then.....mostly when something good is on, the picture and sound breaks up; digital is more dramatic than analogue too. So I purchased a mast head amplifier from JAYCAR, and thought all my worries might be over....not so. With amp in stalled and powered up We get excellent reception on 1/10, 2, all the digi44 channels. Missing are 7,9 sbs SBS digi radio. Remove the mast head amp and the latter channels reappear

. (This is in the Sydney Metro area BTW).
If someone else was telling me this I would tend to doubt them but since it is me....I'm a believer. The first unit was returned as being suspect, but now the replacement unit is doing the same thing.
Hoping someone out there has an answer to this queer fault.

pgc hunter · #2 24-03-2010, 08:26 PM

Same here, it just started happening on channel 7 out of nowhere and I think a few other free to air channels a week or two ago. Foxtel however seems to be completely unaffected.

danielsun · #3 24-03-2010, 09:48 PM

I have had a simalar problem not sure if this will help but I am only using a TV booster amp for the antenna and found that by turning down the gain actually repaired some chanels I previously had trouble with.

Cheers Daniel.

bojan · #4 24-03-2010, 10:30 PM

Amplifiers will not help here.
You guys have a source of interference in vicinity of your houses (or in them.. for example, your neighbour is grinding a coffee with electric grinder), and of course, the interference reaches your antenna and is amplified together with signal.
The only thing that helps is using highly directional antenna mounted on high mast (away from interference.
If the sourse if interference is further away, and in the same direction from where TV signal is ariving (industral areas for example) there is not much you can do.. except to subscribe to cable TV.

pgc hunter · #5 24-03-2010, 10:54 PM

It's odd because we haven't changed anything in our house prior to the interference starting, it literally just happened out of nowhere. I'm getting momentary bouts of pixelation on channel 7 (havent tested the other free to air channels as I dont watch them) at random times, quite often aswell. But only part of the screen gets pixellated, and it freezes for a fraction of a second before returning to normal. Sound is also affected during these episodes.

Foxtel channels are not affected from what I've seen.

Scorpius51 (John) · #6 24-03-2010, 11:50 PM

You don't specifically mention whether you are using an internal or external antenna. The fact you mention a masthead amp, suggests the latter.

TVI is generally via air or mains-borne paths. If it's mains-borne, then a line filter may help. If it's air-borne, then if the masthead amp (I agree with bojan here - I don't think it will help, either) or more directional antenna doesn't help, try changing the antenna position and/or height.

Ground reflections, or reflections off various objects (moving or static) may cause phase cancellation - changing the antenna position/height might reduce or elliminate this (it may also make it worse!).

Do you live in the vacinity of an airport, or flight path?

Good luck!

Cheers
John

Doug · #7 25-03-2010, 12:11 AM

Ok, the Antenna is a Hills Interlaced Yagi type, highest gain they make. I don't live near any airport to speak of.It is mounted roughly(very roughly) on the center of the Tile Roof atop a guyed mast. As for the amp just amping the interference, maybe maybe not, depends on where the interference lies relative to the pass band of the Amp; I should think. The interference is usually on 9, then when really bad, 7 and 2 suffer as well. But the thing that has me stumped most of all is the fact that the amp is passing ch10 and 2 + all of the digi44 channels as well as TVS. It is killing reception of Ch7 and 9 and SBS completely, as though it has some 'T notch' filtering (which is absurd). I have a smaller receiver in the B/room, I will check if I suffer the same channel loss on it. But a T Notch x3 in a mast head amp? I doughnut think so. Thanks for the replies so far,
Doug

Analog6 (Odille) · #8 25-03-2010, 05:47 AM

I'm interested in this too as our digital reception is bloody awful. I will start another topic, don't want to hijack this one.

bojan · #9 25-03-2010, 07:35 AM

It could be the 7 and 9 are strong and amp is clipping the signal (OFDM modulation requires very linear amplifiers, see here: http://en.wikipedia.org/wiki/Orthogo...n_multiplexing)
Try without amp.

Waxing_Gibbous (Peter) · #10 25-03-2010, 12:22 PM

Our analogue reception's been pretty awful over the last week, so perhaps its not confined to digital.

JimmyH155 · #11 25-03-2010, 01:19 PM

Our digital goes funny and pixellated now and then, but it is almost always when there is thunder / lightning around. If the storm gets too close, we have to switch off, it's so bad!!

Doug · #12 25-03-2010, 06:22 PM

Quote:

Originally Posted by bojan

It could be the 7 and 9 are strong and amp is clipping the signal (OFDM modulation requires very linear amplifiers, see here: http://en.wikipedia.org/wiki/Orthogo...n_multiplexing)
Try without amp.

Without the Amp, I get poor reception at times, usually when something good is screening; but yeah at least the stations do get through without the Amp, just thaty sometimes they look like they came via the kitchen blender!

Thanks for that input. You might be right it seems to be about the only logical scenario. Unfortunately the amp I have is fixed at 19db gain. Perhaps I need to swap it for a lower gain indoor booster. In any event it isa no good to me as is.
cheers

bojan · #13 25-03-2010, 06:31 PM

Quote:

Originally Posted by Doug

Without the Amp, I get poor reception at times, usually when something good is screening; but yeah at least the stations do get through without the Amp, just thaty sometimes they look like they came via the kitchen blender!

Thanks for that input. You might be right it seems to be about the only logical scenario. Unfortunately the amp I have is fixed at 19db gain. Perhaps I need to swap it for a lower gain indoor booster. In any event it isa no good to me as is.
cheers

No need to swap..
I had a similar problem (my amp was actually oscillating, because I have WB preamp and distribution amp for 3 TV's in the house) and after adding 6dB pad (made it myself, 3 resistors are needed: 24ohms series, 100ohm parallel to GND, 24 ohms series - see http://www.aubraux.com/design/rf-att...esign-tool.php - make sure you use 75ohms characteristic impedance), oscillations stopped and all is OK since (except when my neighbour is grinding coffee

).
But then again, it may be you have industrial interference in the path of the signal.. it is very hard to give any advice in this case...

Doug · #14 25-03-2010, 06:53 PM

Yeah, my antenna is about 10-15 degrees off bullseyeing a cell tower

bojan · #15 25-03-2010, 06:59 PM

Quote:

Originally Posted by Doug

Yeah, my antenna is about 10-15 degrees off bullseyeing a cell tower

This may explain the problem..
Try with attenuator (between antenna and amp), maybe the amp is saturated occasionally by signal from cell tower.

Also, you may try to change the direction of the antenna, such that tower signal direction coincides with minimum on the antenna polar diagram.

rogerg (Roger) · #16 25-03-2010, 07:20 PM

I've found the set-top-box/tv makes a difference. We often have problems on our $79 digital set top box in the living room but if we use the (also cheap) 19" LCD TV with built in tuner in another room at the same time, it's fine. I've often thought of upgrading to a higher quality set top box in hope of solving the problems in the living room, but it hasn't been worth the money yet.

Roger.

bojan · #17 25-03-2010, 07:39 PM

In Doug's case problem has nothing to do with set-top or TV receiver cheap or expensive.
50W+ from cell tower is enough to saturate any amplifier if antenna is pointed towards it.

TrevorW · #18 26-03-2010, 10:11 PM

take two aspirin and if that doesn't fix it see your Dr ASAP

blindman · #19 29-03-2010, 10:43 PM

Quote:

Originally Posted by Doug

Yeah, my antenna is about 10-15 degrees off bullseyeing a cell tower

Can you imagine what is it doing to your health?

mental4astro (Alexander) · #20 31-03-2010, 09:08 PM

In Sydney's eastern suburbs, I get best (that is minimal interference) from the Wollongong digi channels. If I'm watching the Sydney channels, they react to light switching, and any other powering up or down of appliances.

The flight path is also an issue. I'm not close to it, but on occassion they do come fairly close and there goes reception. Helicopters are the worst offenders.

Oh, and Murphy's Law rules with digi TV.

Bojan offers some solutions, thanks mate.