dtv signal trouble...

tvout · 22-09-2008, 11:13

Hi all,
Please can someone tell me what's going wrong with this...

I have a huge aerial at the back of my house (which was there when I bought it) which points at the local transmitter, which is about 3/4 mile away...
I've checked the angle of the aerial is what it should be etc. but with my USB DTV stick I'm having problems getting channels...

If I connect direct to the cable it shows very good signal strength but nothing for signal quality.
I've been loaned an attenuator and when using that I now get 4 channels but the signal strength has dropped alot although the signal quality is very good...

I can't workout why I only get any level of signal quality when reducing the signal strength.

Should I get a different strength attenuator?

**Chris** · 22-09-2008, 14:31

Too strong a signal is as bad as too weak. If you're only a short distance from your transmitter (does 3/4 mean three-or-four miles, or three-quarters-of-a-mile?) then you almost certainly do not need a 'huge aerial' at the back of your house.

Go to the Wolfbane website, and input your postcode:

http://www.wolfbane.com/cgi-bin/tvd.exe for digital transmissions
http://www.wolfbane.com/cgi-bin/tva.exe for analogue transmissions

The results will tell you which transmitter you should be pointing at, the strength of its broadcasts, and the sort of aerial you should have.

tvout · 22-09-2008, 17:26

Quote:

Originally Posted by Chris

Too strong a signal is as bad as too weak. If you're only a short distance from your transmitter (does 3/4 mean three-or-four miles, or three-quarters-of-a-mile?)

Cheers Chris, I remember finding that website a while ago but couldn't find it again. It says the transmitter is 1 mile from me and it recommends using a set top aerial.

It says I need a C/D aerial set to vertical. Mine's vertical and pointing at that transmitter but not sure what type of aerial it is.

**Chris** · 22-09-2008, 17:28

If it works at all, it's probably the correct group so that is very unlikely to be your problem. The problem is almost certainly the combination of a 'huge aerial' and your location just a mile from the transmitter. The fact that an attenuator improved things somewhat, supports this theory also.

Do you have a set-top aerial? If you can get your hands on one, give it a try and see what happens.

tvout · 22-09-2008, 17:36

I've got a small set top aerial with built in ampifier and that tends to get more channels than the big rooftop one which seems odd.

---------- Post added at 17:36 ---------- Previous post was at 17:34 ----------

Here's a pic of my aerial, not sure if you can say anything from it...

**Chris** · 22-09-2008, 18:17

Quote:

Originally Posted by tvout

I've got a small set top aerial with built in ampifier and that tends to get more channels than the big rooftop one which seems odd.

---------- Post added at 17:36 ---------- Previous post was at 17:34 ----------

Here's a pic of my aerial, not sure if you can say anything from it...

It's not odd at all. It confirms my suspicions that the rooftop aerial is collecting too much signal strength. This is why the strength reading is way up, but the quality reading is way down.

You're right. That is a huge aerial on your roof (although I've seen bigger!).

You need a smaller aerial.

tvout · 22-09-2008, 19:08

I'm thinking about getting a variable attenuator so I can adjust it accordingly as I don't know the strength of the one I'm testing.
I also tried a signal booster and got a few more channels. I can't figure that one out; why I would get channels using an attenuator or booster but not without either...
The other thing to mention about the transmitter is that I can't actually see it from my house as there's a small hill (very small - like about 25 metres high) right behind my house so it's not in line of sight as such. I don't know if the signal would reach over the hill?

Jon T · 22-09-2008, 19:31

You could be getting multipath reception, where you are picking up a strong signal from the transmitter mast direct, and also a signal that's been reflected off the ground(or anything else close to the transmitter) towards your aerial.

tvout · 22-09-2008, 21:42

Quote:

Originally Posted by Jon T

You could be getting multipath reception, where you are picking up a strong signal from the transmitter mast direct, and also a signal that's been reflected off the ground(or anything else close to the transmitter) towards your aerial.

Hmm...good point. Is there much I could do if that's the case?

Jon T · 22-09-2008, 21:56

Difficuilt, raising or lowering the aerial could help.

Matth · 23-09-2008, 00:01

Tweaking DVB is difficult, on analog, you'd just point it the same way as everyone else, then jiggle it to minimise noise or ghosts. With DVB, you get very little feedback, unless the signal is on the borderline. A TV or DVB software may also offer a strength/quality indication, though the calibration can be pretty arbitrary.

To minimize ground reflection, you would cant the aerial up slightly, especially if you have a good line of sight to the transmitter.

The antenna is a 14 element Yagi, variable spacing style, with multiple reflector, not huge, but certainly overkill for a strong signal area, though it may be necessary to use a larger aerial and attenuate it if more directivity is needed to control interference such as ghosting.

http://www.maplin.co.uk/Module.aspx?ModuleNo=223051 - Variable attenuator

If it still gets very few channels, then possibly the aerial aiming is off, or perhaps the aerial is not the correct group.

The other approach, if the signal is that strong, is to try a set top antenna, as you can easily search with it.

Jon T · 23-09-2008, 09:01

Re-reading the first post, i've just thought of something else. The DVB USB you have is probably not designed to accept the signal strength your feeding in to it as a result of your large external aerial and closeness to the transmitter. Your probably overloading the USB sticks "front end"(initial RF stages) with such a strong signal. Has the USB stick got it's own aerial.

The other point that I thought of is that your aerial is vertically polorised, this implies that the transmiitter your on is a repeater(low powered transmitter designed to fill in gaps in coverage). Is it possible that the digital signal doesn't actually come from this transmitter, or that you are picking up a signal from the main transmitter as well(should be on different frequencies, but with DTV i'm not sure).

Do you know the name of the transmitter that your aerial is pointing to at the moment?

**Chris** · 23-09-2008, 09:08

Quote:

Originally Posted by Jon T

The other point that I thought of is that your aerial is vertically polorised, this implies that the transmiitter your on is a repeater(low powered transmitter designed to fill in gaps in coverage). Is it possible that the digital signal doesn't actually come from this transmitter, or that you are picking up a signal from the main transmitter as well(should be on different frequencies, but with DTV i'm not sure).

Do you know the name of the transmitter that your aerial is pointing to at the moment?

The OP has done a postcode check at Wolfbane - I gave him the DTV and analogue checkers so it's possible he's got the results confused, although assuming he's not done that, it would seem that digital signals are indeed coming from the transmitter in question.

He would be able to recover the name of the transmitter from the Wolfbane results.

tvout · 24-09-2008, 21:52

Cheers for the excellent replies everyone. With regards to the USB stick being overloaded it does tend to get very hot but I suspected that was just poor design?
My nearest transmitter is quite small and, like you said about the vertical thing and from the wolfbane results it's shown as smaller in size than the two other nearest transmitters.

If it's a repeater it might explain why my set top box receives all the channels from the 2nd nearest transmitter and not the one 1 mile from me. When scanning all the channels it shows the transmitter name but it's not the one closest to me.

By comparison, the set top box I have seems better at picking up the channels than the usb stick. Not as many channels as it should get but a fair amount more than the usb stick is getting.

I found the USB stick was getting nearly as many channels with the small indoor amplified aerial which is currently in use with the set top box in my bedroom. Far more than with the outdoor aerial.

It says for my local transmitter I need a C/D aerial at vertical. Is there any way I can tell if mine is in that category?

---------- Post added at 21:52 ---------- Previous post was at 21:50 ----------

Thanks also for the link to the variable attenuator at Maplins, I searched on there and could only find two which were at set 6db or 12db, the variable one didn't come up for some reason...

**Chris** · 24-09-2008, 23:23

Quote:

Originally Posted by tvout

It says for my local transmitter I need a C/D aerial at vertical. Is there any way I can tell if mine is in that category?

Your aerial should be colour coded. This is often by means of a coloured plastic plug at the front end that's pointing towards the transmitter. A group C/D aerial should be colour coded green.

The complete list of UHF channels, groups and colours is:

21-37 - A - red
35-53 - B - yellow
48-68 - C/D - green
35-68 - E - brown
21-48 - K - grey
21-68 - W - black

The best suggestion I can make is that you either get a very long ladder so you can get close enough to the aerial to see its colour code, or else use binoculars.