Interference Problem

[Thomas104]

Hi All,

I'm having a bit of trouble with intermittent interference on my set, some evenings and nights but not all. Picture is generally perfect during the day, all bar Virgin 1 and ITV 3 which have a weakish signal (28-40%). The rest hover around 60%.

I have a Triax QR12 aerial, mounted on the front of the house with a Vision V20 1420 Masthead Amp, powered by my Freeview box, all the wall cable is digital, and all my leads are made of spare RG6. I do have one unscreened wall plate (shameful I know) which I will be splicing onto a lead with 2 F connectors to remove this to make sure it's not this.

The installer has fitted the masthead amp in place of my splitter, approx 10-12M from the aerial by the loft hatch. Would I see much of an improvement by replacing the splitter and installing a single output masthead amp where the aerial wire comes into the loft?

Thought I'd ask first before spending £20 on a new amp lol.

Any other suggestions would be most welcome

 

[alan1572]

it's always better to put the outdoor part of the masthead as close to the aerial as possible but at times i have had to do the same as you and have got away with it, when you say it's powered by your freeview box, what exactly do you mean?
Are you using all 4 outputs of the masthead?

 

[Thomas104]

My freeview box is also capable of being a 5V 50ma power supply, there's a setting in the menu you can change to turn it on or off. Saves using the Vision power supply and having more cabling and joints.

I'm using 2 of the 4 outs on the vision amp at the moment (master bedroom and living room). I got it for future proofing at the time as I will eventually cable up the other 2 bedrooms once the kids are a bit bigger.

Since my installer wasn't very good I've had to educate myself a bit. Would a 2 way masthead be better for now? I take it my amp is still splitting the signal 4 ways?

 

[Rodders53]

This is what I'd do first. Turn off the 5V psu in the stb and plumb in the Vision psu (assuming you have it) to see if this makes any difference in levels (although the receiver is rated just adequately for the amplifier it may be struggling). If this solves your problem then great if not we have to look further at your installation.

I assume you have tried adjusting the gain control of the Vision masthead amplifier to different settings? Is it on maximum?

The Triax/Wolsey QR12 isn't a very big aerial and doesn't have a lot of gain, it but does have a rising gain with frequency which is good. You say it is mounted on the front of the house.... at gutter level? or higher?

What transmitter are you pointing toward and where are you? What is between your aerial and the transmitter (does it point through other houses or trees for example)?

Ideally the masthead amplifier should be about 1 metre from the aerial and cables to each outlet run from that point. If it is within 2-3m of the aerial where the cable enters the loft that would probably still be OK - but you would need to extend the cables to the existing points, of course.

 

[Thomas104]

I have tried running the amp off of the Vision power supply, the problem is the same. I stuck with the FB as the Vision PSU increases the noise figure, also gets rid of a few feet of cable and a few joins.

I have tried different amp settings to no avail.

My signal comes from the wrekin and sutton coldfield transmitters.

The aerial is mounted on a T and K and sits just above the gutter. I'm in a middle terrace house with no chimney so I can't really go much higher.

Looking along the aerial I think it runs through a few trees by the roadside but I'm not absolutely sure.

The masthead amp is 12m from where the aerial cable enters the loft. I'm thinking this is too far. Rather than extending the 2 room cables to where the aerial enters the loft can I use a 1 way masthead amp where it enters the loft and a splitter downstream where all the cables meet?

 

[Rodders53]

Right lets assume you're on a C/D transmitter and 10m of cable loses 2dB of signal at the top end.... A passive low loss splitter will lose 3.5-4dB. A staright F-coupler in-line connection will probably lose around 1dB.

So currently you amplify a signal that has lost 2dB by 20dB and then it goes off losing more on the cable runs to the TVs.... say 4dB?

Amplifying the signal closer to the aerial will improve the signal to noise ratio which will help with interference if it isn't being picked up by the aerial but in the cables.

Scenario 1) Move amplifier near aerial use existing cable and split at loft hatch: You will have an improved s/n ratio BUT will lose 4dB of signal to both receivers.

Scenario 2) Move amplifier and run a new 10m lead from it joining each to the two cables to the TVs. S/N is at the loft hatch the same as 1) but now you only lose 1dB extra to each receiver.

3dB of signal when you're already struggling on one mux could be too much.

 

[Thomas104]

This makes more sense now. I will go for scenario 2 I think just to be sure. the way the cable routes through the loft, I would only need to extend the living room cable, I could get away with moving the bedroom cables route.

Would I be better going for a 2 output amp for now, am I losing some signal by using a 4 channel masthead amp?

 

[alan1572]

modern mastheads are fine if you don't use all of the outputs, it doesn't make any difference, some people think/prefer to use a blocker of some sort but i've not seen a difference.

 

[grahamlthompson]

A staright F-coupler in-line connection will probably lose around 1dB.

A f connectors insertion loss is about .3dB at 1Ghz and only about 0.15dB at mid UHF frequencies so a joint will lose approx (a very tiny) 0.3dB at UHF frequencies. This is roughly the same as an extra metre of WF100 cable.

 

[Thomas104]

Thanks for all the advice will give it a try and see what happens, my other approach is the aerial. The installer I had aligned it by eye off of the neighbours aerials.

If the amp work fails I'll get someone to look at the alignment with a signal meter.

 

[Rodders53]

The installer I had aligned it by eye off of the neighbours aerials.

That's a cowboy or window cleaner you got!
Which transmitter is it pointing at?
(The Wrekin is a 'daughter' of Sutton Coldfield so the programmes are the same.)
Plug your postcode into Digital UK - Postcode checker and tick the in the trade box. This will give you predictions of reception now and as switchover progresses. It also tells you how far from the transmitter you are and what the bearing is (0 degrees is true north, 180 south). A compass or even Google maps will give you a clue as to whether your aerial is pointing the right way.
The predictor also lists the multiplex frequencies - manually tuning to each (Wrekin & SC) might improve things for you too. (Some receivers just store the first frequency they find for each and not the best in auto tune mode).

 

[Thomas104]

Cowboy don't cut it he dropped his drill and smashed the roof tiles on my porch!!

Thanks for the info, I assume bearings go clockwise from north to south?

Good in a way though got to learn all about this stuff which will no doubt help when the switchover kicks in.