New loft aerial - problems

Osem · 07-01-2014, 18:41

Our outdoor TV aerial, which looks like this:

http://www.screwfix.com/p/labgear-tv...FaoEwwodRhcATw

blew down in the wind l is now lying on our roof supported by the coaxial cable which luckily is clamped to the mounting bracket. It's pointing roughly 70-90 degrees to where it was and has the brick chimney stack and roof structure lying between it and the transmitter (Crystal Palace) yet it's still providing a good signal on all but a few channels - mainly HD. Currently this aerial is connected into a simple junction box which feeds 3 TVs around the house via 3 cables. As we seem to be in a strong signal area and I'd rather not have an outdoor aerial, I've bought one which I want to install in the loft.

http://www.toolstation.com/shop/p93127

I've run some coaxial cable into the loft and have connected one end to the new aerial via one of the 4 F connections and the other end to a single TV temporarily to see how it performs but am finding that the cable seems to be acting as an aerial itself. As I move it around (with the actual aerial stationary) or even walk past it coiled on the floor, the TV picture comes and goes. So I'm wondering if the cable isn't very well shielded and I need something better to prevent this interference or whether something else is wrong. Ideally I'd like to run a single cable from the loft aerial to the existing junction box to feed the 3 TVs rather than running 3 separate cables all the way from the aerial to each TV but have no idea if this is feasible. Guess it depends on how good the signal is.

Can anyone advise please?

Taf · 07-01-2014, 18:51

Is it a passive "junction box" or an active aerial amplifier/distributor?

Osem · 07-01-2014, 20:14

Passive, Taf, just wires clamped into terminals - no electrical stuff. It's done the job fine serving the 3 sets up until the outdoor aerial went for a burton. Even now with the aerial in the wrong place it's providing a reasonable signal on most stations.

Uncle Peter · 07-01-2014, 22:51

You should only need a modest antenna like a log periodic if you're in a strong signal area. It'll be more than enough and have a relatively small wind loading. I wouldn't buy anything made by labgear though, cheaply made junk IMHO.

Try something like this http://www.blake-uk.com/dml-log-periodic

Matth · 08-01-2014, 01:30

Looks like that Antenna has an amplifier, so if you threw it together temporarily without connecting the amplifier power supply, then you won't get much out of it.

With a gain of 10dB per way, if you use only one of them and split it later, you'll soak up most of that 10dB gain in splitting.

Osem · 08-01-2014, 10:35

Thanks for the replies everyone!

Quote:

Originally Posted by Matth

Looks like that Antenna has an amplifier, so if you threw it together temporarily without connecting the amplifier power supply, then you won't get much out of it.

With a gain of 10dB per way, if you use only one of them and split it later, you'll soak up most of that 10dB gain in splitting.

It comes with a single powered amplifier (one lead in and one out) which the instructions only show being connected from the aerial to a single TV via the supplied white cable. I'm a bit confused as the aerial itself has 4 f connectors for leads to different devices so presumably this amplifier isn't intended to serve up to 4 possible devices, it's been provided to serve just a single set with the option of buying others if more than one set is being connected to the aerial.

I don't know much about these things but I think you're saying at the end of your post is that, regardless of whether I use amplifiers, running separate RG6 cables from the new aerial to each TV should provide a much better signal to each than splitting a single cable to feed them.

---------- Post added at 10:26 ---------- Previous post was at 10:23 ----------

Quote:

Originally Posted by Uncle Peter

You should only need a modest antenna like a log periodic if you're in a strong signal area. It'll be more than enough and have a relatively small wind loading. I wouldn't buy anything made by labgear though, cheaply made junk IMHO.

Try something like this http://www.blake-uk.com/dml-log-periodic

That's very similar to what I've not got.

Interestingly while fiddling around with the rg6 cable last night (10m long) I found that it was providing a TV picture on certain channels even before being connected to the aerial. This is what made me think that the cable isn't well shielded and is acting like an aerial on its own.

---------- Post added at 10:35 ---------- Previous post was at 10:26 ----------

Quote:

Originally Posted by heero_yuy

If the cable is sensitive to you moving about then it is not correcty terminated somewhere. If you just join three cables together in a junction box then none of them are correctly terminated. You need to use a proper splitter, a simple passive one will have several resistors to get the termination correct or a set of baluns to achieve the same thing in the low loss version.

A couple of quid from Amazon etc.

The ideal method is to use a distribution amplifier if there's more than 2 sets.

Yes I have a couple of those Y shaped splitters lying around somewhere. I'm not sure if the problem is a connection one though because it still happens when I'm just walking around the cable and not actually touching it. It's just like when you're watching TV using a cheap indoor aerial and lose the TV signal temporarily when people start moving about in the room.

Uncle Peter · 08-01-2014, 10:57

I would disconnect the cable and pop a continuity check across the conductor and shield and if possible, across the length of the cable to make sure you don't have a break in the braid. Another possiblilty is that you've been unlucky and cut the cable on a current node so may be acting rather too efficiently it's own antenna as you say, perhaps try chopping a few inches off one end and make a new termination. Having the cable coiled up won't help either

Also if you must use a splitter (no problem if the source signal is decent and you're not running miles of downlead), do yourself a favour and get a proper shielded one in a metal enclosure.

Osem · 08-01-2014, 11:57

Right guys, I have located the aerial in what looks like a good spot in the loft and run the cable through the eves through one of the loft hatches into our bedroom. I've connected it to one of the TVs and the picture is very good indeed although I'm conscious that it's a much nicer day today and when the weather worsens it'll probably reduce a bit. I'm just going to try splitting the signal with a Y connector to the other TV in the room and see how much that affects the signal. I feel like progress is being made

qasdfdsaq · 08-01-2014, 13:06

Shielding on TV aerial cables isn't very important which is why lots of cables have terribly poor quality shielding, and the connector standard doesn't even provide particularly reliable contact for the shield. Or anything else for that matter.

Matth · 10-01-2014, 00:50

Quote:

Originally Posted by Osem

Thanks for the replies everyone!

It comes with a single powered amplifier (one lead in and one out) which the instructions only show being connected from the aerial to a single TV via the supplied white cable. I'm a bit confused as the aerial itself has 4 f connectors for leads to different devices so presumably this amplifier isn't intended to serve up to 4 possible devices, it's been provided to serve just a single set with the option of buying others if more than one set is being connected to the aerial.

I don't know much about these things but I think you're saying at the end of your post is that, regardless of whether I use amplifiers, running separate RG6 cables from the new aerial to each TV should provide a much better signal to each than splitting a single cable to feed them.

From the description, I think the powered bit is actually a power over antenna cable system, which probably has to connect to the masthead amplifier/splitter at port 1.
The other ports also provide 10dB gain, so if you had separate runs, each has 10dB gain, while if you use a single run and a splitter that loses 6dB, then you only have 4dB gain. also, a splitter between the power unit and the antenna may not properly pass the power, depending on the design of splitter.

The advantage of a masthead amplifier (in weak signal areas) is that it gets to work on the signal before any has been lost by the cable length, making a big difference if the signal is borderline.

Osem · 15-01-2014, 20:24

Right, it's been a few days. The cable is run in the eaves and the signal strength (as shown on the programme bar) very good on most channels but I'm still having some issues with a few other channels which seem to be related to movement in the room (reflections) and low signal level. Is there any easy way to prevent this? I know it's not down to movement of the coaxial cable because I've isolated it from any physical movement and there doesn't seem to be any problem with the coaxial connection to the actual TV set because I can fiddle with it an that has no effect.

Osem · 16-01-2014, 11:02

OK matey, thanks for that. Going to investigate this further later on, will report back when I've checked it all out.

Taf · 16-01-2014, 11:38

Earthing and bonding of coaxial cables is a subject that can develop into a slanging match between electricians and radio telecomms bods.

New regs oblige installers to bond coaxial cables in communal systems as electrical leakage from equipment can rise to quite an appreciable voltage and current. But the same can be said of a single aerial feeding a single home but with multiple bits of kit attached.

Then there is lightning. One group says bonding will protect against lightning, the other says it attracts lightning.

Then there is the potential (no pun intended) to form interference through earth loops when both ends of a cable are attached to "earth" points at different places.

I'm not a TV techie type, but I seem to remember that old style TV's didn't have an "earth" as such, so coaxial screens from the dipole on the aerial to the TV socket could be at unexpected voltages. Do modern flat screen TV's have earthed aerial connections I wonder?

Osem · 16-01-2014, 15:58

Thanks for the continued advice guys. To rule out a problem with screening I bit the bullet and bought some new pf100 cable and installed that direct from one of the F terminals on the aerial to our second bedroom TV set. It has far more copper braid than the stuff I had been using and seems to have solved the problem with a good strong signal on all but a few lesser channels. I'm wondering whether I can sort this problem out by moving the aerial a little (trial and error) or 'fine tune' those channels to improve things?

The main issue I have now is that if I connect the cable to the set via a normal coaxial plug there is no signal at all. If I disassemble the outer threaded metal barrel however and just push the metal screw-on pin into the rf socket it works fine. I'm at a loss to explain why a simple coaxial socket would cause a total break in the signal - I've fitted quite a few in my time and never had such a problem so I'm stumped as to what might be wrong.

Taf · 16-01-2014, 17:27

Is this aerial and old analogue one? If so you may have to stump up for a wideband one 'cos the digital signals are spread out a bit more AFAIK.

As for the aerial socket be wary of fine threads of screen shorting it all out.