TV coax puzzle

[John Smith]

Our house is cabled with coax to every main room. The aerial in the loft has a cable into the sitting room, and there are two distribution cables that feed wall plates in the front and back of house. It's old cabling for sure as we've been here 20 years and was there when we moved in.

At some point I got rid of satellite feeds also there and sat boxes and put in a signal booster that sits between the aerial cable and the distribution cables. This has worked fine to feed a strong DVB signal to TVs on both distibutions.

But a few months ago the feed for the front rooms went down. The rear rooms are fine. I've checked the booster and all its 4 outputs and they are fine; only 2 are used. I have redone the coax plug for the front distribution cable in case it was faulty but no joy.

Nothing has been disturbed as far as I know - what is a way to trace the fault? I suppose the cable could have broken down somewhere, or a junction has come loose. This will entail carpets and floorboards up - I suppose no alternative.

I suppose the main clue is that all the rooms are down on that cable which means the problem is somewhere between the cable start and splitter, but no idea where the splitter is except i know where the cables disappear into the ground floor celing in a duct (it's a 3 storey house).

Thanks

[Mike Strange]

John, I would suggest using a Time Domain Reflectometer but they are rather expensive!

Is it possible that the splitter is in the back box for one of the outlets?

[Mike Strange]

If it isn't there then this DIY solution for a TDR is rather useful and very cheap, hope you have a decent scope:

[Paul]

"I've checked the booster and all its 4 outputs and they are fine; only 2 are used."

"bqb 11, 2015

If you don't use all four outputs, you need to get some 75 ohm terminators like these for the unused ports. They're only a few dollars for a 10-pack. "

There are many ways to design things.

An easy way to do one of those boxes, is a single amplifier followed by a four way passive splitter. If this were the case, I would equip the unused ports with terminators. Every port should have a 75 ohm load and then there are no weird phantom effects from open ports.

The amplifier could also have four completely separate outputs, in which case you would not need terminators in the interest of not disturbing the other outputs.

Still, it's a good idea to keep resistive terminators around, of RF quality, for this sort of wiring. I've made a couple shitty ones here for home usage, but I don't know what's happened to them.

If the resistors are carbon composition type, they're supposed to be flat up to 1GHz (covers UHF band). Metal film would be unlikely to be purely resistive. The terminator power rating should be sufficient for the application - if you were a ham radio operator and putting a termination on the transmitter, that's called a "dummy load" and those can be the size of a coffee cup or larger. For the tiny amps, the signal amplitude isn't nearly as high, and you can't "boil water with them" :-)

This is the first URL with a picture I found, to show the tiny size of the things. The resistor being in that thing on the end. The manufacturer will use the right type of resistor, so there are practically no reflections.

Paul

[alan_m]

Both ends of the cable? Have you checked all fly leads for short circuits? It may only take a short circuit on one of the splitter outputs to kill (as in up to 100% attenuation) the signal on all other outputs.

Have you tried testing by removing all equipment and/or fly leads from the final outputs of the splitter and then checking with only one splitter output at a time that you get a signal to your TV/STB equipment.

If you have a multimeter set it for continuity (ohms range) and check every destination (wall sockets??) for a short circuit between the centre and outer connector.

[williamwright]

Very clever stuff, but his answer is entirely dependent on the assumption that the velocity factor is 0.66. I expect the VF could be determined exactly though, by a similar method.

Bill

[williamwright]

The price is quoted at $8.40 each. The item is commonly used in the UK in the RF and CCTV industries and can be purchased for £6 per 100.

The general rule for terminating is as follows: Inductive tap-off units with tap values below 16dB need terms. Resistive tap-off units with tap values 20dB and above don't need terms. Most multi-output UHF amps don't need terms, either at the amp or at the outlet. Remember that a long run of relatively lossy coax won't need terminating because bugger all will get back to the source anyway. It's very important to terminate the end of a trunk line and it must be an accurate term value.

Bill

[williamwright]

Total bollocks.

Bill