Electromagnetism vs. The BBC

September 16, 2007 1:55 PM

TV Signal Problem: Co-axial cable suffering EM interference (I think), and it only affects the good channels!

I recently built a Home Theater PC and it works perfectly except for one thing: some of the TV channels suffer from terrible interference, ranging from stuttering to total loss of signal. I'm receiving UK Freeview (from the Crystal Palace transmitter 30-odd miles away) through a Hauppauge WinTV Nova Digital Freeview PCI card. The current cabling is standard co-ax from the roof aerial, which comes into the house and runs through a couple of boosters and then through whatever el-cheapo co-ax cable came with the boosters into the TV card. The same signal also goes direct into my digital TV and provides a flawless reception. The HTPC box is tucked into a corner behind the TV cabinet.

The whole BBC output is affected (i.e. BBC 1 through 4 and News24) plus a few others (Five Life, Five US and abc1 but I'm not really bothered about them). All of the other digital channels are absolutely fine through the TV card.

However... if I pull the HTPC out of the corner, so the co-axial cable is at full stretch and away from all the other cables and power supplies, the signal problem instantly disappears, and all of the channels get a flawless reception. This leads me to think that I might be getting some kind of EM interference when all the cables are bundled up together in the corner. It's not an issue with one particular co-ax cable, as I've tested 3 different ones (although they're all the same standard quality cables).

Repositioning the unit or the route of the cable is not really an option, so I guess that I need a cable with better shielding to keep the interference out (but please correct me if I'm wrong). I've done a load of googling and come across options like 'quad shield RG-6' and 'triaxial', but what do I actually need and where can I get it in the UK? My bullshit meter always goes haywire when I browse any site selling cables and interconnects, so recommendations would be greatly appreciated.

posted by boosh to technology (6 answers total)

Try making a balun or RF choke. I don't know where to get them in the UK, but there's your search terms.
posted by kuujjuarapik at 2:05 PM on September 16, 2007

If you can cross the cables at right angles, that will minimize interference. You can use tape in an X shape to hold the cable at right angles to the others.
posted by Malor at 2:09 PM on September 16, 2007

quad-shield RG6 is your best bet. All that means is coaxial cable, with a 75-Ohm impedance, and an 18 gauge (AWG) copper stinger.
the "quad-shield", simply enough, indicates that the copper is supposed to be shielded 4 times.

As for where you can get it in the UK....no clue, but it's not rare. Any decently equipped electronics store / department is going to have some. (Your UK equivalent of Frys, Radio Shack, even Wal-Mart should have this)

Alternately, you could try stopping the next time you see a cable guy working on a telephone pole - speaking as a former splicer, people in the cable tv industry typically have more coax than they know what to do with, and I certainly wouldn't have minded if someone had come up to me and said "hey, can I have about 4 feet of coax?" I carried the stuff in thousand foot spools.

As for the balun recommended above - you aren't converting the coax into another source (such as going from coax to twisted pair) so a balun seems completely unnecessary.

And crossing the cables at right angles might help in a single intersection, but something tells me that this recommendation doesn't really apply; you aren't talking about long runs of cable which intersect, but instead end runs that are in close proximity to one another.

Hope this helps.
posted by namewithoutwords at 4:03 PM on September 16, 2007

Most of your post is gibberish to me, but when you say that the interference changes when you stretch the cable in a certain direction, that indicates that there may be a bad connection between the cable and either the male or female coax connector it's immediately attached to. Good luck.
posted by JimN2TAW at 4:16 PM on September 16, 2007

You can try repositioning the cables, but namewithoutwords has it. You want quad-shield RG6, in the shortest lengths you can manage, with, and this is important, properly terminated hex-crimp connectors. If you don't terminate the shields to the connector properly they might as well not be there.
posted by markr at 1:20 AM on September 17, 2007

Agree with JimN2TAW - are you sure you don't have a bad connector? What does the signal do if you jiggle the connector on each end of the offending coax cable? Make sure everything is tightened down sufficently.

You can do an experiment by keeping the cable to your computer stationary and moving the other cable(s) nearer and further. This would allow you to confirm if you really are getting EM intereferance.

Also, try swapping the output cables on your splitter to make sure that isn't the problem either. Does the split occur before or after your amps? If the amps are after the split, then those are likely the source of the problem, not the cable. (Get new ones, or figure out how to sheild them - Al foil?)

If you put an amp closer to the antenna, you will significantly improve your signal to noise ratio.
posted by jpdoane at 10:40 AM on September 17, 2007