HardwareBanter - View Single Post - Hum from phone wires running next to mains?

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In alt.engineering.electrical George wrote:
| On Tue, 04 Mar 2008 21:22:30 +0000, phil-news-nospam wrote:
|
| In alt.engineering.electrical wrote:
| | On Tue, 4 Mar 2008 19:13:07 -0000, "Graham." wrote:
|
| |It is quite difficult to induce hum into telephone wiring. Use twisted
| |pair cabling rather than the flat ready-made extension cables.
| |
| | Exactly!
| | The phone company has millions of miles of cable running right below
| | power lines and hundreds literally touching each other in the jacket of
| | the cable. That little twist they put in the pairs is excellent in
| | isolating them from crosstalk.
|
| That twist is a great little means to ensure induced signals, whatever
| they may be, are induced in equal amount on both wires, so they do not
| contribute to the actual intended signal that is a differential between
| those two wires.
|
| However, a risk exists when two different pairs are present next to each
| other and each pair is twisted at the same pitch. The signal carried by
| one can end up being induced differentially on the other. So don't twist
| those power lines, or if you do, twist them at a pitch with a ratio to the
| phone line twist that is not a whole number.
|
| CAT5 cable is an example. It has 4 different pairs twisting along. Each
| of the pairs has a different twist pitch by design (unless you get some
| cheap cable not manufactured correctly).
|
| Very interesting.
|
| Please can you advise on how these twisted pairs compare with
|
| 1. shielded audio cable
| and
| 2. rf coax.
|
| In case 1 both the wanted signal and the noise are in the audio frequency
| range.
|
| In case 2 the electricity supply noise contains harmonics of similar
| frequency to the wanted rf signal.

I don't have specific data on the quality of noise immunity. I'd bet that
kind of research has been done. It most certainly would vary by quality of
construction of the cables in question.

RF coax comes in various levels of quality based on a stated shielding
percentage. I've seen lows of 60% all the way up to 100%. The latter
could be a foil, or a solid metal encapsulation (quite a variety of
different coax types with this).

I've seen cables, including CAT5, with both twisting _and_ shielding around
the whole cable assembly. I don't know how much the effectiveness works
together. I have not had a case where I would consider using it.

--
|---------------------------------------/----------------------------------|
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
| first name lower case at ipal.net / |
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