[mythtv-users] Picture Bars - Was: Electrical power in my cable line? Is that bad?

Brian Wood beww at beww.org
Tue Sep 11 19:19:39 UTC 2007

A JM wrote:
> Actually the information is great, thanks. It's a little over my head
> but I get the gist of what your saying.
> A couple of other quick questions if you don't mind ...
> "A splitter "splits" (obviously) the signal, and each individual path (or
> output port) needs to be terminated properly, just as a single cable
> needs to be."
> What is the correct way to terminate as you suggest above?

If the port is unused you should screw on a 75-ohm "terminator",
available at Radio Schlock and other places.

> Can I do some tests to determine where I have a problem and where I don't?

You're pretty much limited to checking the mechanical integrity of the
cable and connectors, and checking suspect devices by substitution.

There is a device called a TDR (Time Domain Reflectometer) that can spot
irregularities in a signal path. Basically it sends a pulse down the
cable and measures any energy that is reflected back to the source, as
well as the time the reflection took (essentially similar to a radar
set). Given that information you can calculate where the flaw lies, but
TDRs are generally too expensive for consumer use.

> Since 75 OHM is the "magic" number how would I determine that in fact I
> have 75 ohm running through the line? I mean can I take an OHM meter to
> my cable line and make some determinations on the readings that I get?

No. An ohmmeter reads DC resistance, not AC impedance. A 75-ohm
terminator is actually just a 75-ohm resistor, so you could check that,
but the "75-ohm" that refers to coax cable is the "characteristic
impedance" of the cable, which is the combination of DC resistance,
capacitive reactance and inductive reactance of a theoretically
infinitely long piece of that cable, at a certain frequency or range of

The characteristic impedance of a coax cable is determined by several
factors, including the diameter of the center conductor and the shield,
the distance between them and the "dielectric coefficient" of the
insulating material.

If the cable is less than infinitely long, as most are, then terminating
it in its characteristic impedance will cause it to behave electrically
in the same manner as an infinitely long piece.

You can measure the characteristic impedance with a device known as an
impedance bridge, or with a network analyzer, both of which cost more
than many many PVRs. You can also use an RF sweep generator and a return
loss bridge, but I think we're getting a tad OT here.

Hope that's of some help.


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