Re: Detecting where a coax cable goes to

[Bob Bob <bob3bob3@xxxxxxx>]

Mmmmm!

Any unterminated cable looks like reactance to the rest of the circuit.
How much effect it has mainly depends on whats between it and the other
sections. A splitter like what one finds in motels with long runs has
only a small signal going to the TV and most of it continuing to the
next splitter. There is minimal effect leaving this configuration
unterminated. If however you are splitting the power in half (like most
home splitters) the effect will be greater. Worst case it could look
like a short circuit on one output port of the splitter, thus lowering
the other o/p port somewhat. You can also increase line losses by the
increased line VSWR but this is a very small figure next to the coax
resistive losses.

The simple rule is if you think you have a problem, try terminating and
check the effect on all TV channels. If you dont want to buy a
terminator just add an extra bit of coax (to the unterminated one) for a
test. If you have a strong signal in your area you may not need worry.
The biggest problems I have seen with CATV systems is the lack of good
connector earth connections yielding bleedthrough and low s/n
performance. Coax will only radiate or receive signal direct into the
jacket if there is a current inbalance inner to outer conductor. This
generally mean an asymmetric source or load. An open, shorted or high
line VSWR doesnt cause this problem.

An effective "short" or "open" situation only exists where the length of
the unterminated cable is either odd multiple of a 1/4 wavelength
(short) or multiples of a half wavelength (open) for ONLY the frequency
of interest. If the frequency doubles the wavelength halves. That gets
very complex with so many different TV freqs. This is in fact a very
good way of making a filter. If you were for example watching TV from
both a very weak and very strong station and needed a masthead preamp
for the weak one, chances are the strong signal will break through. If
you attach a piece of coax cut to a 1/4 wavelength of the strong station
at the preamp input it will null it by maybe 20-30dB. Note that the
other end of the coax needs to be waterproofed and the length modified
(shorter) by the velocity factor of the coax (between 0.66 and 0.82 or
so. Foam RG6 would be about 0.82)

Apologies for the waffle. I couldnt resist!

Cheers Bob


Charlie Bress wrote:
> Good guess.
> Wrong reason. The unterminated piece of cable, either open  or shorted,
> creates a phenomenon called a standing wave. Depending on the length of the
> offending cable and whether it is opened or shorted, the wave can raise the
> dickens with the right signals. Terminating the cable in the correct
> impedance doesn't let the standing wave be created.