Not a guess. Both reasons are correct. I don't make this stuff up. From
the AW catalog:
F Terminator - 75 ohm terminator properly closes unused coaxial
cable ports on splitters, diplexers, etc. preventing signal leakage and
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
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!
Charlie Bress wrote:
"Other posters have given you reasonable suggestions for identifying
cables. But I'm wondering why it matters as long as each outlet is
the same signal from the same source -- the splitter. "
Interesting you should ask (and, btw, you seem to be the first to do
so...). Anyway, a little off topic, but the reason for the question is
this: the cable company's feed starts from the attic and is being split
into various rooms as I described. What I would like to do is intall in
the attic an HTDV over-the-air antenna and feed the signal from that to
an HDTV in one of the rooms.
I don't want to run a new cable from the attic thru the second floor to
the first floor (where the HDTV is located); I just don't have the
skills (and probably tools) required for that kind of job. So, in my
infinite wisdom, I figured that if I can determine which cable goes to
the room in question, I'll just disconnect it from the splitter and
connect it to the OTA HDTV antenna in the attic (using a coax coupler)
and to the HDTV in the room and voila! OTA HDTV reception.
I understand that by doing that I give up regular cable reception in
that room but (1) it's a problem for another day and (2) it's not
really much of a problem anyway since I don't intend to user that for
the usual cable fare.
So that's the reason....
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