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.
Take a look at
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talks about standing waves in a string, but the concept is similar for liquids and radio waves.
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 ingress problems.
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)
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