cablescoaxcommunicationemissionsRF
My question relates to the accumulated AC voltage that will appear on the screen of a length of coax:
If the cable is properly terminated at one end and fed with data (or any signal) at the other end, what will be the voltage on the screen relative to ground? Will it be ground potential(?) or will it be some other potential(?), and if so, will it be potentially acting as a "radiator" of the data?
With nothing connected to the end of the cable the reflection occurs in phase, so that cancellation occurs in those regions where the positive half cycle of one wave overlaps with the negative half cycle of the other. The duration of this overlap is the round-trip time of the signal through the cable. This is shown in the photograph.
It looks like your transmission line is open circuit!
After the exchange of comments I think I now know what is happening. The coaxial cable is indeed 50 ohm. The resistor used wasn't a 47 ohm (yellow, purple, BLACK) but a 470 ohm resistor (yellow, purple, BROWN). This explains why the reflection looks more like an open circuit cable because it was being terminated by a resistance 10x higher than the impedance of the cable. The 100 ohm was much closer and shows less distortion but it was still twice the impedance. If you look at the 100 ohm trace it has a similar but less pronounced castle shape with an asymmetry between the rising and falling edge of the pulse.
You get the step because this waveform is a superposition (addition) of the pulse and the reflections which are shifted in time by the round circuit trip. When the cable is terminated with 50 ohms these reflections disappear.
It would be interesting to see this happen by using a variable (non inductive) resistance starting high (say 500 ohms) and ending with a short circuit. Measuring the time shift should also give you an estimate of the cables length knowing that the signal travels at about 2/3 speed of light in vacuum. (about 10m per 100nS)
http://web.physics.ucsb.edu/~lecturedemonstrations/Composer/Pages/76.18.html
The coax cable shielded the signal line between the antenna and the receiving circuit, which is how it reduces extraneous pickup. Note that this does nothing about extraneous signals picked up by the antenna itself.
The reason it attenuated signals from the "antenna" is because it adds some capacitance from the signal line to ground. It looks like what you are calling a antenna is really just a capacitive pickup. The capacitance from the source signal to this pickup will be small, so the small capacitance in the coax cable is large enough to form a significant voltage divider.
Best Answer
Plant leakage is a well known effect - The system of cables and connectors in distribution systems is known as "plant" for some reason - and causes signal to leak out and be received via antennas.
In an ideal coax, the wave travels between the conductors and imposes local current eddies and voltage nodes along the length of the transmission line, but most notably on the inner surface of the outer conductor and on the outer surface of the inner conductor (the skin effect). Again ideally, no signals that travel on the outside of the outer conductor can interact with the signals on the inner surface of the outer conductor.
Plant leakage arises from the divergence from this ideal, when you use wound or wrapped coax, poor connector termination and also mismatched termination loads.
In your drawing above, assuming the termination also is properly shielded, the return signal should travel along the inner surface of the outer connector if the system is ideal. So you shouldn't actually see any radiation. Evidence of this is how non-coaxial waveguides (TEM- mode) work - everything flows along the inner surface and there is very low leakage.
If this is beyond a hypothetical situation and you are seeing leakage, it can arise from the quality of the coax, if it is solid foil or braided, and that depends on the frequency you're driving it at too (wavelength vs. mesh size). Connector shield continuity, termination matching all are radiation points. Also, the attached eqt. on the receiving end may inject a current mode return signal into the grounded shield causing the whole outer surface to act as an antenna and if this is a signal derived from the feed signal (amplifier power rail bounce as one example) then it might appear that the coax is leaking.
In general cable television plant leakage arises because of the less expensive cables they use (they have many many miles/KM installed cost really matters), the difficulty in maintaining a connector over many years and damage, nicks and knicks etc.
There are some systems that use gas dielectric and solid conductors that transport 10's of MW to antennae with almost no leakage so ideal performance is approachable.