Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx> Hi Steve,Your question is a good one and deserves serious thought. I think we both agree that the fundamental frequency in the spectra of streamer currents wants to return to the base of the coil and the question to be pondered is what about the other high frequency components of the streamer current. Given the context of the original question was coax feed lines used with PIG powered systems, I think it is prudent to confine operation to outdoor use where surrounding walls and floors are not part of the geometry (keep it simple for now).
I believe the following to be true and certainly welcome any comments: The topload capacitor being charged (as you say) is mostly a capacitance to its surroundings (earth for outdoor use). Once the topload is charged, I believe the secondary can mostly be removed from the picture. One could just mentally think of the topload suspending in space that is charged to some high potential with capacitance to earth ground. Assuming a streamer breaks out and arcs to earth, one can ask what the return path is. The conduction path is the streamer itself. As the streamer depletes the charge on the topload, the voltage decreases rapidly and there is a high dD/dt (displacement current) that seems like it needs to be equal and opposite to the conduction current. The current is limited by the streamer resistance and its inductance and maybe there is an LRC resonance where C is the capacitance of the top load (haven't gone thru the numbers to know).
Anyway, if a streamer hits the coax that is RF grounded instead of mains grounded, it seems that a portion of the current will want to return to the secondary base and the other portion will be seeking earth ground. Since the secondary base and RF ground are connected, the current can seek whatever it wants to and the path is there and everything will be "happy". If the coax was mains grounded and the topload had capacitance to earth ground, seems like all the streamer current will travel thru the mains ground while seeking earth ground and the coil base. It also seems like this would happen even if powered outside with a RF choke in the mains ground path. Any hit to mains ground would just arc around the choke :-(((
Gerry R.
Original poster: Steve Conner <steve@xxxxxxxxxxxx>The advantage I see is if a secondary arc makes it to the cable, the current will be returned to the base directly and not thru the mains ground.Here's something to ponder: Does the current from a strike really want to return to the base of the coil? The coil is an inductor so it will have a very high impedance to the high frequency strike current.I think the strike current (or at least the highest frequency components of it that cause flashovers, EMI etc- it's hard to deny that the fundamental frequency returns to the coil base) actually comes from the topload capacitance, and hence it wants to return to the other "plate" of the capacitor. That would be the surrounding walls and floor (and any wiring in them and any metal objects nearby) To my mind this is a very powerful argument for using a large counterpoise if not a full Faraday cage, when you play with arcs to ground in the vicinity of any sensitive equipment.You might wonder what higher frequency components I'm talking about? A ground strike discharges the topload capacitance very suddenly- in a matter of nanoseconds- and this transient contains a whole band of high frequencies up into the 10s of MHz. It's basically a Hertz type spark transmitter.Steve Conner http://www.scopeboy.com/