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General Science / Re: Horizontal Lightning Conductors
« on: 22/06/2023 18:03:51 »"Skin effect" means that high-frequency components (like lightning impulses) only travel through a thin skin on the outside of the conductor.
- That is why lightning down-conductors are made of a braid of many fine wires (with a large surface area) instead of one big conductor.
All the lightning conductors I ever recall seeing are flat copper strips. Braiding it won't make any significant difference to the skin effect because the strands aren't insulated from each other, which is why radio engineers use Litz wire to reduce skin effect. (At MF/HF, above that the usual ploy is to use silver plated copper.)
Digressing a bit, a novice mistake for radio engineers is to use tinned copper wire, in which most of the current flows through the lossy plating instead of the copper, or bare copper wire, which has the same effect by the time it's oxidised. A common technique for reducing the cost of UHF feeder is to use copper plated steel for the inner conductor, because the current isn't flowing in the steel.
"Lightning impulse" - I'll guess it's not genuine AC with Voltage swings from +V to -V many times. I'll assume it's a single DC spike, from 0 (volts or amps) to +Vmax (or +Amax ) and back to 0 over a short time interval. I've never really looked at a Lightning strike and seen it as a plot of Voltage (or Current) vs. time, maybe it has mutiple sinusoidal components with different frequencies.
The bit highlighted is correct.
If you have a repetitive pulse waveform with a period T1, a pulse width T2, and a rise time T3, then it will have a spectrum with components spaced apart by a frequency equal to 1/T1. At frequencies which are an integer multiple of 1/T2 there will be a null in the envelope of the spectrum where the amplitude reduces to zero, and the highest frequency component will be at 1/T3.
From this is follows that if you have a single pulse, T1 is infinite, and the spectrum will be continuous, without discretely separate frequencies, and with nulls every 1/T2, up to a maximum of 1/T3. (If you had a single infinitely narrow impulse with zero rise time it would have a spectrum which is a horizontal line flat to infinity.) In the case of the pulse in BC's reference, the rise time is ~1.2uS, so the highest frequency component you'd expect to see in the spectrum would be about 800kHz, but since the spectrum is continuous, the majority of the energy will be at much lower frequencies. (More than half the energy will be below 400kHz)
Skin depth in copper is:
Freq. Skin depth (μm)
50 Hz 9220
60 Hz 8420
10 kHz 652
100 kHz 206
1 MHz 65.2