[time-nuts] Lightning arrestors for GPSDO antenna

[Jim Lux]

On 10/18/14, 2:05 PM, Bill Hawkins wrote:
> FWIW, when grounding the metal mast of a boat, three inch wide copper
> strap is used because it is a better RF conductor. The strap is
> available
> in marine supply stores.

It's actually more about being convenient to install, and tradition.

If you have a 1 square centimeter conductor at 1x1 cm, the inductance is 
about 1.5 uH/meter, and when it's 10cmx 1mm (4" wide) the inductance is 
1.15 uH/meter.  That's not a big difference in the voltage due to a 
transient.

What does go down dramatically (at frequencies where the skin effect 
thickness is "small") is the AC resistance, which will be 0.2 for the 
10x0.1 (20cm perimeter) relative to the 1x1 bar (4 cm perimeter).
Skin depth in copper at 1 MHz is about 0.06mm.  so the bar has a 
effective RF cross-section of 2.4 square mm, for a AC resistance of 
about 7 milliOhm/meter.

However, for lightning, with rise times in the microsecond range (e.g. 
MHz kind of frequency), the voltage due to inductance is huge (1 
uH/meter * 10 kA/microsecond = 10kV/meter) compared to the voltage due 
to resistance (10kA*7mOhm/m = 70V/meter)



Now, if that strap is part of an RF ground for your transmitting 
antenna, then the lower resistance of the strap is a good thing. 
Inductance is lossless, but AC resistance sure isn't.