5 May
2005
5 May
'05
7:21 p.m.
I've been told (during military service) that the
dv/dt
of an EMP is so fast a Faraday cage will saturate and the
eddy currents are so strong it re-launches an EM wave
inside the cage.
Not quite. What happens is that the surfaces parallel to
the incident wave behave as capacitance and the surfaces
perpendicular to the incident wave behave as indictors.
The initial pulse sets this LC circuit resonating much
like a struck bell and creates it's own internal EM field
as the energy dissipates in the resistance. This can be
effectively damped by placing low value resistors, 50 to
100 ohms, across surfaces to reduce the Q of the shield
and absorb the energy.
Similar damping Rs are used in high power Tx equipment to
stop parasitic resonance in structural parts. It's not
uncommon to see a couple of 700W 68ohm resistors bolted to
the anode boiler of an HF output tube. Without them the
boiler suffers greatly from burnt supports and melted bolts.
Lee.
.
___________________________________________________________
How much free photo storage do you get? Store your holiday
snaps for FREE with Yahoo! Photos http://uk.photos.yahoo.com