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PICList Thread
'PIC_k You Brains; RF PIC_k-up'
1998\10\10@222655 by paulb

James Cameron wrote:

> I'd heard that explosives experts do _not_ short the cable at the end
> they are rolling out, because that creates a loop circuit in which any
> radio energy will just _have_ to be consumed by the detonator.

 It would be most sensible to terminate it with a resistor about 100
ohms.  This is a reasonable approximation to the characteristic
impedance of the cable, and should prevent resonance, which is what is
being discussed.  Also, it should be much greater than the fuse
resistance so that most of any power received would be dissipated in the
terminator and only a very small proportion in the fuse.

 It depends on the length of wire vs. the wavelength of the RF in
question.  A quarter wave of cable with fuse (shorted in effect) at one
end and open at the other will resonate with maximum current at the
fuse end (bang!).

 A half wave with fuse at one end and shorted at the other can also
resonate with the same effect.  Any other length however will not
resonate, and a *much* greater field would be required to induce
appreciable current.

 Twisted or not, a close-spaced parallel cable represents a
transmission line whose function is *not* to pick up RF.  It will only
do so if the wires are appreciably separated at places subject to an
RF field (you'd be pretty dumb to go blasting/ rocket launching within
a mile of the local AM radio station, or at the same level as the FM one
for example!).  Good practice dicatates keeping the wires together *all*
the way.

 Of course all this is exclusively for the benefit of the *non*- radio
amateurs on the list ;-)
       Paul B.

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