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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Wed, 22 Sep 2010 23:59:25 +0100
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Subject: LF: Re: PA0RDT miniwhip in TX?
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Dear Marco, Roelof, LF Group,

I suppose what you have to consider is what will happen to the JFET when a 
high EMF is induced on the whip due to the nearby TX antenna. Certainly, one 
could expect more than 25V, so reverse breakdown of the gate-channel 
junction would occur. But this does not in itself cause damage, the junction 
is just acting like a zener or avalanche diode, and when the high voltage is 
removed it will return to normal. What would cause damage is if the power 
dissipation (V*I) in the junction was high. If we assume a whip capacitance 
of say 3pF, its reactance at 136k would be about 400kohm. Even if the TX 
antenna induced 1kV EMF on the whip element, the current would then only be 
about 1kV/400kohm = 2.5mA. So if the breakdown voltage of the gate junction 
was a few tens of volts, power dissipation of the order of 10s of milliwatts 
could be expected, which probably is not enough to excessively heat a J310. 
At 500k, the reactance of the whip would be lower, but the TX antenna 
voltage and induced EMF would be lower also, so the current would stay at a 
similar level. So if the whip element is very small and therefore high 
impedance, the JFET will indeed be very difficult to damage. If the JFET 
gate is connected to bigger, lower source impedance antenna elements, 
protection diodes etc. would probably be a good idea.

This would not be true for a MOSFET preamp - in that case, excessive gate 
voltage would puncture the gate oxide and permanently destroy the FET.

Cheers, Jim Moritz
73 de M0BMU