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| To: | <[email protected]> |
|---|---|
| Subject: | LF: Re: PA0RDT miniwhip in TX? |
| From: | "James Moritz" <[email protected]> |
| Date: | Wed, 22 Sep 2010 23:59:25 +0100 |
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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 Moritz73 de M0BMU |
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