Re: LF: IRF510 500kHz PA stability issues - advice please

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Andy,

Thanks for the quick feedback. The 500kHz antenna is a 5m long vertical with a spiral top hat. It is fed via a (thickish) ferrite rod wound with the required inductance to resonate and this is tapped at the 50 ohm point (very low down - just a couple of turns). Well that is the general idea! 

Yes the IRF510 is rated at 100V.

73s
Roger

2009/11/24 Andy Talbot <andy.g4jnt@googlemail.com>

If you've studied the drain waveform when operating inrto a correct
load, and assuming its all within design guidelines, then the trouble
is because of nasty impedances presented by the antenna during the
tuning process.

What sort of antenna matching are you using?  The often suggested idea
of an an adjustable tap onto the loading coil can lead to damage as,
when the antenna is detuned, the small number of turns of the link,
with an untuned coil, appear not far different from a short circuit.
 A Class E PA, when presented with a slightly inductive short circuit
can get a bit stressed.

On my antenna over the years on 73, 137 now 500kHz the system has
always been fed in series, using a ferrite tapped transformer to match
load impedance.   I kept well away from tapped or link windings as
they are not easy to calculate or predict reliably.  WIth a series fed
system,  a detuned antenna results in a high impedance.  This may not
always good news either to some class-E designs, but all my LF/MF PAs
are completely open-circuit-proof voltage fed designs.    The only
true class-E one here was for 1.8MHz, and can be seen at
http://www.g4jnt.com/TopbandPA.pdf   .    This was completely imune to
open and short circuits and seemed very robust; but I did use the
meatier and more robust higher voltage IRF540. Doesn't the IRF510 only
have a 100V rating?

Andy
www.g4jnt.com

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2009/11/24 Roger Lapthorn <rogerlapthorn@gmail.com>:

> I've been using a single IRF510 FET PA stage on 500kHz and it produces
> around 6-7W of RF. But, when adjusting my antenna I've managed to damage
> several devices now.  I think the stage is running somewhere towards class-E
> looking at the PA drain waveform on the scope and I believe either the stage
> is going unstable or it is running in a condition in which too much current

> (and heat) is taken and the device fails.500kHz teh ant

>
> Are there any simple "rules of thumb" or design/biasing/decoupling
> guidelines to ensure that my devices don't destroy themselves with a less
> than perfect load?
>
> Any simple advice would be appreciated.
>
> 73s
> Roger G3XBM
>
>
>
>
>
>
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