Hi Andy it is our experience that a properly tuned single ended Class-E PA
is fairly docile and its power drops if presented with a mis-matched load.
What you MUST NOT DO is tune the PA to the aerial.
The PA should be tuned into its design resistive load and then the aerial
adjusted to provide the right (same) load. Too low or too high and you will
probably not get current draw from the FET. There is a great temptation to
retune the PA at this point, but this certainly can be fatal, and is not the
right thing to do. I have no doubt there are some conditions where the phase
of the reflected impedance from the antenna could give excessive voltages on
the FET but Mike GW4HXO, Finbar and a few other who have used the design
equations, and this procedure, havent lost a FET though aerial mis-match (as
far as I know!). In fact Mike transmitted into an open when his aerial broke
one night.
I advise a simple RF bridge to check that your aerial is what you think it
is....tuning, and tap changing, on aerial current alone is like driving
blind. Once you have the resonance and match right, then all you should need
are slight alterations to take up different weather conditions.
I seem to remember you can some strange parasitic resonances on your tuning
system on 136 at one time which were a bit troublesome?.
Alan G3NYK
----- Original Message -----
From: "Andy Talbot" <[email protected]>
To: <[email protected]>
Sent: Tuesday, November 24, 2009 8:00 PM
Subject: Re: LF: IRF510 500kHz PA stability issues - advice please
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
This email has been scanned for damaging side-effects by the health
and safety police
2009/11/24 Roger Lapthorn <[email protected]>:
> 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
>
>
>
>
>
>
> --
> -------------------------------------------------------------
> http://g3xbm-qrp.blogspot.com/
> http://www.g3xbm.co.uk
> G3XBM GQRP 1678 ISWL G11088
>
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