Not really microwaves and certainly not LF, but hopefully someone can help...

I've been reminded that I promised to finish a 200 Watt 70MHz solid state PA, a project started some time ago and got put on hold.  I'm stuck for a suitable ferrite core for the output transformer.   It needs to match approx 3 ohms load resistance to 50 ohm output.  The windings will be made using either conventional wire as in HF PAs, or made from semirigid coax with shunt outer, series inner.  The latter will need a physiacally larger core to accomodate the wire thickness.

Some cores I did have that 'ought' to have worked taken from a 35 watt broadband PA (0.5 to 1000MHz) failed miserably and got seriously hot no matter what winding configuration was used.  In that original PA they were acting only as common mode chokes and presumably didn't have to carry nearly so much flux.

Does anyone have details of a suitable core/material, or better still have a core going spare   :-)

The PA uses an MRF 141G Mosfet device rated for 300 Watts out with 14dB gain.  The circuit  itself is quite simple - see the data sheet for teh MRF141G -  and would have been finished a long time ago but for the output transformer.

Andy  G4JNT

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