Re: ?- How high SWR before smoke?

Jerry Gaffke
 

Should be fine, gets done all the time.
The drains are operating into a very low impedance, driving the antenna through that step-up transformer.
A little added capacitance there is not terribly significant.
The fact that they are mounted on the same heatsink is not an issue,
generally the heatsink is bolted directly to the chassis and is at RF ground.

Other than no need for heat sink insulators, the other big advantage of the RD16HHF1's
is a somewhat lower gate capacitance, so easier to drive when operating above 20 meters.
But the RD16HHF1's are no panacea, we have bigger problems with the uBitx's gain
through all those 2n3904's than we do with the IRF510's.
And unless you have a cure for Allison's  45mhz-DialFreq  spur,
    https://groups.io/g/BITX20/message/52838
you should be operating at low power for 21mhz and higher anyways.
If at all.

Another big advantage for some is that RD16HHF1's are "Real RF" parts, don't want
their rig soiled by some fool switching FET designed for use in automotive turn signal blinkers. 
If that's an issue for you, by all means go for it.   
But the IRF510's work just fine.

If you are worried about damage from high SWR or high Vcc, the IRF510's are the better choice.
Their max Vds is 100v instead of the wimpy 50v Vds max spec of the RD16HHF1's.

Jerry
 


On Fri, Jul 6, 2018 at 07:42 am, Ralph Mowery wrote:
I understand full well about how to isolate the irf510s electrically.  However has anyone given any thought as to what may happen, if anything, to the RF coupling if the transistors are mounted on the same heat sink ? 
 

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