WA2WVL wrote:Bruce, the first thing you need is the input impedance at the frequency.
I have measured this on some devices but maybe not the type you are using.
This is done by first assuming that the real (resistive part) is 1 ohm and calculating an "L" network to match this up to 50 ohms. Using a bridge
(I use an MFJ259B) to adjust the network for a perfect match. No circuit just connecting to the Gate to Source.
When you get the match take the shunt C value in the computer network back into the computer program and adjust the R to find out what the R really was. I use the ARRL program TL which comes with the Handbook. Forget the C because it is series tuned out by the L in the network. Now using Xc=1/2PIxFxC calculate the series Xc of the FET. For the IRF540N this comes out to be 1.51-j2.8 @ 28.5MHz.
Make sense?
Floyd
WA2WVL wrote:Bruce, KF1Z I am sorry that I have not been reading my spots for some time. If you still need info please send me a message or Email.
I think I have figured out how to Broadband match LARGE FETS over 1.8-29Mhz. Operation would be class AB linear.
I just bought some IRFP90N20D FETS which I hope to use. These are 200V, 94A, 580Watt devices ($5.50 ea) They have very low feedback capacity which becomes very importain on 10 meters. (about 2.8% of Cin)
Floyd WA2WVL
PS: I will keep the Forum up on my progress
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