S21 parameter of HJFET
My question is why does this decrease S21?


The S-parameters are measured with the input supplied with a impedance-matched source, and the outputs terminated with a matched impedance. That is, both ports are terminated with the characteristic impedance of your system.

So, to measure the \$S_{21}\$ of your circuit you'd set it up like this:







simulate this circuit &ndash; Schematic created using CircuitLab

(This is the ac circuit only. The full circuit would need some dc bias elements as well, but these would normally be filtered so as not to affect the ac performance.)

If the output of the MESFET is not a perfect current source, then there is essentially a voltage divider formed between the load impedance and the combination of R2 and the MESFET output impedance. As R2 increases, the share of the output voltage developed across R3 decreases, so \$S_{21}\$ decreases.