On Feb 11, 5:38 am, wrote:
I can't seem to get much gain
out of two separate designs I've tried. I built AA3SJ's amp that's
on the front end of his transverter (http://www.qsl.net/aa3sj/Pages/50MHz-Xverter.html)

That design looks OK to me, though I haven't tried calculating the
resonant frequency of the tuned circuits. Do you get nice resonant
peaks in gain when varying input and output caps ? The 2 pF output
coupling capacitor is intended for a very high impedance load (the 2nd
BF998/mixer) so may not work for a 50 ohm load if that is what you
need. I would probably try a one or two turn link on L2 for output
coupling to 50 ohms.

and I even tried Hamtronics' design (http://www.hamtronics.com/pdf/Manuals/LNK%20Preamp.pdf

Very similar design except it seems the output is untuned and very
heavily loaded (by the large coupling capacitor C6), and has a 47 ohm
resistor in the output for stabilization.

The BF998 has good gain past 1 GHz. For stability, it is probably
necessary to design this like a microwave circuit: very short leads,
use only chip capacitors for bypassing (and chip resistors, at least
AA3SJ's R3 and Hamtronics' R4), double sided circuit board with full
ground plane, ground the source lead of the package within 0.1" of the
body of the BF998, either directlly to the ground plane or to a top-
side pad which is grounded with at least two wires or eyelets or
plated through holes to the ground plane, within 0.2" of the BF998.

I assume you haven't got gates 1 & 2 reversed ? (input to gate 1, 3 or
4V or so bias to gate 2)

Other things to check:
-is the gate 2 bias voltage correct (roughly 3-4 V) ?
-is the drain current on the order of 10 mA ? (that is where the BF998
noise figure is specified)
-are the inductors used exactly as specified ?(absolutely must be
either toroids or in shield cans to avoid feedback, I think)
-is the amplifier oscillating ? (look for RF voltage on the output or
hold an antenna connected to a frequency counter or spectrum analyzer
up to the coils, or look to see if the supply current jumps when you
tune the input or output.)

Good luck. I'd be interested to hear what works as I've just acquired
some BF998s to play with !

73,
Steve VE3SMA