[HomeBrew] Grounded gate puzzle

[Ian Wilson]

I am building what was going to be a simple converter for 6m :).

This uses a DG MOSFET mixer and a 22MHz BJT xtal oscillator, to convert down
to 28MHz. All this is working fine.

However, the front end consists of an MPF102 JFET grounded-gate amplifier.
This is not behaving as I expect. To check, I have reproduced the circuit on
a
separate piece of PCB (the whole thing is built ugly style, so capacitances
are
pretty low).

Here are the two pieces of behavior that I don't understand:
  1. I am seeing a voltage gain of only about 1.5 between input (source) and
output (drain)
  2. If I turn the power supply off,  the output drops only to about the
input level

Here's the circuit:
  - MPF102 JFET (checked, more than 1 behave identically)
  - source to ground through 270 ohm; gate to ground
  - +12V supply. Measured about 1.2V across the source resistor => drain
current =~ 4.4mA
  - input is 100 ohm to ground, 0.01uF to source
  - output is 0.21uH inductor from drain to bypass cap and +12V
  - tuning capacitance (including scope probe) of about 50pF (variable) from
drain to ground
  - Q of inductor is pretty good, measured X=66 ohm R=1 ohm on MFJ analyzer
at 50MHz
  - adding a 10 ohm stopper resistor at the drain makes no difference (in
case it's oscillating)

Even if you assume a Q of 50 for the inductor (parallel tuned circuit R =
3.3k) and gm of the
device of 5mA/V I would expect to see a voltage gain in the double digits,
not this feeble 1.5x.

What am I doing wrong?

My scope may not be ideal at 50MHz but I have done some basic sanity checks
and in any
case I'm comparing input vs output levels.

Thanks for any help!

73, ian K3IMW