I have constructed a Mixer for a VHF receiver (150MHz).
The IF frequency is 45 MHz and LO is 105 MHz.

Having not made a mixer before, I copied a typology directly from one of
my fundamental electronics books.

The layout is...
RF signal into Gate 2 of BF998.
Source has 100R Par with 1n to ground.
G1 has 47K to ground and 100p to LO signal (I am using a sig gen for
testing).

The drain has a Tuned circuit consisting of 100nH and 125pF.
A 1k resistor is across the coil to provide approx. 800 Ohms needed
to interface to a 4-pole crystal filter.

For testing I used a tap-C down to 50 Ohms from the 800R tank in the
drain of the mixer BF998.

I observe the frequencies coming out of the mixer when I inject the 150
MHz RF signal and find I can tune the 45MHz for a peak no problem.
There do not appear to be any instabilities.
The gain of the preceding stages (Input single tuned filter, MOSFET RF
amp and Double Tuned Circuit) is around 24dB.

When I add the mixer I find the total system gain is the same (24dB),
The mixer seems to be mixing but adding no gain or loss. I also wonder
where the mixer MOSFET is getting bias (from the LO ?) because the mixer
stage doesn't appear to be drawing any current. I found the max. gain at
the mixer output was when the LO level was anywhere from -5 to 0dBm.

Would someone please mind explaining how the bias of the mixer should be
applied and how much gain/loss I should expect. ( i hope I am asking the
right questions here as have no previous experience with a discrete mixer).

Thank in advance.

Regards

David