VLF up-converters and MOSFETs

Today I built a MOSFET up-converter, converting 7-9kHz VLF to around 20MHz. However, I was disappointed with the results: low conversion gain and sensitivity when using either a 3N211 and a BF981 device. I used an 82mH choke with a 4n7 cap as the tuned circuit on G1 (tapped capacitance for max sensitivity with 50 ohm input) with LO injection on G2. Despite optimising everything I couldn't better the sensitivity achieved with the SBL1 based up-converter that appears on my website.