I have had the same problem with my Airmar WX120. It consistently seems to read about 1/2 of the real wind speed based on weather apps (and common sense). I'm in southern California and there is a red flag warning today with wind speeds of about 20 knots, and my Airmar reads between 6-8 knots, with gusts up to 10. I see no way to calibrate the unit and no help in the user's manual or FAQs.
I also don't think I have a dead air issue with my setup, although the VHF antenna is probably closer than it should be. But not a lot of options on my crowded radar arch. I would love to know if anyone has found a solution.