Try making a new loop from 22awg wire or something similar. Keep the spacing at around 1ft as you had previously. If that doesn't change anything, look for any machinery or CFL lights in your shop that could be inducing noise in the environment. Try turning off lights or machines/tools. Also, do you have any neighbors that operate Ham radio gear, large antennas outside their house?

Our decoder sits around 17-20 with the loop and coax connected when we run at an indoor gym.