Most incorrect power readinsg are due to RF on the outside of the coax. It might help to put an isolator at the antenna port (ferrites over the coax). Also if you are using coax to feed your antenna, a BALUN could help. RF in the shack is the bane of many amateurs. Putting ferrites on the earth lead would not be a good idea as it would impede the flow of RF currents to ground.