Today, finally, the rain is gone from my area, and the late afternoon conditions were overcast. This was perfect for testing a few items which you can use to protect the front of your jammer heads from damage by sand and road rocks.
My test setup was simple: I used my home built laser gun simulator to first find the maximum range at which it will alert the rear 9500ci shifter. The maximum alert distance which consistently produced an alert was 9 feet. I then tested a couple of items which could be used to protect the fronts of jammer heads.
First, I tested two layers of Scotch brand clear Long Lasting Storage Tape. These two layers of tape reduced the maximum detection range for my laser simulator by about 5%. So it seems that this stuff is a quick and convenient way to protect the fronts of your jammer heads from sand and other small road debris.
Next, I tested a 1/16" thick sheet of clear acrylic which would provide better protection. The results were virtually identical as the results for the two layers of storage tape.
One note about potentially installing clear acrylic in front of your jammer heads:
It occurs to me that if the clear acrylic sheet is mounted such that there is a gap between the front of the jammer head and the acrylic sheet, then a strong potential exists for feedback from the jammer's IR LED or laser emitters into the jammer's IR sensor(s). Thus it would be best to mount a protective clear acrylic sheet such that it is either in contact with the front of the jammer head, or very nearly in contact with the front of the jammer head. This should greatly minimize the possibility of any feedback.