Chimney mounted antennas can be a problem for the very reason you mention. As suggested, you can try to drill thru the mast and fix the location, or use the hose clamps, but either method doesn't always work over time. You could install a rotor so when the antenna moves, you could tweak its orientation from the comfort of your living room. Our antenna is mounted 15' above the roof apex using 3 sets of guy wires and a rotor. The antenna has never come down or lost reception in almost 30 years of use,and we rarely have to use the rotor.