Simple. 1/2 wave antenna.
Get a piece of copper rod 3'-10 1/4" long.
Solder it to the center lead of coax. You can use a piecebof PVC to hold it or fabricate something fancier. Works great with #12 Romex too.
Make four equal length rods for a ground plane. Cinnect all four and solder those to the shield of the coax. Four sections of any metal will work. Copper pipe, rods, etc.
Should be resonant at 121.500 Mhz.
Leave it up to you to make the mount.
I've made antennas with aluminum arrow shafts too.
Makes a great lightning rod so don't have it connected during a storm!