PMR radios work in the 446Mhz range. Lambda length for 446Mhz is around 62cm however an antenna can be 1/4 lambda so around 15/16cm would work in theory. That's for the TX side for the receive side the longer the better is my experience (I once managed to talk to a ship anchored in Cyprus from Brize using the worlds longest slopping V HF antenna which was a bit of a surprise.).
The TX/RX will probably be shit to be honest as you are working with 0.5W output power so the range is always going to be a bit pants. factor in the site you play at having trees, buildings, etc then the range will drop significantly.
Thinking into it a bit more the best chance would be to thread 16cm lengths up and down and across the full molle on a chestrig would in theory give you multiple 16cm antennas which might work better.