For some reason, I lucked out and my first R15 came with the RF antenna in the box. However, my son in law's R15 came without it, so I decided to make an antenna for him.
My 'factory' antenna is 6-1/2" inches long. After lots of trial and error, I found that making the new antenna 11" long worked the best for me.
Just take a length of RG-6 coax and put an "F" connector on one end. About 1" out from the connector, use a plastic tube or a pencil to help you make a nice 90 degree bend in the coax. (In other words, don't make a sharp 90 degree bend that breaks the inner conductor.) Just above the bend, strip away the outer cable sheath, the ground braid and any foil wrapped around the inner conductor. You are left with the center conductor with a clear plastic covering over it. Removing the clear plastic cover doesn't improve the antenna at all.
If you want to experiment, just make your antenna extra long. Using trial and error, repeat operating the R15 from a distance. Keep a log as you keep snipping off an inch of so of the antenna. Again, in my case, 11" gave me the best range and reliability, but your environment may be different.
Once you've gone "too far" in snipping off the antenna length, you'll need to endure the expense of another scrap piece of coax and another connector.
It not only works great, but it looks as good, if not better, than the factory supplied unit.