What's the best way to "mod" the antenna. Is there an easiest one? One for a newbie like me?
I did the "reflector" mod, which is the simplest one in my opinion.
You need an 18.5" length of wire (I used #12 electrical wire, but a length of straightened coat-hanger is fine, too).
Simple attach it alongside the CM15a's antenna, making them even at the bottom. Use some tape, cable ties, or some tubing to hold the two together.
There is no electrical connection between the wire and the CM15A's antenna. The wire acts as a 1/2 wavelength reflector.
In my case, it has improved my RF reception enough to give me coverage in my entire house.
A side benefit is that it doesn't void the warranty.