I've been wrestling with where to put the comm antenna. Some builders have opted to put it on the tail boom. I've seen more than a couple who even installed a large flat piece of aluminum on the boom directly below it to act as a ground plane (maybe it's not effective on the boom without it?). I've decided to place the antenna underneath the cabin to the back right side. There shouldn't be any ground plane issues if it's mounted on the cabin sheet metal. With the oil reservoir on the left side, I thought there would be less chance for interference on the right. I'm not real crazy about the amount of ground clearance provided, so I guess I won't be straddling any logs where I land. The XCOM radio has a full 6 watt output so maybe the placement will be OK.
