Hi Jeff;
I don't know if you got your answer yet or not what I learned from
the Navy.
Your radar sends out a fairly directional beam of energy straight
out the front of the antenna, then receives the reflection of the
other ships (usually large and metal) and shows you the blip on the
screen. Ideally that's all you'll see. But radar antennas aren't
perfect and they also transmit a tiny bit of energy off to each side
of the main beam at about 45 degrees relative to the main beam. So the
arc you see starts as the first lobe sweeps the big ship and the
antenna sees that as the beginning of the arc. Then the main beam
sweeps the ship and you get your blip. Then the other lobe sweeps the
ship and that's the arc on the other side of the blip your seeing.
This all happens as the antenna is rotating pass the large metal ship.
Well I hope that helps.
Steve Hicks