The Radar uses a device called a TR switch that prevents you from pouring all your transmitted power right into your own receiver (especially since you broadcast and then "listen" with the same antenna). There is a momentary delay that keeps you from receiving a return signal too quickly; this also makes you radar "blind" too close to your own source. So the end effect is you cannot spot things in the water within several tens of yards from your boat, nor can you spot your own mast or other gear right on your boat.

The new wideband radars may work differently. The TR switch is a device that exists on basic pulse type radars that most of us still use.