Chuck:
We mounted ours about a foot under the spreaders and would do this again in a second. I agree that above the spreaders is rather high and not needed. Below the spreaders is not a problem with the stay sail, it will tack just fine, plus most use a kato marine ss mount which has a rub rail protecting the unit. Still, it is not a problem for the stay sail.
Our Maine dealer, Norm Pierce, mounts his radar units below the spreaders and places them about 12' off the deck. This way he says it is also easy to reach with a long extension ladder! Now that is an interesting idea. So below the spreaders is a great place. I do not have any problem seeing targets right up to the boat at this height.
Back in 2002 when we commissioned Island Spirit, I published detailed photo albums on the installs. The radar docs are here:
fhayden.com/rockhallfleet/islandspirit/radar.htm
This is also the same URL as:
IslandSpirit.us
Hayden