The radius of throw of a rotor or spray head is measured in feet and describes the distance the water will be thrown forward, away from the sprinkler head itself. Anotherwords, if the I-20 rotor specs state that the radius of coverage is 17' to 47', that means the furthest distance the water will be thrown will be 47' and the shortest distance will be 17' feet. The different distances or radius are achieved by installing smaller or larger nozzles into the rotor head (complete nozzle sets come with each rotor) and by turning down or up an adjustment screw on the rotor head.
The Arc is the amount of a circle that the rotor will rotate to provide coverage ( how much of a circle is covered by the rotor spray). The arc is very simple to increase or decrease. The left or right side of a rotor, depending on which brand you choose, will be the fixed side that you line up with the boundary (sidewalk). You can then use the adjustment screw on the rotor to open or close the arc thus increasing or decreasing the degree of a circle covered.
I do not suggest you use an I-20 rotor for throwing 16 feet. You should use a rotor that is designed for that small of throw, Hunter PGM rotor or Rain Bird 3500 series or 5004 series rotor. The I-20 would have to be turned down too much which would mess up the uniformity of spray.
For more on radius and arc adjustments see the following link on our website for the I-20 rotor:
http://www.sprinklerwarehouse.com/manuals/I-20install.htm
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