The Mk-series and Skymaster 3-element designs are very popular due to their overall size and performance characteristics.  However, there are a few mechanical considerations to keep in mind when planning to install one of these systems.  While three elements might be a more economical option than a larger 4-element model due to shorter matching cables, lighter support structures and smaller rotator units, keep in mind the location of the driven elements and their feed points.  The driven element wires almost always hang down below the top plate of tower structures.  This isn’t a problem and has little to no effect on the performance of full-wave loops, however what could be an issue is the relative clearance from the support structure and the driven elements themselves.  If mounted too closely, the driven elements and/or the feed points could come into contact with the tower in high wind conditions.  We’ve done our best to mitigate this risk by offsetting the driven element spreader set by 18” from the mast, which is adequate for most installations.  However, if you have a wide stance in your tower legs (i.e. Rohn 55G or other large width tower), this space becomes critical.  What’s more, if you intend to use a trolley system, such as a Hazer or RT Innovations Razer, this is very critical, especially when the antenna is rotated opposite to the side carrying the trolley.  Just something to consider as you plan your project.  Many are using the 3-element designs with good results, but planning for guy wires and other structural or mechanical items will ensure better overall success with your project.