If the elements are mounted above the boom, the center of gravity of
the beam is above the boom and the antenna tends to rotate (example:
the KLM designs). This makes the antenna difficult to hold horizontal
when mounting, and also makes it tough to keep from tipping over while
being hauled up a catenary or tram. On antennas like the Hy-Gains,
the small droop of the elements helps to keep the CG below the boom
center, but there's absolutely no reason why the elements can't be
below the boom. The first Yagi I built when I was a kid had this by
accident, but my first experience with KLM's underscored the advantages.
73 de Dave, W6NL (ex-QHS)
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