A few years ago there was some discussion on r.r.a.a. about helically-
wound, normal-mode monopoles, and the rather common expectation that
they had higher gain than a linear monopole of the same physical
height (and with other things equal).

A recent NEC-2 analysis of this topic might be of interest:
http://i62.photobucket.com/albums/h8...r_Monopole.gif
..

Also this link to a page from John Kraus' ANTENNAS FOR ALL
APPLICATIONS, 3rd Edition: http://i62.photobucket.com/albums/h8...ndVertical.gif
..

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