Performance data and EZNEC Modeling
EZNEC models of the low band five band hexbeam and the high band five band hexbeam were constructed using the wire lengths and end
spacing recommended on the preceding page and below are performance charts. These charts are based on a hexbeam 30 feet above
average ground. Only the graphs for 20 meters are shown; the other bands' charts are similar in their basic characteristics.

Study these charts. They show you where you will be with the wire dimensions recommended here for low band and high band. They also  
show that
    (1) SWR tends to be high on the lower edge of the best operating range.
    (2) Gain is fairly flat across the band and
    (3) Front to back performance varies a lot with frequency. The wire lengths, mainly the reflector wires, were measured to place the
    peak front/back performance at the frequency points indicated on these charts. You can move your "sweet spot" to the frequency
    range of your own choice by changing wire lengths as discussed on the Specifications and FAQs page.

You can get a good explanation of this by perusing the hexbeam site of G3TXQ.
Site publication date 12/2005
Page revision date 10/2007
Five Band Hexbeam  low band design
Five Band Hexbeam high band design
20 meter elevation pattern
20 meter azimuthal pattern
Measured SWR with Hex beam elevated 30 ft
This hex beam uses the low band wire lengths
These radiation patterns are for the design frequency with a hex beam at 30 feet over average ground. Performance at other
frequencies will not be as good. That's just the nature of any hex beam antenna, or any yagi beam for that matter.
Hex beam by K4KIO