[Antennas] 40 m vertical beam

[ARDUJENSKI at aol.com]

I noted reference to a 40m vertical beam (vertical antenna with a  reflector).

http://www.chem.hawaii.edu/uham/index.html

I modeled  it on EZNEC and it appears to show good front-to-back ratio and 
reasonable gain  (F/B 10 dB and gain 3.8 dB). I modeled it with the parasitic as 
both a director  and reflector with about equal values for performance.

I was  wondering in practice if it would be doable to set the two verticals 
the same  height and adjust one or the other (driven or parasitic) electrically 
with a  coil/variable cap to adjust the height to take advantage of the set 
up?  

Since the parasitic is grounded how critical are the radials?

Alan KB7MBI
Woodinville, WA