BV155CA.YW, optimized 26'boom                     
21.0   21.225   21.45 MHz
 5 elements, inches
             1.045    0.875    0.500    0.438
    0.000    2.500   78.250    3.250   64.000
   48.000    2.625   66.130    3.250   65.500
   96.000    2.500   66.250    3.250   63.875
  178.750    2.500   66.250    3.250   61.625
  306.000    2.500   66.250    3.250   55.000

This element taper is that of the Hy-Gain 155BA or
155CA. Note that the .5" o.d. section is modeling
the taper swaging of the .875" o.d. element tubing,
and the length of the 0.5" section subtracts from
the overall 84" or 72" length of the first section
-- as does the modeling of the element clamp and
boom also, represented by the 1.045" first section.

This design was optimized for F/R over band > 20 dB,
and SWR over the band < 2:1, with a mid-band free-
space gain of 9.9 dBi. Torque compensator is 2.5" OD 
PVC, 39.625" long, mounted 12" behind Director 3, 
on flat metal plate with U-bolts. 
 
Match frequency: 21.225 MHz
Driven-element tip: 65.5 inches
Cable Z0: 50.0 ohms
Hairpin rod diameter:  0.25 inches
Hairpin rod spacing:  6.0 inches
Hairpin inductance = 0.30 uH
Hairpin length = 7.50 inches