Practical_Antenna_Handbook_0071639586

506 P a r t V I : A n t e n n a s f o r O t h e r F r e q u e n c i e s
The beamwidth of the helical antenna is
q =
K NS
(22.5)
where q = beamwidth, in degrees
N = number of turns
S = pitch, in wavelengths
C = circumference, in wavelengths
K is 52 for the –3-dB beamwidth and 115 for the beamwidth to the first null in the pattern.
The short section between the helix and the ground plane is terminated in a coaxial
connector, allowing the antenna to be fed from the rear of the ground plane. The feedpoint
impedance is approximately 140 Ω.
Multiple Helical Antennas
Stacking helical antennas allows a radiation pattern that is much cleaner than the normal
one-antenna radiation pattern. It also provides a good way to obtain high gain with
only a few turns in each helix. If two helixes are stacked, then the gain will be the same
as for an antenna that is twice the length of each element, while for four stacked antennas
the gain is the same as for a single antenna four times as long. Figure 22.6 shows a
side view of the stacked helixes.
Figure 22.6 Dual/quad helical antenna.