Practical_Antenna_Handbook_0071639586

C h a p t e r 2 2 : R a d i o A s t r o n o m y A n t e n n a s 505
The construction details of the DDRR are so similar to those of the ring radiator that
the same diagram can be used (see Fig. 22.3 again).
The normal attitude of the DDRR for communications is horizontal. However, for
maximum signal when monitoring Jupiter, the entire antenna assembly, including the
ground-plane screen, can be tilted to track Jupiter’s position in the sky.
Helical Antennas
The helical antenna (Fig. 22.5) provides moderately wide bandwidth and circular polarization.
Thanks to the latter, some find the helical antenna particularly well suited to
radio astronomy reception. The antenna (of diameter D) will have a circumference C of
0.75l to 1.3l. The pitch of the helix (S) is the axial length of a single turn, while the overall
length L = NS (where N is the number of turns). The ratio S/C should be 0.22l to
0.28l. At least three turns are needed to produce axial-mode main lobe maxima.
The diameter or edge of the ground plane G should be on the order of 0.8l to 1.1l if
the conducting surface is circular or square, respectively. The offset between the ground
plane and the first turn of the helix is 0.12l.
The approximate gain of the helical antenna is found from
2
Gain = 11.8 + 10 log ( C NS)
dBi
(22.2)
The pitch angle f and turn length g for the helical antenna are given by
–1 ⎛ πD
⎞
φ = tan ⎜ ⎟
(22.3)
⎝ S ⎠
and
2 2
g = (π D) + S
(22.4)
Figure 22.5 Helical antenna.