Practical_Antenna_Handbook_0071639586

504 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 horizontal section is an open-circuited loop of diameter D, which varies with frequency.
The optimum conductor diameter E for the DDRR is at least 0.5 in at 28 MHz, increasing
to 4 in at 4 MHz. Thus, for monitoring Jupiter, a diameter of 1 in or slightly less
is appropriate.
Because of the loop, some people call this the hula hoop antenna. One author recommends
using a 2-in automobile exhaust pipe bent into the correct shape by a cooperative
automobile muffler mechanic. The far end of the loop is connected to ground
through a small-value tuning capacitor C 1 . The actual value of C 1 , which is used to resonate
the antenna at a particular operating frequency, is found experimentally.
The feedline of the DDRR antenna is coaxial cable; its shield is grounded at the bottom
end of the vertical section. The center conductor is connected to the ring radiator a
distance F from the vertical section in a circular variant of the autotransformer or gamma
match. The length F is determined by the impedance that must be matched. The radiation
resistance is approximated by
R
H
= 2620 2 (22.1)
l
RAD 2
where R RAD = radiation resistance, in ohms
H = height of antenna above ground plane
l = wavelength
H and l must be in the same units.
The approximate values for the various dimensions of the DDRR are given as follows
in general terms, with examples in Table 22.1:
D 0.078l
H 0.11D
F 0.25H
E 0.5 to 4 in
G See Table 22.1
Band (MHz)
Dimension 1.8 4 7.5 15 22 30 50 150
G (in) 16 7 5 3 2.5 2 1.5 1
C 1 (pF) 150 100 75 35 15 12 10 6
F (in) 12 6 6 1.5 1.5 1 1 0.5
H (in) 48 24 11 6 4.75 3 1.5 1
D (ft) 36 18 9 4.5 3.33 2.33 1.4 6
E (in) 5 4 2 1 0.75 0.75 0.5 0.25
Table 22.1 Examples of Dimensions for DDRR