Practical_Antenna_Handbook_0071639586

C h a p t e r 1 9 : V H F a n d U H F A n t e n n a s 435
at VHF because it tends to broaden the SWR bandwidth of the antenna’s input impedance
and because the native feedpoint impedance of Yagis is often very low. For this
construction project, two ¾-inch aluminum tubes, spaced 6.5 in apart, were used.
The antenna can be directly fed with balanced 300-W line or from 50-W or 75-W coax
through a 4:1 balun. If broadbanding is not important, the folded dipole of the driven
element can be replaced with a conventional dipole and the antenna fed directly from a
52- or 75-W feedline.
Two-Meter Yagi
Figure 19.8 shows the construction details for a six-element 2-m Yagi beam antenna.
This antenna is built using a 2- × 2-in wooden boom and elements made of either brass
or copper rod. Threaded brass rod simplifies assembly, but is not strictly necessary. The
job of securing the elements (other than the driven element) is easier with threaded rod
F
E
D
C
B
A
30 30 30.5 31 50
cm cm cm cm cm
22
Wood
Director no. 4
Director no. 3
Director no. 2
Director no. 1
Driven element
Coax
(see text)
Reflector
Tie
wire
Solder
Mast
A:102 cm
B:98 cm
C:91.5 cm
D:90 cm
E:89 cm
F:88 cm
22 Boom
Solder
Figure 19.8 Two-meter vertical beam.