Practical_Antenna_Handbook_0071639586

268 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s
A
B
/2
/4 /4
L 1 L 2
L 1 L 2
L 3
To transmitter
Figure 11.2A Feeding a phased array antenna in phase.
Its effect is to cause a line that is electrically l/2 to be much shorter than l/2 physically.
In particular:
L = v L
(11.1)
PHYSICAL
F
ELECTRICAL
Example 11.1 Assuming the use of standard RG-8U coaxial cable with a velocity factor
of 0.67, a half-wavelength section of cable for 3.6 MHz is
vF
Ll (feet) = 492 = 91.57 ft = 91 ft 7 in (11.2)
2 PHY F (MHz)
If we are interested only in the end-fire pattern of Fig. 11.1B, we can feed the array
as shown in Fig. 11.2B, where it is understood that the lengths shown for the segments
of coaxial cable are electrical lengths, not physical lengths. If, instead, we wish to be able
to switch back and forth easily between broadside and end-fed modes, we can use the
circuit of Fig. 11.3. Here two convenient, but equal, lengths of coaxial cable (L 1 and L 2 )
are used to carry RF power to the antennas. One segment (L 1 ) is fed directly from the