THE THICK LINEAR ANTENNA

The thin linear antenna is frequency-sensitive. In practical communication scenarios, the transponders use wideband sig­nals to increase the channel capacity and therefore needs an­tennas that can handle a large band of frequencies. One way of increasing the bandwidth is to use electrically thick wires.

A thick cylindrical dipole (Fig. 16) is the inexpensive way to increase the bandwidth of linear antennas. The increase in thickness leads to circumferential component Ц of the other­wise linear current. This can be handled with the integral equation formulation. Figure 17(a) and Fig. 17(b), respec­tively, show the variation of input resistance and reactance of the dipole with l/2a ratios 25 (thick), 50, and 10,000 (thin), where 2a is the diameter of the wire.

10"

Near fields:

H<p component at z = 1^5 m Radiated power = 8J75 mW at f = 1 MHz

10"

E

<

33

о

ф

MININEC

. „ NEC

Antenna:

Height = h = 75 m Radius = a = 0^3 m 15-Segment model

100 200 300 400

Radial distance (m)

500

MININEC compared to NEC

4

10c

10-

І10-

>10-

10-4

Antenna:

Height = h = 75 m Radius = a = 0.3 m 15-Segment model

Antenna: z – Component at wire surface Radiated power =

8.775 mW at f = 1 MHz

THE THICK LINEAR ANTENNA

Near fields:

3

Vertical and radial components of E-fields z = 1.5 m

Power radiated = 8.775 mW f = 1 MHz

2

Antenna:

ф

CL

Height = h = 75 m Er Radius = a = 0.3 m

4 15-Segment model

Ez

0

50 100 150 200 250 300 350

Radial distance (m)

0

400 450 500

Figure 11. Percentage difference in electric fields Ez and Er versus radial distance.

THE THICK LINEAR ANTENNA

0

144

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