Practical_Antenna_Handbook_0071639586

C h a p t e r 6 : D i p o l e s a n d D o u b l e t s 185
4 : 1
A
3.5 : 1
VSWR
3 : 1
2 : 1
1.5 : 1
1 : 1
D
Lower
band
edge
B
C
Design
frequency
E
Upper
band
edge
FREQUENCY
Figure 6.5 VSWR versus frequency for several cases.
comes from the fact that the frequncy per unit length varies from one band to another.
Here is a simple procedure for calculating this figure:
1. Calculate the length required for the upper band edge.
2. Calculate the length required for the lower band edge.
3. Subtract the first length from the second.
4. Determine the width of the band in kilohertz by subtracting the lower band
edge (in kilohertz) from the upper band edge.
5. Divide the bandwidth obtained in Step 4 by the difference in length obtained in
Step 3; the result is the variation in kilohertz per unit length.
Example 6.3 Calculate the frequency change per unit of length for 80 m and for 15 m.
Solution For 80 m (3.5 to 4.0 MHz in the United States):
468
1. 1.
L
UPPER
= = 117 ft
4
468
2.
2.
L
133.7 LOWER
= = ft
3.5
3. Difference in length: 133.7 ft – 117 ft = 16.7 ft