Physical Principles of RFID Systems - RFID: Fundamentals and Applications in Contactless Smart Cards and Identification - page 124

l = l /2

R s = 73 Ω

l = l /2

d 2
d 1

D

R s = 240 ... 280 Ω

l = l /2

d 2
d 1
d 2

D

D

R s = 540 ... 2000 Ω

Figure 4.67 Different dipole antenna designs — from top to bottom: simple extended dipole,
2-wire folded dipole, 3-wire folded dipole

the single λ/ 2 dipole ( R r = 240 - 280 ). According to Rothammel (1981) the follow-
ing relationship applies:

lg 4 D 2
d 1 · d 2

2

lg 2 D
d 2

R r = 73 . 2 · ( 4 . 91 )

A special variant of the loop dipole is the 3-wire folded dipole. The radiation
resistance of the 3-wire folded dipole is greatly dependent upon the conductor diameter
and the distance between the λ/ 2 line sections. In practice, the radiation resistance of
the 3-wire folded dipole takes on values of 540 - 2000 . According to Rothammel
(1981) the following relationship applies:

lg 4 D 3
d 1 · d 2

2

lg D
d 2

R r = 73 . 2 · ( 4 . 92 )

The bandwidth of a dipole can be influenced by the ratio of the diameter of the λ/ 2
line section to its length, increasing as the diameter increases. However, the dipole must
then be shortened somewhat in order to allow it to resonate at the desired frequency.
In practice, the shortening factor is around 0.90 - 0.99. For a more precise calculation