Loop Antenna

A loop antenna has simple structure but its analysis is not easy to perform. Since a loop antenna is a dual pair of a dipole antenna, we can adopt the analysis of a dipole for a loop based on the duality theorem. By stacking a number of loops, we can increase the antenna gain and radiation resistance very easily.

1. Loop Antenna
Antenna EngineeringAntenna Engineering
CHO, Yong HeuiCHO, Yong Heui

2. Antenna EngineeringAntenna Engineering
EM Wave LabEM Wave Lab2
Small loop antenna
1. Loop antenna
Duality theorem
Electric dipole antenna

JEjH
MHjE
+=´Ñ
--=´Ñ
we
wm


3. Antenna EngineeringAntenna Engineering
EM Wave LabEM Wave Lab3
Magnetic dipole moment
1. Loop antenna
Electric dipole moment vs. magnetic
dipole moment

nISmdQp ˆ=Û=
zIzI
m
DÛD
( ) ( )zQ
dt
d
zQ
dt
d
m
DÛD
mpmÛ HMEP
me
cce =Û=
0

4. Antenna EngineeringAntenna Engineering
EM Wave LabEM Wave Lab4
Near field components
1. Loop antenna
Constant current
Duality theorem:

ISjzI
m
wm=D
fq
p
ˆ
sin
1
4
ú
û
ù
ê
ë
é
+
D
-=
-
r
jk
r
zeI
E
jkr
m
qq
ph
q
ph
ˆ
sin
1
4
ˆcos1
2
2
2
ú
û
ù
ê
ë
é
-+
D
+
ú
û
ù
ê
ë
é
-
D
=
-
-
kr
j
r
jk
r
zeI
r
kr
j
r
zeI
H
jkr
m
jkr
m

5. Antenna EngineeringAntenna Engineering
EM Wave LabEM Wave Lab5
Far field components
1. Loop antenna
Observation point: infinity
fq
p
ˆ
sin
4
jk
r
zeI
E
jkr
m
-
D
-=
qq
ph
ˆ
sin
4
jk
r
zeI
H
jkr
m
-
D
=
[ ]
*
Re
2
1
HES ´=

6. Antenna EngineeringAntenna Engineering
EM Wave LabEM Wave Lab6
Radiation power and resistance
1. Loop antenna
Effective antenna: # of turns N
 ( )
rr
RIISkP
222
2
2
1
12
==
p
h
( )
2
2
3
2
2
3
8
6
÷
ø
ö
ç
è
æ
==
l
h
p
p
h S
SkR
r
Single turn
( )
2
2
3
2
2
3
8
6
÷
ø
ö
ç
è
æ
==
l
h
p
p
h NS
NSkR
r
Multiple turns

7. Antenna EngineeringAntenna Engineering
EM Wave LabEM Wave Lab7
Loop-stick antenna
1. Loop antenna
Ferrite core and multiple turns
Large radiation resistance
AM receiver


8. Antenna EngineeringAntenna Engineering
EM Wave LabEM Wave Lab8
One-wavelength loop antenna
1. Loop antenna
Continuous current distribution
-Not uniform current
-Compared with standing wave antenna
-Traveling wave antenna
Square loop antenna: gain = 3.09 dB

lde
r
lI
A
rrkj
ò
¢-·
=
loop
)(
4
)(
p
m

9. Antenna EngineeringAntenna Engineering
EM Wave LabEM Wave Lab9
Crystal loop antenna
1. Loop antenna
Efficient antenna: multiple turns
Direction of current, feeding point


10. Antenna EngineeringAntenna Engineering
EM Wave LabEM Wave Lab10
Circular loop antenna
1. Loop antenna
Gamma matching: variation of T-matching
Tuning: capacitor, resonant frequency

Gamma matching part
Tuning part

11. Antenna EngineeringAntenna Engineering
EM Wave LabEM Wave Lab11
Gamma matching
1. Loop antenna
Folded dipole antenna
Capacitor needed


12. Antenna EngineeringAntenna Engineering
EM Wave LabEM Wave Lab12
Tuning: capacitor
1. Loop antenna
