Bandwidth characteristics of folded structures with very small
normal-mode helical antennas
Katsuya Mouri, Keita Ochiyama, Naobumi Michishita, and Yoshihide Yamada
National Defense Academy; 1-10-20 Hashirimizu Yokosuka, Japan;
e-mail: yyamada@nda.ac.jp
1. Introduction
Recently, very small antennas are requested in RFID tags and tire attached sensors of the tirepressure monitoring system (TPMS). In these applications, normal-mode helical antennas(NMHA) are interested that can achieve high antenna efficiencies at very small size such as 1/100
wavelengths. Moreover, NMHA can increase antenna gains in the metal plate proximity use.
However, NMHA have serious defect of very narrow bandwidths in very small sizes. In order toexpand bandwidths, employing additional antennas are often considered.
In this paper, employment of folded structures with NMHA is studied. In studying antenna
electrical characteristics, electromagnetic simulations are conducted. As for folded structures,
antenna input impedances increases of NMHA were studied [1]. However, effects on bandwidths
were not shown. Therefore, band widths characteristics of a fundamental NMHA is studied atfirst. Then, calculated results of folded structures are shown.
D2. Fundamental antenna characteristics
aAntenna structural parameters of the NMHA is shown in Fig.1. The～VH
most important condition for efficient radiations of a NMHA is theN=5
self-resonant condition. Self-resonant structures are determined by thedfollowing equation [2]. Antenna structures of Eq. (1) and simulation
Fig.1 Structure of NMHA
are shown in Fig.2. Relations of antenna length (HA)
and diameter (DA) are determined for a given parameter
0.035
of number of terns (N).
D 2)

DH9

 20

279
N (0.920.030HH



(1)
D )2



At the self-resonant structures, bandwidths are shownby following expressions [3]. First of all, the fractional
bandwidth (FBW) is given by the equation..
(2) u ( s)   l ( s)FBW ( 0 ) (ka)CBA

N = 150.010

f0=315MHz
d=0.55 mm

0.005

0.02 0.04 0.06 0.08 0.10
H/
Fig.2 Self-resonant structures

f0＝315MHｚ2 skaHere, a indicates a radius of the sphere enclosing anantenna as shown in Fig.1. Comparisons of Qlb and Q ofNMHA are shown in Fig. 3. Q(0) are obtained atstructures of A, B and C that are shown in Fig.2. As forVSWR, s = 2 is selected.

Qlb

QOn the other hand, smallest antenna Qlb is given by thenext expression.
(4)
113

N = 100.020

Here, 0 indicates center frequency. u(s) and l(s)indicate upper and lower frequencies. s indicates theVSWR value. Next, Q factor is given by the nextequation.
10000
2 s 1,(3)Q(0 )  Qlb Simulation
Eq.(1)0.015

0FBW (0 )N=5D0.025

D/60019.7 N (AB

1000
Q(ω0)
C0.0075 0.0100 0.0125 0.0150 0.0175 0.0200 0.0225

a/λFig.3 Q factors compared with Chu limit

CFI

Q(0) of A and B becomes smaller than Qlb. The cause of small Q values is owing to the ohmic
loss of NMHA. However, Q values of exceeding 1,000 are still very large.

w

Fig.4 Folded structure
543210314.0

2.0

316.0

VSWR

30.81 MHz

215.0

4.05W

-0.2j

315.5

45.0j

1.13W

1.0315.0(a) Single NMHA

2.0j

0.5

314.5

Frequency [MHz]

1.0j

0.2

0.47MHz

50.5j

0.2jf0+ff0=315MHz

VSWR

3. Band width characteristics of folded structures
The folded structure is shown in Fig. 4. The antenna
structure of D shown in Fig.1 is used. This case, wire diameteris set to d =1 mm. The feed and parasitic antennas have theresonant frequencies of f0 and f0+f, respectively. VSWR
characteristics of single and folded NMHA are shown in Fig.5.
In Fi5(a), bandwidth at VSWR=2 is 0.47MHz. Thiscorresponds to Q = 470. The result of Fig. 5(b) shows the caseof a folded structure. The bandwidth at VSWR=2 becomes
0.81MHz. Although, f was change from zero to 0.5 MHz,achieved bandwidths could not change. By employing thefolded structure, bandwidth is expanded approximately 1.7
times.
The input impedances of single and folded NMHA are shownin Fig.6. The input resistance at the resonance is increased
approximately 4 times. This increase agree very well with thetheoretical value.
As a result, it is shown that the folded structure is useful in
expanding bandwidth and input resistance increase. However,
only the fundamental effect of the folded structure isunderstand in this study. Further studies of folded structures
are necessary.

-5.0j0314.0

314.5

315.0

315.5

316.0

Frequency [MHz]
-0.5j

-2.0j
-1.0j

Fig.6 Input impedances(b) Folded NMHA
Fig.5 Bandwidth characteristics

4. Conclusion
The effect of folded structure for expanding bandwidth is shown. Further studies are necessary
to clarify the abilities of folded structures.
References[1] W.H.Jung, Y.Yamada and N. Michishita, “ Electrical Performances of a Folded Structureswith Very Small Normal Mode Helical Antennas”,Tr. on the Comm. Soc. of IEICE, Vol. J90-B,No.9, pp.906-910, 2007
[2] Nguyen Quoc Dinh, Naobumi Michishita, Yoshihide Yamada and Koji Nakatani. “DeterministicEquation for Self-Resonant Structures of Very Small Normal-Mode Helical Antennas ,” IEICE Trans. onCommun., Vol.E94-B, No.5, pp.1276-1279, May2011[3] J.L.Volakis, ANTENNA ENGINEERING HANDBOOK (Fourth Edition), McGraw-Hill
Companies, Ch.6, pp.6-11/6-16, 2007