A Half-Wave Hula-Hoop Antenna for GSM Mobile Applications

A Half-Wave Hula-Hoop Antenna
for GSM Mobile Applications
Here is a low-profile antenna option for applications requiring
low visibility, durability and a uniform radiation pattern
V. Stoiljkovic, A. D. Spencer and G. Wilson
Centurion International, Inc.
his article describes an investigation into
the input impedance bandwidth and radiation characteristics of a half-wave hulahoop antenna. The antenna is designed to work
in the GSM frequency band of 860-960 MHz.
Theoretical and experimental results are presented, showing the antenna’s capability of producing nearly isotropic radiation patterns. The
basic antenna structure is metallic and does not
involve any dielectric materials, enabling an
inexpensive and easy-to-construct low-profile
antenna design.
Low-profile antennas have been used in specialized and covert applications in Professional
Mobile Radio (PMR) for many years. They were
initially designed to satisfy the need for almost
indestructible antennas. The aesthetic appeal of
low-profile designs and their durability are their
main marketing advantages. On the other hand,
GSM is increasingly employed in mobile location services, especially tracking and remote
services. Tracking services, for example, are
used to locate and monitor cargoes, and remote
services are used for long-distance control of
vehicle functions, etc. most of these applications
require robust low-profile antennas with an
omni-directional radiation pattern. This article
introduces one such design, which is inexpensive to produce and has good RF performance.
T
Structure of the antenna
The hula-hoop antenna, also known as a
directional-discontinuity ring-radiator (DDRR)
gained a lot of attention in the early 1960s [1-3].
The original version consists of a quarter-wavelength long piece of wire bent into a circular
shape over a conductive ground plane, as illustrated in Figure 1a. The position of the loop is
42 · APPLIED MICROWAVE & WIRELESS
d=λ/4π
φ
h
feedpoint
(a)
d=λ/2π
φ
h
feedpoint
(b)
▲ Figure 1. Hula-hoop antennas: (a) the quarterwave loop, and (b) the half-wave loop.
parallel and close to the ground plane. The loop
is driven at one of its ends and open at the other.
The antenna resonant frequency can be lowered
by capacitive tuning at the open end. The basic
idea of this design is to replace a quarter-wave
monopole antenna with a shorter but wider
radiating structure with similar electrical characteristics. The quarter-wave DDRR has, however, been proven to have the input impedance
bandwidth of only one to two percent [4].
In order to increase the input impedance
bandwidth, a closed half-wave loop structure
has been proposed (Figure 1b) [4]. A half-wavelength long piece of wire is bent into a circle and
short-circuited to a ground plane at one point.
Loop
Loop
Resonant
Bandwidth
Diameter (mm) Height (mm) Frequency (MHz)
(MHz)
50
50
50
50
60
60
60
60
60
70
70
70
70
70
80
80
80
80
80
30
35
40
45
25
30
35
40
45
25
30
35
40
45
25
30
35
40
45
1050
975
910
855
1040
960
900
840
795
940
880
825
775
735
870
760
760
720
685
90
85
70
60
120
105
80
70
65
50
65
70
60
60
45
50
55
60
60
▲ Table 1. Antenna resonant frequency and bandwidth as a
function of loop diameter and height.
▲ Figure 3. Radiation pattern (azimuth) at 925 MHz.
Triangles are vertical polarization, squares are horizontal
polarization and diamonds are the reference horn antenna.
The antenna is fed by a coaxial cable at another point.
The position of the feed point is usually 100° to 140°
away from the short-circuit point. By changing the original quarter-wave long, open hula-hoop design into a
half-wave long closed-loop design, a noticeable improvement in the input bandwidth can be achieved.
The antenna was modeled using WIPL [5]. Initial
numerical results showed that the antenna impedance
bandwidth was strongly dependent on the size of the
ground plane. For practical reasons, the ground plane
was chosen to be circular with a diameter of 240 mm.
The loop diameter and its height above the ground
plane were then changed and the antenna resonant frequency and input impedance bandwidth were calculated. The numerical results are shown in Table 1. As
expected, the resonant frequency decreases with an
increase of both loop diameter and its height above the
ground plane.
In order to cover the whole of the GSM band (890-960
MHz), the loop diameter was chosen to be 60 mm and its
height was chosen to be somewhere between 30 and 35
mm. The final value for the height was to determined
experimentally.
Antenna electrical characteristics
▲ Figure 2. Measured antenna input reflection coefficient
vs. frequency.
44 · APPLIED MICROWAVE & WIRELESS
A prototype antenna was built and tested. the antenna’s input reflection coefficient as a function of frequency is shown in Figure 2. The bandwidth (–10 dB) is about
90 MHz.
The antenna radiation patterns in the azimuth and
elevation planes are shown in Figures 3 and 4, respectively. The antenna has an almost omni-directional radiation pattern. The peak gain is 0.2 dBi in the azimuth
plane and 2.0 dBi in the elevation plane. The gain was
measured using a double-ridged horn as a reference
antenna. The horn peak gain was 6.3 dBi. The fair
amount of cross-polarization in the elevation plane is
considered to be beneficial for applications involving
multipath fading.
Conclusion
A half-wave hula-hoop antenna has been described.
The antenna operation was analyzed both theoretically
and experimentally. It has been shown that the antenna
has an almost omni-directional radiation pattern. The
antenna also has a low profile and is inexpensive and
easy to manufacture. The design presented here is suitable for GSM applications.
■
References
▲ Figure 4. Radiation pattern (elevation) at 925 MHz.
Triangles are vertical polarization, squares are horizontal
polarization and diamonds are the reference horn antenna.
1. J. M. Boyer, “Hula-Hoop Antenna: A Coming
Trend?” Electronics, Vol. 11, Jan. 1963, pp. 44-46.
2. R. W. Burton and R. W. P. King, “Theoretical
Considerations and Experimental Results for the HulaHoop Antenna,” Microwave Journal, Nov. 1963, pp. 8990.
3. S. Egashira and J. Iwashige, “Analysis of HulaHoop Antenna and Consideration of its Radiation
Resistance,” IEEE Trans. on Antennas and
Propagation, Vol. AP-23, Sep. 1975, pp. 709-713.
4. M. Boella, C. Cugiani, A. Villa and R. Zich, “ThinWire Loop Antennas,” Electronics Letters, Vol. 1, Sep.
1965, pp. 183-184.
5. B. M. Kolundzija, J. S. Ognjanovic, T K. Sarkar and
R. F. Harrington, WIPL: Electromagnetic Modeling of
Composite Wire and Plate Structures: Software and
User’s Manual, Artech House, 1995.
Author Information
Vladimir Stoiljkovic is Lead Engineer, and A. D.
Spencer and G. Wilson are members of the engineering
staff at Centurion International, Inc., 2A, Alton Business
Park, Gatehouse Way, Aylesbury, Buckinghamshire, HP
19 3XU, UK; by fax at +44 1296 339 808; or by e-mail at:
vladimirs@centurion-int.co.uk
Wanted: Your Ideas!
A good idea doesn’t have to be
the size of a major technical paper!
Short, concise articles that detail
a single technical topic are very
important to your fellow engineers.
Send us that “clever idea” you
have developed — to at the address
at the bottom of page 8!
46 · APPLIED MICROWAVE & WIRELESS