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A Compact Printed Spiral FM Antenna
Abraham Loutridis1,2,Kansheng Yang1,2,Matthias John2,and Max Ammann1
1Antenna & HF Research Centre, Dublin Institute of Technology, Dublin 8, Ireland
2CTVR — The Telecommunications Research Centre, Trinity College Dublin, Dublin 2, Ireland
Abstract—In this work,a compact printed spiral monopole antenna operating in the FM
frequency band (88–108 MHz) is reported. The antenna is printed on a 100mm ×50 mm PCB
layer providing more than 20 MHz bandwidth at −2 dB threshold and is easily fabricated with
low manufacturing cost. The antenna was also measured and simulated on 900 mm ×900 mm
ground plane which is representative of a vehicle roof
1. INTRODUCTION
Electrically Small Antennas (ESAs) are desired and essential for many applications and especially
at lower frequencies such as in the HF and VHF bands. Nowadays, compact antennas have become
standard for radio receivers on vehicles and mobile terminals which lead to new requirements for
small, efficient and low cost designs. Good performance of a radio receiver is heavily depended
on the antenna performance. A variety of FM antenna types have been reported for automo-
tive and portable applications including, active [1] and short meander line monopoles [2], fractal
Hilbert curve antenna [3], chip antennas [4], window-printed active antennas [5] and the shark type
antennas [6].
The frequency range for the FM radio band which defined from FCC regulation is from 88MHz
to 108 MHz and the respective wavelength λ0for the centre frequency f0= 98 MHz is around 3
meters. The height of a quarter wavelength FM monopole antenna is around 750mm. In order
to reduce the size of the monopoles, helix antennas with a height of 80 mm are used for FM radio
receivers. In this paper a compact spiraled monopole antenna is reported with an overall volume
of 100 mm ×50 mm ×1.5 mm. A five element network matching circuit is embedded in orderto
increase the bandwidth to more than 20 MHz. The antenna can be easily integrated into compact
volumes, is low cost and easily fabricated.
2. COMPACT SPIRAL ANTENNA
The antenna was designed and printed on a 100 mm ×50 mm double sided FR-4 substrate (εr= 4.3,
tan δ= 0.025, thickness = 1.5 mm) with copper metallization thickness of 0.035 mm (Figure 1(a)).
The 67.9 mm ×50 mm metal ground plane is on the rear of the same PCB board. The antenna
is fed by a 50 Ω microstrip line of 3 mm width which is connected to a SMA connector. The
miniaturization is based on its spiral structure occupying a 33 mm (0.01λ0)×50 mm (0.02λ0) area
on the PCB board (Figure 1(c)). In order to improve the antenna bandwidth, a five element π-
matching network circuit (Figure 1(b)) is added so that the impedance bandwidth of antenna can
be wider, covering the frequency range of 31.5 MHz bandwidth at VSWR 8.5 : 1 (Figure 2). As
shown in Figure 1(b) the matching network consists of two parallel capacitors of 68 pF and 82 pF,
two serial inductors of 100nH and 1.5 nH and a parallel inductor of 68 nH.
3. SIMULATED & MEASURED RESULTS
Figure 2 illustrates the measured S11 results of the final model and the simulated S11 results of the
antenna with and without the matching network. The measurements were obtained using a Rohde &
Schwarz ZVA 40 vector network analyzer. For the simulation model without the matching network
the antenna resonates at 102.9 MHz with a −2 dB impedance bandwidth of 6.3 MHz. For the
antenna with the matching network the simulation results indicate a −2dB impedance bandwidth
of 22.3 MHz (86.2–108.5 MHz), while for the measured design the impedance bandwidth at −2 dB
threshold is 31.5 MHz (76.8–108.3 MHz).
The antenna was also measured and simulated on 900 mm ×900 mm ground plane which is
representative of a vehicle roof (Figure 3). The spiral monopole (33 mm ×50 mm) is located on
a metallic ground plane while the other part of the antenna with the microstrip line is located
2
(a)
(b)
(c)
Microstrip Line
SMA connector
Z
X
Y
Figure 1: (a) Antenna general view, (b) π-matching network, (c) spiral monopole.
Figure 2: Simulated and measured S11 results.
below it. In Figure 4 the simulated and measured results are depicted. The antenna resonates at
91.5 MHz with a −2 dB bandwidth of 30.5 MHz (77.3–107.8 MHz) which is compared to the results
in Figure 2. The simulated results provide a −2 dB bandwidth of 18 MHz (83–101 MHz).
In Figure 5 the simulated z-xand x-yplane radiation patterns at 98 MHz are illustrated.
3
900 mm
900 mm
GND
Monopole
33 mm
50 mm
Figure 3: The FM antenna on large ground plane.
Figure 4: Simulated and measured results with the
antenna on a large ground plane.
Figure 5: The simulated Phi (ϕ) components for
the zx and xy-plane at 98 MHz.
The patterns are illustrated both in 10 dB/division scaled plot. The Phi (ϕ) component provides
omnidirectional characteristics in both planes. The simulated maximum realized gain and total
efficiency is −24.9 dBi and 2% at 98 MHz respectively.
4. CONCLUSIONS
In this work, a compact spiral FM monopole antenna is described. The proposed antenna is
an electrically small antenna (<0.037λ0) offering a 32% fractional bandwidth (−2 dB) over the
centre frequency (f0= 98 MHz) and covers the whole FM frequency band (88 MHz–108 MHz).
The antenna is low cost, easily fabricated with omnidirectional radiation characteristics suitable
to embed into housing. The antenna still operates with a decent wide bandwidth on large sized
ground plane which makes it usable for automotive and vehicle applications.
REFERENCES
1. Negut, A., L. Reiter, J. Hopf, and S. Lindenmeier, “Performance of a 20 cm short active
AM/FM monopole antenna for automotive application,” IEEE European Conference on An-
tennas and Propagation (EuCAP), 2009.
2. Perri, E. B. and S. Forcellini, “Very short meander monopole antennas,” IEEE Antennas and
Propagation Society International Symposium AP-S, 2008.
3. Borja, C., J. Anguera, C. Puente, and J. Verg´es, “How much can be reduced the internal
FM antenna of mobiles phones,” IEEE European Conference on Antennas and Propagation
(EuCAP), 2010.
4. Song, S.-M., L. Jin, Y. Zheng, and G.-Q. Yang, “A miniaturized FM chip antennas for handset
devices,” International Workshop on Microwave and Millimeter Wave Circuits and System
Technology (MMWCST), 2012.
5. Schaffner, J. H., H. J. Song, A. Bekaryan, H.-P. Hsu, M. Wisnewski, and J. Graham, “The
impact of vehicle structural components on radiation patterns of a window glass embedded
FM antenna,” IEEE Transactions on Antennas and Propagation, Vol. 59, No. 10, 3536–3543,
2011.
6. Chang, D.-C., F.-Y. Lin, B.-H. Zeng, and J. Chen, “Compact size antenna for car FM radio,”
PIERS Proceedings, 223–224, Taipei, Mar. 25–28, 2013.
