Directional antennas provide angle-of-arrival information, which can be used for localization and routing algorithms in wireless sensor networks. We briefly describe three classical, major types of antennas: 1) the Adcock-pair antenna, 2) the pseudo-Doppler antenna, and 3) the electronically switched parasitic element antenna. We have found the last type to be the most suitable for wireless sensor networks, and we present here the early design details and beam pattern measurements of a prototype antenna for the

2.4-GHz ISM band, the SPIDA: SICS Parasitic Interference Directional Antenna

Nilsson, Martin

English

Software institutes' Online Digital Archive

Directional Antennas for
Wireless Sensor Networks
Martin Nilsson
SICS, P.O.B.1263, SE-164 29 Kista, Sweden
from DOT adhoc09 AT drnil DOT com
Abstract—Directional antennas provide angle-of-
arrival information, which can be used for localization
and routing algorithms in wireless sensor networks.
We briefly describe three classical, major types of
antennas: 1) the Adcock-pair antenna, 2) the pseudo-
Doppler antenna, and 3) the electronically switched
parasitic element antenna. We have found the last type
to be the most suitable for wireless sensor networks,
and we present here the early design details and beam
pattern measurements of a prototype antenna for the
2.4-GHz ISM band, the SPIDA: SICS Parasitic Inter-
ference Directional Antenna .
Keywords and phrases: Smart antenna, directional,
direction finding, DF, RDF, wireless sensor network,
localization, AOA, ESPE, ESPAR, parasitic element.
I. Introduction
Antennas for wireless sensor networks (WSN) need to
satisfy a number additional of properties, besides those
that are desired from all antennas, such as the antenna
efficiency. Since there are many WSN nodes, low cost of
the antenna is important. WSN nodes are small , which
means antennas must also be small. The RF transceiver
stage must be designed to save power , and advanced signal
processing capabilities cannot generally be expected . How-
ever, for WSN, it is reasonable to assume that the antenna
can be committed to narrow-band at one of the ISM-fre-
quencies 434 MHz (wavelength λ=69 cm), 868 MHz (λ=35
cm) or 2.4 GHz (λ=12.5 cm).
In light of these considerations, we have compared three
classical, major types of antennas that appear to be espe-
cially relevant candidates. These antenna types are 1) the
Adcock-pair antenna, 2) the pseudo-Doppler antenna, and
3) the electronically switched parasitic element antenna.
We will refer to these antenna types as the AP, PD, and
ESPE antennas. We have found that the ESPE type is
especially suitable for use in WSN. We have built and
measured a prototype of such an antenna–SPIDA, for
SICS Parasitic Interference Directional Antenna–and in
this paper, we present the detailed design as well as some
measurement results.
Radio direction finding (RDF) antennas can compute
the angle of arrival (AOA) by measuring either the ampli-
tude of the incoming signal, or the phase (or both). An
advantage of measuring the phase is that it is less affected
by noise, compared to amplitude. However, measuring
phase usually requires at least two input amplifiers and
fast electronics with a highly stable timebase (cf. ultra-
wideband, UWB). Since this is difficult to achieve for a
WSN node, at least at the time of this writing, the main
emphasis here is on methods using amplitude measure-
ments.
Fig. 1. SPIDA 2.44-GHz prototype.
II. Related work
Directional antennas have been intensively studied for
many years, and all major relevant types of antennas
appear to already have been invented several decades
ago. Nevertheless, there is a flurry of suggestions in
the literature on new types of small (i.e. diameter <λ)
antennas. Despite this activity, the requirements imposed
by WSN applications as described above are satisfied by
surprisingly few classical designs. A thorough review of
electrically small antennas, and a dismissal of most of
them, is given in [12]. Considering the requirements above,
three remaining potential candidate classes of antennas
still feasible for WSN are described below.
A. The Adcock-pair Antenna
An Adcock antenna consists of a pair of parallel, vertical
dipoles, spaced close together (<λ/2) . The difference of
the signal from the two antennas indicates the angle of
arrival of an incoming signal. The lobe shape will be a
1
figure-of-eight, close to two circles if the dipoles are closely
spaced. The Adcock-pair uses two Adcock antennas at
a 90◦ angle. One pair gives the sine of the angle, while
the other gives the cosine. This method was invented
by Robert Watson-Watt, also the inventor of Radar, in
the 1930s. It was invented as an improvement over loop
antennas, frequently used for low-frequency radio direc-
tion finding (RDF) at that time.
The dipoles should not be tuned to the signal frequency,
because the lobe shape is very sensitive to this tuning.
Unfortunately, taking the difference of two closely spaced,
non-tuned, parallel elements will make the antenna rather
inefficient. It is also difficult to adjust the antenna, since
the dipoles must have very similar properties. On the other
hand, this method appears to produce the highest accu-
racy for direction finding. A good description of how to
construct a simple AP antenna can be found in [1]. A thor-
ough description of the Watson-Watt method for direction
finding can be found in [4].
B. The Pseudo-Doppler Antenna
The PD antenna was invented by Servo Corporation
in the 1940s. A PD antenna consists of a ring of dipole
antennas spaced typically λ/2 apart. In turn, each dipole
is switched to the input amplifier at a high frequency.
This produces a sine wave at the RF receiver, and the
phase of this wave will show the angle of arrival. A simple
description with constructional details is available in [2].
The minimal number of active elements of a PD antenna
is four, but it is usually used with 8 or 16 elements. Since
the spacing is half a wavelength, the antenna becomes rel-
atively large. However, due to the large aperture, the PD
antenna is robust against noise. Detailed comparisons of
PD and AP antennas can be found in [3] and [5].
C. The Electronically Switched Parasitic Element Antenna
The ESPE antenna consists of a central monopole,
surrounded by a number of monopole-like parasitic ele-
ments spaced approximately λ/4 apart. In its simplest
form, the parasitic elements are switched between ground,
when they work as reflectors , and isolation, when they
work as directors . In a more advanced form, the reac-
tance between the elements is controlled. This can be done
simply by biasing a capacitance diode with a controlled
DC voltage.
The ESPE antenna principle appears to first have been
published in 1979 [7]. This type of antenna has been dis-
cussed in detail in [6]. Recent work in this area has been
performed by teams at ATR in Japan and at Griffith Uni-
versity in Australia [8, 9]. An interesting and attractive
feature of the ESPE antenna is that the parasitic elements
are not involved in the RF chain, greatly simplifying the
impedance matching problem. The ring of parasitic ele-
ments on a ground plane can be added to a monopole,
and each parasitic element can be controlled by a micro-
processor output. When implemented by an FET, such a
switch will not consume any DC power.
In order to reduce the size of the antenna, the space
between the elements can be filled with a dielectric. With
a relative permittivity of εr , the theoretical size reduction
of the antenna is εr
√
. Descriptions of such antennas can
be found in [8, 9]. Many practical pieces of information
relevant for a 2.4 GHz antenna are given in [10]. Descrip-
tions av ESPE antennas and an extensive bibliography is
given in [11].
III. The SPIDA design
SPIDA is a type of ESPE antenna, designed primarily
for the 2.4 GHz ISM band. Distinguishing features are
that SPIDA is simple and inexpensive to manufacture,
consisting of a small circuit board and 1-mm copper wire;
and that the beam pattern in the horizontal plane well
approximates an offset circle, without any significant side-
lobes, despite an 11-dB (7 dB effective) gain difference
between 0◦ and 180◦ directions.
The SPIDA prototype consists of a small (≈3 cm in
diameter) hexagonal disc made of copper-clad FR-4 circuit
board, on which six legs made of 1-mm copper wire have
been soldered (fig. 1). An SMA-connector is mounted cen-
trally on the disc. A sensor network node can be attached
directly to this connector, eliminating the need for a feed
line.
The antenna was originally designed for the purpose of
localization, implying that the antenna gain as a func-
tion of direction should ideally approximate the direction
cosine. The simulation package Nec-2 [13] was extensively
used for designing and optimizing the antenna with this
target in mind. The 3D radiation pattern generated by
Nec-2 is shown in figure 2.
Fig. 2. Theoretical 3D gain pattern from Nec-2 simulation.
The antenna is directed along the x-axis.
Here, both the distance from the origin and the shading
indicates the gain in dB. An interesting observation in
the process was that the smoothness of the radiation dia-
gram in the horizontal plane could be improved by rather
counter-intuitively reducing the size of the ground plane
hexagonal disc.
The parts cost for SPIDA is marginal, and completely
dominated by the SMA connector. The antenna can be
built in a short time, and can be easily tuned, thanks to
the radiation elements being ordinary copper wires, which
can be bent and cut with good precision after soldering.
The antenna does not require any additional amplifiers
beyond those needed by an ordinary dipole. The parasitic
elements can be switched at low frequency, without con-
suming DC power. This SPIDA prototype was built with
the reflecting elements soldered to the ground legs, and the
directing element permanently isolated (glued), but in an
application, these elements would be connected to ground
via electronic switches. Construction details are given in
the appendix.
IV. Measurement results
The vertical gain in the horizontal plane for SPIDA has
been measured both in simulation and in a damped mea-
surement chamber. The ideal, smooth circular radiation
diagram was well achieved in simulation (fig. 3).
Fig. 3. The SPIDA horizontal beam pattern approximates an
offset circle (from simulation).
This also agreed reasonably well with the
physical measurements (fig. 4).
Fig. 4. SPIDA horizontal radiation pattern as measured in
measurement chamber.
V. Conclusions
We have reviewed plausible candidates for directional
antennas that can be used by WSN nodes. Due to its sim-
plicity and low cost, the proposed SPIDA instance of the
ESPE antenna structure appears to be an attractive can-
didate. The special structure of SPIDA produces a beam
pattern well suited to efficient signal processing. However,
the quality of measurements (i.e. RMS error in angular
measurements) is also an important consideration, which
is hard to predict theoretically. In order to investigate this,
antennas will have to be tested in their intended environ-
ments.
VI. Acknowledgments
The author is grateful to Erik Björnemo and Prof.
Anders Rydberg at the Ångström laboratory, Uppsala
University, who performed the antenna measurements in
their damped measurement chamber.
This work has been carried out within the SICS Center
for Networked Systems, funded by VINNOVA, SSF, KKS,
ABB, Ericsson, Saab Systems, TeliaSonera and T2Data.
This paper was prepared with TeXmacs.
VII. References
The URIs below were valid February, 2009.
[1] G. Andrews: The Adcock HF RDF Antenna.
(http://members.aol.com/BmgEngInc/Adcock.html)
[2] G. Andrews: Theory of the Amateur Radio Doppler.
(http://members.aol.com/BmgEngInc/Doppler.html)
[3] Servo Corporation of America: Radio Direction Finder
Antenna Systems. (http://www.servo.com/radio.html)
[4] RDF Corporation: Basics of the Watson-Watt Radio
Direction Finding Technique. Web note WN-002.
(http://www.rdfproducts.com/wn002_apl_01.pdf)
[5] RDF Corporation: A Comparison of the Watson-Watt
and Pseudo-Doppler DF Techniques. Web note WN-004.
(http://www.rdfproducts.com/wn004_apl_01.pdf)
[6] D.V. Thiel and S. Smith: Reactively controlled directive
arrays. Artec House, Inc. 2002. ISBN 1-58053-154-7.
[7] R. F. Harrington: Switched Parasitic Antennas for Cellular
Communications. IEEE Trans. Antennas and Propagation,
vol. AP-26, no. 3, pp. 390-395, May 1978.
[8] Q. Han, B. Hanna, T. Ohira: A Compact Espar Antenna
with Planar Parasitic Elements on a Dielectric Cylinder.
IEICE Trans. Commun., Vol. E88-B, No. 6, June 2005.
[9] J. Lu, D. Ireland, R. Schlub: Dielectric Embedded ESPAR
(DE-ESPAR) Antenna Array for Wireless Communica-
tions. IEEE Trans. Antennas and Propagation, Vol. 53,
No. 8, August 2005.
[10] R. Schlub, J. Lu, T. Ohira: Seven-Element Ground
Skirt Monopole ESPAR Antenna Design From a Genetic
Algorithm and the Finite Element Method. IEEE Trans.
Antennas and Propagation, Vol. 51, No. 11, November
2003.
[11] C. Sun: What is a smart antenna?
(http://www.wel.atr.jp/~sun/SmartAntennas.html)
[12] R.C. Hansen: Electrically Small, Superdirective, and Super-
conducting Antennas. John Wiley, 2006. ISBN 978-0-471-
78255-1.
[13] G.J. Burke and A.J. Poggio: Numerical Electromagnetics
Code (NEC): Method of Moments. Lawrence Livermore
Laboratories, USA. Technical Report UCID-18834. Jan-
uary 1981.
VIII. Appendix: SPIDA drawing
Figure 5 shows a drawing of the 2.44-GHz prototype
of SPIDA. The board can be made from standard copper
laminate. Double-sided is practical, but not necessary.
Wires are 1-mm copper wire. It is a good idea to make
legs a little longer, and then bend and cut to length after
everything has been soldered. The hole in the middle
is intended for an SMA male-to-coaxial connector. The
monopole is mounted as the central lead. All parasitic
elements (27mm) are ground, except one, which should
be isolated (glued).
Fig. 5. SPIDA 2.44-GHz prototype dimensions.


Directional antennas for wireless sensor networks

Swedish Institute of Computer Science Publications Database

A Compact Espar Antenna with Planar Parasitic Elements on a Dielectric Cylinder.

A Comparison of the Watson-Watt and Pseudo-Doppler DF Techniques. Web note WN-004. (http://www.rdfproducts.com/wn004_apl_01.pdf)

Basics of the Watson-Watt Radio Direction Finding Technique. Web note WN-002. (http://www.rdfproducts.com/wn002_apl_01.pdf)

Dielectric Embedded ESPAR (DE-ESPAR) Antenna Array for Wireless Communications.

Harrington: Switched Parasitic Antennas for Cellular Communications.

Reactively controlled directive arrays.

Servo Corporation of America: Radio Direction Finder Antenna Systems.

Seven-Element Ground Skirt Monopole ESPAR Antenna Design From a Genetic Algorithm and the Finite Element Method.

The Adcock HF RDF Antenna.

The URIs below were valid

Theory of the Amateur Radio Doppler.

What is a smart antenna? (http://www.wel.atr.jp/~sun/SmartAntennas.html)

Directional antennas provide angle-of-arrival information, which can be used for localization and routing algorithms in wireless sensor networks. We briefly describe three classical, major types of antennas: 1) the Adcock-pair antenna, 2) the pseudo-Doppler antenna, and 3) the electronically switched parasitic element antenna. We have found the last type to be the most suitable for wireless sensor networks, and we present here the early design details and beam pattern measurements of a prototype antenna for the 2.4-GHz ISM band, the SPIDA: SICS Parasitic Interference Directional Antenna

Digitala Vetenskapliga Arkivet - Academic Archive On-line

RISE – Research Institutes of Sweden

http://soda.swedish-ict.se/3605/1/adhoc2009_final_paper.pdf

Directional antennas for wireless sensor networks

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Software institutes' Online Digital Archive

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RISE – Research Institutes of Sweden