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Making Quarter Wavelength Notch Antennas Wideband
P. J. Massey1 , K. R. Boyle1 , A. J. M. de Graauw2 , M. Udink2 , and D. L. Raynes3
1
2
NXP Semiconductors, UK
NXP Semiconductors, The Netherlands
3
Ansoft Corporation, UK
Abstract— Very compact yet efficient UWB antennas are created by utilizing the ground plane
as the antenna. There are notches in the ground plane, some to act as the coupling element(s),
and some for optimizing the antenna bandwidth. The performance of the antennas compares
favorably with wideband dipole designs occupying the same area.
DOI: 10.2529/PIERS061007111933
1. INTRODUCTION
Notch antennas have a long history of being used as compact designs, having originally been
integrated into missiles and aircraft flying surfaces [1, 2]. More recently notch antennas have been
proposed as compact structures for UWB (Ultra WideBand) applications [3–5]. Some designs
achieve high bandwidth using flared notches [3, 6], and others use relatively wide notches [4]. The
very compact structures discussed in [5] conserve the ground plane area that can be shared with
circuitry by using relatively narrow notches. This paper re-examines the structures discussed in [5],
describes the geometrical and electromagnetic features used their design, and discusses the reasons
for the exceptionally wide bandwidth achieved in their limited size.
2. FEATURES USED IN NARROW WIDTH NOTCH UWB ANTENNAS
Figure 1 illustrates how the impedance of a notch antenna is affected by ground plane dimensions,
and the principles of how a very wideband notch antenna is designed. The notch configuration
is shown on the left of the figure. The feed is 5 mm away from the short end of the notch, and
for discussion purposes, there is no dielectric substrate. The 18 mm notch length is a quarter
wavelength long at about 4 GHz.
The first Smith chart shows the response when the ground plane is very large. Increasing the
notch width from 1 mm to 2 mm would result in the loop reducing in diameter. Introducing a
dielectric substrate would both change the centre frequency of the loop, increase the loop diameter,
and move the loop downwards and to the left of the chart. When the feed is moved further away
from the notch’s shorted end, the loop in the Smith chart grows and moves to the downwards and to
right. If the feed is far enough away from the shorted end, the loop crosses the real impedance axis,
and therefore for a relatively narrow bandwidth, the response is well matched to a real impedance.
In the past notch antennas have been matched using these dielectric substrate and feed location
mechanisms.
As the Smith chart to the top right of the figure shows, providing the ground plane depth is
significantly greater than the notch length, reducing the depth of the ground plane has relatively
little effect upon the antenna response. However, as shown on the bottom right of the figure,
adjusting the width can make a huge difference to the response. Finite ground plane widths
introduce a second loop into the response curve. As this loop tends to be centered on a frequency
at which the ground plane width is close to half wavelength across, it is believed to be associated
with a dipole like response in the ground plane (the notch acting as the separation between the two
dipole elements. When the ground plane width is about twice the electrical length of the notch, the
two loops are centered on similar frequencies. This results in the impedance varying little over a
wide bandwidth, with slight reduction in reactance with increasing frequency over this bandwidth.
Fortunately, this reactance slope can easily be compensated for using a series capacitor.
The bottom left Smith chart shows the response when the antenna has been matched with a
series capacitor. This preserves the relative constancy of the impedance between 2.5 and 5.5 GHz,
and achieves a return loss of better than 10 dB between these frequencies. Beyond this frequency
range, the impedance diverges to the edges of the Smith chart.
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Figure 1: Variation in impedance with ground plane dimensions of a notch antenna.
Note that the constructive combination of the dipole and notch resonance mechanisms relies
upon the notch’s open end lying close to halfway across the width of the dipole. Offsets of around
1/6th of the ground plane width have been shown to detrimentally widen the response shown in
the figure’s lower right Smith chart.
The conference presentation includes charge and field plots that identify and illustrate the
contributions of the dipole and notch resonance mechanisms to the antenna’s behavior.
3. PRACTICAL IMPLEMENTATIONS
For clarity of presentation, the above description has used a straight notch with no substrate. In
practice, to save space, the notch can be folded into an L shape, and the ground plane’s depth can
be reduced. Also the ground plane can be a PCB that includes conventional dielectric substrates.
Reference [5] describes some examples of practical designs, including the use of two fed notches
and choke notches in achieving coverage of the FCC UWB 3.1 to 10.6 GHz band, while suppressing
coupling to the 5.5 GHz WLAN band. The choke notches act to restrict the effective width of the
ground plane in situations where the overall width of the ground plane is larger than that required
for wide bandwidth antenna operation.
ACKNOWLEDGMENT
The original work on notch antennas for consumer devices was undertaken as a joint development
between Philips Semiconductors Nijmegen and the antennas team in Redhill at Philips Research
UK. Later work specifically on UWB notch antennas was undertaken at Redhill with the support
of Philips Semiconductors’ San Jose (USA) branch. All these organizations are now part of NXP
Semiconductors.
REFERENCES
1. Johnson, W. A., “The notch aerial and some applications to aircraft radio installations,” Proc.
IEEE (London), Vol. 102, Part B, 211–218, 1955.
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2. Johnson, R. C. and H. Jasik, Antenna Engineering Handbook, McGraw-Hill, New York, 1961.
3. Foster, P., “UWB antennas issues,” Proceedings of IEE Seminar on “Ultra Wideband Communications Technologies and System Design”, 69–83, London, UK, July 2004.
4. Latif, S., L. Shafai, and S. K. Sharma, “Bandwidth enhancement and size reduction of microstrip slot Antennas,” IEEE Trans. AP, Vol. 53, No. 3, 994–1003, 2005.
5. Massey, P. J., K. R. Boyle, A. J. M. de Graauw, M. Udink, and D. L. Raynes, “Optimised
UWB notch antennas for miniaturized consumer electronics applications,” Proceedings of IET
Seminar on “Ultrawideband Systems, Technologies and Applications”, 263–267, London, UK,
April 2006.
6. Burberry, R. A., VHF and UHF Antennas, Peter Peregrinus, London, 1992.