Wireless communication applications have really taken off over the past two decades. In addition to voice services, wireless systems are now also handling an increasing volume of data traffic, with a demand for high transmission capacities. To provide these capacities, mobile radio cells (AMPS, GSM, PCS and UMTS) are steadily shrinking, allowing multiple uses of frequency bands, which in turn enables capacity to be increased.
In addition, new standards in new frequency bands are increasing transmission capacity and possible applications. Typical examples are the 2.4 and 5 GHz bands covering the 802.11 a-h standards. To satisfy this demand for WLAN and mobile radio (GSM/AMPS and UMTS networks) for indoor applications a vast forest of antennas would be required if they were to operate solely in the relevant frequency bands.
To minimize the number of antennas needed, Huber + Suhner set out to develop an antenna that would cover the entire frequency range from 800 MHz to 6 GHz as completely as possible. In addition to high technical performance, the aim was to get away from conventional discrete, gray, cylindrical antennas and satisfy aesthetic criteria to produce a stylish antenna. The result is the Sencity™ ART ultrabroadband antenna.
The Development Process
The starting point was the premise that an antenna operating in the frequency range of 800 to 6000 MHz, for example, corresponds to a bandwidth ratio of 1 to 7.5. Therefore, the development goal was to consistently achieve a VSWR of < 1.5 for this bandwidth ratio.
Antennas described in previous work1 achieved a VSWR of < 1.5 for a maximum frequency range of 3.75 to 11.5 GHz. This translates into a bandwidth ratio of just 1 to 3.1. It was found that a modification, especially of the upper half of the antenna, appreciably improved the lower frequency range. Some of the possible forms considered are shown in Figure 1 .
Here, if the surface area of the round disk shown with dashed lines is identical to that of the forms drawn with solid lines, then similarly, the resonance frequencies will be almost identical. The lower resonance frequency can then be approximated by the function
where
a = disk radius in cm
This equation only applies if the conductive base area has a minimum diameter of one wavelength at the lowest operating frequency. If it is reduced to below this value, the lower operating frequency will shift towards higher frequencies. Also, the size of the base area additionally influences the vertical diagrams especially at the upper end of the operating frequency band.
In the above example, the upper half of a round disk was modified. These modifications, of course, can also be applied to a horizontal or vertical elliptical disk with a ratio between the main and secondary axis of approximately 1.1 to 1.3. Any larger ratio will result in the reduction of the bandwidth.
Another important consideration is that the material of the base surface must be highly conductive. Therefore, it is preferable to use aluminum or brass to avoid further losses; its thickness should be considerably greater than the penetration depth of the skin effect. The geometry of the base surface is of secondary significance as it may be square, circular or polygonal. Circular geometries are preferable because they produce rounder horizontal diagrams. The same applies to the selection of the material and thickness of the radiator.
The height (h) of the radiator above the base surface is in the range of 0.3 to 1 mm. Its surface can also be selectively interrupted by openings, which may be circular, elliptical, square or polygonal in shape. Skillful arrangement of these apertures may improve antenna matching in certain frequency ranges of the operating band. Referring again to the figure, the monopole is fed through a coaxial connector, which is a widespread method. But the monopole can also be fed through a separate network arranged on the top or bottom side of the base surface. Such a feed network, which may also include filter structures or active elements, connects the external interface (preferably a coaxial connector) with the monopole.
The overall height of the disk antenna described in the study1 amounts to approximately 62.5 percent of the wavelength of the lower operating frequency (3.75 GHz) with a VSWR of < 1.5. Rescaled to the example of a lower operating frequency of 800 MHz, this would translate into an overall height of 234 mm - an unacceptable value for antennas used in indoor applications. Therefore, subsequent design efforts focused on reducing this value appreciably, with the solution shown in Figure 2 (only the side view is shown). The difference being that the plane surface of the monopole is now curved.
These are merely examples, but the important point is that the surfaces are curved and the specific geometry itself is of secondary significance. The radiator surface can additionally be curved in a plane perpendicular to the plane shown. In addition, it is possible to bead the surfaces, which may increase the stability of the radiator, especially if its surface strength is reduced by openings.
For a bandwidth ratio of approximately 1 to 8 at a VSWR of < 1.5, it is advantageous to observe the following dimensions:
l = 0.2 - 0.35
b = 0.02 - 0.06
c = 0.07 - 0.13
where
= wavelength of the lowest operating frequency
Any departures from these values will restrict the broadband capabilities of the antenna.
As a result, the dimensions of an antenna that cover the frequency range of 800 MHz to 6 GHz and has a VSWR of < 1.5 are
l = 95 mm, b = 16 mm, c = 35 mm, h = 0.5 mm and a = 50 mm
The radiator forms selected were Form 2 from Figure 1 and Form 2 from Figure 2, with a circular base, 200 mm in diameter.
The result is an ultrabroadband antenna where the radiation characteristics have been designed to ensure optimal coverage of a given building. Its unique broadband capabilities cover all globally used mobile radio frequencies (AMPS, GSM900/1800, PCS and UMTS), as well as the most common WLAN standards (802.11 a-h). This means it has a range of application covering all frequencies from 806 MHz through 5.9 GHz.
Styling
For this project, performance was not the only goal, as aesthetics was considered to be just as important. Normally, antenna radiators are covered by radomes with an inconspicuous and sleek appearance. One of the aims of the Sencity ART was to eliminate the need for such a radome.
Therefore, in addition to its unique electrical characteristics, the novel radiator design offers an innovative base from which to develop stylish designs. Taking into consideration that lightweight and careful selection of form and color are fundamental to the design of products for indoor applications, the geometry of the radiator has been modified so that it resembles a withering leaf, making it appear more like a work of art than a product of engineering.
Conclusion
An indoor ultrabroadband antenna has been developed for wireless applications in the frequency range of 806 MHz to 5.9 GHz. These broadband characteristics reduce the number of antennas required in a given application, slashing installation time and cutting materials and logistics requirements. In addition, with its curved and stylish form and a selection of different colors, it is a modern interior design object rather than a purely functional indoor antenna. What is certain is that whether it is installed in a modern office complex or in straightforward industrial buildings the Sencity ART will not be recognized as a WLAN or any other mobile radio antenna.
Reference
1. N.P. Agrawall, G. Kumar and K.P. Ray, "Wide-band Planar Monopole Antennas," IEEE Transactions on Antennas and Propagation , Vol. 46, No. 2, February 1998.
Huber + Suhner , Herisau, Switzerland +41 71 353 4111, e-mail: hmeier@hubersuhner.com.Circle No. 303