Very Broadband Radiating Element Eugen Arnold, Ingo Walter

EADS Deutschland GmbH, Woerthstrasse 85, 89077 Ulm /Germany, e-mail: eugen.arnold@eads.com

Abstract — An antenna is presented with an impedance

bandwidth ratio which exceeds 3:1. The dimensions of the base of the antenna are 0.22*λlow x 0.24*λlow, the antenna height is 0.12*λlow, where λlow denotes the wavelength belonging to the lower frequency limit. The antenna looks similar to a patch antenna. Actually it is an open TEM waveguide whose aperture is shunted by inductors. The lumped elements have only slide inductance. The antenna can be mounted on a metallic plane

I. INTRODUCTION

For future applications small antennas with large bandwidth are required. Bandwidth ratios of 3:1 or 5:1 are desirable, with the bandwidth ratio being defined as the ratio between the upper frequency limit and the lower frequency limit. These antennas can be used for simultaneous reception of signals over large frequency bands, simultaneous transmission of signals of several transmitters or for increased interception security by frequency hopping. Antennas with very small dimensions compared to the wavelength of the lower frequency limit are well known. Examples are spiral or sinuous antennas, but due to their efficiency of less than 50% they are not suitable as transmit antennas. On the other hand broadband antennas with good efficiency are usually not small. A well known example for a very broadband antenna is the open TEM waveguide – however this antenna is rather large. In this article an antenna is introduced which is small with a very simple construction. The antenna uses no dielectric or ferrite material.

II. THE ANTENNA AND ITS DEVELOPMENT

Starting point of the development was the open TEM waveguide over a perfect electrically conducting (pec) ground-plane. The aim was to change the design in that way, that the lower frequency limit is shifted down as far as possible. By using inductors, which were connected between the TEM waveguide and the ground-plane, this aim could be achieved successfully. The necessary inductances of the inductors are very low. They can be realized by using a low number of windings of thin wire or by printing them on a substrate. Fig. 1 shows a typical antenna and Fig. 2 shows the VSWR of the antenna as a function of frequency. The antenna was designed for the UHF-Band with a lower frequency limit of 250 MHz. It is trivial that the antenna can be scaled with frequency.

Fig. 1: View to a typical antenna under discussion.

Fig. 2: VSWR of the antenna of Fig. 1.

Beside the VSWR property, the radiation pattern as a function of frequency is a very important property. The radiation patterns are shown in Fig. 5 to Fig. 7. The dimensions of the antenna are height 0.12*λlow , length (feed to aperture) 0.22*λlow , width at the aperture 0.24*λlow. In the case of a lower frequency limit of 250 MHz, λlow is 1.2 m, the antenna height is 14.3 cm, the length (from feed to aperture) is 26.7 cm and the width is 29 cm.

The large bandwidth together with the small size of the antenna are astonishing. We can not give a valid physical explanation of this property, even not with the transmission line model shown in Fig. 3. Nevertheless we use this model, because the transmission line model makes the explanation of the antenna construction easier.

feeding point

inductor

ground-plane x y

z

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Fig. 3: Transmission line circuit diagram of the antenna.

In Fig. 3 the open TEM waveguide is represented by the transmission line parts of the model. We start at the aperture of the antenna , which is located on the left side of Fig. 3 and move step by step to the feeding point at the right side. The two resistors at the left side, which shunt the transmission line, represent the complex radiation resistance of the aperture and the inductance of the inductors. The adjacent section describes that piece of the TEM waveguide, where its width and its height are constant. This part is represented by the homogeneous transmission line section. Next to the right follows the raising part of the TEM waveguide. That part is represented by the inhomogeneous transmission line section. The characteristic impedance of this line at the aperture side is about 107 Ω and at the right side 85 Ω. At the right side of Fig. 3 is the feed of the antenna. For fine tuning of the antenna matching a small capacitance is shunted at the feed point. The impedance of the feed point should be 50 Ω. As mentioned above the transmission line model of Fig. 3 cannot describe this good matching shown in Fig. 2. Probably the transmission line model lacks the fact, that only the TEM waveguide mode is taken into consideration. It is trivial that the TEM waveguide carries several modes. A better theoretical model with more modes was not created, because models with more than one mode are rarely synoptic. To show the effect of the inductors connected to the aperture, we first show the radiation resistance of the aperture with these elements disconnected. Fig. 4a shows the model: A waveguide port feeds a small piece of the TEM-waveguide. Fig. 4b shows the complex radiation resistance plotted into the Smith chart. The reference plane is the aperture. Please notice, that the reference impedance of the diagram is Z0=120 Ω. The radiation resistance looks very inconvenient. At the lower frequency limit its value is (107.3-j181.4) Ω. Considering the Smith chart in Fig. 4b the first idea is to improve the impedance of the radiating aperture by an inductor which is connected in series to the aperture. But because no current flows at the open end of the TEM waveguide, the inductive resistor would not be effective. The second idea is, to connect an inductive resistor shunt to the aperture. Fig. 4c shows this result using the set of inductive resistors which lead to the result shown in Fig. 2. The curve of Fig. 4c looks much more convenient than the curve of Fig. 3. Especially at the lower frequencies matching of the antenna feed to 50 Ω seems to be possible.

Fig. 4a: Model with 5 inductors for calculation of the complex resistance of the aperture (Fig. 4).

Fig. 4b: Smith Chart showing the complex resistor of the aperture (inductors disconnected), Z0=107.3 Ω.

Fig. 4c: Smith Chart showing the complex resistor of the aperture (inductors connected), Z0=107.3 Ω.

III. INDUCTORS

The inductors are important parts of the antenna. They have different inductances. For frequencies at the lower frequency limit of the antenna, their inductive reactance are as following. centre inductor j 52.0 Ω, inductors adjacent j 94.5 Ω and for the outer inductors j105.0 Ω. In case of a lower frequency limit at 250 MHz

ground-plane

inductive resistor

end of open TEM waveguide at the aperture

aperture region

inductors

homogeneous transm.line

inhomogeneous transmission line

antenna feed

Z0=107 Ω Z0=107Ω Z0=85 Ω

200 MHz 300 MHz x

x 600 MHz 1000 MHz

Z0=107 Ω

Z0=107 Ω

x

x

300 MHz

600 MHz

200 MHz

1000 MHz

waveguide port

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the centre inductor must have 34 nH, each of the 2 adjacent inductors must have 60 nH and the 2 outer inductors must have 67 nH. Calculations showed that 1 winding with radius of 11 mm has an inductance of about 40 nH. Therefore no difficulties are expected using these inductors.

IV. RADIATION PATTERNS

The antenna does not belong to the class of so called frequency independent antennas. Therefore the current distribution on the antenna changes as a function of frequency. Because of the large frequency range we have to expect, that the radiation pattern changes also as a function of frequency. Fig. 5 to Fig. 7 show the function of directivity at frequencies of 250 MHz, 450 MHz and 650 MHz. Beside a 3D-diagram, the diagrams for one cut in elevation and for the 2 main cuts in azimuth are shown. For the definition of elevation and azimuth refer to the coordinate system within Fig. 1. The x-axis points to az=0°, the y-axis to az=90° and el=0° is the xy-plane.

elevation=0°

azimuth=90° azimuth=0° Fig.5: Farfield pattern (directivity) of the antenna at 250 MHz.

elevation=0°

azimuth=90° azimuth=0° Fig. 6: Farfield pattern (directivity) of the antenna at 450 MHz.

elevation=0°

azimuth=90° azimuth=0° Fig. 7: Farfield pattern (directivity) of the antenna at 650 MHz..

At 250 MHz the pattern looks similar to the pattern of a monopole above a pec-ground-plane (see Fig. 5). If the frequency approaches 450 MHz the zero of the monopole diminishes and the pattern looks similar to the pattern of a patch antenna (see Fig. 6). At frequencies above 550 MHz the pattern changes slightly to the pattern of an open TEM waveguide. For all frequencies up to 1000 MHz the pattern is very broad and covers a large part of the hemisphere surrounding the antenna.

III. A VARIATION OF THE ANTENNA

It is sometimes desirable to have an antenna with a radiation pattern which changes only slightly with frequency over a large frequency range. For this purpose the antenna shown in Fig. 8 has been designed.

Fig. 8: Variation of the antenna of Fig. 1.

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The dimensions of this antenna are height 0.123*λlow , length (feed to aperture) 0.094*λlow , width at the aperture 0.230*λlow.

The antenna in Fig. 8 is still smaller than the antenna in Fig. 1. Nearly the same inductors are used. But the impedance of the feed point is 100 Ω instead of 50 Ω. Fig. 9 shows the function of the VSWR. It looks as good as that of the 1st antenna, actually the impedance bandwidth is still larger.

Fig. 9: VSWR of the antenna of Fig. 8.

Fig. 10, Fig. 11 and Fig. 12 show the radiation patterns for frequencies of 250 MHz, 500 MHz and 800 MHz.

elevation=0°

azimuth=90° azimuth=0° Fig. 10: :Farfield pattern (directivity) of the antenna of Fig. 8 at 250 MHz.

elevation=0°

azimuth=90° azimuth=0°

Fig. 11: Farfield pattern (directivity) of the antenna at 500 MHz

elevation=0°

azimuth=90° azimuth=0°

Fig. 12: Farfield pattern (directivity) of the antenna at 800 MHz The farfield patterns show for all frequencies very

good coverage in the elevation=0° plane. The diagrams look similar to the diagram of a monopole.

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