WO2009050325A1

WO2009050325A1 - Reflectionless lens

Info

Publication number: WO2009050325A1
Application number: PCT/FI2008/000109
Authority: WO
Inventors: Olli Luukkonen; Pekka Alitalo; Sergei A. Tretyakov; Joni Vehmas; Liisi JYLHÄ; Jukka Venermo
Original assignee: Aalto Korkeakoulusaatio sr
Current assignee: Aalto Korkeakoulusaatio sr
Priority date: 2007-10-16
Filing date: 2008-10-13
Publication date: 2009-04-23
Anticipated expiration: 2010-04-16
Also published as: WO2009050325A8; FI20070774A7; FI20070774A0; FI20070774L

Abstract

The invention is a novel way of building electromagnetic lenses that enables us to match the lenses with free space and minimize the power losses due to reflections. The electromagnetic lens comprises transmission-line networks and a separate matching layer. The networks can be designed to have the characteristic impedance which equals to the free space wave impedance in a certain frequency band. The matching layer ensures the power transmission between free space and the network and thus minimizes the reflections from this new type of lens. The phase velocity of the electromagnetic wave traveling in the transmission-line networks can be varied by changing the design parameters of the network (cf. refractive index).

Description

REFLECTIONLESS LENS

1. For which purpose the invention has been invented

There are many applications for lenses in the field of radio technology. In antennas lenses are used to change the direction of the electromagnetic beams in terms of geometrical optics. The change in the direction of the beam happens when the beam is refracted from the interface of two media. In measurement setups lenses can be used to focus incident field to a wanted area.

Commonly lenses have been manufactured from conventional or artificial dielectric materials. The artificial dielectric materials are as a rule composed of periodical metallic or dielectric components, which effectively behave as an uniform dielectric medium when the wavelength of the incident field is much larger than the period of the structure. References [1] and [2] are patents for particular type of artificial dielectric materials. These materials can be used, for instance, to reduce the weight or the wind load of antennas. In reference [3] an artificial dielectric material composed of thin metallic strips is used to enhance directivity of a base station antenna. In addition to refraction, also reflection occurs on the interface of each dielectric lens. Conventional lenses have therefore been manufactured from a low-loss dielectric material, that has a low dielectric constant, such as teflon ( ε r = 2.06). Depending on the distance between the two interfaces, 0 - 12 % of the incident power is reflected from _λa lens that has been manufactured from teflon [4]. The minimums in the reflected power are due to the Fabry-Perot resonance. Other possible types of lenses are multicellular lens and metal plate lens. Multicellular lens has been described in [5] and examples of metal plate lenses are available in [4] and [6].

The reflectionless lens operates as the conventional lenses. By changing the shape and the effective refraction index of the lens, the properties of the lens can be changed to fit the particular application in hand. In addition to this, the reflectionless lens is matched to free space on a broad frequency band and the reflected power is minimized.

2. How the corresponding problem has been solved before

Earlier reflections from dielectric lenses have been minimized by using quarter wavelength matching layers. With these matching layers the matching of the lens to free space is achieved only for a particular frequency and hence the bandwidth of such matched lenses is minimal [4]. Instead of having just one dielectric layer, it is possible to use more layers with different dielectric constants [7]. The benefit of this is impedance matching with free space on a wider frequency band.

3. How does the invention work?

The reflectionless comprises three components: 1. Transition layer 2. Transmission-line network 3. Supporting structure The transition layer is used to connect the transmission-line network to free space. This transition layer is composed of an antenna array, for instance, that couples the incident power to the transmission-line network. The impedance of the transmission-line network itself can be matched with surrounding medium. This is why the back scattering is minimized. For example ordinary window glass does have back scattering. This can be seen as mirror reflections from the window.

The transmission-line network can be designed to be two- or three-dimensional, depending on the application. For some applications the network can be simplified and the third dimension is not needed. The three dimensional realization corresponds to three dimensional transmission-line network, which has three di- mensional connections. Two dimensional network has connections in a plane. The transmission-line network is constructed in that manor that it is matched with the surrounding material over a broad bandwidth. The properties of the network can be changed so that it effectively would correspond to a conventional dielectric lens with a given dielectric constant.

The supporting structure is placed inside the transmission-line network and does not have any effect on the operation of the lens. The purpose of the supporting structure is to hold the transmission-line network and the transition layer in place.

It is also possible to embed a source or multiple sources inside the lens, so that the lens creates a wanted electromagnetic field distribution, such as a plane wave from the embedded source. The sources can-be realized e.g. by voltage sources that are connected to the transmission-line network. The feeding can be easily realized since the structure of the lens allows the insertion of feeding cables between the transmission lines of the network. Instead of sources also detectors can be embedded into the network. In this case the lens would operate as a receiver.

4. Could the invention be applied in some other field of technology?

The invention can be utilized in every application in which the back reflections have to be minimized and the incident field needs to be refracted.

5. Provide the results with some experiments or experimental material

The properties of the reflectionless lens relate closely to those of the invisibility cloak. Therefore the results shown here offer only the proof-of -concept for the properties which are claimed in this disclosure, that is, the refractive properties equal to those of the reference lens and reflection coefficient for the proposed lens is minimized with respect to the reference lens.

The operation of the reflectionless lens is verified by simulations. In order to proof the operation of the invention we have simulated the proposed reflectionless lens and a conventional dielectric lens, and calculated the reflection constant from these simulations. The refraction index of the reflectionless lens equals to that of the dielectric lens. The dimensions of the lenses are the same.

In Fig. 1 the reflectionless lens is designed so that its refraction index equals to that of a dielectric lens with a dielectric constant ε r = 2. In Fig. 1 the magnitude of the electric field components is shown. Similar is done for the dielectric lens in Fig. 2. The calculated reflection constants are shown in Table 1. It can be concluded from the results that the reflectionless lens indeed reduces the back scattering as compared to the dielectric lens.

To prove the feasibility of the invention we have designed another lens whose refraction index equals to that of a dielectric material having ε r = 4.66. In the case of reflectionless lens the magnitude of the electric field components at 3.6 GHz is shown in Fig. 3. In the reference case the dielectric constant of the conventional lens is ε r = 4.66. The magnitude of the electric field components for

Table 1: The results of the HFSS-simulations for the reflectionless and dielectric lens when the dielectric constant of the reference lens is ε r = 2.

Reflectionless lens Dielectric lens

SI l (@ 5.5 GHz) -14.0 dB -11.6 dB

the reference case is shown in Fig. 4. The reflection coefficients calculated from the simulations are shown in Table 2. In order to prove the large bandwidth of the invention the reflection coefficients have been calculated at 4.0 GHz as well. Table 2: The results of the HFSS-simulations for the reflectionless and dielectric lens when the dielectric constant of the reference lens is ε r = 4.66.

Reflectionless lens Dielectric lens

SIl (@ 3.6 GHz) -13.1 dB -6.1 dB

SI l (@ 4.0 GHz) -8.6 dB -6.2 dB

6. Do you have any obstacles for implementation of the invention

- Any state-of-the-art patents or existing publications which may totally or partially prevent or interfere the patenting process

7. Other

-Strategic and economical aspects

Figures :

Figure 1: The magnitude of the electric field components in the case of reflec- tionless lens at 5.5 GHz.

Figure 2: The magnitude of the electric field components in the reference case of 1. The dielectric constant of the lens is ε r = 2 and the frequency of operation is 5.5 GHz.

Figure 3: The magnitude of the electric field components in the case of reflec- tionless lens at 3.6 GHz.

Figure 4: The magnitude of the electric field components in the reference case of 3. The dielectric constant of the lens is ε r = 4.66 and the frequency of operation is 3.6 GHz.

References

[1] R. E. Diaz, Method for making a material with artificial dielectric constant,

US Patent 5,385,623, 1995. [2] J. D. Lilly, D. T. Auckland, and W. E. McKinzie JH, Reduced weight artificial dielectric antennas and method for providing the same, US Patent

6,075,485, 2000.

[3] P. Ikonen, C. Simovski, and S. Tretyakov, Compact directive antennas with a wire-medium artificial lens, Microwave and Optical Technology Letters, Vol. 43, No. 6, pp. 467 - 469, 2004.

[4] A. Lehto, ja A. Raisanen, Millimetriaaltotekniikka, Otatieto Oy, Helsinki,

1997.

[5] I. Lindell, ja K. Nikoskinen, Antenniteoria, Otatieto Oy, Helsinki, 1995.

[6] F. Galle, G. Landrac, and M. M. Ney, Artificial lens for third-generation automotive radar antenna at millimetre- wave frequencies, Antennas and

Propagation, IEE Proceedings, 2003.

[7] T.-K. Wu, Two layer matching dielectrics for radomes and lenses for wide angles of incidence, US Patent 5,017,939, 1991.

Claims

1. Reflectionless lens, characterized in that it encompasses the following layers and/or parts: transition layer, transmission-line network, and support structures.

2. Reflectionless lens according to claim 1 , characterized in that the transition layer comprises radiating elements, advantageously antennas, in order to couple the impinging electromagnetic radiation to the transmission-line network.

3. Reflectionless lens according to claims 1 or 2, characterized in that the transmission-line network is advantageously two- or three-dimensional.

4. Reflectionless lens according to one of the above claims, characterized in that the transmission-line network is advantageously one-dimensional .

5. Reflectionless lens according to one of the above claims, characterized in that the transmission-line network operates as the support structure.

6. Reflectionless lens according to one of the above claims, characterized in that the efective index of refraction of the lens is achieved by filling the transmission-line network with a dielectric or magnetic material.

7. Rellectionless lens according to one of the above claims, characterized in that the effective index of refraction of the lens is achieved by loading the transmission-line network e.g. with capacitive or inductive loads.

8. Reflectionless lens according to one of the above claims, characterized in that the effective index of refraction of the lens is advantageously electrically or magnetically controllable by loading the transmission-line network by electrically controllable elements, such as varactors, or by magnetically controllable elements.

9. Reflectionless lens according to one of the above claims, characterized in that the effective index of refraction of the lens is inhomogeneous by loading the transmission-line network e.g. with capacitive, inductive or electrically controllable loads, or by filling the transmission-line network inhomogeneously with dielectric or magnetic materials.

10. Reflectionless lens according to one of the above claims, characterized in that inside the lens, there is an embedded source or multiple embedded sources, from which the lens creates a wanted electromagnetic field distribution, such as a plane wave.

1 1. Reflectionless lens according to one of the above claims, characterized in that inside the lens, there is an embedded detector or detectors, which are used to measure the incoming radiation.