The invention relates to a directional coupler configured to providing a forward and/or backward signal derived from a high frequency signal.

Directional couplers are passive devices used in high frequency applications. They are used to derive a defined amount of electromagnetic signal power from a main transmission line, which transports a high frequency signal, to a measuring port for analyzing purposes in another circuit. They are constructed from two coupled lines arranged close enough such that the signal

transported through the main line is coupled to the coupled line.

Directional couplers are used in a huge amount of

applications including: Providing a signal sample for measurement or monitoring or feedback or combining feeds to and from antenna or antenna beam forming or providing taps for cable distributed systems or separating

transmitted and received signals or identifying

mismatching circuits or identifying structural damages in the transmission line and so on.

Patent application DE 10 2010 009 227 Al describes a directional coupler for measuring power of a forward and/or backward high frequency signal in a coaxial waveguide. This directional coupler comprises a voltage divider coupled to the main line. Since the voltage divider is built with resistors, the directional coupler can be driven with high current loads without using impedance converters . To compensate the frequency response characteristics of a directional coupler, capacitance elements are used which are galvanically coupled between the coupled line and reference potential of the high frequency signal, especially ground. Disadvantageously, those capacitances can only compensate the frequency response of the

directional coupler within a small bandwidth. To achieve broader bandwidth compensation, so called chip capacitors are used. Since these chip capacitors have to be bonded and bonding technologies include additional manufacturing steps, like fixing, cleaning and gilding of substrate, these chip capacitors are very expensive.

It is an objective of the invention to reduce the costs of manufacturing directional couplers on the one hand and on the other hand to increase the precision of providing the derived high frequency signals. The directional coupler should have small deviations in the magnitude of the frequency response and high power functionality within a broad frequency bandwidth. The directional coupler should be applicable for monitoring broadband amplifier and EMC applications .

The above-identified objective is solved by the features of independent claim 1. Advantageous embodiments of the invention are described in the dependent claims. The objective is solved especially with a directional coupler configured to providing a forward and/or a

backward signal derived from a high frequency signal. The directional coupler comprises a coupling element coupled to a main line, wherein the main line serves for

transporting the high frequency signal. The directional coupler comprises a signal line connecting the coupling element to a measuring port for providing the forward and/or backward signal. The signal line of the directional coupler according to the invention comprises a defected ground structure, wherein the defected ground structure is inserted between the coupling element and the measuring port . A coupling element according to the invention preferably is an element, which is placed close enough to the main transmission line to derive a forward and/or backward signal from the main transmission line.

The main line according to the invention preferably is a transmission line comprising a feeding port for feeding a high frequency signal to a tapping port of the main line for tapping the high frequency signal. The main line is a waveguide for transporting a high frequency

electromagnetic signal, e.g. a micro strip line, a coplanar line, a substrate integrated waveguide, a slotted line and/or a hollow conductor. Especially the main line is a coaxial waveguide.

A defected ground structure is preferably built in a metallic ground plane connected to reference potential of the signal line. Such a defected ground structure in the metallic ground plane leads to a disturbance of the shield current distribution in the ground plane caused by the defect in the ground. This disturbance changes the

characteristics of the signal line such as line

capacitance and line inductance. Any defect in the

metallic ground plane of the signal line gives rise to increasing effective capacitance and inductance. A first extension and a second extension increase the route length of current and the effective inductance of the signal line. A non-metallic bar accumulates charge and increases the effective capacitance of the signal line.

The equivalent circuit of a defected ground structure preferably is a parallel RLC resonator circuit, also called oscillating circuit. To insert a defected ground structure into the metallic ground plane of the signal line therefore can electrically be expressed by insertion of a parallel RLC circuit in series in the signal line between the coupling element and the measuring element. According to the frequency response characteristics of the RLC resonator circuit the frequency response characteristics of the directional coupler is compensated therewith that signals with lower frequencies, e.g. 100 MHz to 5 GHz, can be derived from the measuring port without significant deviations, especially lower than ldB, in the magnitude of the frequency response. Since the defected ground structures are easily producible, the manufacturing costs of such a directional coupler

according to the invention are highly reduced.

In a preferred embodiment, the directional coupler further comprises a substrate comprising a first surface and a second surface. The second surface is opposite to the first surface. The signal line is arranged on the first surface and the defective ground structure is built in a metallic ground plane arranged on the second surface of the substrate. The metallic ground plane is connected to a reference potential of the high frequency signal.

Preferably the defected ground structure is placed on the second surface in a manner that a center of the defected ground structure is placed underneath the signal line.

Such an arrangement advantageously leads to a band stop filter structure for compensation of the frequency

response of the directional coupler. Since frequencies below 5 GHz are strongly affected with high transmission losses of the derived high frequency signal at the

measuring port the filter characteristics of such a defected ground structure compensates the frequency response of the directional coupler.

In a preferred embodiment a resistor element is arranged in the defected ground structure. The resistor element comprises a first contact pin galvanically coupled to a first metallic extension of the defected ground structure. The resistor element further comprises a second contact pin galvanically coupled to a second metallic extension of the defected ground structure. The first metallic extension and the second metallic extension are separated via a non-metallic bar of the defected ground structure.

The preferred insertion of a resistor element as described above electrically leads to a parallel circuit of the resistor element and the parasitic resistor of the RLC- resonator. Since the resistor values of the arranged resistor element is low-resistive compared to the

parasitic resistor, the insertion of a resistor element advantageously reduces the quality factor of the

equivalent circuit of the parallel RLC-resonator of the defected ground structure. The reduction of the quality factor of the oscillator circuit leads to a smoother frequency response gradient of the RLC-resonator and therefore to a better compensation of the frequency response. Therefore the defected ground structure affects the frequency response characteristics in a broader bandwidth which leads to a broader frequency compensation of the directional coupler.

In a preferred embodiment the directional coupler

comprises a resistor element. The resistor element is arranged on the first surface of the substrate. Therefore the resistor element comprises a first contact pin

galvanically coupled to the signal line and the resistor element. A second contact pin is galvanically coupled to the reference potential of the high frequency signal.

By placing the resistor element in the described manner, a transformation of the resistance occurs, leading to a parallel RLC-resonator in parallel with the resistor element. The above identified technical effects of smoothing the frequency response characteristics of the equivalent RLC-resonator also apply within such an

arrangement .

Since the signal line is preferably built on a first surface and the defected ground structure is preferably built on a second surface of the substrate, the insertion of a resistor element on the first surface instead of the second surface of the substrate advantageously leads to decrease of manufacturing costs, since no fixing, no gilding and/or bonding of the second surface of the substrate of the directional coupler is necessary.

In a preferred embodiment a second resistor element is arranged on the first surface of the substrate and the second resistor element comprises a first contact pin galvanically coupled to the signal line and the second resistor element comprises a second contact pin

galvanically coupled to the reference potential of the high frequency signal.

Preferably, the resistor is arranged on a first side of the signal line on the first surface of the substrate wherein the second resistor is arranged on an opposite second side of the signal line on the first surface of the substrate. This advantageously leads to a symmetrically loaded signal line which provides a better high frequency shielding of the derived high frequency signal.

In a prefered embodiment the signal line further comprises a second defected ground structure, wherein the second defected ground structure is inserted between the defected ground structure and the measuring port. The use of a second defected ground structure advantegously leads to a better frequency compensation.

In a preferred embodiment a capacitor element is arranged in the defected ground structure. The capacitance element comprises a first contact pin galvanically coupled to a first metallic extension of the defected ground structure and a second contact pin galvanically coupled to a second metallic extension of the defected ground structure and wherein the first metallic extension and the second metallic extension of the defected ground structure are separated via a non-metallic bar of the defected ground structure .

The arrangement of the capacitance element in such a manner advantageously leads to a galvanic coupling of the capacitance element in parallel to the equivalent RLC- resonator circuit. This leads to an influence of the resonation frequency of this oscillating circuit.

Therefore, the frequency compensation of the frequency response is adjustable and improved.

In a preferred embodiment a capacitance element is arranged on the first surface of the substrate and the capacitance element comprising a first contact pin

galvanically coupled to the signal line. The capacitance element comprises a second contact pin galvanically coupled to the reference potential of the high frequency signal . According to the resistance element, also the capacitor element is arranged on the first surface of the substrate of the directional coupler. As described above, a fixing, gilding, and bonding manufacturing step on the second surface of the substrate can be avoided leading to a reduction of manufacturing costs.

In a preferred embodiment a second capacitance element is arranged on the first surface of the substrate. The second capacitance element comprises a first contact pin

galvanically coupled to the signal line. The second capacitance element comprises a second contact pin

galvanically coupled to the reference potential of the high frequency signal. Preferably, the capacitance element is arranged on a first side of the signal line on the first surface of the substrate wherein the second capacitance element is arranged on an opposite second side of the signal line on the first surface of the substrate. This advantageously leads to a symmetrically loaded signal line which provides a better high frequency shielding of the derived high frequency signal.

In a preferred embodiment the signal line further

comprises an inductance element galvanically coupled in series between the coupling element and the measuring port. The insertion of an inductance element is

electrically interpreted as a series RLC-resonation circuit. This series resonation circuit advantageously has a frequency response characteristic as a high pass filter. Choosing the resonation frequency of this series RLC- resonator high above the maximum derived electromagnetic signal frequency, the frequency response of the

directional couplers measuring port is further improved.

Advantageously the signal line further comprises a second inductance element arranged in series to the inductance element. This leads to a second series RLC-resonator . By choosing different inductance values for the inductance element and the second inductance element, two high pass filters are obtained for further influencing and

compensating the frequency response characteristics.

Preferably the ratio of the inductance element and the second inductance element is in the range between 1:2 and 1:10, most preferably at about 1:5.

For further improving the frequency compensation, a frequency trimming element can be inserted in the signal line of the directional coupler.

In a preferred embodiment the capacitance element, the second capacitance element, the inductance element, the second inductance element and/or the trimming element are arranged as conductive path elements build on the first surface of the substrate, e.g. as micro strip elements. This drastically reduces the costs of manufacturing the directional coupler.

In a preferred embodiment the directional coupler further comprises a second signal line connecting the coupling element to a second measuring port for providing the forward and/or backward signal. The second signal line comprises a defected ground structure, wherein the

defected ground structure is inserted between the coupling element and the second measuring port. Advantageously, the directional coupler can provide forward and backward signals .

In an alternative solution, the directional coupler comprises a second coupling element and a second signal line for connecting the second coupling element to a second measuring port for providing the forward and/or backward signal. The second signal line comprises a defected ground structure, wherein the defected ground structure is inserted between the second coupling element and the second measuring port.

Advantageously, the first measuring port is configured to provide the forward signal wave, wherein the second measuring port is configured to provide the backward signal wave. Additionally each coupling element comprises a termination port, wherein the termination port is terminated with a matching termination element. In the following, embodiments of the invention are

described with reference to the figures of the drawing by way of example only. Reference signs are used for the same technical features in different figures. In the drawing: Fig. 1 shows a directional coupler according to prior art; Fig. 2 shows the frequency response of the

directional coupler according to Fig. 1 ;

Fig. 3a-3d show defected ground structures according to the invention and its electrical equivalent circuit;

Fig. 4a shows a cross-section of an embodiment of the inventive directional coupler

Fig. 4b is a top view of a first surface of the

inventive directional coupler according to Fig. 4a; Fig. 4c is a top view of a second surface of the inventive directional coupler according to Fig. 4a;

Fig. 5 is a top view of an alternative second

surface of the inventive directional coupler according to Fig. 4a;

Fig. 6 is a top view of another alternative second surface of the inventive directional coupler according to Fig. 4a;

Fig. 7 shows a frequency response of a defected ground structure with an inventive resistor element ;

Fig. 8 shows a frequency response of a defected ground structure with an alternative inventive resistor element according to Fig. 7;

Fig. 9 is a top view of an alternative first

surface of the inventive directional coupler according to Fig. 4a; is a top view of another alternative first surface of the inventive directional coupler according to Fig. 4a; is a top view of another alternative first surface of the inventive directional coupler according to Fig. 4a; is a top view of a first surface of an alternative inventive directional coupler; shows an electrical equivalent circuit of the embodiment of an inventive directional coupler according to Fig. 11; and shows the frequency response of an

inventive compensated directional coupler. Fig. 1 shows the functional principle of a directional coupler according to prior art. A main transmission line 2 comprises a feeding port 2a for feeding a high frequency signal and a tapping port 2b for tapping the high

frequency signal. To monitor or measure the high frequency signal via main line 2, which is transported, a coupling element 3 is placed near the main line 2 to derive a defined amount of the signal. The coupling element 3 comprises a measuring port 5 and a second measuring port 10 to provide the derived signal to another circuitry.

Such an arrangement might be used when a signal generator or transmitter feeds a signal to the main line and another circuit, e.g. an antenna.

Disadvantageously, such directional couplers are normally tuned to high frequency signals with a defined small frequency bandwidth. In Fig. 2, the magnitude of the scattering parameter from measuring port 5 to feeding port 2a according to a directional coupler of Fig. 1,

hereinafter called S (5,2a), is illustrated from 500 MHz to 8 GHz. The transmission loss 21 between 500 MHz and 8 GHz deviates between 50 dB and 34 dB . As can be achieved from Fig. 2, the S - parameter S (5,2a) - is only linear in a very small bandwidth from five to six Gigahertz.

Especially in the frequency band from 500 MHz to 5 GHz the frequency response of the directional coupler 1 is highly non-linear. Such a directional coupler 1 according to Fig. 1 cannot be used for broadband high frequency applications without a compensation of the frequency response.

A compensation of the frequency response according to the invention is achieved by defected ground structures 6, short DGS.

In Figures 3a-3c, different DGS 6 are illustrated. A DGS 6 according to the invention comprises a first metallic extension 61 and second metallic extension 62. The first extension 61 is separated from the second extension 62 by a non-metallic bar 63.

Since in high frequency applications a metallic ground plane is arranged on a second surface 72 of a substrate 7 for RF-shielding purposes, the DGS 6 can easily be built by removing areas of the ground plane, e.g. through chemical etching or electrolytic etching methods. Figures 3b and 3c show different embodiments of a DGS 6. Not shown but not excluded from the invention is an arrangement of micro strip, coplanar etc. transmission lines . In Fig. 3d an electrically equivalent circuit of a DGS is shown. The equivalent circuit of a DGS can be seen as a parallel RLC-resonator circuit comprising an inductor L, a capacitor C and a resistor R. Any defect in the metallic ground plane of the signal line 4 changes the characteristics of that signal line 4 and give rise to increasing effective capacitance C and inductance L of the signal line 4. Especially the first extension 61 and the second extension 62 increase the route length of current and the effective inductance L. the larger the extensions, the higher the effective inductance L becomes which leads to a higher cut-off frequency of the RLC-resonator and vise versa. The non- metallic bar 63 accumulates charge and increases the effective capacitance C of the signal line 4. In case the non-metallic bar 63 decreases in its width, the effective capacitance C increases and vise versa.

In Fig. 4a a cross-section of an inventive directional coupler 1 is shown. The directional coupler 1 comprises a main line 2. The main line 2 comprises a feeding port 2a and a tapping port 2b. Since the directional coupler 1 can be used in both directions, the feeding port 2a and the tapping port 2b can be interchanged. The main line 2 herein is a coaxial waveguide with an inner conductor surrounded by air with a specific dielectric constant ε _Γ = 1. Such a main line 2 is able to transport broadband signals without high transmission losses. Other types of main lines are not excluded here.

The directional coupler 1 according to Fig. 4a further comprises a coupling element 3. The coupling element 3 is arranged on a second surface 72 of a substrate 7 of the directional coupler 1. This second surface 72 is face to face with the main line 2. The substrate 7 further

comprises a first surface 71 which is opposite to the second surface 72 of the substrate 7. The coupling element 3 has a first connection pin 31 and a second connection pin 32 on the first surface 71 of the substrate 7. The connection pins 31 and 32 are integral elements of the coupling element 3. The coupling element 3 according to Fig. 4a is placed close to the main line 2 to derive a forward and/or backward signal from the transported high frequency signal of the main line 2.

The directional coupler 1 has four ports. A feeding port 2a is the input port where the signal to be transported is applied. The measuring port 5 is the coupled port where a defined portion of the signal applied to the feeding port 2a appears. The tapping port 2b is the transmitted port where the signal from feeding port 2a is output.

The directional coupler 1 is preferably symmetric. Thus, there also exists an isolated port, which is not shown in Fig. 4a. A defined portion of the signal applied to the tapping port 2a will be coupled to the isolating port. However, the directional coupler 1 is not normally used in this mode and the isolated port is usually terminated with a matching load 13. This termination is typically arranged on the first surface 71 of the substrate and is therefore not accessible to the user. Effectively, this results in a 3-port directional coupler 1. According to Fig. 4a the directional coupler 1 comprises a signal line 4 to connect the coupling element 3 via the connection pin 31 to a measuring port 5. Beneath the signal line 4 a DGS 6 according to Fig. 3 is arranged on the second surface 72 of the substrate 7. The first surface 71 and the second surface 72 of the substrate also comprise a ground plane GND between the arranged

components for RF shielding purposes. The ground plane of the first surface 71 is not shown in Fig. 4a. Fig. 4b illustrates a top view of a first surface 71 of a substrate according to intersection line A-A' of Fig. 4a. Elements which are placed on the second surface of the directional coupler are shown in dotted lines; see for instance the coupling element 3 and the DGS 6. The first connection pin 31 and the second connection pin 32 of the coupling element 3 are arranged on a first surface 71 of the substrate 7 of directional coupler 1. The connection pin 32 is terminated with a termination element 13 being the above described matching load, e.g. an impedance of 50 Ohm. The connection pin 31 is connected to the signal line 4. Therefore the signal line 4 connects the coupling element 3 and the measuring port 5. The signal line 4 in this embodiment is a micro strip line and according to the invention comprises a DGS 6 built on the second surfaces 72 of the directional coupler 1.

In Fig. 4c a top view of a second surface 72 of the substrate 7 of the directional coupler 1 of the invention at intersection line A-A' is illustrated. As shown the coupling element 3 and the defected ground structure 6 are arranged on the second surface 72. The defected ground structure 6 comprises a first extension 61 and a second extension 62 of a conductive material, especially of a metallic material. Advantageously the first extension 61 and the second extension 62 are part of a ground plane, as illustrated with specific texture in Fig. 4c. For

explanation purposes, the signal line 4 and the measuring port 5 are shown in dotted lines.

The DGS 6 in the metallic ground plane of the micro strip signal line 4 changes the characteristics of the signal line 4 and give rise to increasing effective capacitance and inductance of a signal line. Since the signal line 4 has a parallel equivalent circuit according to Fig. 3d in its signal path, this RLC-resonator circuit in series in the signal path has the characteristic of a band stop filter. It therefore compensates the frequency response of the S-Parameter S (5,2a) shown in Fig. 2.

In Fig. 5 a top view of an alternative embodiment of the second surface 72 of the inventive directional coupler 1 according to Fig. 4a is shown. Advantageously the DGS 6 further comprises a resistor element 8. A first pin 81 of the resistor element 8 is connected to the first extension 61 of the defected ground structure 6. A second connection pin 82 of the resistor 8 is connected to a second

extension 62 of the defected ground structure 6. This resistor element is arranged as a discrete element.

Preferably the resistor element 8 crosses the non-metallic bar 63 of the DGS 6. The value of resistance of the resistor element 8 is for instance 100 Ohm. This resistor element 8 is inserted for reducing the quality factor of the defected ground structure 6. This is achieved by arranging the resistor element 8 in parallel to the electrically equivalent RLC-resonator circuit. This arrangement of a resistor parallel to the electrically equivalent RLC-resonator circuit leads to a lowering of the high-resistive parasitic resistor value R according to Fig. 3d, since a parallel connection of the high-resistive parasitic resistors R and the low-resistive resistor element 8 leads to a total resistance lower than the low- resistive resistor element 8, e.g. lower 100 Ω. Lowering the Q-factor leads to smoother frequency response

gradients in the resulting equivalent RLC-circuit.

Therefore the compensation of the frequency response is improved.

In Fig. 7 the resulting frequency response of a DGS 6 with a parallel resistor element 8 according to Fig. 5 is shown. The resistor element 8 has a value of 100 Ω, resulting in a total resistance of the equivalent RLC- resonator of less than 100 Ω. As can be seen in Fig. 7, the frequency response of such an DGS 6 with 100 Ω

resistor element 8 in parallel has the behavior of a low pass filter with a 3 dB cut-off frequency f _c = 2,6 GHz. Thus, the frequencies from 1 GHz to 3 GHz are passed with less deviation in the transmission loss characteristic than the frequency above 3 GHz. This leads to a higher linearity of the frequency response. In Fig. 8 an alternative frequency response according to Fig. 7 is shown. The DGS 6 according to Fig. 8 comprises a resistor element 8 with a resistive value of 180 Ω. As can be seen in Fig. 8, the frequency response of such an DGS 6 with 180 Ω resistor element 8 in parallel has the behavior of a low pass filter with a 3 dB cut-off frequency f _c = 2,5 GHz. Furthermore this resistor element with 180 Ω leads to an increasing frequency response at frequencies above 8 GHz. In comparison with Fig. 2 a higher linearity of the frequency response is achieved using a 180 Ω resistor element 8.

In Fig. 6 a top view of an alternative second surface 72 of the inventive directional coupler 1 according to Fig. 4a is shown. In contrast to the above provided

explanations on directional couplers, the isolating port in Fig. 6 is not terminated directly on the substrate 7 leading to inaccessibility for a user. In contrast Fig. 6 shows an inventive directional coupler 1 with a first measuring port 5 and a second measuring port 10.

This is to provide a directional coupler 1 which does not have to be turned to provide the forward signal and the backward signal to another circuit. Since the above described directional coupler 1 has only one measuring port 5 wherein the isolating port is terminated

permanently, the directional coupler 1 can either measure the forward signal or by turning the directional coupler at 180 degrees of a horizontal axes measure the backward signal derived of the transported high frequency signal.

In some arrangements such a 180 degree turning is

impossible or highly inconvenient. Also it might be difficult to obtain a coupling behavior between coupling element 3 and main line 2. Therefore the directional coupler 1 according to Fig. 6 comprises a second measuring port 10 and a second signal line 19 in the second surface 72. Basically the first measuring port 5 and the second measuring port 10 are built equivalently . In case of providing a forward signal of a high frequency signal transported via main line 2 and applied to feeding port 2a of the directional coupler 1, the second measuring port 10 is terminated via a matching terminal element 13.

Accordingly, measuring point 5 now provides the forward signal derived from the applied signal. Additionally, the directional coupler 1 can be used in a reciprocal manner. Therefore, the measuring port 5 is terminated via

termination element and the backward signal is provided on the second measuring port 10. The directional coupler 1 does not need to be turned. The coupling element 3 stays coupled to the main line 2.

The embodiment according to Fig. 5 and 6 show resistor elements 8 arranged on the second surface 72 of the substrate 7. Since the second surface does not comprise other bonded or soldered elements, the second surface 72 is normally not gilded. Since the directional coupler 1 should be manufactured at low cost, it is advantageously suggested to arrange the resistor elements 8 on the first surface 71 of the substrate 7.

Therefore, Fig. 9 shows a resistor element 8 arranged on the first surface 71 of the directional coupler 1. The resistor element 8 is connected via a first contact pin 81 to the signal line 4. The resistor 8 is connected via second connecting pin 82 to the reference potential ground. Since a ground plane GND is also build on the first surface 71 the connection of the second connection pin 82 of the resistor 8 can be achieved without

additional vias or additional transmission lines.

The resistance value R of the resistor element 8 according to Fig. 9 has to be transformed and recalculated to obtain a parallel resistor element with a value of 180 Ω

according to Fig. 5, Fig. 6, and Fig. 8.

Fig. 9 shows a top view of an alternative first surface 71 of the inventive directional coupler 1. Fig. 9 shows a capacitance element 11 arranged in parallel to the

resistor element 8. Therefore, a first connection pin 110 of the capacitance element 11 is connected to the signal line 4. Additionally, the second connection pin 111 of the capacitance element 11 is connected to ground potential GND. This capacitance element 11 further compensates the frequency response of the directional coupler 1.

The disadvantageously gilding of the second surface 72 and an expensive bonding process for arranging resistor element 8 and capacitance element 11 on the second

substrate 72 can therefore be avoided.

Furthermore the coupling element 3 comprises a second connection port 32. This second connection port 32 is terminated via termination element 13. Advantageously determination element 13 comprises a value matching with the impedance of the main line 2, for instance 50 Ω. Fig. 10 shows a top view of an alternative first surface 71 of the inventive directional coupler. In contrast to Fig. 9, the directional coupler 1 of Fig. 10 comprises a DGS 6 and a second DGS 14. The cascading of DGS leads to an inclination of depth and bandwidth of the band stop filter characteristics of the equivalent LRC-resonator . Therefore further compensation of the frequency response of the directional coupler 1 according to Fig. 2 is achieved . A further difference between Fig. 10 and Fig. 9 is the insertion of a second resistor element 12. According to Fig. 10 the resistor 8 is galvanically coupled with a first connection pin 81 on a first side of the signal line 4 and with a second connection pin 82 to ground potential GND. The second resistor element 12 is galvanically coupled with a first connection pin 81 to an opposite second side of the signal line 4 and with a second

connection pin 82 to ground potential GND. This leads to a symmetric current loading of the signal line 4 and

therefore improves the transmission characteristics of the signal line 4. The resistance values of the resistor element 8 and the resistor element 14 are equal and total resistance of the resistor element 8 and the resistor element 14 is equal to the transformed resistance value to obtain a parallel resistance value of approx. R = 180 Ω in the RLC-resonator circuit according to Fig. 3d. To further improve the directional couplers frequency response the total resistance value R of the resistor element 8 and the second resistor element 12 should be lower than 130 Ω and ideally be R = 120 Ω.

Respectively a second capacitance element 15 is

galvanically coupled to a first side of the signal line 4, wherein the capacitance element 11 is galvanically coupled to the second side of the signal line 4. This improves the transmission characteristics of signal line 4. To further improve the directional couplers frequency response the total capacitance value C of the capacitance element 11 and the second capacitance element 15 should be lower than 1 pF and ideally C = 0.2 pF.

Since the first surface 71 comprises a ground plane GND no additional vias or transmission lines are necessary to galvanically couple the resistor elements 8, 12 or the capacitance elements 11, 15 to the ground potential GND.

In Fig. 11 a top view of another alternative first surface of the inventive directional coupler 1 according to Fig. 4a at intersection line B-B' is shown. In Fig. 11 the resistor element 8 and the second resistor element 12 are arranged according to Fig. 10 for symmetric current loading. The capacitor element 11 and the second capacitor element 15 are arranged as conductive path capacitance elements instead of discrete elements according to Fig. 10. Therefore the first connection pin 110 of the

capacitance element 11 is built as a stud of the signal line 4. The second connection pin 111 of the capacitance element 11 is built as a stud of the ground plane.

Accordingly the first connection pin 110 of the second capacitance element 15 is built as a stud of the signal line 4. The second connection pin 111 of the second capacitance element 15 is built as a stud of the ground plane. The total capacitance value of the capacitance element 11 and the second capacitance element 15 should be lower than 1 pF and ideally C = 0.2 pF.

An inductance element 16 is arranged in series in the signal line 4 between the measuring port 5 and the

resistor elements 8 and 14. The inductance element is arranged as conductive path inductance. Alternatively the inductance element 16 can be arranged as a discrete element. The inductance element 16 builds another LRC- resonator circuit for further compensation of the

frequency response. The LRC-resonator with inductance element 16 has the electrical equivalent characteristics of a high pass filter with a cut-off frequency of more than 20 GHz. This leads to a slight improvement of the transmission characteristics of the derived high frequency signal . A second inductance element 17 is arranged in series in the signal line 4 between the resistor elements 8 and 14 and the coupling element 3. The second inductance element 17 is arranged as conductive path inductance.

Alternatively the second inductance element 17 can be arranged as a discrete element. The second inductance element 17 builds another LRC-resonator circuit for further compensation of the frequency response. The second inductance element 17 has a 5 times higher inductance value than the inductance element 16. Therewith two different high passes with different cut-off frequencies are achieved to compensate the frequency response. Alternatively and not shown in Fig. 11 the second

inductance element 17 is arranged in series in the signal line 4 between the measuring port 5 and the inductance element 16. The inductance element 16 has an inductance value of L = 3.55 nH. The second inductance element 17 has an inductance value of L = 1.56 nH.

Additionally the embodiment of Fig. 11 further comprises a frequency trimming element 22. This trimming element is built as a conductive path element and comprises a

conductive stud on the signal line 4 and an unconnected stud of conductive path. The trimming element 22 can be adjusted in two ways. In case the conductive stud on the signal line 4 is too short to obtain a correct

compensation of the frequency response, the stud is lengthened with the unconnected stud by a conductive bridge. In case the conductive stud on the signal line 4 is too long to obtain a correct compensation of the frequency response, the stud is shortened by a cutting tool. With such a trimming element the compensation can be achieved very precise and in low cost manner. Alternative a varicap is inserted as a trimming element.

Building the inductance elements 16, 17, the capacitance elements 11, 15 and the trimming element 22 as conductive paths on the first surface 71 of the directional coupler 1 the manufacturing costs are drastically reduced.

In Fig. 12 a top view of a first surface 71 of an

alternative inventive directional coupler 1 is shown. The directional coupler 1 according to Fig. 12 comprises a second coupling element 18. The directional coupler 1 according to Fig. 12 is build mirror symmetric, shown through dotted line S. The coupling element 3 and the second coupling element 18 are built in a similar manner.

Both coupling elements 3 and 18 comprise a connection port 32 which is terminated via termination element 13 or 20. The connection port 31 of the coupling element 3 is connected to a measuring port 5. The connection port 31 of the second coupling element 18 is connected to a measuring port 10. The connections to the respective measuring ports 5 or 10 are realized with signal line 4 or 19 and are built equivalently . All the features described in above Figures 3 to 10 are applicable to the embodiment of Fig. 12. In Fig. 12 a directional coupler 1 is shown which provides a forward signal and a backward signal of the high

frequency signal transported through the main line 2.

Therefore two coupling elements 3 and 18 are arranged within the directional coupler 1. Since both coupling elements 3 and 18 are terminated on second connection port 32, the forward signal and the backward signal can be directly derived from the high frequency signal without electromagnetic influences between each other. As a result, a highly precise directional coupler with low manufacturing costs is achieved. Since all elements are arranged on the first surface 71 of the directional coupler 1, no gilding or bonding of the second surface 72 of the substrate 7 of the inventive directional coupler 1 is to be realized.

In Fig. 13 an equivalent circuit of a directional coupler 1 according to the preceding Figures 4 to 12 is shown. A main line 2 transports a high frequency signal from the feeding port 2a to the tapping port 2b. The coupling element 3 derives a backward signal from the fed signal and provides the derived signal on measuring port 5. In case the signal is applied to port 2b instead, a forward signal can be derived and provided at measuring port 5. A termination element 13 is placed on the isolated port of the coupling element 3. The coupling element 3 comprises a signal line 4 and a measuring port 5. The signal line 4 comprises a first parallel RLC-resonator circuit which represents the DGS 6. As can be seen, the resistor element of the equivalent circuit of DGS 6 has a resistance value of R = 140 Ω which is achieved with resistor element 8 and/or second resistor 12. In series to the parallel RLC- resonator of DGS 6 a second parallel RLC-resonator circuit representing the second DGS 14 is inserted in the signal line 4. Also the resistor element R of the second DGS 14 is lowered via a parallel resistor element. Finally the trimming element 22 as well as inductance elements 16, 17 and capacitor elements 11 and 15 are represented by a third parallel RLC-resonator circuit.

In Fig. 14 a frequency response of the compensated

directional coupler 1 according to the invention is shown. As an electrical model the equivalent circuit of Fig. 13 is used. As can easily be obtained, the deviation 21 in the frequency response of the magnitude of the S-Parameter S (5,2a) is highly decreased in the frequency band of 500 MHz to 6 GHz.

The directional coupler 1 according to the invention comprises a deviation 21 in the frequency response of the magnitude of the S-Parameter S (5,2a) of less than 0.6 dB in a frequency band from 800 MHz to 6 GHz. It is achieved deviation 21 in the frequency response of the magnitude of the S-Parameter S (5,2a) of less than 0.4dB in a frequency band of 800MHz to 3 GHz and also in a frequency band of 2.5 GHz to 6 GHz. The described directional coupler 1 can be used in signal generation or signal amplification applications with transmission power signals higher 800 Watts. Al l described features of each embodiment are combinable with technical features of other embodiments .