WO2005093896A1

WO2005093896A1 - Directional coupler

Info

Publication number: WO2005093896A1
Application number: PCT/FI2005/050066
Authority: WO
Inventors: Jarkko MÄNTYNIEMI
Original assignee: Filtronic Comtek Oy
Priority date: 2004-03-25
Filing date: 2005-03-07
Publication date: 2005-10-06
Also published as: FI121516B; FI20040450A0; FI20040450A

Abstract

The invention relates to a implementation way of a directional coupler used in radio frequency circuits. Coupling to the transmission path of the signal to be measured takes place at one point through a measuring line (421, 422). The terminal resistor (425) of the measuring line is placed preferably in the direction of the transmission path in the field (EMF) of the signal to be measured so that it functions as part of the measuring strobe sensing the field strength. In addition, the strobe has a piece of conductor surface (424), which belongs to one conductor of the measuring line and is located on the side where the signal to be measured is coming, with respect to the resistor. The directivity is based on such an asymmetric structure of the strobe. As viewed from the transmission path, the outer end of the measuring line functions as the measuring port (P3) of the directional coupler thus formed. A good directivity is achieved on a very wide frequency range. In addition, the level of the measuring signal in relation to the level of the signal to be measured is relatively constant, and the reflection attenuation of the input port of the directional coupler is high on a wide frequency range. The directional coupler is small-sized, and it can be implemented in connection with some other structural part.

Description

Directional coupler

The invention relates to an implementation way of a directional coupler used in radio frequency circuits.

A directional coupler is a measuring device, which gives a signal proportional to the strength of an electromagnetic field propagating in a certain direction on the transmission path. The field propagating in the opposite direction on the transmission path does not influence the level of said signal in principle. A directional coupler has at least three ports: The energy coming to the input port is guided almost entirely through the coupler to the output port, and a small part of this energy is transferred to the third port, or measuring port. The part of the directional coupler between the input and output port is at the same time a part of the transmission path of the radio device, which continues to the antenna of the transmitter, for example. In that case a measuring signal proportional to the real strength of the field propagating towards the antenna is given from the measuring port, and this meas- uring signal can be used for the purpose of adjusting the transmitter. If the directional coupler has been connected to the transmission path the other way round, a measuring signal proportional to the strength of the field reflected from the antenna is given from the measuring port, and this measuring signal, as well, can be used for the purpose of adjusting the transmitter. The accuracy of the adjustments is partly dependent on the quality of the directional coupler, i.e. how well the effect of the field propagating in the opposite direction to the field to be measured becomes eliminated.

A simple directional coupler can be formed on a circuit board by microstrips. An example of such a known directional coupler is seen in Fig. 1. The lower surface of the circuit board PCB is conductive and functions as the signal ground GND. On the upper surface of the board there is a straight first microstrip 110, the head end of which together with a conductor pad connected to the signal ground forms the input port P1 of the directional coupler. Correspondingly, the tail end of the first microstrip together with the signal ground forms the output port P2 of the direc- tional coupler. On the upper surface of the board PCB there is also a second microstrip 120 parallel with the first microstrip, the length of which second microstrip 120 is a quarter of the wavelength λ on the operating frequencies of the directional coupler. The distance between the microstrips 110 and 120 is one-tenth of their distance from the ground, for example. The second microstrip 120 continues from its ends away from the first microstrip. The first extension 121 ends at the measur- ing port P3. When the directional coupler is in use, there is a circuit connected to the measuring port, the impedance Z of which circuit is the same as the characteristic impedance Z₀ of the transmission lines formed by the microstrips of the directional coupler together with the signal ground and the medium. The second exten- sion 122 of the second microstrip ends at the fourth port P4. So the directional coupler of the example, like most other directional couplers, has four ports.

Because of the electromagnetic coupling between the first and the second micro- strip, part of the energy fed to the input port is transferred to the circuit of the second microstrip, to the load impedances of ports P3 and P4. When the frequency of the coming field is such that the λ/4 condition mentioned above and marked in Fig. 1 is satisfied, the energy transferred to the measuring port P3 is at the highest, and the energy transferred to the fourth port P4, or the isolated port, is at the lowest. In an ideal coupler the latter energy is zero, because the even and odd waveforms that occur in the coupler cancel each other out at the end of the isolated port of the transmission line based on the second microstrip 120. The directivity of the coupler is based on this fact. Namely, if a field of the same frequency propagates from the output port towards the input port, due to the symmetrical structure hardly any of its energy is transferred to the measuring port P3. The quality of the directivity is expressed as the relation of the signal level of the measuring port to the signal level of the isolated port. This is the same thing as the relation of the level of the signal caused by the field propagating from the input port to the output port in the measuring port to the level of the signal caused by the field propagating from the output port to the input port in the measuring port, when the fields propagating in opposite directions have the same frequency and the same strength.

The drawback of structures according to Fig. 1 is a relatively poor directional isolation. This is due to that the even and odd waveforms do not entirely cancel each other on the side of the isolated port, because the odd waveform propagates to a greater extent in the air, in addition to the dielectric medium, in which case its speed is higher. The directivity of the structure is improved by arranging both mi- crostrips within a dielectric plate, on the both sides of which plate there is a ground plane. Another way of improvement, known from the patent publication US 6549089, is to increase the inductance by a suitable amount between both the first and the second microstrip and the ground. This can be done, for example, by shorted transmission line branches that are shorter than a quarter-wave. However, all directional couplers based on microstrips have the drawback of a narrow band, i.e. they function satisfactorily only on a relatively small frequency range. An ex- ample of this is in Fig. 2, which presents the curves of two transmission coefficients as a function of frequency. Curve 21 shows the change of the signal level of the measuring port in relation to the level of the input signal, and curve 22 shows the change of the signal level of the isolated port in relation to the level of the input signal. The difference between the coefficients in decibels expresses the value of the directivity. From the curves appears that the directivity is about 20 dB at the highest, which value is, however, valid only on a relatively narrow frequency range. The directivity exceeds the value 10 dB in the range 1.8-2.45 GHz, the relative width of which is 30%. It also appears from curve 21 that on the operating range of the directional coupler, the signal level in the measuring port is about 25 dB lower than the level of the signal that goes through the coupler. This means that the coupler causes an attenuation of 0.014 dB in the signal passing through.

Fig. 3 presents a known structure of a directional coupler, in which coupling to the transmission path of the signal takes place in a point-like manner by means of measuring strobes. The transmission path is a coaxial line, which includes an inner conductor 311 and a sheath-like outer conductor 312. In the figure, the outer conductor is cut open at the directional coupler arrangement. There are two strobes, the first 321 and the second 322 strobe. They extend through the outer conductor 312 of the coaxial line to the space between the outer and inner con- ductor, in which space the electromagnetic field containing the energy of the signal propagates. The distance between the strobes is a quarter of the wavelength of the field on the operating frequencies of the directional coupler. The first strobe 321 is connected to a intermediate line 330, and the second strobe 322 is connected to another intermediate line 340. In this description and the claims, the name "measuring line" is used for such intermediate lines starting from a strobe. The measuring lines are galvanically connected together at their tail ends, and the connecting point forms a measuring port P3. There is no isolated port in this structure. In this structure, the length of the first measuring line 330 is half a wave, and the length of the second measuring line 340 is a quarter-wave. The second meas- uring line is terminated at the end of the measuring strobe by a 50-ohm resistor. The first line does not interfere with this matching, because it is seen by the measuring port as a parallel resonance circuit, i.e. a very high impedance. In Fig. 3, the direction of the signal is marked so that when propagating, at first it reaches the first strobe 321 and then the second strobe 322. Thus the fields propagating through the strobes and the measuring lines have the same phase in the measuring port and thus strengthen each other. If a field propagating in the opposite direction occurs in the coaxial line, the parts branching from it to the strobes have opposite phases in the measuring port, thus cancelling each other in theory. In this way, the structure functions as a directional coupler. However, it also has the drawback that the directivity is good only on a relatively narrow frequency range. Similarly, the reflection attenuation as viewed from the input side of the signal is good only on a relatively narrow frequency range.

It is an objective of the invention to reduce the above mentioned drawbacks of the prior art. A directional coupler according to the invention is characterized in what is set forth in the independent claim 1. Some preferred embodiments of the invention are presented in the other claims.

The basic idea of the invention is the following: Through a measuring line, the coupling to the transmission path of the signal to be measured takes place at one point. The terminal resistor of the measuring line is placed in the field of the signal to be measured so that it functions as a part of the measuring strobe sensing the field strength. In addition, the strobe comprises a piece of conductor surface, which belongs to one conductor of the measuring line and is located on the side of the input direction of the signal to be measured with respect to the resistor. The directivity is based on such an asymmetric structure of the measuring strobe. As viewed from the transmission path, the outer end of the measuring line functions as a measuring port of the directional coupler thus formed.

The invention has an advantage that the frequency dependency of the directional coupler according to it is minor: A good directivity is achieved, and the level of the measuring signal in relation to the level of the signal to be measured is relatively constant on a very wide frequency range. The reflection attenuation in the input port of the directional coupler is also high on a very wide frequency range. Fur- thermore, the invention has the advantage that the directional coupler according to it is small-sized and can be implemented at a relatively low cost in connection with some other structural part.

In the following, the invention will be described in more detail. Reference will be made to the accompanying drawings, in which

Fig. 1 presents an example of a prior art directional coupler,

Fig. 2 presents an example of characteristics of a prior art directional coupler,

Fig. 3 presents another example of a prior art directional coupler,

Fig. 4 presents the principle of a directional coupler according to the invention, Fig. 5 presents an example of a directional coupler according to the invention,

Fig. 6 presents another example of a directional coupler according to the invention,

Fig. 7 presents an example of a coaxial structural part belonging to the trans- mission path of the signal, which structural part includes a directional coupler according to the invention,

Fig. 8 presents an example of a directional coupler according to the invention in the structure shown by Fig. 7,

Fig. 9 presents a second example of a directional coupler according to the invention in the structure shown by Fig. 7,

Fig. 10 presents a third example of a directional coupler according to the invention in the structure shown by Fig. 7,

Fig. 11 presents an example of the directivity of directional couplers according to the invention, and Fig. 12 presents an example of the effect of the invention on the reflection coefficient of the directional coupler.

Figs. 1 , 2 and 3 were already discussed in connection with the description of the prior art.

Fig. 4 illustrates the principle of the directional coupler according to the invention. It shows a transmission path, which is defined by the conductors 411 and 412. The former conductor 411 is called the centre conductor and the latter conductor 412 is called the ground conductor according to the structure generally used. However, the "centre conductor" need not be located just in the middle of some transmission path structure. The name "ground conductor" comes naturally from the fact the conductor in question is generally connected to the signal ground, or ground GND, like in Fig. 3. The electromagnetic field EMF of the signal to be measured propagates in the space between the centre conductor 411 and the ground conductor 412 from the input port P1 to the output port P2. The structure also includes a measuring line extending to the transmission path, which measuring line com- prises a first 421 and a second 422 measuring conductor. The outer end of the measuring line functions as the measuring port P3 of the directional coupler. At the end of the measuring line on the side of the transmission path there is coupled a resistor 425, the first end of the resistor coupled to the first measuring conductor 421 and the second end to the second measuring conductor 422. These couplings are galvanic in Fig. 4, but they can also be capacitive, if galvanic isolation is needed. The resistance R of the resistor has the same value as the characteristic impedance Z₀ of the measuring line, e.g. 50Ω. Thus the resistor functions as a terminal resistor of the measuring line in a manner known as such. According to the invention, the resistor 425 is located on the transmission path within the sphere of influence of the field EMF, in which case it also functions as a part of the strobe sensing the field strength. The strobe also includes a sensor conductor 424, which is connected to the first end of the resistor 425 and to the first measuring conductor 421. Thus the sensor conductor belongs structurally to the first measur- ing conductor. The total extent of the strobe in any direction is at least one order smaller than a quarter of the wavelength of the field to be measured.

The sensor conductor 424 has a certain conductive surface towards the transmission path, the second measuring conductor starting from the second end of the resistor having no such conductive surface. The conductive surface is located be- side the resistor closer to the input port P1 than the resistor. The substantial feature is that the centre of the sensor conductor, or the centre of the conductive surface mentioned above is located closer to the input port than the centre of the resistor. On such an asymmetry is based of the directivity in the directional coupler according to the invention. A field of the TEM (Transverse ElectroMagnetic wave) form propagating from the input port to the output port causes an alternating voltage proportional to the field strength in the measuring port P3, and energy is transferred to the impedance loading the measuring port. On the contrary, energy is transferred from the field propagating in the opposite direction, or from the output port to the input port, to the resistive mass of the resistor 425, but hardly any to- wards the measuring port. The directivity achieved is good, and it does not even presume a certain frequency, like in the known directional couplers.

Fig. 5 shows an example of a directional coupler according to the invention in practice. The directional coupler has been formed on a circuit board, like the coupler of Fig. 1. In this example, the circuit board PCB is a multi-layer board. In one of its intermediate layers, there is a straight first micro strip 511 , or the centre conductor of the transmission path, and parallel with it a ground strip 512 belonging to the signal ground GND. On the other side of the centre conductor there could well be another ground strip parallel with it. In addition to the ground strip 512, the ground conductor of the transmission path comprises the conductive lower surface of the circuit board, or the ground plane. The transmission path between the input port P1 and output port P2 of the directional coupler is thus formed of the first micro- strip 511 , the ground partially surrounding it according to the above description, and the dielectric material of the circuit board. On the upper surface of the board PCB, there is a chip resistor 525 above the first microstrip 511 and the ground strip 512 in the direction of the transmission path. The first end of the resistor on the side of the input port P1 has been connected to a strip-like first measuring conductor 521 , and the second end of the resistor on the side of the output port to the strip-like second measuring conductor 522. The measuring line formed by the measuring conductors leads to the measuring port P3 of the directional coupler. The resistor 525 on the transmission path functions as a terminal resistor of the measuring line and as a part of the strobe sensing the strength of the electromagnetic field propagating on the transmission path. A second substantial part of the strobe is formed of a sensor conductor 524, which is an enlargement of the first measuring conductor beside the first end of the resistor.

Fig. 6 shows another example of a directional coupler according to the invention in practice. The directional coupler has been formed on a multi-layer circuit board PCB in the same way as in Fig. 5. The difference compared to the coupler of Fig. 5 is that the resistor 625, which functions as a part of the measuring strobe, is now transverse to the transmission path, on top of it. The second substantial part of the strobe, or the sensor conductor 624, is for its most part beside the resistor, on the side of the input port P1. According to the invention, the sensor conductor has been connected to the first end of the resistor, and so it extends at the resistor in the direction of the transmission path. However, the centre of the sensor conductor is clearly closer to the input port than the centre of gravity of the resistor. Another difference compared to the coupler in Fig. 5 is that the second end of the resistor 625 has been connected by a via directly to the ground plane. Then the signal ground on the lower surface of the circuit board functions as the second measuring conductor.

In the structures according to Figs. 5 and 6, the circuit board can naturally also have only one layer. In that case the strobe can be raised above the upper surface of the circuit board by means of a small dielectric additional board, for example.

Fig. 7 shows an example of a structural part belonging to the transmission path of the signal, which structural part includes a directional coupler according to the invention. The structural part is a cylindrical piece, e.g. a coaxial connector. It has a centre conductor, which is not seen in this drawing, and an outer conductor 712 with a relatively thick wall. The outer conductor is the ground conductor of the transmission path. For the directional coupler the outer conductor has a recess, which receives the measuring line 720 of the directional coupler. The recess is covered by its lid 750, which has a via for the measuring line. The lid supports the measuring line and at the same time functions as a part of the outer conductor. The signal to be measured is brought to the structural part in question with a coaxial line 705, the conductive sheath of which is connected to the outer conductor 712 and the inner conductor to said centre conductor. Fig. 7 also shows an enlarged extension 713 of the outer conductor, which is placed e.g. against some planar surface in the finished product.

Fig. 8 shows an example of a directional coupler according to the invention in a coaxial structure like the one presented in Fig. 7. The structure is presented as a longitudinal section, wherein in addition to the outer conductor 812, the centre conductor 811 is also seen of the transmission path of the signal. With regard to the directional coupler, the left end of the structure in Fig. 8 functions as the input port P1 and the right end as the output port P2. On the bottom of the recess 830 in the outer conductor, there is a small circuit board 860. On the outer surface of this circuit board there is a strobe according to the invention, the substantial parts of which are the resistor 825 and the sensor conductor 824. On the bottom of the recess there is an opening 831 to the cavity of the transmission path, which opening, as viewed from the cavity, reveals the resistor and the sensor conductor on the signal frequencies. The electromagnetic field propagating in the cavity can thus influence the measuring strobe. The coaxial measuring line 820 extends to the recess through the via on its lid 850. The inner conductor 821 of the measuring line, or the first measuring conductor, has been connected through the sensor conductor 824 to the first end of the resistor, or the end on the side of the input port, and the conductive sheath 822, or the second measuring conductor, to the second end of the resistor. These connections are seen in the auxiliary drawing, which presents the circuit board 860 from above. The sensor conductor 824 is a microstrip of a suitable extent beside the first end of the resistor. The second end of the resistor has been connected to the second microstrip 861, to which the sheath of the measuring line is connected. As viewed from the cavity, the second microstrip 861 is mostly "in the dark", or outside the opening 831 , for the di- rectivity.

The strobe according to Fig. 8 can be modified in the following manner, for example: A via is made from the microstrip 824 to the lower surface of the circuit board 860, where, as an extension of the via, some conductive surface is formed, the surface being defined against the cavity of the transmission path and functioning as a part of the sensor conductor. The part of the sensor conductor on the side of the upper surface can then be correspondingly smaller. Fig. 9 presents another example of a directional coupler according to the invention in a coaxial structure like the one shown in Fig. 7. The structure is presented as a longitudinal section, except for the measuring line 920. The transmission path includes an outer conductor 912 and a centre conductor 911. The structure is similar to the one in Fig. 8, with the difference that the resistor 925 that functions as the strobe is now on the lower surface of the circuit board 960, i.e. on the inner surface with respect to the coaxial transmission path. The resistor is then located in an opening 931 on the bottom of the recess 930 of the outer conductor. On the lower surface of the circuit board 960 there is also a microstrip 924 functioning as a sensor conductor. This is connected to the first end of the resistor 925, or the end on the side of the input port P1 , and through a via of the circuit board 830 to the inner conductor 821 of the measuring line. The second end of the resistor is connected to a second micro- strip 961 , which is in this example connected to the outer conductor 912 by a screw, which presses the circuit board 960 against the bottom of the recess 930.

Another difference between the structures in Figs. 9 and 8 is that in Fig. 9, the conductive sheath of the measuring line is connected to the outer conductor of the transmission path through a bush 951 and a lid 950.

Fig. 10 shows a third example of a directional coupler according to the invention in a coaxial structure like the one shown in Fig. 7. In this case, the recess A30 of the outer conductor A12 is a cylindrical hole from the outer surface of the outer conductor down to the inner surface. In the middle of the hole there is a measuring pin A21 extending to the cavity of the transmission path, the lower end A24 of which pin functions as the sensor conductor of the strobe. The measuring pin is surrounded and supported by a dielectric mass that fills up the hole A30. The resistor A25 belonging to the strobe is in the cavity of the transmission path with its first end connected to the measuring pin and the second end connected to the inner surface of the outer conductor A12. The resistor is located parallel with the transmission path with its second end on the side of the output port P2 of the directional coupler as viewed from the measuring pin. The hole A30 has been covered with a conductive lid A50, except for the via of the measuring conductor, in order to avoid a leakage of the high-frequency electromagnetic field. A small part of the energy of the field propagating on the transmission path is, of course, intentionally taken out to the measuring port P3 of the directional coupler by the measuring line A20, which includes an extension conductor of the measuring pin A21 and the second measuring conductor connected to the outer conductor of the transmission path. The measuring line can be a coaxial line so that the measuring pin is part of its inner conductor, and the dielectric mass filling up the hole A30 is part of the insulating mass between the inner conductor and the sheath of the measuring line. In that case the sheath can extend to the level of the lower edge of the hole, and the second end of the resistor A25 can be connected to the sheath instead of the ground conductor of the transmission path.

Fig. 11 presents an example of characteristics of directional couplers according to the invention. Curve B1 shows the change of the signal level of the measuring port in relation to the level of the input signal as a function of frequency in directional couplers according to Figs. 8 and 10; both give approximately the same result. Graph B2 shows, as a function of frequency, the change of the signal level of the measuring port in relation to the level of the signal propagating from the output port towards the input port in a directional coupler according to Fig. 8, and graph B3 shows the change of the signal level of the measuring port in relation to the level of the signal propagating from the output port towards the input port in a directional coupler according to Fig. 10. (Relatively dense variation as a function of frequency occurs on the grey areas of graphs B2 and B3.) Curve B1 corresponds to curve 21 in Fig. 2, and graphs B2 and B3 curve 22 in Fig. 2. The difference between the coefficients in decibels thus expresses the value of the directivity.

The graphs indicate that the directivity is good in a very wide frequency range. The improvement compared to the prior art presented by Fig. 2 is very remarkable. In the directional coupler of Fig. 8, the directivity exceeds the value 20 dB in the range of approx. 1-4 GHz. In the directional coupler of Fig. 10, the corresponding range is approximately 0.2-3 GHz. For example, in the range 1.8-1.9 GHz, the directivity is approximately 28 dB in the coupler of Fig. 8 and approximately 23 dB in the coupler of Fig. 10.

It also appears from curve B1 that at about 2 GHz, for example, the signal level in the measuring port is about 40 dB lower than the level of the signal that goes through the coupler. This means that the measuring signal causes an attenuation of only 0.0003 dB in the signal going through.

Fig. 12 shows another example of the characteristics of a directional coupler ac- cording to the invention. Curve C1 presents the reflection coefficient of the input port as a function of frequency in a directional coupler according to Fig. 10. The coupler of Fig. 8 gives a substantially similar result, but in the ranges 0.5-1.5 GHz and 3-3.5 GHz its reflection coefficient is about 10 dB worse. For comparison, curve C2 presents the reflection coefficient of the input port in a known quarter- wave directional coupler. It is seen that in a directional coupler according to the invention, also a good reflection attenuation is achieved in a very wide frequency range. The reflection coefficient is about -30 dB or smaller from the zero frequency up to the frequency of 3 GHz, after which the coefficient deteriorates to about -20 dB continuing to the frequency of 5 GHz. In the prior art directional cou- pier, the reflection coefficient is almost -30 dB at the best. However, it is under - 20 dB only in the frequency range 1.95-2.2 GHz, which is 12% as a relative bandwidth.

The qualifiers "upper" and "lower" in the claims refer to the position of the directional coupler presented in Figs. 5 to 10, and they have nothing to do with the posi- tion in which the devices are used.

Structures of the directional couplers according to the invention have been described above. The details of their implementation ways can differ from those described. In addition to a chip resistor, the resistor used in the strobe can be, for example, a carbon film resistor, a thin film resistor or a thick film resistor, and its resistance can vary. The inventive idea can be applied in different ways within the scope set by the independent claim 1.

Claims

1. A directional coupler comprising an input port (P1), output port (P2), measuring port (P3), a transmission path with a centre conductor (411; 511; 611 ; 811; 911 ; A11) and a ground conductor (412; 512; 612; 712; 812; 912; A12) for guiding a signal to be measured from the input port to the output port and a measuring line (620; 720; 820; 920) coupled to the transmission path to guide a measuring signal to the measuring port, comprising a first (421 ; 521; 621; 821; 921 ; A21) and a second (422; 522) measuring conductor and being terminated at its end on the side of the transmission path by a resistor (425; 525; 625; 825; 925; A25) so that the first measuring conductor is coupled to the first end of the resistor and the second measuring conductor to the second end of the resistor, characterized in that coupling to the transmission path for measurement takes place only at one point by a strobe, which comprises, defining against a propagating space of the field of the signal to be measured, said resistor and a sensor conductor (424; 524; 624; 824; 924; A24), which sensor conductor belongs to the first measuring conductor and its centre is located closer to the input port (P1 ) than the centre of the resistor to implement a directivity.

2. A directional coupler according to Claim 1 , characterized in that its transmission path is coaxial, the ground conductor (712; 812; 912; A12) of which having a recess (830; 930; A30), which starts from the outer surface and opens to a cavity of the transmission path for placing the sensor conductor (824; 924; A24) and the resistor (825; 925; A25) into the transmission path so that an electromagnetic field propagating in the cavity has an effect on them.

3. A directional coupler according to Claim 2, characterized in that it comprises in said recess a small circuit board (860; 960), on which the resistor is mounted and where a microstrip on its surface forms the sensor conductor at least partly.

4. A directional coupler according to Claim 3, characterized in that the sensor conductor (824) and the resistor (825) are on the upper surface of said circuit board (860), at an opening (831 ) on the bottom of the recess (830).

5. A directional coupler according to Claim 3, characterized in that the sensor conductor (924) and the resistor (925) are on the lower surface of said circuit board (960), in an opening (931 ) on the bottom of the recess (930).

6. A directional coupler according to Claim 2, characterized in that in the middle of said recess (A30) there is a measuring pin (A21) extending to the cavity of the transmission path, which measuring pin is a part of the first measuring conductor and is at its lower end coupled to the first end of said resistor (A25).

7. A directional coupler according to Claim 6, characterized in that the lower end (A24) of the measuring pin forms said sensor conductor.

8. A directional coupler according to Claim 6, characterized in that the second end of the resistor (A25) is coupled to the ground conductor (A12) of the transmission path.

9. A directional coupler according to Claim 6, in which the measuring line is coaxial and its sheath extends to the level of lower edge of said recess, characterized in that the second end of the resistor is coupled to the sheath of the measuring line.

10. A directional coupler according to Claim 6, characterized in that a space of the recess (A30) surrounding the measuring pin (A21) is at least partly filled by dielectric support material.

11. A directional coupler according to Claim 2, characterized in that said recess is covered by a conductive lid (750; 850; 950; A50).

12. A directional coupler according to Claim 1, characterized in that the centre conductor of its transmission path is a microstrip (511 ; 611 ) belonging to a circuit board (PCB), and the ground conductor comprises at least a ground plane (GND) of the circuit board, and said sensor conductor (524; 624) and resistor (525; 625) are above the transmission path.

13. A directional coupler according to Claim 12, characterized in that the centre conductor of the transmission path is in an intermediate layer of the circuit board, and the sensor conductor and the resistor are on upper surface of the circuit board.

14. A directional coupler according to Claim 12, characterized in that the centre conductor of the transmission path is on upper surface of the circuit board, and the sensor conductor and the resistor are above the upper surface of the circuit board, separated from the upper surface by a dielectric layer.

15. A directional coupler according to Claim 1 , characterized in that said resistor is one of the following: chip resistor, thin-film resistor, thick-film resistor, carbon- film resistor.