Abstract:
A directional coupler for radio frequency application, comprising: an input ( 110 ) for receiving a radio frequency input signal; a port ( 120 ) for delivering a radio frequency output signal; a first elongated conductor ( 150; 150:1 ), suspended in air between two ground planes, for connecting the input ( 110 ) with the port ( 120 ); the first conductor ( 150 ) comprising a sandwich structure with a first upper conductive strip ( 150 A), a first intermediate layer comprising a dielectric material and a first lower conductive strip ( 150 B); a second elongated conductor ( 200; 200:1 ), suspended in air between two ground planes, the second elongated conductor ( 200:1 ) comprising a sandwich structure with a second upper conductive strip ( 200:1 A), a second intermediate layer comprising a dielectric material and a second lower conductive strip ( 200:1 B); said first elongated conductor ( 150; 150:1 ) and said second elongated conductor ( 200; 200:1 ) being substantially parallel; said first upper and lower conductive strips and said second upper and lower conductive strips, respectively, having conductive interconnections ( 190, 210, 158 ).

Description:
TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to a directional coupler, an antenna interface unit, and to a radio base station having an antenna interface unit. 
   DESCRIPTION OF RELATED ART 
   A communications network for mobile radio units such as mobile phones, comprises radio base stations for establishing radio contact with mobile units within a certain range from the radio base station. The area covered by one radio base station, i.e. the range within which radio contact with sufficient quality is obtained, depends among other factors on the power of transmission from the radio base station. In order to ensure that a radio base station has an adequate level of output power, the power of the transmitted signal is often measured, within the radio base station at a point close to the antenna. Such measurement, however, should not contribute more than absolutely necessary to the losses in the system. Also, the reflected power from the antenna is preferably measured for the purpose of ensuring that the antenna is working properly. 
   SUMMARY 
   An aspect of the invention relates to the problem of providing a directional coupler for a radio base station, having high performance characteristics at a reduced cost. 
   This problem is solved, in accordance with an embodiment of the invention, by providing a directional coupler for radio frequency application, comprising:
         an input for receiving a radio frequency input signal;   a port for delivering a radio frequency output signal;   a first elongated conductor, suspended in air between two ground planes, for connecting the input with the port; the first conductor comprising a sandwich structure with a first upper conductive strip, a first intermediate layer comprising a dielectric material and a first lower conductive strip;   a second elongated conductor, suspended in air between two ground planes, the second elongated conductor comprising a sandwich structure with a second upper conductive strip, a second intermediate layer comprising a dielectric material and a second lower conductive strip;   said first elongated conductor and said second elongated conductor being substantially parallel;   said first upper and lower conductive strips and said second upper and lower conductive strips, respectively, having conductive interconnections; wherein   said port for delivering a radio frequency output signal is also arranged to deliver electric power supply to active circuitry connected to said port.       

   This solution advantageously eliminates the need for a separate conductor in order to deliver electric power supply to active circuitry connected to the port. Such active circuitry may be positioned at some distance from the directional coupler, and therefore the elimination of a conductor leads to simplified installation of a radio base station, as well as reduced costs. The solution enables the delivery of the radio frequency output signal and the electric power supply on the same conductor. Therefore the costs are reduced both on account of lower materials costs—one conductor eliminated- and lower labour costs, since fewer conductors need to be installed. 
   Another aspect of the invention relates to a directional coupler for radio frequency application, comprising:
         an input for receiving a radio frequency input signal;   a port for delivering a radio frequency output signal;   a first elongated conductor, suspended in air between two ground planes, for connecting the input with the port; the first conductor comprising a sandwich structure with a first upper conductive strip, a first intermediate layer comprising a dielectric material and a first lower conductive strip;   a second elongated conductor, suspended in air between two ground planes, the second elongated conductor comprising a sandwich structure with a second upper conductive strip, a second intermediate layer comprising a dielectric material and a second lower conductive strip;   said first elongated conductor and said second elongated conductor being substantially parallel;   said first upper and lower conductive strips and said second upper and lower conductive strips, respectively, having conductive interconnections. The conductive interconnections substantially eliminates any electrical field in the dielectric material between them.       

   According to an embodiment of the invention the directional coupler is modified in that air is replaced by inert material or vacuum. 
   According to an embodiment of the directional coupler the first elongated conductor comprises at least one further electrically conductive strip embedded in said first intermediate dielectric layer. The at least one further electrically conductive strip is electrically connected to said first upper and lower conductive strips by means of said conductive interconnections. The provision of this intermediate electrically conductive strip advantageously improves the performance of the directional coupler. 
   According to an embodiment of the directional coupler said port for delivering a radio frequency output signal is connected to a lightning protection device. The provision of a lightning protection device advantageously protects any circuitry coupled to the directional coupler from the electric pulse caused by flashes of lightning hitting the radio base station antenna. 
   A further elongated conductor is connected to the said first elongated conductor, said further elongated conductor being designed such as to cause full reflection of any radio frequency transmission signal T x , whereas the electric pulse caused by a flash of lightning is delivered from said first elongated conductor to the lightning protection device. The lightning protection device is advantageously designed so as to lead said electric pulse to ground, thereby protecting the circuitry coupled to the directional coupler from the electric pulse caused by flashes of lightning. 
   An embodiment of the directional coupler comprises:
         said further elongated conductor suspended in air between two ground planes, the further elongated conductor comprising a sandwich structure with a further upper conductive strip, a further intermediate layer comprising a dielectric material and a further lower conductive strip;   said further elongated conductor making electrical contact with said first elongated conductor; wherein   said further elongated conductor is provided with a reflecting impedance at a distance from said first elongated conductor. The reflecting impedance provides a matched input for radio frequency signals within a certain bandwidth. The reflecting impedance may comprise a capacitive load at a certain distance, along the conductor, from said first elongated conductor. The reflecting impedance may advantageously be adapted to cause full reflection of a radio frequency transmission signal T x .       

   The dielectric substrate may be provided with cut out portions in the region adjacent to the sides of said further elongated conductor. Therefore the electric fields in that region will propagate in air (or in another inert material or vacuum), rather than in a dielectric substrate material. The radio frequency losses in the circuitry are dependent on the dissipation factor of the material through which the electric field propagates. Hence, there will be very low losses in said further elongated conductor. This is advantageous since it reduces losses for the signal T x  as it travels to the reflecting impedance and back again. 
   According to an embodiment said further elongated conductor widens to form a patch just after the reflecting impedance, as seen from said first elongated conductor. According to an embodiment this patch is a multi-layer patch; said multi-layer patch being provided with a plurality of conductive interconnections providing electrical contact between plural conductive layers of said patch. This advantageously minimizes the power generated at said patch in connection with a flash of lightning. 
   When a flash of lightning hits an antenna connected to the port for delivering a radio frequency output signal, a large current is to be drained from that port to the lightning protection device. The power generated in a conductor depends on the current and the resistance, as defined e.g by Ohms law: P=U*I=R*I 2 . The further elongated conductor advantageously comprises a plurality of conductive strips, thereby reducing the resistance between the port for delivering a radio frequency output signal and the widened part of the further elongated conductor. Hence the power, and the corresponding heat, generated in the further elongated conductor is minimized. 
   According to an embodiment said port comprises a patch which is provided with a plurality of conductive interconnections providing electrical contact between plural conductive layers of said patch. This advantageously minimizes the power generated at said port in connection with a flash of lightning. 
   Advantageously the further elongated conductor comprises more than two conductive layers. 
   According to an embodiment the directional coupler comprises
         a strip line for coupling said first elongated conductor to said input for receiving a radio frequency input signal. According to one version of the invention the directional coupler further comprises   a high pass filter connected between said strip line and said first elongated conductor. Said high pass filter is adapted to permit the passage of said radio frequency input signal.       

   An embodiment of the invention relates to an antenna interface unit comprising
         a first directional coupler, and   a second directional coupler; said first and second directional couplers being provided on a common printed circuit board.       

   According to an embodiment of the antenna interface unit
         the first directional coupler has a first port for delivering a radio frequency output signal, said first port being arranged to deliver electric power supply to first active circuitry connected to said first port; and   the second directional coupler has a second port for delivering a radio frequency output signal, said second port being arranged to deliver electric power supply to second active circuitry connected to said second port.       

   According to an embodiment of the antenna interface unit
         said first port and said second port are connected to a common input for receiving a DC power signal. In one version of this antenna interface unit said second port is connected to said common input by mans of a conductor including at least a portion positioned in an intermediate conductive layer. Advantageously this conductor can provide delivery of said DC power signal from said common input to said second port via the intermediate conductive layer which is separate from said strip line for coupling said first elongated conductor to said input for receiving a radio frequency input signal. This solution provides a compact circuit for handling the RF signals, the power supply as well as lightning protection.       

   An embodiment of the directional coupler further comprises:
         a third elongated conductor, suspended in air between said ground planes, the third elongated conductor comprising a sandwich structure with a third upper conductive strip, a third intermediate layer comprising a dielectric material and a third lower conductive strip;   said first elongated conductor and said third elongated conductor being substantially parallel;   said third upper and lower conductive strips having conductive interconnections for substantially eliminating any electrical field in the dielectric material between them; wherein   said third conductor is shaped and positioned such as to provide a coupled output indicative of a power of a radio frequency signal propagating in a direction from said a port towards said input. According to an embodiment said a third elongated conductor is separate from said second elongated conductor.       

   According to an embodiment of the directional coupler
         said second elongated conductor is provided along one side of said first elongated conductor; and
 
said third elongated conductor is provided along another side of said first elongated conductor.
       

   Further variations and embodiments of the invention are provided in the enclosed specification and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For simple understanding of the present invention, it will be described by means of the examples and with reference to the accompanying drawings, of which: 
       FIG. 1  illustrates a radio base station having an antenna placed on high ground for providing good radio coverage to mobile units in the geographic neighbourhood. 
       FIG. 2  is a schematic block diagram illustrating a transceiver/receiver unit having an input for receiving a message to be transmitted. 
       FIG. 3  is top plan view of an embodiment of the antenna interface unit including an embodiment of the directional coupler. 
       FIG. 4  is a cross-sectional view taken along line A—A of  FIG. 3 . 
       FIG. 5  is an enlarged view of a part of  FIG. 3 , showing the third conductor. 
       FIG. 6  illustrates a multi-layer embodiment of the antenna interface unit shown in  FIGS. 4 and 3 . 
       FIG. 7  shows a schematic block diagram of another embodiment of the radio base station parts shown in  FIG. 2 . 
       FIG. 8  is a top plan view of a printed circuit board (pcb) in an antenna interface unit according the embodiment described in  FIG. 7 . 
       FIG. 9  is a cross-sectional view taken along line B—B of  FIG. 8 , additionally showing a corresponding cross-section of the casing with lid for the sake of improved clarity. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   In the following description similar features in different embodiments will be indicated by the same reference numerals. 
     FIG. 1  shows a radio base station  10  having an antenna  20  placed on a hill for providing good radio coverage to mobile units  30  in the geographic neighbourhood. 
     FIG. 2  is a schematic block diagram illustrating a transceiver/receiver unit  40  having an input  50  for receiving a message to be transmitted. The transceiver unit  40  has an output  90  for providing a radio frequency transmission signal, modulated with the message, to the antenna  20 . The output  90  of the transceiver unit  40  is connected to the antenna  20  via an antenna interface unit  100 . Hence, the antenna interface unit  100  has an input  110  coupled to the output  90  of the transceiver unit  40 , and a port  120  for providing the radio frequency transmission signal to the antenna  20 . The antenna interface unit  100  includes a directional coupler  122  having an output  130  for a feedback signal. The output  130  is coupled to a feedback input  140  of the transceiver unit  40 . 
   The feedback signal T xmeasure  received on the output  130  is indicative of the power of the transmission signal delivered from the port  120  of the antenna interface unit. Hence, the feedback signal T xmeasure  can be used in the transceiver unit  40  for controlling the transmission power of the radio base station  10  so as to provide radio coverage to mobile units  30  in an area of a desired size in the geographic neighbourhood. 
   The radio frequency transmission signal may have any frequency suitable for radio communication. According to some embodiments of the invention the radio frequency transmission signal may have a frequency of 350 Mhz or higher. 
   According to preferred embodiments of the invention the frequency may be higher than 800 MHz. 
     FIG. 3  is top plan view of an embodiment of the antenna interface unit  100  including an embodiment of the directional coupler  122 . The directional coupler  122  includes a substrate  142  mounted in a casing  144 . The substrate  142  is provided with an elongated electrically conductive strip  150 A, connecting an input patch  110 A to a port patch  120 A. The substrate in combination with the conductors and other components forms a printed circuit board (pcb) 143 . The input  110  may include a coaxial contact having a centre conductor  154  for contacting input patch  110 A at the end of the conductor  150 , as illustrated in  FIG. 3 . Similarly, port  120  may include a coaxial contact having a centre conductor  156  for contacting input patch  120 A at the opposite end of the conductive strip  150 A. Input patch  110 A and port patch  120 A are densely provided with plated through openings  158  providing good electrical contact between the conductive layers at the two opposite ends of the conductor  150 . 
     FIG. 4  is a cross-sectional view taken along line A—A of  FIG. 3 . As shown in  FIG. 4 , the pcb  143  rests on shoulders  160  in the casing  144 . The pcb  143  may be firmly attached to the casing by means of screws (not shown) introduced through suitable openings in the casing lid  170  ( FIG. 5 ) and through openings  180  ( FIG. 3 ) in the pcb  143  and casing  144 . 
   The casing  144 , and lid  170  can be made of an electrically conductive material, such as an aluminium alloy. When the lid  170  is attached to the bottom part  144  of the casing the pcb  143  will be confined in a closed chamber. The conductive walls of the chamber are connected to ground so as to provide ground planes in relation to conductors on the substrate  142 . The chamber may be filled with air, or another inert material. The inert material may be an inert gas. Alternatively, there may be a vacuum, instead of inert material, in the chamber. 
   The conductive strip  150 A is electrically connected to another conductive strip  150 B on the opposite side of the dielectric substrate  142  by means of plated through openings  190  ( FIGS. 3 and 4 ). Hence, the conductive strips  150 A and  150 B form an elongated conductor  150  connecting the input  110  with the port  120 . Since the strips  150 A and  150 B are interconnected they will have the same electrical potential, and hence there will be substantially no electrical field in the dielectric substrate between the strips  150 A and  150 B. Instead, when a radio frequency transmission signal T x  is supplied to the input  110 , there will be an electric field extending between the conductive strip  150 A and the ground plane formed by lid  170 . Additionally an electric field will extend between the conductive strip  150 B and the ground plane formed by inner wall  192  of casing  144  ( FIG. 4 ). 
   The antenna interface unit  100  also includes a second elongated conductor  200 , having conductive strips  200 A and  200 B on opposite sides of the substrate  142 , as illustrated in  FIGS. 4 and 3 . The elongated conductive strips  200 A and  200 B are interconnected by plated through openings  210  ( FIGS. 3 and 4 ). The interconnection of the strips  200 A and  200 B provides for a common electric potential, thereby substantially eliminating any electric fields in the substrate between the strips  200 A and  200 B, as mentioned above in connection with strips  150 A and  150 B. 
   Each one of the conductive strips  150 A,  150 B,  200 A,  200 B may comprise a metal layer, such as e.g. copper, aluminium or gold. The conductive plating in the openings  190 ,  210  is preferably made in the same material as the corresponding metal strip. 
   The pcb  143  is provided with a cut out portion  220  in the region between the conductor  200  and the conductor  150 . Therefore the electric fields in that region will propagate in air (or another inert material or vacuum), rather than in a dielectric substrate material. The losses in the circuitry are dependent on the dissipation factor of the material through which the electric field propagates. In vacuum the dissipation factor equals zero, rendering vacuum a medium without any loss. The dissipation factor of a substrate made by glass fibre reinforced epoxy resin typically has a value in the range from 0,003 to 0,2. Air has a dissipation factor very close to that of vacuum, i.e. very near zero. In this context the term “very near zero” is a value significantly smaller than 0,003. 
   With reference to  FIG. 3 , the second conductor  200  has a first conductor portion  230  parallel with a portion  240  of the first conductor  150 . The second conductor  200  also has a single second conductor portion  250  extending in a direction perpendicular to the extension of the first conductor portion  230 . The second conductor portion  250  includes an output patch  260  connected to the output  130  of the antenna interface unit via a strip line  252 . The output  130  may include a coaxial contact having a centre conductor  254  for contacting a pad  256  at the end of the strip line  252 , as illustrated in  FIG. 3 . 
   In operation, when a transmission signal propagates from the input  110 , via the first conductor  150 , to the port  120 , a certain proportion of the transmission signal will be coupled to the second conductor  200 . The coupled signal propagates via the second conductor portion  250  to the output  130  of the antenna interface unit. 
   The cut out portion  220  extends along the side of the first conductor portion  230  facing towards the first conductor  150 . The cut out portion  220  also extends along the side of the second conductor portion  250  such that an electric field in the vicinity of the second conductor  200  on the sides facing the first conductor  150  and the input patch  110 A will propagate in air (or in another inert material or vacuum). The fact that the cut-out portion provides a gap along the whole length of the side of conductor  200  advantageously lowers losses. 
   A problem related to the radio base stations, in particular when placed at a high position in relation to the geographic neighbourhood, is that high objects such as antennae are prone to attract flashes of lightning  265  ( FIG. 1 ) when there are thunderstorms. A large proportion of the energy of such a flash passes through the casing  267  of the radio base station tower ( FIG. 1 ), but a certain amount of energy often travels along the transmission/reception (T x /R x ) signal path from the antenna  20  towards the transceiver unit  40  ( FIG. 2 ). This energy may appear as a pulse having a duration of e.g. 350 microseconds and a rise time of some 10 microseconds. The energy pulse from a flash of lightning may generally be in the one megahertz frequency band, which is to be considered a low frequency band in relation to the frequency of the transmission signal T x . 
   In order to protect sensitive parts in the radio base station, the antenna interface unit  100  is therefore provided with a set of lightning protection devices. According to an embodiment of the invention the antenna interface unit  100  includes a third conductor  270  connected to the port patch  120 A ( FIG. 3 ). The third conductor  270  has conductive strips  270 A and  270 B on opposite sides of the substrate  142 , as illustrated in  FIGS. 4 and 3 . The elongated conductive strips  270 A and  270 B are interconnected by plated through openings  280  ( FIGS. 3 and 4 ). The interconnection of the strips  270 A and  270 B provides for a common electric potential, thereby substantially eliminating any electric fields in the substrate between the strips, as mentioned above in connection with strips  150 A and  150 B. 
     FIG. 5  is an enlarged view of a part of  FIG. 3 , showing the third conductor. The third conductor  270  is connected to the port patch  120 A and designed such as to cause full reflection of any radio frequency transmission signal T x , whereas the electric pulse from a flash of lightning is forwarded to a lightning protection unit  290 . The lightning protection unit  290  is designed so as to lead said electric pulse to ground. 
   According to an embodiment of the invention, the third conductor  270  is provided with a capacitive load  300  at a distance D, along the conductor, from the port patch  120 A ( FIG. 3 ) in order to cause full reflection of any radio frequency transmission signal T x . The capacitive load may comprise two capacitors  300 , as illustrated in  FIGS. 3 and 5 . 
   The dielectric substrate  142  is provided with cut out portions  310 ,  320  in the region adjacent to the sides of the conductor  270 . Therefore the electric fields in that region will propagate in air (or another inert material or vacuum), rather than in a dielectric substrate material. The radio frequency losses in the circuitry are dependent on the dissipation factor of the material through which the electric field propagates. Hence, there will be very low losses in the conductor  270 , which is advantageous since it reduces losses for the signal T x  as it travels between patch  120 A and reflecting impedance  300 . 
   Just after the load  300 , as seen from the port patch  120 A, the conductor  270  widens to form a patch  302 . 
   When a flash of lightning  265  hits the antenna  20  ( FIG. 1 ), a large current is to be drained from port  120  to lightning protection unit  290 . The power generated in a conductor depends on the current and the resistance, as defined e.g by Ohms law: P=U*I=R*I 2 . The conductor  270  advantageously comprises a plurality of conductive strips, as described above, thereby reducing the resistance between port  120  and patch  302 . Hence the power, and the corresponding heat, generated in conductor  270  is minimized. 
   Moreover, the patch  302  is densely provided with plated trough openings  304  providing interconnections between the plurality of conductor layers. A dense provision of plated openings  304  in patch  302  minimize the resistance, thereby enabling the supply of relatively high peak currents from the other conductive layers to the top layer  302 A. 
   According to an embodiment the lightning protection unit  290  comprises a gas-filled surge arrester  290 , such as e.g. SIEMENS Type A81-C90XMD. According to an embodiemnt the surge arrester  290 , acting as a primary protection unit, cooperates with secondary protection units, such as overvoltage arresters. The lightning protection unit  290  has a first terminal coupled to the patch  302 A, and another terminal connected to a ground patch  324 . The patch  324  is a portion of a large ground layer, which is densely provided with plated trough openings  305  providing interconnections with other conductive layers having ground potential. The dense provision of plated openings  305  in ground patch  324  minimises the resistance, thereby enabling the supply of relatively high peak currents from the first terminal of the lightning protection unit via the patch  302 A to the other conductive layers of ground patch  324 . 
   According to a preferred embodiment the distance D is substantially one quarter of a wavelength of the radio frequency transmission signal. The distance D may also be: 
   
     
       
         
           
             
               
                 
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   Since, according to an embodiment of the invention, the dielectric substrate  142  is provided with cut out portions  310 ,  320  along the sides of the conductor  270  any signal in conductor  270  will propagate through air. Hence, for the purpose of defining the distance D, ε r  will be the dielectric constant for air. Air has a dielectric constant of 1,00059, whereas a substrate made by glass fibre reinforced epoxy resin typically has a dielectric constant value of about 3,3. 
     FIG. 6  illustrates a multi-layer embodiment of the antenna interface unit  100  shown in  FIGS. 4 and 3 . Hence,  FIG. 6  is a cross-sectional view taken along line A—A of a multi-layer embodiment of the antenna interface unit shown in  FIG. 3 . In addition to conductor layers A and B, there is provided intermediate layers C and D, also interconnected by means of plated through openings. 
     FIG. 7  shows a schematic block diagram of another embodiment of the radio base station parts shown in  FIG. 2 . A first transceiver unit  40 : 1  includes a modulator unit  330 : 1  having an input  340 : 1  for receiving a message to be transmitted. The modulator unit  330 : 1  has an output  350 : 1  for providing a radio frequency transmission signal, modulated with the message, to an adjustable attenuator  360 : 1 , which in turn delivers the attenuated signal to a power amplifier  370 : 1 . The output T x  of the power amplifier  370 : 1  is delivered to an input  110 : 1  of an antenna interface unit  100 . 
   The antenna interface unit  100  has a port  120 : 1  for providing the radio frequency transmission signal to the antenna  20 : 1 . The antenna interface unit  100  includes a directional coupler having an output  130 : 1  for a feedback signal T xmeasure  indicative of the power of the output signal delivered on the port  120 : 1 . The directional coupler also includes another output  380 : 1  for a signal indicative of a signal T xreflected:1  reflected from the antenna  20 : 1  to the antenna interface unit  100 . The power of the signal T xreflected:1  is compared to a reference value, and if it deviates from certain limit values the controller  395 : 1  delivers an alarm signal to an alarm unit  372 : 1 . 
   The output  380 : 1  is coupled to a feedback input  390 : 1  of a control unit  395 : 1 . The output  130 : 1  is coupled to a feedback input  400 : 1  of the control unit  395 : 1 . The controller  395 : 1  receives, on an input  410 : 1 , a signal indicative of the power of the radio frequency signal delivered from the modulator  330 : 1  to the attenuator  360 : 1 . 
   A problem in connection with radio base stations is that the total attenuation or amplification of the signal, counted from the output  350 : 1  to the antenna  20 : 1 , varies in dependence on temperature and other variable factors. I order to compensate for this variation the controller adjusts the total amplification of  360 : 1 ,  370 : 1  by controlling attenuator  360 : 1  so as to maintain a pre-determined output power level to the antenna  20 : 1 . For this purpose the controller delivers a control signal on an output  420 : 1  to a control input  430 : 1  on the attenuator. Hence, the controller adjusts the attenuation in dependence on the signals received on inputs  410 : 1  and  400 : 1  such that the power level of the signal T xmeasure:1  is kept equal to a reference value. Since T xmeasure:1  is indicative of the signal power delivered to the antenna  20 : 1 , this solution will eliminate or significantly reduce the undesired variation of the output signal power. 
   A second transmitter unit  40 : 2  functions in the same manner for another message delivered on an input  340 : 2 , in relation to another antenna  20 : 2 . 
   A DC Power supply unit  440  delivers a power supply voltage to a DC power input  450  of the antenna interface unit  100 . The antenna interface unit  100  is advantageously adapted to enable provision of a DC power signal on the ports  120 : 1 ,  120 : 2 , i.e. on the same port as the radio frequency transmission signal T x1  and T x2 , respectively. The DC power supply signal delivered on the port  120 : 1  is separated from the radio frequency transmission signal T x1  by a filter  452 : 1 , and the DC power signal is delivered to the power input  460 : 1  of an amplifier  470 : 1  (often referred to as tower mounted amplifier, TMA). The filter  452 : 1  may be embodied by a capacitor, just like capacitor  540 : 1  in  FIG. 8 . The amplifier  470 : 1  operates to amplify the signal R x  received by the antenna  20 : 1 . A filter  480 : 1  is adapted to deliver the signal R x  received by the antenna  20 : 1  to the amplifier  470 : 1 , and the amplified R x  signal is delivered to the contact  120 : 1 , via another filter  482 : 1 , so that the received signal R x  can propagate through the antenna interface unit in the direction opposite of the T x  signal. A filter  490 : 1  in transceiver  40 : 1  separates the signal R x  and delivers to the circuitry  500 : 1  designated for demodulation etc. 
     FIG. 8  is a top plan view of a printed circuit board (pcb)  510  in an antenna interface unit  100  according the embodiment described in  FIG. 7 . The pcb  510  includes a conductive pad  520  connected to the input  450  ( FIG. 7 ) for receiving the DC power signal. A conductor  530 : 1  ( FIG. 8 ) delivers the DC signal to the patch  302 : 1 , which is connected to the T x  signal output port  120 : 1  via the flash pulse protection conductor  270 : 1 . Hence, the DC power signal is provided to the DC separation filter  452 : 1  as described with reference to  FIG. 7  above. 
   Another conductor  530 : 2  delivers the DC signal from the pad  520  to the patch  302 : 2 , which is connected to the T x  signal output port  120 : 2 . Hence, the DC power signal is provided to the DC separation filter  452 : 2  as described with reference to  FIG. 7  above. As illustrated in  FIG. 8  the conductor  530 : 2  includes a portion  532  where it runs in an intermediate conductive layer, i.e. in a conductive layer between the top conductive layer A and bottom conductive layer B. 
   In order to prevent the DC power signal from propagating to the first T x  signal input  110 : 1  ( FIG. 7 ) of the antenna interface unit, there is provided a high pass filter  540 : 1 . The high pass filter  540 : 1  functions as a DC-blocker and to let the RF signal pass. According to the illustrated embodiment the DC-blocker  540 : 1  is embodied by a surface mounted capacitor having a capacitance selected so as to permit the passage of the T x  signal and the R x  signal. As illustrated in  FIG. 8 , the DC blocker  540 : 1  is provided between the stripline  560 : 1  and the conductor  150 : 1 . Similarly, there is another DC-blocker  540 : 2  provided between the stripline  560 : 2  and the conductor  150 : 2 . Therefore the DC power supply delivered via conductors  532 ,  530 : 2  and  270 : 2  to port  120 : 2  is prevented from reaching the RF input  550 : 2 . 
   The T x  signal input  110 : 1  ( FIG. 7 ) is connected to a pad  550 : 1  by means of a centre conductor  154 : 1  (like the centre conductor  154  described in connection with  FIG. 3  above). Pad  550 : 1  connects to a stripline  560 : 1  adapted to deliver the T x  signal to a patch  110 A: 1  which is densely provided with plated through openings  158 : 1  providing good electrical contact between the conductive layers of conductor  150 . 
   With reference to  FIG. 9  a conductive strip  150 A: 1  is electrically connected to another conductive strip  150 B: 1  on the opposite side of the dielectric substrate  542  by means of plated through openings  190 . The conductive strips  150 A and  150 B, sandwiched together with intermediate layers of dielectric material and conductor layers  150 C: 1  and  150 D: 1  form an elongated multi-layer conductor  150 : 1  connecting the input  110 : 1  with the port  120 : 1 . In the same manner as described with reference to  FIG. 3  above there will be substantially no electrical field in the dielectric substrate. 
   The antenna interface unit  100  also includes a second elongated multi-layer conductor  200 : 1  ( FIG. 8 ), having conductive strips  200 A: 1  and  200 B: 1  (Not shown) on opposite sides of the substrate  542  and intermediate conductive strips  200 C: 1  and  200 D: 1  (Not shown). The elongated conductive strips  200 A and  200 B are interconnected by plated through openings  210 : 1  ( FIG. 8 ) providing for a common electric potential, thereby substantially eliminating any electric fields in the substrate between the strips  200 A and  200 B. 
   Additionally, the antenna interface unit  100  also includes another elongated multilayer conductor  570 : 1  ( FIG. 8 ), having conductive strips  570 A: 1  and  57013 : 1  (Not shown) on opposite sides of the substrate  542  and intermediate conductive strips  570 C: 1  and  570 D: 1  (Not shown), interconnected in the same manner as described above. The conductor  570 : 1  is shaped in a similar way to conductor  200 : 1 , but is positioned such as to provide a coupled output indicative of the power of the T x  signal which is reflected from the antenna  20 : 1 . Conductor  570 : 1  has patch  580 : 1  dense with plated through openings connecting to a stripline  590  leading the coupled signal T xreflected  to a contact pad  600 : 1 . Contact pad  600 : 1  connects to a coaxial contact embodying the output  380 : 1  which is described above in connection with  FIG. 7 . As mentioned above this signal may be used for error detection purposes, including the generation of an alarm in case of detected abnormal reflected signal values. 
   The elongated conductive strips  570 A: 1  and  570 B: 1  are interconnected by plated through openings  602 : 1  ( FIG. 8 ) providing for a common electric potential, thereby substantially eliminating any electric fields in the substrate between the strips  570 A: 1  and  570 B: 1 . 
   Each one of the conductive strips may comprise a metal layer, such as e.g. copper, aluminium or gold. The conductive plating in the openings is preferably made in the same material as the corresponding metal strip. 
   The pcb  510  is provided with a cut out portions forming gaps on both sides of conductor  150 : 1 , on both sides of conductor  200 : 1  and on both sides of conductor  570 : 1 . As illustrated in  FIG. 8 , the pcb  510  is provided with a cut out portions forming gaps on both sides of conductor  270 : 1  as well. In  FIG. 8  the cut out portions-or gaps-of the pcb  510  are indicated by dotted areas. Shaded areas in  FIG. 8  indicate bare dielectric material providing isolation from other neighbouring conductors or ground planes. 
   Therefore the electric fields in that region will propagate in air (or another inert material or vacuum), rather than in a dielectric substrate material. The radio frequency losses in the circuitry are dependent on the dissipation factor of the material through which the electric field propagates. In vacuum or free space the dissipation factor equals zero, rendering free space a medium without any loss. The dissipation factor of a substrate made by glass fibre reinforced epoxy resin typically has a value in the range from 0,003 to 0,2. Air has a dissipation factor very close to that of vacuum, i.e. very near zero. In this context the term “very near zero” is a value significantly smaller than 0,003. 
   The bandwidth of the conductor  270 : 1  depends on the width of the conductive strips, the distance D (described in connection with  FIG. 5  above) and the capacitance in the capacitive load  300 . By decreasing the width of the conductor  270 : 1 , the bandwidth will be increased. The provision of cut out portions forming gaps on both sides of conductor  270 : 1  renders a higher impedance in conductor  270 : 1  than the case would be with solid dielectric material near the sides of conductor  270 : 1 . This has to do with the value of the relevant dielectric constant. Advantageously, the provision of cut out portions forming gaps on both sides of conductor  270 : 1  also improves the bandwidth of conductor  270 : 1 . Tests indicate that the radio frequency bandwidth of conductor  270 ,  270 : 1  increases more than 15 percent when dielectric material near the sides of conductor  270 : 1  is removed so as to be replaced by cut out portions forming gaps. 
   Improved Directivity 
   With reference to  FIG. 8  the directive coupler formed by conductors  150 : 1 ,  200 : 1  and  570 : 1  provides an advantageously good directivity, thereby providing for accurate signal measurements. With regard to the primary conductor  150 : 1  along which radio signal T x  travels from pad  550 : 1  to port  120 : 1 , the conductor  200 : 1  is a secondary conductor. Due to the geometry and the fact that conductor  200 : 1  is parallel to primary conductor  150 : 1  the degree of coupling between the conductors is predictable. The fact that the pcb can be produced in a rational manner, by etching pressing, drilling the cut outs, and plating/etching before finally milling, provides a stable production method rendering a low cost antenna interface unit. The fact that the flash protection circuitry is integrated on the pcb additionally reduces the number of separate circuits and casings needed, thereby further improving the cost benefit of the present solution. 
   The coupling between conductors  150 : 1  and  200 : 1  is such that the signal T x  travelling from pad  550 : 1  to port  120 : 1  is coupled so as to produce a measured signal T xmeasure  at the upper end of the conductor  200 : 1  as seen in  FIG. 8 . Similarly a certain proportion of a reflected signal T xreflected , travelling along conductor  150 : 1  in the direction from port  120 : 1  to pad  550 : 1  generates a signal in the lower end of conductor  200 : 1  as seen in  FIG. 8 . In order to eliminate any interference in the measurements from this undesired signal, there is provided a balanced termination impedance  610 : 1 . The termination impedance  610 : 1  is connected from the end of conductor  200 : 1  to ground. Ground is provided as a large conductive layer in the top or A-layer of the pcb  510 . 
   The value of the impedance  610 : 1  is preferably selected to a value identical to the impedance seen when looking into the coupler from the end of conductor  200 , i.e. when looking from the position of impedance  610 : 1 . In a preferred embodiment the value of the impedance  610 : 1  will be 50 ohm. Due to the advantageous fact that the conductors are surrounded only by air such that all coupled electric energy has passed through the same medium- air- the coupled signal will be of substantially one single phase. This in turn provides for a resulting high degree of directivity. 
   The air, mentioned above, may be replaced by another inert material or vacuum while maintaining the advantageous properties. 
     FIG. 9  is a cross-sectional view taken along line B—B of  FIG. 8 , additionally showing a corresponding cross-section of the casing  144  with lid  170  for the sake of improved clarity. 
   As illustrated on the left hand side in  FIG. 9  conductor  150 : 1  includes four conductive layers, sandwiched by dielectric layers and interconnected by plated openings  190 . At the portion with an extra high concentration of plated openings  158 : 1  the four layer conductor is transformed to a strip line  630 : 1  leading to DC stop capacitor  540 : 1 . The surface conductive layer is interrupted under the surface mounted capacitor  540 : 1  so as to hinder DC current from flowing to strip line  560 : 1 .