Abstract:
A radar level gauge using electromagnetic signals to determine a filling level of a product in a tank, comprising an electronics unit including transceiver circuitry, a signal propagation device arranged to direct electromagnetic signals into the tank and to return electromagnetic signals reflected from the tank, and a directional coupler. The gauge further comprises a wave guiding structure comprising a rod of a dielectric material. A mounting portion of the electronics unit defines an opening and is detachably mountable on the dielectric rod so that the rod extends through the opening and is at least partly surrounded by the opening. The mounting portion is further arranged to secure the circuitry portion relative the rod so that said directional coupler is in a position to couple signals between the transceiver circuitry and the rod. 
     The radar level gauge according to the present invention facilitates replacement of an electronics unit, e.g. in case of malfunction or upgrades. At the same time, the dielectric rod enables a very simple, cost-effective and reliable feeding to and from each electronics unit.

Description:
FIELD OF THE INVENTION  
       [0001]    The present invention relates to a radar level gauge using electromagnetic waves to determine a filling level of a product in a tank. The gauge comprises an electronics unit including transceiver circuitry for transmitting and receiving electromagnetic waves, and processing circuitry connected to the transceiver circuitry and adapted to determine the process variable based on a relation between transmitted and reflected waves. 
       BACKGROUND OF THE INVENTION  
       [0002]    Radar level gauging (RLG) to measure the level of a filling material, such as a liquid or a solid like a granulate is an increasingly important method for level gauging in tanks, containers, etc. Many different types of RLG systems are previously known. An example of such a systems is disclosed in U.S. Pat. No. 7,106,247 assigned to Rosemount Tank Radar, and includes an antenna inside the tank and an electronics unit containing transceiver circuitry and processing circuitry outside the tank. There are many ways to connect the electronics unit with the antenna in the tank, typically including some sort of wave guiding structure or a coaxial connector, 
         [0003]    However, in some applications it is useful to arrange the electronics unit of the RLG so as to be easily replaceable and exchangeable. Also, in some applications, it is desirable to connect several electronics units providing functionally independent measuring channels to the same antenna. Independent radar level gauges using the same physical antenna but otherwise electrically independent provides redundancy at a low cost. For example, to connect more than one sensor to one antenna is a very cost effective way to implement a system with a level sensor and an independent overfill alarm, etc, and has gained wide acceptance among users and authorities. 
         [0004]    An example of such a system using several channels for feeding one and the same antenna is disclosed in WO 03/025523, assigned to Rosemount Tank Radar.  FIG. 1  shows an example of a prior art arrangement of this kind, with three electronics units  101 ,  102 ,  103  connected to a waveguide  104  connected to an antenna (not shown). The electronics units are connected to the waveguide by means of a connection device, for example a turnstile junction  105 . The electronics units are connected to the connection device using conventional terminals, such as coaxial connectors. One of the electronics units is connected to a first probe  106  coupling energy having a first polarization into the waveguide. The other two electronics units are connected to a second probe  107  via a power divider  108  which is part of the turnstile junction. 
         [0005]    Another example is disclosed in U.S. Pat. No. 7,701,385, assigned to Rosemount Tank Radar. In this case, two or more electronics units are combined to form a pineapple slice shape, the hole of which may form part of the wave guide, 
       GENERAL DISCLOSURE OF THE INVENTION  
       [0006]    It is an object of the present invention to provide a radar level gauging system with one or several easily replaceable electronics units connected to the same antenna. According to an aspect of the present invention this object is achieved by a radar level gauge using electromagnetic signals to determine a process variable of a product in a tank, comprising an electronics unit comprising a mounting portion and a circuitry portion including transceiver circuitry for transmitting and receiving electromagnetic signals, processing circuitry connected to the transceiver circuitry and adapted to determine the filling level based on a relation between transmitted and received signals, and a directional coupler. The gauge further comprises a signal propagation device arranged to direct electromagnetic signals into the tank and to return electromagnetic signals reflected from the tank, and a wave guiding structure arranged to guide the electromagnetic signals between the electronics unit and the signal propagation device, the wave guiding structure comprising a rod of a dielectric material. The mounting portion defines an opening and is detachably mountable on the dielectric rod so that the rod extends through the opening and is at least partly surrounded by the opening. The mounting portion is further arranged to secure the circuitry portion relative the rod so that said directional coupler is in a position to couple signals between the transceiver circuitry and the rod. 
         [0007]    By “detachably mountable” is intended to mean that the unit may be replaced by a technician in the field, without requiring extensive equipment. The radar level gauge according to the present invention facilitates replacement of an electronics unit, e.g. in case of malfunction or upgrades. Mounting is not critical as there are no metallic connections. Each mounting portion can simply be pushed on or pulled off the dielectric rod. It should be noted that in this case where the tank contains pressurized or toxic contents, and is hermetically sealed from the outside, the electronics unit may detached from the rod under maintained sealing of the tank. 
         [0008]    At the same time, the dielectric rod enables a very simple, cost-effective and reliable feeding to and from each electronics unit. 
         [0009]    The directional coupler ensures that the transmitted power is directed downwards to the antenna and that reflected power can be received the same way. By the directional coupling structure two, or possibly three, units with the same polarization can be stacked on each other but with an acceptable power loss and assuming there is sufficient distinction of some of the signal parameters. It is noted that polarization typically provides signal isolation of around 18-20 dB, while a directional coupling typically provides signal isolation of around 15-20 dB. 
         [0010]    The mounting portion and the circuitry portion may be formed as one integrated unit, even further facilitating any handling of the electronics unit. However, it is also possible to have the mounting portion as a separate element, to which the circuitry portion is attachable. In such a case, it may be possible to detach the circuitry portion from the rod without removing the mounting portion. 
         [0011]    According to one embodiment, the dielectric rod is adapted to act as a surface wave guide. In this case, the rod is not surrounded by any conducting enclosure, and this design therefore effectively prevents any gas leakage along the waveguide. In this case, the antenna may be a rod antenna and the dielectric rod may be an extension of the antenna. This provides a mechanically very simple and robust system, 
         [0012]    Alternatively, the wave guiding structure may further comprise an intermediate transition structure for matching the dielectric rod surface wave guide to a hollow wave guide. A transition to a hollow wave guide may be desirable in connection to certain types of antennas. 
         [0013]    By “hollow” waveguide is here intended a waveguide where the conducting material encloses a hollow space, which can be empty or filled with a suitable dielectric material. The hollow waveguide can be tubular with a suitable cross section, and in a preferred embodiment it has a circular cross section. 
         [0014]    According to another embodiment the dielectric rod is surrounded by a conducting enclosure so as to form a hollow (but dielectric fined) waveguide. In this case, a wall of the mounting portion delimiting the opening may form part of the conducting enclosure. 
         [0015]    The present invention may be useful when several electronics units are connected to the same antenna. In this case, the dielectric rod should have a length allowing several electronics units to be arranged along the rod on top of each other. The units are thus stacked on top of each other in the axial direction of the rod. The units may rest on each other, or be separated by suitable distance element(s), or be secured at desired locations by means of suitable fastening elements. 
         [0016]    Each electronics unit may provide a functionally independent measurement channel. For example, in radar level gauging systems for tank ships, at least one overfill alarm that is functionally independent of the level measuring system is required. Functional independence here means that a fault in one channel should not make another channel inoperable, so that a level measurement may still be performed in the other channel. 
         [0017]    A channel is here defined as all the electronics, including microwave transmitter and receiver, that is needed to generate, transmit, distribute and receive the microwave signals up to the microwave waveguide. Such independence can be achieved by ensuring that there are no common electrical circuits and cabling. However, fixed mechanical constructions for example, which cannot normally go wrong, may be shared. 
         [0018]    The microwave signals used in the separate channels must be distinguishable from each other. This may be arranged by generating microwave signals for the different channels to have, for example, different polarization, different modulation, different frequencies or being separated in time. 
         [0019]    In order to ensure functional independence, each electronics unit may be electronically and galvanically separated from other electronics units. The expression electronically and galvanically separated should here be understood to mean that the electronic units are electrically separated and isolated from each other. Typically, but not necessarily, the electronic units are also formed as physically individual units, that are individually mounted on the dielectric rod. 
         [0020]    Two electronics units may be arranged to transmit waves having different polarization to enable separation of channels. For example, two electronics units, adapted to transmit linearly polarized radiation may be rotated 90 degrees with respect to each other. This ensures that electromagnetic waves emitted from a first of these electronics units have a polarization orthogonal to electromagnetic waves transmitted from a second of these electronics units. 
         [0021]    If circular polarization is implemented, reflected waves will have an opposite polarization compared to the emitted waves due to the reflection in the surface interface. In this case, therefore, all units will typically transmit in the same polarization (e.g. RHCP) and receive in the same polarization (e.g. LHCP). Separation of channels will then be accomplished by the directional coupling and possibly by additional distinguishable features. 
         [0022]    In some cases, for example when more than two electronics units are connected to the rod, electromagnetic waves transmitted from a first electronics unit may be distinguishable from electromagnetic waves transmitted from a second electronics unit by means of a feature other than polarization. Examples of such features comprise timing, separated frequency ranges, frequency modulation, etc. 
         [0023]    In a situation where three electronics units are arranged on the rod, they may be rotated 120 degrees with respect to each other. This enables separation of the channels using a combination of polarization and at least one other distinguishable feature. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    The present invention will be described in more detail with reference to the appended drawings, showing currently preferred embodiments of the invention. 
           [0025]      FIG. 1  is a schematic illustration of how several electronics units are connected to the same wave guide, according to prior art. 
           [0026]      FIG. 2  shows a plane view of a radar level gauge implmenting a first embodiment of the present invention. 
           [0027]      FIG. 3  shows a plane view of a radar level gauge implmenting a second embodiment of the present invention. 
           [0028]      FIG. 4-6  shows various examples of couplings between the electronics unit suitable for the radar level gauges in  FIGS. 2 and 3 . 
           [0029]      FIG. 7  is a schematic illustration of how several electronics units are connected to a waveguide according to an embodiment of the present invention. 
           [0030]      FIG. 8  shows schemically a geometric design of an electronics unit according to a further embodiment of the present invention, 
           [0031]      FIG. 9  shows schematically an embodiment with only one electronics unit. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0032]      FIG. 2  shows schematically a radar level gauge  1  according to an embodiment of the present invention, mounted to the roof of a tank  2 . 
         [0033]    In brief, the gauge  1  is an exemplary radar level gauge for determining a filling level L of a filling material  3  contained in the tank  2 . The filling material  3  may be products such as oil, refined products, chemicals and liquid gas, or may be a solid material in powder or granular form, such as grain, pellets or coal. The tank  2  may be stationary or arranged on a moving vehicle, such as on a tanker. The working principle of the gauge is to transmit microwave signals into the tank and to receive reflected signals from the tank. The microwave signals are reflected by an interface between the filling material and surrounding atmosphere  4  (i.e. the upper surface  5  of the filling material), or between different filling materials (e.g. oil and water). The reflected signal is analyzed to determine the distance to such an interface. 
         [0034]    The radar level gauge  1  comprises one or several, in the illustrated case two, electronics units  6 ,  6   a,    6   b,  described in more detail with reference to  FIGS. 4-6 . The electronics unit comprises a circuitry portion including transceiver circuitry for transmitting and receiving microwave signals and processing circuitry for determining the filling level of the container based on received signals reflected from the tank. 
         [0035]    The electronics units  6   a,    6   b,  as well as any additional circuitry such as communication interface and power supply and/or power management circuitry (not shown), are surrounded by a housing  7 , preferably providing an airtight sealing of the circuitry. 
         [0036]    The system further comprises an antenna  8  arranged inside the tank  2  for transmitting microwave signals into the tank  2  and receiving reflected signals, and a wave guiding structure  9  for guiding microwave signals between the electronics units  6   a,    6   b  and the antenna  8 . The antenna  8  may be used both as a transmitter for emitting the output electromagnetic waves and as a receiver for receiving the reflected echo signals. The antenna  8  in  FIG. 2  is a rod antenna. However, other types of antennas or signal propagation devices can be used as well, including free propagating antennas such as a horn antenna (see  FIG. 3 ), a fixed or movable parabolic antenna, a conical antenna, and wave guides, such as a still pipe, a transmission line or probe. 
         [0037]    The radar level gauge  1  is typically arranged outside the tank  2 , and the wave guiding structure  9  then protrudes into the container through a hole  10  in a mounting structure  11  mounted in the roof of the tank  2 . If required, the opening  10  is provided with a sealing  12 , arranged to allow the electromagnetic signals to pass through the wall of the container while maintaining an air tight seal, so as to prevent container contents from escaping from the container. 
         [0038]    The wave guiding structure  9  comprises a rod  13  of a dielectric material, extending from the electronics unit  6  towards the antenna  8 . Each electronics unit  6  has a mounting portion, defining an opening  14  making it detachably mountable on the rod  13 . In the illustrated case, the mounting portion is formed by the housing itself. For example, as in the illustrated case, the unit  6  can be annular, with a central opening  14 . The dielectric rod  13  is not necessarily completely surrounded by the unit  6   a,    6   b,  which may have for example a partially annular shape. 
         [0039]    Alternatively, the mounting portion is a separate component, for example in the form of a ring, and the circuitry portion of the electronics unit is then attached, possibly releasably, to the mounting portion. 
         [0040]    The unit  6   a  is mounted on the rod  13 , so that the rod  13  extends through the opening  14 . In the illustrated case, the opening  14  in the unit  6   a  surrounds the rod  13 , and the unit  6   a  is thus mounted from the top onto the rod  13 . If the relative rotation of the units  6   a  is important, such as in the case of orthogonal polarizations, the orientation of the units may preferably be ensured by suitable guiding means. As illustrated in  FIG. 2 , the rod  13  may for example be provided with groves  21  in its axial direction, and the units  6   a,    6   b  may be provided with protrusions  22  fitting in these grooves. 
         [0041]    In the case of an opening  14  which only partially surrounds the rod  13 , the unit  6  may alternatively be mounted from the side. This is possible for example in the case with a rod  13  having a square or rectangular cross section, and a unit  6  with a rectangular opening located in one of its sides. This is illustrated in  FIG. 8 . One advantage with this embodiment is that a relative rotation between the electronics units  6   a,    6   b  of 90 degrees can be easily ensured by mounting the units from different sides of the rectangular rod  13 . 
         [0042]    The electronics units  6   a,    6   b  are arranged to direct electromagnetic waves, e.g. microwave signals, into the rod  13 , and receive reflected signals. For this purpose, the unit  6   a,    6   b  may be provided with a directional coupler connected to the transceiver circuitry, and adapted to couple transmitted microwave signals into the dielectric rod  13  extending through the opening  14  and to out-couple reflected microwave signals from the dielectric rod  13 . This will be described in more detail below with reference to  FIGS. 4-6 . 
         [0043]    In order to provide several level measurements, independent of each other, several electronics units  6   a,    6   b  may be arranged on the rod  13 . Each unit  6   a,    6   b  is then arranged to transmit microwave signals on a separate channel, i.e. the signals from different units  6   a,    6   b  are distinguishable from each other, and do not interfere with each other. 
         [0044]    Suitable matching may be provided between the electronics units  6   a,    6   b,  Such matching may be formed as a tube or sleeve of a dielectric or metal material mounted on the rod  13  between two adjacent units  6   a,    6   b,  and will be described in more detail with reference to  FIGS. 4-6  below. Such sleeves may be separate from the units  6  or integrated therewith. 
         [0045]    As the dielectric rod in  FIG. 2  is arranged to act as a surface wave guide, it is not enclosed by any conducting sleeve. The insides of the openings  14  may be open or closed by a suitable dielectric wall. In order to separate the units  6   a,    6   b  from each other by a suitable distance, a distance element  20 , such as a plastic ring or sleeve, may be arranged between adjacent units  6   a,    6   b.  This will be described in more detail with reference to  FIGS. 4 and 5 . 
         [0046]    A wave guide termination  15  may be arranged in a distal end  16  of the rod  13  with respect to the antenna  8 . The termination  15  is adapted to dampen, or absorb, the microwave energy that reaches the distal end  16  of the rod  13 , so as to avoid a reflection in the end causing a possible interfering echo to be received by the electronics units  6   a,    6   b.  The termination  15  may also be adapted to lock, or fixate, the electronics units  6   a,    6   b  that are mounted on the rod  13 . 
         [0047]    When, as in the embodiment illustrated in  FIG. 2 , the antenna  8  is a rod antenna, the dielectric rod  13  may be simply an extension of the antenna. In other cases, as will be illustrated in  FIG. 3  depicting a horn antenna, or in case of a parabolic antenna, the wave guiding structure  9  may advantageously include an intermediate transition structure, arranged between the rod and a different type of wave guide. 
         [0048]    Further, the dielectric rod  13  may extend through a ventilated space  17  between the tank  2  and the housing  7 . The rod  13  then passes through an opening  18  in the bottom wall of the housing  7  into the ventilated space  17 , and then continues into the tank  2 . Just like the opening  10  in the tank  2 , the opening  18  may be sealed by a sealing  19 , allowing passage of electromagnetic signals but protecting the electronics inside from the outside environment. The ventilated space  17  may be formed for example by a perforated ring  23  attached to the roof of the tank  2  and supporting the housing  7 . Alternatively, the ring  23  may be replaced by other suitable suiports, such as pins, that automatically create a ventilated space. 
         [0049]    Adjacent to the tank opening  10 , a suitable matching element  24  may be arranged, to ensure matching between surrounding conducting material having different thickness. 
         [0050]    It should be noted that one of the units  6   a,    6   b  may be replaced by an annular distance element having approximately the same thickness as the electronics unit. This may be advantageous for example if the rod  13  is manufactured in a standard length adapted for two units. The annular element  25  can then be mounted on the rod  13  below or above a unit  6  so as to create the same total axial extension, thereby still allowing the termination  15  to lock the unit  6 . This is illustrated in  FIG. 9 . 
         [0051]    The basic principle of the radar level gauge illustrated in  FIG. 3  is similar to that illustrated in  FIG. 2 . However, in this case, the antenna  31  is a horn antenna, and the dielectric rod  13  here forms the filling of a hollow wave guide  32 . 
         [0052]    By “hollow” waveguide is intended a waveguide where the conducting material encloses a hollow space, which can be empty or filled with a suitable dielectric material. Electromagnetic waves are guided across the entire cross section of a hollow wave guide. In a preferred embodiment the cross section is 90-degree symmetrical, e.g. an essentially circular cross-section. 
         [0053]    In the illustrated example, the rod  13  extends all the way to the antenna  31 , and ends at the upper end of the horn. The lower tip  33  of the rod is pointed, to form a matched transition between the wave guide and the antenna. The horn of the antenna may further be sealed by a dielectric material (not shown). Alternatively, the wave guide structure  9  may include a transition to a different type of hollow wave guide, such as an empty hollow wave guide. 
         [0054]    In  FIG. 3 , the opening  14  of each electronics unit  6   a,    6   b  has an inner wall  34  of conducting material, and any distance elements  35  are also formed with an inner wall, facing the rod, made of a conducting material. As a result, the dielectric rod  13  is completely surrounded by a conducting layer, so as to form the hollow wave guide  32 . This will be described in more detail in  FIG. 6 . 
         [0055]    Several directional coupling structures are well known in the art and a few examples of suitable implementations will be discussed with reference to  FIGS. 4-6 . The embodiments described in relation to  FIGS. 4 and 5  are suitable in an implementation as disclosed in  FIG. 2 , where the dielectric rod acts as a surface wave guide (not surrounded by a conducting layer). The embodiment described in relation to  FIG. 6 , on the contrary, is suitable for an implementation as disclosed in  FIG. 3 , where the dielectric rod forms part of a hollow wave guide. 
         [0056]    In  FIG. 4   a , each unit  6  includes a flexible printed circuit board (PCB)  41  supporting the required circuitry (e.g. transceiver circuitry and processing circuitry), which is housed in a housing  42  of non conducting material, such as plastic. The PCB  41  includes a lip portion  43  extending through an opening in the inner wall  44  of the housing, so as to extend into the opening  14 . As illustrated in  FIG. 4   b , the lip  43  has a circuit path forming a transmission line a quarter wave length long. (The “wave length” here relates to the wave length in the rod material at the operating frequency of the radar level gauge.) 
         [0057]    The lip  43  is bent so as to be arranged along the outer surface of the rod  13  when the unit  6  is mounted on the rod  13 . The quarter wave transmission line will thus extend in the axial direction of the rod, and act as a directional coupling. 
         [0058]    The bending of the lip  43  may be achieved by the rod  13  at the time of mounting, or by a separate sleeve  45  which may be inserted into the opening  14  before mounting. In this case the lip  43  will be sandwiched between the inner wall  44  and the sleeve  45 . In order to allow bending of the PCB  41  with reduced risk of damage of the circuit pattern, the wall  44  may have a smooth lower edge  47 , against which the flexible PCB  41  may rest. 
         [0059]    The sleeve  45  may also advantageously act as a guide in the radial direction, ensuring a correct alignment of the unit  6  on the rod  13 . The sleeve  45  may further include a flange portion  46  below the bottom of the housing  42 , serving as a distance element between adjacent units  6  (corresponding to distance element  20  in  FIG. 2 ). 
         [0060]    In  FIG. 5   a , the unit  6  again includes a PCB  51  and a housing  52 , similar to those in  FIG. 4 . In this embodiment, a coupling element  53  is mounted on the PCB so as to be located close to the dielectric rod  13  when the unit is mounted thereon. The coupling element  53  may be of a type known per se, and is adapted to provide a directional coupling of electromagnetic waves from the circuitry on the PCB  51  to the rod  13  acting as a surface wave guide. In order to ensure satisfactory coupling between the coupling element  53  and the rod  13 , the housing  52  in this embodiment does not have any inner wall facing opening  14 . 
         [0061]    In order to ensure satisfactory guiding in the radial direction of the rod  13 , the unit may further comprise a guiding sleeve  54 , attached to the bottom of the housing  52 . 
         [0062]    As indicated in  FIG. 5   b  a second coupling element  53 ′ may be arranged at 90 degrees with respect to the first element. Such an arrangement will enable transmission and reception of signals with circular polarization, which may be advantageous in some implementations. 
         [0063]    In  FIG. 6   a , the unit  6  again includes a PCB  61  and a housing  62 , similar to those in  FIGS. 4 and 5 , and again a coupling element  63  is mounted on the PCB so as to be arranged adjacent to the rod  13  when the unit is mounted thereon. In this embodiment, however, the dielectric coupling element comprises two feeding points  64 ,  65  having a vertical distance of a quarter wavelength, and arranged to protrude into the opening  14  of the unit  6 . The two feeding points are fed 90 degrees out of phase, thereby ensuring transmission of electromagnetic waves in one direction. 
         [0064]    The rod  13  is here provided with a groove  68 , in which the feeding points  64 ,  65  may extend into. This grove will also act as a guide, ensuring correct radial orientation of the unit  6 , as discussed above with reference to  FIG. 2 . 
         [0065]    The direction of the transmitted waves will be from the feeding point leading in time (i.e. connected by the shortest transmission line) to the feeding point lagging in time (i.e. connected by the longest transmission line). For this reason, it may be advantageous to arrange the PCB  61  in the roof of the unit  6 , so that the coupling element  63  protrudes downwards, toward the tank. The feeding point  65  located closest to the tank will then also be the feeding point with the longest transmission line to the PCB  61 . 
         [0066]    The housing  62  in this embodiment preferable includes a conducting inner wall  66 , surrounding the opening  14 . The feeding points  64 ,  65  then extend through openings in this sleeve  66 , and are isolated therefrom. 
         [0067]    The housing may further be provided with a conducting sleeve portion  67 , arranged below the bottom of the housing  62 . Just like the flange portion  46  in  FIG. 4   a , this sleeve  67  will act as a guide, ensuring correct alignment of the unit  6 , and also as a distance element ensuring correct distance between adjacent units  6 . 
         [0068]    With conducting inner walls  66  and conducting sleeve  67 , the units  6  will form a continuous conducting enclosure around the dielectric rod  13 , thus creating a hollow wave guide, as discussed in relation to  FIG. 3 . 
         [0069]    As indicated in  FIG. 6   b  a second coupling element  63 ′ with a second pair of feeding points  64 ′ may be arranged at  90  degrees with respect to the first pair. In this case, the rod  13  of course needs to be provided with a second groove  68 ′. Similar to the embodiment in  FIG. 5   b , this allows transmission and reception of signals with circular polarization. 
         [0070]    As illustrated in  FIG. 4-6 , the directional coupling structure  43 ,  53 ,  63  may be arranged on the same printed circuit board (PCB)  41 ,  51 ,  61  as the transceiver circuitry. The concentration of the coupling structure to one PCB may enable better performance (insulation and matching), and is relatively cost-effective and simple to produce. Thus, the collection of all critical microwave functions for each channel to one PCB is a very cost effective solution. Further, there is a very short microwave path from the TX/RX-modules to the antenna, which is an important property e.g. for good measuring performance at small distances. 
         [0071]    The directional coupler structure should be designed for a suitable degree of coupling between the transceiver circuitry and the antenna. This can be quantified as the attenuation of an incoming upward wave (reflected signal) before it reaches the input on the PCB. This attenuation is a typical specification parameter of any directional coupler used as a component. In case of a rather weak coupling (such as −10 dB) the signal received by the lowest electronics unit will be decreased by −10 dB. At the next electronics unit the signal in the waveguiding structure is lower so the signal delivered to the second unit will be decreased by −10.5 dB. The third unit (if all three uses the same polarisation) will only receive −11 dB as the signal in the waveguide is still a bit weaker. 
         [0072]    In order not to lose too much signal the coupling should be stronger but that will also decrease the signal to next unit. One way to get an optimum division of the available signal is to arrange a coupling which is different along the waveguide. With the coupling shown in  FIG. 6   a / 6   b  this is fairly simple to arrange as the coupling is strongly dependent on the penetration dept of the probes  64 . If the dielectric rod  13  is covered by a specially designed plastic sheet it can eb arranged to be slightly excentric in a non-uniform way, so that the probes of the lowest unit has less penetration (=less coupling) than the next unit which in its turn has less coupling than the uppermost one if three are stacked. A suitable variation in coupling strength can typically be accomplished by varying the pentration depth in the order of 1 mm. 
         [0073]    If the rod is designed for three identical electronic units the coupling may be arranged so that the lowest unit gets ¼ of the reflected echo from the antenna, the next unit ⅓ of the rest and the uppermost unit gets ½ of what is left. Then each unit and also the termination  15  gets ¼ each of the total signal power. 
         [0074]    With two units stacked (with the same polarization) the sleeve can be arranged so that ⅓ of the power gets to the lowest unit and ½ of the rest to the upper unit. The two units and the termination in this case get ⅓ each. 
         [0075]    For another optimization the sleeve can be designed have a strong coupling to the lowest unit and weaker to the second and third as the use for high level alarm is active at high level when the signal is strong. 
         [0076]      FIG. 7  schematically illustrates the connection of three electronics units  6   a,    6   b,    6   c  to a common wave guide according to an embodiment of the present invention. Compared to the prior art illustrated in  FIG. 1 , the wave guide comprises the dielectric rod  13  which guides electromagnetic signals from all three electronics units  6  towards and antenna (not shown in  FIG. 7 ). The directional coupling of each electronics unit  6  enables separation of each measurement channel associated with each unit  6  respectively. 
         [0077]    In  FIG. 7 , two of the units  6   a,    6   b  can be arranged to have different polarization (e.g. orthogonal linear polarization). The third unit  6   c  has the same polarization as one of the other units, and signals from this unit are preferably distinguishable from signals from the other units by means of a characteristic other than polarization. Such features include time and frequency. It is also possible that all three units employ the same polarization, such as left or right hand circular polarization. 
         [0078]    The radar level gauge disclosed herein is easily mounted on a tank. First, the signal propagation device and the wave guiding strucuter are mounted to a suitable mounting structure in the roof of the tank, preferably a mounting flange intended for this purpose. The flange may be provided by a suitable sealing. The dielectric rod is arranged to extend essentially vertically from the roof of the tank, and on the rod one or several electronics units are mounted so that the rod extends through the opening in each unit. The directional couplers of each unit will then be secured in relation to the rod to enable directional coupling of signals transmitted from the unit and received from the tank. After installation, the unit(s) may easily be removed from the rod in the field without requiring special equipment, for maintenance or replacement. 
         [0079]    In use, the transceiver circuitry transmits microwave signals which are coupled into the dielectric rod  13  by the directional coupler  43 ,  53 ,  63 . The signals are guided through the rod  13  through the container roof  11  and to the antenna  8 ,  31 , which emits the signals into the tank  2 . The microwave signals propagate into the tank  2  and are reflected by any impedance transitions present in the tank. In particular, the microwaves are reflected by the surface  5  of the material  3  in the tank. This reflection is referred to as a surface echo. 
         [0080]    The reflected microwaves, including any surface echo, is received by the antenna  8 ,  31 , and guided by the wave guiding structure  9  back to the directional coupler  43 ,  53 ,  63 , and transmitted via the transceiver circuitry to the processing circuitry. The processing circuitry determines the distance to the surface  5  based on transmitted and reflected signals. The received signals can be processed by a processor with software for analyzing the signals in order to determine the filling level, and the processor is preferably a microprocessor based circuit. The functions and algorithms implemented by the signal processor, some of which can be embodied in hardware and some of which can be embodied in software, may be per se known in the art and will not be discussed further in this application. The radar level gauge may be coupled to a remote location (for example a control room) via a signal wire or the like. 
         [0081]    The person skilled in the art realizes that the present invention by no means is limited to the preferred embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. For example, the geometric form and design of the electronics unit  6  may be different than the illustrated example, and may be for example rectangular, semi annular, or any other shape defining an opening through which the dielectric rod is insertable. 
         [0082]    Further, the coupling of signals between the units  6  and the rod  13  may be achieved in alternative ways. In a case where the mounting portion of the electronics unit  6  is detachable from the circuitry portion, the mounting portion is arranged to allow the directional coupler to be in operational contact with the rod.