Multidielectric microstrip filter
BACKGROUND OF THE INVENTION 
     This invention relates to an electrical filter of the type employed to attenuate harmonic spurs prior to radio transmission. More particularly, this invention relates to a microstrip-type filter based upon a substrate that includes a first dielectric material to produce an optimum capacitive coupling between a radial pad for attenuation of the harmonic spurs and a second dielectric material underlying a transmission line to provide a distinct capacitive coupling with regard to the primary signal. 
     In a high power radio transmitter of the type employed in a base station of a communication system, an electrical signal is passed through a filter en route to the antenna for broadcast. The filter passes the primary signal of the desired broadcast frequency, while removing signals at higher frequencies, most notably signals at harmonic frequencies, referred to as harmonic spurs. For this purpose, it is desired to minimize insertion loss of the primary signal while maximizing attenuation of the harmonic spurs. 
     A conventional high power filter comprises a wire transmission line along a central axis of a cylindrical metal housing. The transmission line is connected to a series of circular metal plates that perpendicularly intersect the line. The plates are circumferentially spaced apart from the housing, which is grounded, by a dielectric material to create a capacitive coupling effective to shunt the harmonic spurs. Such filters have been generally satisfactory for suppressing harmonic spurs, but are massive in construction and expensive. 
     Microstrip filters are known that comprise a transmission line and intersecting pads affixed to a surface of a dielectric board opposite a ground plate. The resonator pads are capacitively coupled to the ground plate and sized and located along the transmission line to attenuate the harmonic spurs. Microstrip filters have been generally suitable for low power transmissions, but suffer from a high insertion loss that reduces the power of the primary signal and is not desirable for high power transmissions. 
     SUMMARY OF THE INVENTION 
     This invention contemplates an electrical filter comprising an elongated transmission line and one or more radial pads overlying a first surface of a dielectric support opposite a ground plate. The support is formed of a first dielectric material to achieve a desired capacitive coupling between the pad and the ground plate effective for attenuating harmonic spurs. The support further comprises a region underlying the transmission line and containing a second dielectric material having a dielectric constant distinct from the first material to modify the capacitive coupling between the primary signal carried by the transmission line and the ground plate. Preferably, the second material is selected that reduces the capacitive coupling and further exhibits a low loss tangent relative to the first material, and thereby minimizes insertion loss of the primary signal. 
     In one aspect of this invention, this invention contemplates a radio broadcast system comprising the electrical filter operatively connected to a power amplifier and the antenna for attenuating harmonic spurs while passing the primary signal with minimal insertion loss. 
     In a preferred embodiment, a multidielectric electrical filter of this invention comprises the dielectric board having opposite upper and lower major surfaces. A transmission line is mounted to the board upper surface and includes an input end and an output end. The filter further comprises one or more radical pads affixed to the board upper surface and laterally disposed relative to the transmission line. Each pad is electrically connected to the line spaced apart from the conductor ends. The size, shape and location of the one or more pads are selected to optimize attenuation of signals at harmonic frequencies above the primary signal. The filter also includes an electrical ground plate affixed to the lower substrate surface opposite the transmission line and the pad. In accordance with this invention, the board is formed of a solid material having a dielectric constant greater than air and disposed intermediate the pad and the ground plate to produce a desired capacitive coupling effective for harmonic frequency attenuation. The board also defines a channel underlying the transmission line, which contains a second dielectric material having electrical properties distinct from the first material. Preferably, the channel forms an air space between the transmission line and the ground plate. The air features a low dielectric constant to reduce capacitive coupling between the transmission line and the ground plate. Furthermore, the air exhibits a minimal loss tangent to reduce conduction and thermal losses due to induced current. As a result, the preferred filter of this invention reduces transmission loss of the primary signal through the transmission line, which would otherwise reduce the strength of the signal supplied to the antenna. Nevertheless, the filter provides effective attenuation of the harmonic spurs. This combination is particularly advantageous for high power radio transmitters. In addition, the electrical filter features a relatively simple microstrip construction comprising elements attached to a board surface and is relatively inexpensive to manufacture, particularly in comparison to conventional high power coaxial filters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     This invention will be further described with reference to the drawings wherein: 
     FIG. 1A is a perspective view of an electric filter in accordance with this invention; 
     FIG. 1B is an enlarged view of an end portion of the filter in FIG. 1A: 
     FIG. 2 is a cross sectional view of the filter in FIG. 1 taken along the line 2--2 in the direction of the arrows; and 
     FIG. 3 is an elevational view of a portion of the filter in FIG. 1A showing a radial pad. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In a preferred embodiment, referring to the figures, a multidielectric electrical filter 10 in accordance with this invention is adopted for transmitting a high power electrical signal on the order of about 300 watts and having a predetermined frequency within a range about 900 MHz. The filter passes relatively low frequency signals, including the primary signal, while attenuating high frequency signals, including harmonic signals that accompany the primary signal at integer multiples of the primary frequency. Thus, high power, low pass filter 10 is particularly well suited for filtering signals for radio broadcast purposes, for example, in a base station of a land-mobile communications system. 
     Filter 10 comprises a dielectric board 12 having opposite, planar upper and lower surfaces 14 and 16, respectively. Board 12 is composed of a glass fiber-reinforced polymeric material, such as polytetrafluroethylene polymer or the like, having a dielectric constant of about 2.55. In accordance with this invention, board 12 defines a series of slots 18a-f that extend between surfaces 14 and 16 aligned along an axis 20 parallel to upper surface 14 and are separated by partitions 22a-g that are part of the board material. 
     Affixed to upper surface 14 is a series of radial pads 24a-e formed of metallic copper plated onto surface 14. As illustrated in FIG. 3 for central pad 24c, each pad is formed in a fan-shaped pattern symmetrical about centerline 32 and having an apex 26 proximate to axis 20 and an accurate edge 28 remote from the axis at a constant radius from apex 26. Each pad 24a-e is located adjacent a corresponding partition 22 and includes an extension 30a-e onto the partition. In the illustrated arrangement, pad 24c intersects axis 20 at a midpoint equidistant from the ends of board 12 and extends in a first direction. Pads 24b and 24d intersect axis 20 equidistant about center pad 24c and extend in a direction opposite pad 24c, whereas pads 24a and 24e are also disposed equidistant about center pad 24c, but extend in the direction of pad 24c. This provides a symmetrical, compact arrangement of the pads on surface 14. Surface 14 also carries copper plated end pads 34a and 34b overlying end partitions 22a and 22g. 
     Filter 10 further comprises a nickel-clad copper wire transmission line 38 axially mounted onto board surface 14 and affixed by solder bonds to each extension 30a-e and each end pad 34a and 34b. The solder bonds not only physically attach line 38 to board 12, but also provide electrical connections between the line and pads 24a-e. Line 38 includes an input end 41 and an output end 42. The diameter of wire 38 is sized small relative to the width of slots 18a-f, so that the slots extend laterally about the wire for communication with ambient atmosphere. 
     A copper plate 40 is affixed to board lower surface 16 and is adapted for electrical connection to ground potential. Ground plate 40 is coextensive with the lower board surface and extends to close the lower openings of slots 18. Thus, ground plate 40 is disposed opposite pads 24 spaced apart by dielectric board 12 to provide a first capacitive coupling therebetween. Furthermore, ground plate 40 is disposed opposite transmission line 38 and, except at partitions 22, is spaced apart by air within slots 18 to provide a reduced capacitive coupling therebetween. 
     For operation, filter 10 is incorporated into a radio base station with inlet 41 of transmission line 38 operatively coupled to a power amplifier 50 that generates a high power electrical signal at a predetermined frequency within a range about 900 MHz for broadcast, as well as one or more harmonic secondary signals. Output end 42 of transmission line 38 is operatively connected to an antenna. As the signals are transmitted through wire 38, the pads 24 cooperate with ground plate 40 to form a capacitive shunt for attenuating the high frequency spurs. However, the primary frequency signal is passed with reduced insertion loss. This is attributed in large part to the low dielectric constant and the low loss tangent of the air within slots 18. This reduces the capacitive coupling between line 38 and the ground plate 40. In addition, the lower dielectric constant extends the effective wavelength of the signal through filter 10 to increase the uppermost frequency at which the filter becomes ineffective for attenuating signals, referred to as the flyback frequency, to provide harmonic suppression over a greater frequency range. As a result, filter 10 provides more efficient transmission at the primary frequency, and provides more effective filtering over the expanded high frequency range. This is accomplished by a filter 10 having a relative simple microstrip construction, particularly in comparison to conventional coaxial filters. 
     In the described embodiment, the filter comprises a series of five radial pads symmetrically disposed along the transmission line. The specific performance characteristic of the filter depends upon the combination of several factors in accordance with well known principles, including the nature and thickness of the dielectric material forming the board, the thickness of the copper plate forming the pad, the location and shape of the pads, the size of the transmission line, and the characteristics of the electrical signals applied to the filter. It is believed that conventional methods may be employed to calculate pad size and placement for a particular application, while obtaining the benefits of a reduced insertion loss due to the air spaces underlying the transmission line in accordance with the preferred embodiment of this invention. 
     In the described embodiment, the filter comprises a dielectric polymer composite underlying the pads and a second dielectric, e.g., air, underlying the majority of the transmission line and having low dielectric properties that are advantageous for minimizing insertion loss and is preferred. Alternately, the slots may contain a gas other than air, such as nitrogen or a noble gas. Furthermore, this invention may be suitably adapted utilizing a second dielectric other than a gas. Thus, the slots may be suitably filled with a second polymer having a dielectric constant distinct from the board polymer. The slots may be filled following construction of the resonator pads and mounting of the wire to the board. Alternately, the slots may be filled prior to forming the pads or affixing the transmission line. Thus, in an alternate embodiment, the board may comprise a continuous channel that is filled with a second material, such as a polymeric material, that cooperates with the board to provide a continuous planar upper surface to which the pad and transmission wire is affixed, thereby eliminating the intermittent partitions in the described embodiment for supporting the pad extensions. Furthermore, the transmission line may be suitably formed of a copper plate applied to the surface of the channel material, as opposed to a preformed wire. While this invention is particularly well suited for providing a material having a low tangent loss and a reduced capacitive coupling underlying the transmission line, this invention may be adapted to provide a distinct dielectric material having a relatively higher loss tangent and producing a relatively higher capacitive coupling with the transmission line for providing insertion loss characteristics that are optimum for a particular application. 
     While this invention has been described in terms of certain embodiments thereof, it is not intended that it be limited to the above description but rather only to the extent set forth in the claims that follow.