Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
1999-06-17
2001-08-21
Bettendorf, Justin P. (Department: 2817)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S204000, C333S202000, C333S134000
Reexamination Certificate
active
06278341
ABSTRACT:
TECHNICAL FIELD OF INVENTION
The present invention relates in general to a filter and more particularly to a microstrip filter.
DESCRIPTION OF RELATED ART
The continued growth of subscribers in different cellular systems has led to a situation where the call intensity, in peak traffic hours, reaches a maximum number of simultaneous calls due to limited frequency band availability. Thus, an operator might be restricted in adding new subscribers to the cellular system. This is solved by introducing new frequency bands used for communication. For instance, the mobile telephone system band 900 Mhz is supplemented in some areas by PCS/GSM 1800 Mhz. A subscriber having a dual band cellular phone may thus use any of the two frequency bands for communication resulting in less congestion, and the operator may add more subscribers to the combined system and will thus reach even more revenues.
The use of two different frequency bands by one operator may require the operator to use two different antennas, one for each frequency band. However, it is beneficial for the operator to use the same cables to and from the two antennas, or a dual band antenna, serving a specific geographic area, and thus the need for a combiner or a filter for separating the different frequency bands arises.
Conventionally, such high-power filters may, for instance, be designed using cavity resonator filters according to known techniques. Such filters have generally been satisfactory in function but are bulky, expensive and require manual tuning for optimal performance. Another example of 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 grounded housing by a dielectric material to create a capacitive coupling effective to shunt high frequencies. Such filters have also been generally satisfactory for suppressing harmonic spurs or high frequencies, but are massive in construction and expensive. It would be beneficial if a filter could be designed which is easy to manufacture, which does not require manual tuning, and which is smaller and less costly.
These requirements could be met by a microstrip, or a stripline, filter. However, microstrip filters are generally not suitable for high-power filter applications due to, for instance, high insertion loss, which reduces the power of the signal beyond acceptable values.
In U.S. Pat. No. 5,153,541 to Johnson et al and titled ‘Multidielectric Microstrip Filter’, assigned to Motorola Inc, a high-power microstrip filter is disclosed. Two different dielectric materials separate a transmission line and the ground plane, and radial pads, and the ground plane, respectively. The radial pads cooperate, through a first of the two dielectric materials, with the ground plane for attenuating harmonic spurs. Along the transmission line are several apertures positioned, filled with a second of said two dielectric materials, preferably air, which reduces the capacitive coupling with the ground plane.
The use of several apertures interposed with support parts for supporting the transmission line results in a somewhat cumbersome solution. The support parts are necessary, according to the cited patent, for supporting the transmission line and for enabling conductive coupling between the transmission line and the radial pads. One problem which might occur with this arrangement is that the support parts also introduce an increased capacitive coupling, caused by the circuit board at those points where high currents occur and where, possibly, the best need for a reduced capacitive coupling is present. It would be beneficial if this capacitive coupling, at the support parts, could be reduced.
It may also be very difficult to selectively vary the characteristic impedance along the transmission line, which in turn may impede the freedom of design. It would be beneficial if the freedom in designing the filter characteristics could be increased.
The design in U.S. Pat. No. 5,153,542 to Tai et al is also a broad-band design meaning that the filter is used for attenuating signals in a very broad frequency band above the low-pass frequency band. This filter design requirement may cause restrictions on the building practice.
SUMMARY OF INVENTION
The main object of the present invention is to achieve a high-power filter for filtering a first frequency band. The filter should be small, capable of handling high-power applications, and easy and inexpensive to manufacture.
Another object of the present invention, according to a preferred embodiment, is to provide a high-power filter which has an increased freedom in design of filter characteristics.
In more detail, the problems described above, concerning how to achieve a small, low cost filter for high-power applications, are solved by providing a filter comprising a first and a second signal port arranged to pass signals of the first frequency band and at least a first ground plane means, the filter further comprising an elongated conductor coextending substantially in parallel with the ground plane means providing a first signal path for the frequency band between the first and second ports, at least a first conductive segment having first radio frequency characteristics and being connected to the signal path at a first interconnection point and where the first characteristics and the position of the first interconnection point being selected such that radio frequencies outside the first frequency band are effectively attenuated in the first signal path and the filter being characterized in that the elongated conductor and the conductive segment are formed partly by a planar dielectric material, having a relative dielectric constant substantially greater than one, provided with a conductive pattern, and partly by a self supporting conductor, the first interconnection point and the ground plane means being separated by a dielectric in the form of a gas.
According to one embodiment of the invention, the radio frequency characteristics are selected such that signals in a second frequency band above the first frequency band is effectively attenuated.
The objects of the present invention, according to one embodiment, are obtained by providing a filter where the signal path has narrower and broader parts to form different characteristic impedance along the signal path and where the characteristic impedance of the signal path cooperates with the frequency characteristics of the conductive segment to attenuate frequencies in at least a second frequency band above the first frequency band.
An advantage with the present invention is that a small, low-cost filter is achieved.
A further advantage is that a relative robust conductive material can be used where the current has its peak values and a material of high dielectric strength can be used where the electric field reaches high values. Thereby, a further advantage is achieved, namely that the losses can be kept low and the power handling capabilities can be kept high.
If the dielectric material is a printed circuit board, then high mechanical tolerances can be maintained which is a basis for making a tuning free filter.
Another advantage is that a transmission line, which is capable of handling high-power signals, is achieved.
Another major advantage is that the relation between the distance to the ground plane and the width of the conducting portions can be chosen according to need or desire.
REFERENCES:
patent: 2945195 (1960-07-01), Matthaei
patent: 2964718 (1960-12-01), Packard
patent: 3104362 (1963-09-01), Matthaei
patent: 5153541 (1992-10-01), Johnson et al.
patent: 5153542 (1992-10-01), Tai et al.
Allgon AB
Bettendorf Justin P.
Jacobson & Holman PLLC
Jones Stephen E.
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