GB2305547A - Temperature compensation using a composite resonator in a coaxial cavity signal transmission filter - Google Patents

Abstract

A signal transmission filter for filtering a signal transmitted therethrough includes a housing 112 and a cover 114 forming a filter cavity 128 enclosing a composite filter resonator connected to the base of the housing. The composite filter resonator includes first and second portions 130,132 of first and second different materials, respectively. The filter cover 114 has an aperture disposed therein, and a tuning element 118 is partially inserted in the aperture. The tuning element extends into the filter cavity 128 and partially protrudes above the filter cover 114. Thermal compensation is provided by selection of an appropriate ratio of the first and second resonator materials.

Description

TEMPERATURE COMPENSATION USING A COMPOSITE RESONATOR IN A COAXIAL CAVITY SIGNAL TRANSMISSION FILTER - The present invention relates generally to improved temperature compensation in signal transmission filters, and more particularly, to improved temperature compensation using a composite resonator in a coaxial cavity signal transmission band-pass filter.

Tuned cavities of various types have been used in various high frequency communications applications for many years. Two common types of cavities include coaxial cavities and waveguide filters. Such cavities are commonly connected in well known configurations to provide bandpass filters, notch filters, composite bandpass/notch reject filters and combiners (including duplexers, multiplexers and transmitter-combiners).

A tuned cavity is subject to thermal expansion and contraction of its housing and internal resonator, potentially varying the resonant frequency as the temperature of the tuned cavity varies. Tuned cavities have, therefore, been designed to compensate for varying operating temperature by manufacturing the housing of a material commonly known as Invar Invar is a metallic compound having a very low positive temperature coefficient, and that does not expand compared to other materials, such as copper, when subjected to increasing temperatures.

However, the electrical conductivity of Invarkis far too low for it to be satisfactory for use as the inner surface material of a tuned cavity, because the inner surface must have an extremely high electrical conductivity to provide satisfactory signal transmission performance. Therefore, temperature compensated waveguide filters or cubic filters usually have their interior Invar surfaces silver or gold plated in order to provide the required high conductivity. The necessity of providing silver or gold plated inner surfaces obviously is very expensive.

Furthermore, as the need for economical high "Q" (low loss) tuned cavities has increased in the microwave art, it has been sometimes necessary to increase the volume, and thus, the interior surface area of tuned cavities. Accordingly, as the tuned cavities have increased in size, so has the cost of the materials and machining.

Coaxial tuned cavity devices have been made relatively temperature insensitive by using threaded Invar tuning rods in the construction of adjustable length, copper sleeve-type resonators, the length of which determines the resonant frequency. However, in some instances, even a slight change in length of the Invar tuning rods causes unacceptable variation in resonant frequency with temperature change. Expensive, inconvenient techniques, such as attaching the upper end of the Invar rod to a large bracket attached to the top of the cylindrical housing, are used to physically counteract the temperature variation in the length of the Invar rod as the temperature changes.

For example, U.S. Patent Number 3,577,104 (Fig. 1), discloses a compact microwave band-pass filter that utilizes a sequence of thick capacitive irises 1, 2, 3, 4 ... stationed along the interior 5 of waveguide 6 to form a series of directly coupled resonant chambers.

The spacing between consecutive issues is Xg/4 or less, where Ag is the upper cutoff frequency of the filter.

The close iris spacing causes the second harmonic pass band for the fundamental mode to be far above the main pass band.

U.S. Patent Number 3,617,954 relates to a band pass filter for use in the VHF and UHF regions (Fig. 2).

The band pass filter employs a digital line in which the length of the digits 20, 21, 22, 23 and 24 can be as little as one twelfth of the wavelength at the filter's midband frequency. The resulting band pass filter structure is small compared to the wavelengths in the pass band while obtaining the advantage of the low loss of a digital line. The filter has an elliptic function and is particularly suitable for narrow band applications. The digital line is disposed between and spaced from a pair of ground plane plates 7 and 19 and the line is formed by parallel digits which are short circuited at one end in the manner of a "comb" line.

At the opposite end is a network of lumped element capacitors 9, 10, 11, 12 and 13 interconnecting adjacent digits of the line and coupling the end of each digit to ground. Further, each pair of adjacent digits, at their open circuited ends, is capacitively coupled by a capacitor 15, 16, 17 or 18. The input and output connections to the digital array preferably are made through capacitors 8 and 14 which are connected to terminal digits 20 and 24, respectively.

U.S. Patent Number 3,597,709 involves a band pass filter constructed from arrays of resonant elements (Fig. 3). The resonant elements are coupled in a manner enabling their control to be simultaneously affected over the filter's amplitude and group delay characteristics. The resonant cavities are formed by a sequence of irises located within a hollow rectangular waveguide. A first serial array of resonant cavities 50, 51, 52, 53 is formed by the irises 26, 27, 28, 29.

The irises are spaced along the hollow rectangular waveguide 30. The irises partition the hollow rectangular waveguide into a tandem sequence of resonant chambers which are directly coupled in consecutive order by apertures 31, 32, 33, 34 in the irises. A second serial array of resonant cavities 54, 55, 56, 57 is similarly formed by the irises 36, 37, 38, 39 in the hollow rectangular waveguide 40. The cavities in the second array are directly coupled, in consecutive order, by the apertures 41, 42, 43, 44 in the irises. Waveguides 30 and 40 are separated -longitudinally by a common narrow wall 45. The resonant cavities in the two arrays are disposed so that each resonant cavity in one array is opposite and aligned with a different resonant cavity in the other array.Cross coupling between opposite resonant cavities in the two arrays is provided by coupling apertures 46, 47, 48, 49 in the common narrow wall 45.

U.S. Patent Number 3,882,434 relates to a band pass filter constructed from a group of direct-coupled resonant elements (Fig. 4). The resonant elements form resonant cavities arranged in consecutive order, and at least one alternative path is formed by a cross coupling. The resonant cavities are formed by a sequence of irises located within a hollow rectangular waveguide. A first serial array of resonant cavities 62, 64, 66, 80 is formed by the iris plates 58, 63, 65, 73, 67. The iris plates are spaced along the hollow rectangular waveguide 59 and partition the hollow rectangular waveguide into a tandem sequence of resonant chambers.

A second array of resonant cavities 81, 82 is similarly formed by the iris plate 77 in the hollow rectangular waveguide section 74. The waveguide section 74 is closed at both ends by plates 75, 78, and the plate 77 has an aperture 76 providing direct coupling between the resonant cavities 81, 82. The waveguides 67, 74 are separated longitudinally by a common narrow wall 79. The resonant cavities 64 and 66 are disposed opposite the respective cavities 81, 82.

Coupling between these opposite resonant cavities in the two arrays is provided by coupling apertures 69, 71 in the common wall 79. Signal energy is coupled into the filter at port 60 of waveguide 59, and the output 6f the filter is obtained at the output port 68 - associated with the other end of waveguide 59. The main path taken by the signal through the filter is sequentially through cavities 62, 64, 81, 82, 66, 80, but a cross coupling is provided between cavities 64 and 66 by means of iris plate 70.

U.S. Patent Number 4,423,398 relates to a waveguide filter 93 (Fig. 5). Waveguide filter 93 includes two major opposed faces 89 (top face) and 88 (bottom face), and also includes two rectangular end faces 83 and 90.

The front and rear faces have the same dimensions as end faces 83 and 90. The top and bottom faces 89 and 88 are square. The height of waveguide filter 93 determines its "Q," and the length dimension of the rectangular faces determines the resonant frequency of the tuned cavity.

The front and rear, right and left, and top and bottom faces of waveguide filter 93 are all composed of copper or other highly conductive material such as aluminum, because of its very high electrical conductivity. A coaxial feedthrough connector 86 is connected on the geometric center of the outer surface of right side 83. An electric field probe 85 extends through a small hole in right side 83 into the interior volume of waveguide filter 93. A similar coaxial connector and electric field probe 91 are disposed on the left side 90.

Since the dimension of edge 84 determines the resonant frequency of waveguide filter 93, and since the most economically suitable high conductivity materials have a relatively large thermal expansion coefficients, the "natural" resonant frequency of waveguide filter 93 decreases as the temperature increases. In order to temperature compensate for this decrease in the resonant frequency of waveguide filter 93, a bi-metallic temperature compensating element 87 is attached to the inner surface of bottom 88.

U.S. Patent Number 4,249,148 discloses an electrical cube that functions as three independent bandpass filters, each having a Q" determined by the volume (Fig. 6). Cubical apparatus or device 98 includes six square sides connected together to form a cube. The sides include vertical sides 95, 97, 99 and 104, top side 105, and bottom side 94. All six sides have conductive inner surfaces which are electrically connected together. The sides can be constructed of copper sheet material or of other conductive or nonconductive materials lined with electrically conductive material. A plurality of ordinary coaxial connectors 101, 106 and 96 are mounted precisely in the center of first, second and third mutually perpendicular sides 99, 105 and 95, respectively.The center conductor of each coaxial connector is connected to a straight electric field probe which extends into volume bounded by cubical apparatus 98. An output electric field probe is connected to coaxial connector 103.

Each of the above filter designs, however, do address or relate to temperature compensation of a filter cavity. Further, each of the above filter designs do not take advantage of the ability to obtain temperature compensation of the filter cavity using the resonator disposed therein.

Accordingly, it is desirable to maintain tight electrical specifications for signal transmission filters over a range of operating temperatures.

It is further desirable to insure that the cavities used in filters are accurately temperature compensated.

It is also desirable to provide an accurate filter that is reliable and easy to manufacture.

It is further desirable to provide a filter constructed of material that facilitates its manufacture and construction.

It is a feature and advantage of the present invention to maintain tight electrical specifications for signal transmission filters over a range of operating temperatures.

It is another feature and advantage of the present invention to insure that the cavities used in filters are accurately temperature compensated.

It is also a feature and advantage of the present invention to provide an accurate filter that is easy to manufacture and reliable.

It is another feature and advantage of the present invention to provide a filter constructed of material that facilitates its manufacture and construction.

The present invention is based, in part, on the identification of the problem or need to effectively and efficiently filter signals under various operating temperatures. More specifically, in accordance with one aspect of the invention, it has been determined that, for coaxial cavity filters, the short circuit section of a transmission line acts as an inductor, and the area and spacings between the top of the resonator and the cover and tuning screw acts as a capacitor.

This combination of the two creates a resonant cavity.

As the length of the filter components increases (e.g., due to temperature), the resonator has more inductance which decreases the frequency of the cavity. Also, as the gap between the top of the resonator and the cover and tuning screw increases, the capacitance decreases thereby increasing the frequency of the cavity.

It has been further determined that as temperature rises, the coefficient of thermal expansion (CTE) of the housing is greater than or equal to the CTE of the resonator. Therefore as the temperature rises the distance between the cover and the top of the resonator increases. Since the CTE of the tuning element is less than or equal to that of the resonator and housing, the gap between the resonator and the tuning screw increases. Thus, the increase in resonator length over temperature tends to decrease the resonant frequency of the cavity. The increase in the gap size between the resonator and the tuning screw and cover tends to increase the resonant frequency of the cavity.

When decrease in frequency due to the change in resonator length is balanced by increase in frequency due to the increase in capacitive gap, the filter cavity is temperature stable. The increase in resonator length generally dominates, and the filter drifts down as temperature rises, and up as temperature falls.

The present invention is cognizant of various possibilities of attempting to temperature compensate a filter cavity to provide more stable frequency transmissions. For example, one filter design is considered to reduce resonator length sufficiently such that capacitive loading becomes large and sensitive enough to cancel out the resonator length effect. A second filter design drills a hole in the resonator slightly larger than the tuning screw diameter, into which a tuning screw penetrates. This increases the sensitivity of the tuning screw to temperature variations and compensates for the resonator length effect. In most cases, a combination of these two methods will be necessary to compensate a cavity.A third filter design considered uses verv low CTE

materials such as Invarkfor all components, decreasing the magnitude of the temperature variations without actually balancing or the temperature variations.

I have learned that the first filter design introduces a disadvantageous increase in the loss of the filter with decrease in resonator length. Also, the gap between the top of the resonator and the tuning screw becomes so small that the device becomes susceptible to arcing under power. While the second filter design does not increase filter loss, the second design cannot handle required signal power. Further, the dimensional tolerances between the screw and the resonator make the second design difficult to manufacture. Finally, due to Invar's expense, and machining and weight characteristics, the third filter design is too difficult and expensive to manufacture, and is heavy.

The present invention solves the above problems of designing an accurate filter that is easy to manufacture and reliable. In accordance with the present invention, a temperature compensated coaxial filter is provided for filtering a signal transmitted therethrough. The signal transmission filter includes a filter base with side walls projecting upward therefrom forming a filter cavity, and a composite filter resonator connected to the filter base. The composite filter resonator includes first and second portions of different materials. The signal transmission filter further includes a filter cover, having an aperture disposed therein, connected to the side walls of the filter cavity and covering the composite filter resonator.A tuning element, partially inserted in the aperture of the filter cover, extends into the filter cavity and partially protrudes above the filter cover.

An another aspect of the invention, a method of manufacturing the filter comprises the steps of milling the filter cavity n a first solid body comprising a first aluminum alloy forming the filter base with the side walls projecting upward therefrom, generating a first hole in the filter base, and placing a first solder on the filter base in a first area adjacent to the first hole. The method also includes the steps of securing the first portion of the composite filter resonator to the filter cavity via a first screw inserted in the first hole and threadedly engaged with a first threaded socket in the first portion, and placing a second solder on the upper surface of the first portion of the composite filter resonator.The method further includes the steps of securing the second portion of the composite filter resonator to the upper surface of the first portion via a second screw inserted in a second hole in the second portion and threadedly engaged with a second threaded socket in the upper surface of the first portion, and heating the signal transmission filter a sufficient amount to secure the first solder to the filter base and to the first portion and to secure the second solder to the first and second portions.

In accordance with another embodiment of the invention, a method of manufacturing comprises the steps of milling the filter cavity in a first solid body comprising a first aluminum alloy forming the filter base with the side walls projecting upward therefrom and forming a cylindrical portion protruding from the bottom of the filter base comprising a first resonator portion, and forming a threaded socket in the upper surface of the first resonator portion of the composite filter resonator.The method also includes the steps of placing solder on the upper surface of the first resonator portion of the composite filter resonator, securing the second portion of the composite filter resonator to the upper surface of the first portion via a screw inserted in a hole in the second portion and threadedly engaging with the threaded socket in the upper surface of the first portion, and heating the signal transmission filter a sufficient amount to secure the solder to the first and second portions.

In order that the present invention may be more readily understood, reference will now be made to the accompanying drawings, in which: Figs. 1-6 are illustrations of various prior art filter structures; Fig. 7 is a conceptual illustration of the various characteristics of a filter structure; Fig. 8 is an illustration of an embodiment of a filter structure with a composite resonator in accordance with the invention; Figs. 9 and 10 are respective isometric and top views of the composite resonator structure with cover removed; Fig. 11 is an illustration of the composite resonator structure with cover applied; and Fig. 12 is an illustration of another embodiment of a composite resonator filter structure.

In order to maintain tight electrical specifications over temperature, it is necessary to insure that the cavities used in signal transmission filters are accurately temperature compensated. In a coaxial cavity filter, the short circuit section of the transmission line acts as an inductor, and the area and spacings between the top of the resonator and the cover and tuning screw acts as a capacitor. The combination of the inductive and capacitive functions creates a resonant cavity. As the length of the filter components increase, the resonator has more inductance, decreasing the frequency of the cavity. Also, as the gap between the t-op of the resonator and the cover and tuning screw increases, the capacitance decreases, increasing the frequency of the cavity.

As illustrated in Fig. 7, as the temperature rises the following three phenomena occur in the filter cavity 110: 1. Housing length 120, resonator length 122, and protruding screw portion 124 increase.

2. Housing length 120 > resonator length 122, and the coefficient of thermal expansion (CTE) of the housing 112 and cover 114 is greater than or equal to the CTE of the resonator 116. Therefore, as the temperature rises the distance between the cover and the top of the resonator, distances 124 and 126, increases.

3. Since the CTE of the tuning screw or element 118 is less than or equal to that of the resonator 116 and housing 112, the gap 126 between the resonator 116 and the tuning screw 118 increases.

The increase in resonator length 122 over temperature tends to decrease the resonant frequency of the cavity 110. The increase in the gap size 126 between the resonator 116 and the tuning screw 118 and cover 114 tends to increase the resonant frequency of the cavity 110. When the decrease in frequency due to the change in resonator length 122 is balanced by the increase in frequency due to the increase in capacitive gap 126, the cavity is temperature stable. Generally, the increase in resonator length 122 dominates, and the filter drifts down as temperature rises, and up as temperature falls.

There are, however, a number of crude ways to create a compensated cavity. The first way is to decrease the resonator length 122 so much that the capacitive loading becomes so large and sensitive that it cancels out the variable resonator length effect. A second way is to drill a hole in the resonator 116 slightly larger than the tuning screw diameter, into which the tuning screw 118 would penetrate. This increases the sensitivity of the tuning screw 118 to temperature variations and compensates for the resonator length 122 effect. In most cases, a combination of the above methods will be necessary to compensate filter cavitv 110. A third wav is to use verv low CT

materials such as Invarlor all filter cavity 110 components, thereby decreasing the magnitude of the effect without actually balancing it.

It has been determined, however, that the first filter design introduces the problem that the decrease in resonator length 122 increases the loss of the filter 110. Also, the gap 126 between the top of the resonator 116 and the tuning screw 118 becomes so small that it becomes susceptible to arcing under power.

While the second filter design does not increase the filter loss, the second design cannot handle the required signal power. Further, the dimensional tolerances between the screw 118 and the resonator 116 make the second design difficult to manufacture.

Finally, due to Invar'szexpense, and its machining and weight characteristics, the third filter design is too difficult and expensive to manufacture, as well as being extremely heavy.

A modified filter design manufactures a compound structure in which the resonator is formed of two different materials. The filter cavity and the base of the resonator are machined from, for example, one piece of aluminum, and the upper portion of the resonator is made substantially or completely of, for example, Invar. The upper portion of the resonator is attached to the lower portion of the resonator via, for example, conventional means such as a screw and/or soldering. A stainless steel screw connects the upper and lower portions of the resonator structure. Standard solder SN60 or SN62 are acceptable. This modified resonator structure decreases the overall effective CTE of the resonator.

According to this modified structure, the filter cavity is compensatable by simply changing the ratio of Invar to aluminum, i.e., the ratio of the upper and lower portions of the resonator structure. This compound structure is therefore compensatable regardless of the size of the capacitive gap between the tuning screw and the resonator. The modified filter structure enables temperature compensation to occur without increasing loss or becoming susceptible to arcing under different power levels. Various other combinations of the modified filter structure are also possible. For example, the base of the resonator can be machined separately from the housing or be integral with the housing.

Fig. 8 is an illustration of a modified filter design in accordance with the invention. In Fig. 8, the modified filter cavity 128 includes housing 112 and cover 114. Tuning screw 118 penetrates cover 114, and is used to tune the resonator disposed in the filter cavity. The resonator is formed of at least two different materials. The filter cavity 128 and the base 132 of the resonator are machined from, for example, one piece of aluminum, and the upper portion lrn nf the resonator is made substantiallv or

completely ot, tor example, Invart The upper portion 130 of the resonator is attached to the lower portion 132 of the resonator via, for example, conventional means such as a screw and/or soldering. Lower portion 132 is generally of a solid cylindrical shape.Upper portion 130 is also of a cylindrical shape, preferably having its center hollowed to facilitate connection with the lower portion 132 via a screw.

According to this modified structure, the filter cavity is compensatable by simply changing the ratio of Invar to aluminum, i.e., the ratio of the upper and lower portions 130, 132 of the resonator structure.

This compound structure is therefore compensatable regardless of the size of the capacitive gap between the tuning screw 118 and the resonator. The modified filter structure enables temperature compensation without increasing the loss or becoming susceptible to arcing under different power levels. Various other combinations of the modified filter structure are also possible. For example, the base 132 of the resonator can be integral with the housing 112. Different materials can be used for the upper and lower sections of the resonator. The tuning screw can comprise brass or steel. The specific brass alloy 6 has been found suitable for use in the tuning screw. The upper resonator Dortion can be comprised of, for example.

invarar aiumirium.X p > lLZ v alloy ie nas been found suitable for use in the upper resonator portion. The lower resonator portion can be comprised of, for example, aluminum or brass. The specific brass alloy T706060 has been found to be suitable in some filter applications for the lower resonator. - Note that the specific alloys and materials mentioned herein are for illustrative purposes only and are not considered to limit the composite resonator structure described herein. Rather, any combination of materials can be used for the resonator. that exhibit the complimentary characteristics described above in connection with the tuning screw and resonator structure. The signal transmission filter can be adapted for various applications.

Figs. 9 and 10 are respective isometric and top views of the composite resonator structure with the cover removed therefrom. In Figs. 9 and 10, the coaxial cavity filter 134 includes two cavities 136, 138. Cavity 136 includes composite resonator 140 having upper and lower resonator portions 142 and 144.

Cavity 138 of the second resonator cavity includes composite resonator 146 with upper resonator portion 148 and lower resonator portion 150. Screw holes 152 are disposed along the periphery of cavities 136 and 138 and are used to secure the cover of the filter thereto. While Figs. 9 and 10 illustrate use of the composite resonator for a double coaxial cavity, a composite resonator structure can also be used for a filter structure with a single filter cavity, or with three or more filter cavities.

Fig. 11 is an illustration of the composite resonator structure with the cover disposed thereon.

In Fig. 11, modified filter 134 includes cover 154 secured to the coaxial filter cavities 136 and 138. In addition, filter 134 includes input and output connectors 156 and 158 for inputting the signal to be filtered, and for outputting the filtered signal therefrom.

The following is a comparison of uncompensated and compensated filter structures with respect to the modified filter structure: 1. Uncompensated filter structure with aluminum resonator and housing, and brass tuning screw. The following filter characteristics were determined: @ -30 C Fo - 923.173 MHz @ +25 C Fo = 922.000 MHz @ +60 C Fo = 921.255 MHz Total Drift 1.918 MHz Cavity Q = 6200 (inversely proportional to loss) Voltage Protection = 8000 V The dimensions of the two chamber coaxial filter with aluminum housing was approximately 3"x3"x6". The height of the one-piece resonator made of Alum was approximately 2.135". The diameter of the resonator was approximately 0.750".

2. A compensated filter structure using the same materials as in a previous example, but without a composite resonator. The following filter characteristics were determined: @ -30 C Fo = 922.588 MHz @ +25 C Fo = 922.000 MHz @ +60 C Fo = 921.625 MHz Total Drift= 0.963 MHz Cavity Q = 5400 Voltage Protection = 500 V The dimensions of the two chamber coaxial filter with aluminum housing were substantially identical to example 1.

3. Compensated structure using a composite resonator, with materials identical to those of examples 1 and 2, except with respect to the resonator, as indicated below. The following filter characteristics were determined: @ -30 C Fo = 922.001 MHz @ +25 C Fo = 922.000 MHz @ +60 C Fo = 921.998 MHz Total Drift = 0.003 MHz Cavity Q = 6200 Voltage Protection = 8000 V The dimensions of the two chamber coaxial filter with aluminum housing were substantially identical to example 1.The height, of the upper portion of the

resonator maae or lnvarqwas approximateiy U. /bU". Tne height of the lower portion of the resonator made of aluminum was approximately 1.375". The diameter of the upper and lower portions of the resonator was similar to examples 1 and 2.

Fig. 12 is an illustration of another embodiment of a filter structure with a composite resonator. In Fig.

12, filter structure 160 includes filter cover 114 and tuning screw 118 as in the previous embodiment.

However, the composite resonator and filter cavity has a modified structure. In particular, the lower portion of the composite resonator 162 is integral with the filter cavity structure 160. The upper portion of the resonator 128 is connected to the lower portion 162 described previously. In this manner, there is no additional requirement to connect the lower portion of the resonator cavity to the base of the filter cavity, eliminating one step of the manufacturing process described in detail below. Thus, in this additional embodiment of the filter structure, the lower portion of the composite resonator is machined or milled at the same time that the filter cavity is formed in the filter structure. This is explained in further detail below.

The composite resonator filter structure described herein is manufactured or produced in accordance with the following procedure. For the composite resonator having the lower portion which is not integral with the filter cavity, first a solid rectangular block of material, for example, aluminum, as discussed above, is milled using a standard milling machine to form the basic filter cavity. Once the basic filter cavity is formed, the lower portion of the composite resonator is formed with female groove connections/threads on both ends. The lower resonator portion, as well as the upper resonator portion of the resonator, can either-be formed prior to the milling of the resonator cavity or afterward.

To prepare connection between the resonator cavity and the lower resonator portion, a hole is drilled in the resonator cavity coinciding with the positioning of the lower portion of the resonator on the lower base area of the cavity. Solder is then placed in the juncture between the lower resonator portion and the resonator cavity, and the lower resonator portion is secured to the base of the resonator cavity via a screw which is inserted from the bottom of the resonator cavity into the female threaded hole or socket of the lower resonator portion.

The upper resonator portion includes a larger hole/cavity on the upper area for permitting the tuning screw to be inserted therein, and an additional tapered hole on the opposite end for connection with the lower resonator portion. The upper portion of the resonator is then placed on top of the lower portion of the resonator, with solder disposed therebetween. The upper portion of the resonator is then secured to the lower portion of the resonator using a screw which is inserted into the female threaded hole or socket of the lower resonator portion. The assembly is then heated until the solder liquifies and further secures the connection between the upper and lower resonator portions and the lower resonator portion and the base of the filter cavity.

A similar process is applicable when the lower resonator portion is integral with the base of the filtered cavity. In this situation, the filter cavity is milled around the lower resonator portion, leaving a cylindrical portion protruding from the bottom of the resonator cavity. A female threaded socket is then formed into the lower resonator portion as described above. The upper resonator portion is then secured to the lower resonator portion as described previously.

Claims (14)

1. A signal transmission filter for filtering a signal transmitted therethrough, said signal transmission filter comprising: a filter base with side walls projecting upwardly therefrom forming a filter cavity; a composite filter resonator disposed on said filter base, said composite filter resonator comprising first and second portions, the first portion comprising a first material and the second portion comprising a second material, and the first material being different from the second material; a filter cover, having an aperture disposed therein, connected to the side walls of the filter cavity and enclosing said composite filter resonator; and a tuning element, partially inserted in the aperture of said filter cover, extending in the filter cavity and partially protruding above said filter cover.

2. A signal transmission filter according to claim 1, including solder disposed between the first and second portions of said composite filter resonator.

3. A signal transmission filter according to claim 2, wherein the signal transmission filter is heated to secure the first and second portions together via said solder.

4. A signal transmission filter according to claim 1, 2 or 3, wherein the second portion of said composite filter resonator is integral with said filter base.

5. A signal transmission filter according to claim 1, 2, 3 or 4, wherein the first portion of said composite filter resonator includes a cavity, the second portion of said composite filter resonator includes a threaded socket, and the first portion is connected to the second portion via a screw that is inserted in the cavity and threadedly engaged in the -threaded socket of the second portion.

6. A signal transmission filter according to any preceding claim, wherein said filter base comprises a first aluminum alloy, the first portion of said composite filter resonator comprises one or an Invar

alloy and a second aluminum alloy, and the second portion of said composite filter resonator comprises one of a third aluminum alloy and a brass alloy.

7. A signal transmission filter according to any preceding claim, wherein the signal transmission filter and the filter cavity are compensated by a prescribed ratio of the first and second materials of the first and second portions of said composite filter resonator.

8. A signal transmission filter according to any preceding claim, wherein when said tuning element is partially inserted in the aperture of said filter cover and extends in the filter cavity, a capacitive gap is formed between said tuning element and said composite filter resonator, and whereby the signal transmission filter and the filter cavity are compensatable independently of the size of the capacitive gap between said tuning element. and said composite filter resonator.

9. A signal transmission filter according to any preceding claim 1 to 6, wherein the signal transmission filter and the filter cavity are compensated by a prescribed ratio of the first and second materials without increasing loss of the signal transmitted therethrough or without becoming susceptible to arcing under varying power levels.

10. A signal transmission filter according to any preceding claim, wherein the signal transmission filter comprises a coaxial signal transmission filter with at least two filter cavities connected in series.

11. In a signal transmission filter for filtering a signal transmitted therethrough, said signal transmission filter including a filter base with side walls projecting upward therefrom forming a filter cavity, a composite filter resonator comprising first and second portions, the first portion comprising a first material and the second portion comprising a second material, and the first material being different from the second material, a filter cover having an aperture disposed therein, and connected to the filter cavity, and a tuning element, partially inserted in the aperture of said filter cover and extending in the filter cavity and partially protruding above said filter cover, a method of manufacturing the signal transmission filter, comprising the steps of:: (a) milling the filter cavity in a first solid body comprising a first aluminum alloy forming the filter base with the side walls projecting upward therefrom; (b) generating a first hole in the filter base; (c) placing first solder on the filter base in a first area adjacent to the first hole; (d) securing the first portion of the composite filter resonator to the filter cavity via a first screw inserted in the first hole and threadedly engaged with a first threaded socket in the first portion; (e) placing second solder on the upper surface of the first portion of the composite filter resonator; (f) securing the second portion of the composite filter resonator to the upper surface of the first portion via a second screw inserted in a second hole in the second portion and threadedly engaged with a second threaded socket in the upper surface of the first portion;; (g) heating the signal transmission filter a sufficient amount to secure the first solder to the filter base and to the first portion and to secure the second solder to the first and second portions.

12. In a signal transmission filter for filtering a signal transmitted therethrough, said signal transmission filter including a filter base with side walls projecting upward therefrom forming a filter cavity, a composite filter resonator comprising first and second portIons, the first portion comprising a first material and being integral with the filter base, and the second portion comprising a second material, and the first material being different from the second material, a filter cover having an aperture disposed therein, and connected to the filter cavity, and a tuning element, partially inserted in the aperture of said filter cover and extending in the filter cavity and partially protruding above said filter cover, a method of manufacturing the signal transmission filter, comprising the steps of:: (a) milling the filter cavity in a first solid body comprising a first aluminum alloy forming the filter base with the side walls projecting upward therefrom and forming a cylindrical portion protruding from the bottom of the filter base comprising a first resonator portion; (b) forming a threaded socket in the upper surface of the first resonator portion of the composite filter resonator; (c) placing solder on the upper surface of the first resonator portion of the composite filter resonator; (d) securing the second portion of the composite filter resonator to the upper surface of the first portion via a screw inserted in a hole in the second portion and threadedly engaging with the threaded socket in the upper surface of the first portion; and (e) heating the signal transmission filter a sufficient amount to secure the solder to the first and second portions.

13. A method of manufacturing a signal transmission filter substantially as hereinbefore described with reference to Figures 8 to 12 of the accompanying drawings.

14. A signal transmission filter constructed substantially as hereinbefore described with reference -to Figures 8 to 12 of the accompanying drawings.