GB2259411A

GB2259411A - Coaxial resonator

Info

Publication number: GB2259411A
Application number: GB9218159A
Authority: GB
Inventors: Muurinen Jari
Original assignee: Nokia Mobile Phones Ltd
Current assignee: Nokia Oyj
Priority date: 1991-08-30
Filing date: 1992-08-26
Publication date: 1993-03-10
Anticipated expiration: 2012-08-26
Also published as: FI89645B; GB2259411B; FI914091A; GB9218159D0; FI89645C; FI914091A0

Abstract

A tunable coaxial resonator comprises tuning means for tuning the resonator to the required frequency. The tuning means is provided at one end of the resonator and acts to provide a short circuit between the central conductor 3 and outer sheath 2 of the resonator at a selected point along the length of the resonator. By selecting the appropriate point, the resonator can be tuned to the required resonance frequency. The short circuit is provided by a pair of contacts 4, by a bushing, or a coaxial cable. The short circuit may be provided by contacts (7, fig 2) which contact the sheath on its outer surface. <IMAGE>

Description

A Tunable coaxial resonator This invention relates to a tunable coaxial resonator.

A coaxial resonator comprises an inner conductor and an outer shield preferably consisting of a metal sheath with a dielectric layer between them. The dielectric does not extend quite to the end of the resonator, so that, at the ends of the resonator, there is a space between the inner conductor and the sheath without any dielectric which creates a loading capacitance between the shield and the inner conductor of the coaxial resonator. The resonance frequency of the coaxial resonator is determined by the length of the resonance tube and the loading capacitance, i.e. the resonator is equivalent to a parallel coupling of an inductance and a capacitance.

In order to tune a coaxial resonator to the required frequency a number of tuning methods are known.

In one well-known solution the inner conductor is soldered to a ground sheet, and, then the resonator is tuned by cutting the ground sheet. This has the problem that it is inaccurate and difficult, and the further problem that it is only possible to tune the resonator in one direction only.

In another known method the resonator is tuned with a tuning capacitor. However, this has the disadvantage that the capacitor is relatively expensive.

According to the present invention, there is provided a tunable coaxial resonator comprising a central conductor encapsulated along a substantial portion of its length by a dielectric material and having an unencapsulated portion at an end thereof, a sheath surrounding the dielectric material and the central conductor, the sheath extending, in a lengthways direction, beyond the dielectric material to define a space between the unencapsulated portion of the central conductor and the sheath, the unencapsulated portion of the central conductor, the space and the extended portion of the sheath forming a loading capacitance of the coaxial resonator, and tuning means for tuning the coaxial resonator to a desired resonance frequency, the tuning means comprising means for providing a short circuit between the central conductor and the sheath at point along the length of the loading capacitance forming portion of the coaxial resonator, the point being adjustable to select the desired resonance frequency.

This has the advantage that the coaxial resonator is easily tuned and is tunable in both directions.

An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which: Figure 1 illustrates a first embodiment of a tunable resonator in accordance with the invention; Figure 2 illustrates a second embodiment of a tunable resonator in accordance with the invention; Figure 3 illustrates a third embodiment of a tunable resonator in accordance with the invention; and Figure 4 illustrates a fourth embodiment of a tunable resonator in accordance with the invention.

As illustrated in Figures 1,2,3, and 4 a tunable resonator in accordance with the invention comprises a central conductor 3 which is encapsulated along its length - apart from a portion at its end - with a dielectric material 1. The dielectric 1 is then further encapsulated by a metallic sheath 2. Because the dielectric does not extend along the whole length of the resonator, the ends have a space 12 between the central conductor 3 and the sheath 2, which creates a loading capacitance between the central conductor 3 and the sheath 2. In this respect, the resonator is no different from known coaxial resonators described in the preamble.

Provided at the end of the coaxial resonator is a tuning means 13 which is used to tune the coaxial resonator to the required resonance frequency. The tuning means 13 comprises a connector body 5 from which two pairs of electrically conducting contacts 4 extend.

These contacts 4 are used to create a short circuit between the central conductor 3 and the sheath 2 at a defined point along the length of the coaxial resonator.

In the first embodiment illustrated in Figure 1, the contacts 4 are inserted into the space 12 at the end of the coaxial resonator so that one contact 4 of each pair of contacts, contacts the inner conductor 3 and the other contact 4 contacts the outer sheath 2. The contacts 4 are formed to make contact with the central conductor 3 and the sheath 2 at the defined point. In the embodiment illustrated in Figure 1, the contacts 4 are arcuate to contact the central conductor 3 and the sheath 2 respectively in a tangential manner. The contacts 4 and the connector body 5 are made of any suitable material so that when they contact the central conductor 3 and the sheath 2 as described a short-circuit is created between the two i.e. the central conductors 3 and the sheath 2 are electrically coupled at the defined point.

A change in the resonance frequency is provided by sliding the connector 5 in or out, so changing the defined point at which the contact between the contacts 4 and the central conductor 3 and sheath 2 are made.

In a second embodiment of a coaxial resonator in accordance with the invention, illustrated in Figure 2, a tuning means 14 comprises a connector body 8 and, again, two pairs of contacts 6,7. In this case, however, a first contact 6 of each pair is configured to contact the central conductor 3 while a second contact 7 of each pair is configured to contact the sheath 2 on the outside of the coaxial resonator. In this second embodiment each contact 6,7 is provided with a protruding lip to provide the point of contact with the central conductor 3 and sheath 2.

Again, a change in the resonance frequency is effected by sliding the tuning means 14 in or out to change the defined point at which the short is effected.

In a third embodiment of a tunable resonator in accordance with the invention as illustrated in figure 3, the tuning means 15 comprises a bushing 11 which is screwed into the resonator tube, the bushing 11 having threads both on its inner surface 10 and on its outer surface 9. The outside threads cut into the shield 2 and the inside threads cut into the inner conductor 3 The resonance frequency will change when the bushing 11 is screwed inwards or outwards. The threaded contact of the connector will be readily fixed to soft copper.

When the coaxial resonator is a suitably rigid cable, e.g. a so-called semi-rigid cable, the embodiment illustrated in Figure 4 may be used. Here, the tuning means 13 of the first embodiment is connected to a coaxial cable 16 and the tuning means 13 and coaxial cable are inserted into a coaxial connector or casing 17 of the standard, known type. Instead of a coaxial cable 16 a printed circuit board (not shown) can be used.

The sliding contacts 4 at the output side are shorted for instance by soldering the inner conductor pin 18 of the coaxial cable 16 or the printed circuit board, to the casing 17 of the connector. In this embodiment it could be established by measurements that the frequency of a resonator with a center frequency of 850 MHz can be adjusted within the limits of +20 MHz.

The phase noise was measured as -121 dBc/Hz and the output power as 3,5 dBm.

It will be clear from the foregoing description, to a person skilled in the art, that various modifications are possible, within the scope of the present invention.

Claims

1. A tunable coaxial resonator comprising: a central conductor encapsulated along a substantial portion of its length by a dielectric material and having an unencapsulated portion at an end thereof; a sheath surrounding the dielectric material and the central conductor, the sheath extending, in a lengthways direction, beyond the dielectric material to define a space between the unencapsulated portion of the central conductor and the sheath, the unencapsulated portion of the central conductor, the space and the extended portion of the sheath forming a loading capacitance of the coaxial resonator; and tuning means for tuning the coaxial resonator to a desired resonance frequency, the tuning means comprising means for providing a short circuit between the central conductor and the sheath at point along the length of the loading capacitance forming portion of the coaxial resonator, the point being adjustable to select the desired resonance frequency.

2. A tunable coaxial resonator according to claim 1 wherein the tuning means comprises connection body having a pair of contacts, a first contact of the pair being configured to contact the central conductor at the point along the length of the loading capacitance forming portion of the coaxial resonator, and a second contact of the pair of contacts being configured to contact the sheath at the point along the length of the loading capacitance forming portion of the coaxial resonator, the first and second contacts being electrically coupled such that a short-circuit is created at the point along the length of the loading capacitance forming a portion of the coaxial resonator.

3. A tunable resonator according to claim 2 wherein the second contact of the pair is configured to contact the sheath on its inner surface.

4. A tunable resonator according to claim 2 wherein the second contact of the pair is configured to contact the sheath on its outer surface.

5. A tunable resonator according to any of claims 2 to 4 wherein the tuning means comprises two pairs of electrically conducting conducts.

6. A tunable coaxial resonator according to claim 3 wherein the tuning means includes a coaxial cable.

7. A tunable coaxial resonator according to claim 3 wherein the tuning means is electrically coupled to a printed circuit board.

8. A tunable resonator according to claim 1 wherein the tuning means comprises a bushing having threaded portions on both the inner and outer surfaces of the bushing. The bushing being receivable in the space such that the outer surface of the bushing contacts the inner surface of the sheath and the inner surface of the bushing contacts the central conductor to form the short circuit.

9. A tunable coaxial resonator as herein before described with reference to Figure 1, Figure 2, Figure 3, or Figure 4.