GB1571383A

GB1571383A - Micorwave multiplexer

Info

Publication number: GB1571383A
Application number: GB226876A
Authority: GB
Original assignee: Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1976-01-21
Filing date: 1976-01-21
Publication date: 1980-07-16
Also published as: DE2612789C3; DE2612789B2; DE2612789A1

Description

(54) MICROWAVE MULTIPLEXER (71) We. THE MARCONI COMPANY LIMITED, a British company, of Marconi House. New Street, Chelmsford, Essex, CMI 1PL do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a microwave multiplexer, that is to an arrangement for coupling two or more electro-magnetic microwave signals of differing frequencies to a single electro-magnetic wave transmission line. Because a multiplexer is a reciprocal arrangement it will be understood that it may similarly be employed to separate electro-magnetic microwave signals of differing frequencies.

A known type of multiplexer, more normally referred to as a diplexer since it combines or separates only two different frequencies, utilises hybrid couplers.

A hybrid coupler is essentially a lossless device with its four terminals arranged in two pairs such that when the four arms are completely matched no power flow takes place between the arms of a pair and such a pair of arms is herein termed a conjugate pair of arms. A further property of a hybrid coupler is that when power enters one of the arms of either conjugate pair it may be arranged to divide equally, or with some other predetermined ratio, and flows out of the arms of the other conjugate pair; the relative phase angle of the two equal outputs depending upon the type of hybrid junction used. Such couplers are well known in the art, for example the magic-Tee coupler and the so-called 3dB Riblet coupler. The known type of multiplexer referred to comprises two four terminal hybrid couplers having a conjugate pair of arms of each coupler interconnected by two waveguide sections of transmission line having different lengths. The lengths of the waveguide sections determines the two frequencies which will be combined or separated and the difference between the lengths of the transmission line waveguide sections must be an even number of guide wavelengths at one frequency and an odd number of half wavelengths at the other frequency, thereby determining the bandwidth of each of the two diplexed frequencies. The bandwidth is thus a function of the path length between the two hybrid couplers and the greater the difference in path lengths the greater is the phase sensitivitv between the two transmission line waveguide sections as a function of frequency and the smaller is the operable bandwidth.

Normallv the multiplexer is machined from a solid block of material as in our UK Patent No. 1,345,537. With such a construction however, where high power signals are applied there is a tendencv for a shift of centre frequency to occur. As an example, where the diplexer was manufactured from a block of aluminium and had six signal frequency channels each designed for a üMHz bandwidth centred on a frequency in the region of 6GHz. with a 3Kw input the centre frequency was found to shift downwardly by 13MHz. It will be readilv appreciated that such a large shift in centre frequency, in comparison with the frequency bandwidth. results in an effective decrease in the useful operating bandwidth of each channel.

The present invention seeks to provide a multiplexer in which the foregoing disadvantage is substantially reduced.

According to this invention a microwave frequency multiplexer includes at least two four terminal hybrid couplers having a conjugate pair of arms of each interconnected through two waveguide sections of different lengths, the coefficient of expansion of the longer waveguide section being less than that of the shorter section thereby eliminating or reducing changes of the Icngtll difference caused by changes of temperature.

The invention is based upon the belief that the shift in frequency hereinbefore referred to was due to the differing absolute expansion of one of the transmission line waveguide sections compared to the other.

By selecting the waveguide sections to be of materials having different coefficients of expansion there may be provided a multiplexer which is comparatively insensitive to temperature changes.

In a preferred embodiment where each channel is fed with a 3Kw signal of4()MHz bandwidth centered on 6GHz one of the sections is of a copper material and the other section is of an aluminium material.

The invention will now be described, by way of example, with reference to the drawing accompanying the Provisional specification which shows one form of a diplexing type of multiplexer to illustrate the present invention.

The diplexer shown in the drawing is of the multipole type and comprises two four terminal hybrid couplers 1, 2 having a conjugate pair of arms of each interconnected by further four terminal hybrid couplers 3, 4, and associated with a conjugate pair of arms of the couplers 3, 4 transmission line waveguide sections 5, 6. The waveguide sections 5, 6 act as transmission lines and are short circuited at their respective ends remote from the couplers 3 or 4: ulounted inside the sections 5, 6 may be inductive posts for tuning the signals propagated thol cin, by "squaring off" the frequency/coupling response. The coupler 1 has a conjugate pair of arms which are each connected to a respective one of a pair of input terminals 7 and the coupler 2 has a conjugate pair of arms connected such that one arm is terminated in n load 8 and the other arm is connected to an output terminal 9.

The lengths of the waveguide sections 5, 6, from their associated couplers 3, 4 respectively to the short circuits associated with the sections 5, 6 are now calculated. The required operational frequency regime is obtained by suitably dimensioning the lengths of sections 5, 6 designated L1 and L2 respectively, together with their associated equal rectangular waveguide internal broad dimension, designated A1 and A2. In operation, with power flow through these waveguide sections 5, 6, an increase in the lengths and widths of the sections 5, 6. due to thermal transfer, is designated to be AL1, AL2 and Anal, AA2 respectively. If no steps were taken to prevent it, i.e. the sections 5, 6 are made from a contiguous block of material, and if the length L1 is twice the length L2 then L, will be approximately twice L2, the waveguide A dimensions will increase equally and the deviation in centre frequency of each channel, Af, may be expressed as

fcl is the cut-off frequency of the section 5 fc2 is the cut-off frequency of the section 6 Al is the internal broad wall dimension of the rectangular waveguide of section 5 A. is the internal broad wall dimension of the rectangular waveguide of section 6 c is the velocity of light in free space The present invention provides that the waveguide sections 5 and 6 are physically formed from different materials selected so that overall no matter what the change in the lengths of sections 5, 6 (ALl AL.) combined with the change in broad wall dimension (hAl, AA2) the diplexer produces a substantially zero deviation from the desired channel centre frequencies. It has been found that the selection of a copper material for the longer length (LI) and an aluminium material for the shorter length (L) produces a substantially zero deviation of channel centre frequency Af, where each of six channels is fed with a 3Kw signal of 40MHz bandwidth centred around 6GHz. The material required for the channels 5, 6 is calculated with reference to their coefficients of expansion with the ratios being inversely proportional to the lengths of the sections.

It is pointed out that the illustrated embodiment of the invention can be modified by omitting the hybrid couplers 3 and 4 in which case the ends of the waveguides 5 and 6 can be connected directlv to the couplers 1 and 2.

WHAT WE CLAIM IS: 1. A microwave multiplexer including at least two four terminal hybrid couplers having a conjugate pair of arms of each interconnected through two waveguide sections of different lengths, the coefficient of expansion of the longer waveguide section being less than that of the shorter section thereby eliminating or reducing changes of the length difference caused by changes of temperature.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. changes of the Icngtll difference caused by changes of temperature. The invention is based upon the belief that the shift in frequency hereinbefore referred to was due to the differing absolute expansion of one of the transmission line waveguide sections compared to the other. By selecting the waveguide sections to be of materials having different coefficients of expansion there may be provided a multiplexer which is comparatively insensitive to temperature changes. In a preferred embodiment where each channel is fed with a 3Kw signal of4()MHz bandwidth centered on 6GHz one of the sections is of a copper material and the other section is of an aluminium material. The invention will now be described, by way of example, with reference to the drawing accompanying the Provisional specification which shows one form of a diplexing type of multiplexer to illustrate the present invention. The diplexer shown in the drawing is of the multipole type and comprises two four terminal hybrid couplers 1, 2 having a conjugate pair of arms of each interconnected by further four terminal hybrid couplers 3, 4, and associated with a conjugate pair of arms of the couplers 3, 4 transmission line waveguide sections 5, 6. The waveguide sections 5, 6 act as transmission lines and are short circuited at their respective ends remote from the couplers 3 or 4: ulounted inside the sections 5, 6 may be inductive posts for tuning the signals propagated thol cin, by "squaring off" the frequency/coupling response. The coupler 1 has a conjugate pair of arms which are each connected to a respective one of a pair of input terminals 7 and the coupler 2 has a conjugate pair of arms connected such that one arm is terminated in n load 8 and the other arm is connected to an output terminal 9. The lengths of the waveguide sections 5, 6, from their associated couplers 3, 4 respectively to the short circuits associated with the sections 5, 6 are now calculated. The required operational frequency regime is obtained by suitably dimensioning the lengths of sections 5, 6 designated L1 and L2 respectively, together with their associated equal rectangular waveguide internal broad dimension, designated A1 and A2. In operation, with power flow through these waveguide sections 5, 6, an increase in the lengths and widths of the sections 5, 6. due to thermal transfer, is designated to be AL1, AL2 and Anal, AA2 respectively. If no steps were taken to prevent it, i.e. the sections 5, 6 are made from a contiguous block of material, and if the length L1 is twice the length L2 then L, will be approximately twice L2, the waveguide A dimensions will increase equally and the deviation in centre frequency of each channel, Af, may be expressed as fcl is the cut-off frequency of the section 5 fc2 is the cut-off frequency of the section 6 Al is the internal broad wall dimension of the rectangular waveguide of section 5 A. is the internal broad wall dimension of the rectangular waveguide of section 6 c is the velocity of light in free space The present invention provides that the waveguide sections 5 and 6 are physically formed from different materials selected so that overall no matter what the change in the lengths of sections 5, 6 (ALl AL.) combined with the change in broad wall dimension (hAl, AA2) the diplexer produces a substantially zero deviation from the desired channel centre frequencies. It has been found that the selection of a copper material for the longer length (LI) and an aluminium material for the shorter length (L) produces a substantially zero deviation of channel centre frequency Af, where each of six channels is fed with a 3Kw signal of 40MHz bandwidth centred around 6GHz. The material required for the channels 5, 6 is calculated with reference to their coefficients of expansion with the ratios being inversely proportional to the lengths of the sections. It is pointed out that the illustrated embodiment of the invention can be modified by omitting the hybrid couplers 3 and 4 in which case the ends of the waveguides 5 and 6 can be connected directlv to the couplers 1 and 2. WHAT WE CLAIM IS:

1. A microwave multiplexer including at least two four terminal hybrid couplers having a conjugate pair of arms of each interconnected through two waveguide sections of different lengths, the coefficient of expansion of the longer waveguide section being less than that of the shorter section thereby eliminating or reducing changes of the length difference caused by changes of temperature.

2. A microwave multiplexer as claimed in claim 1 wherein one of the sections is of a

copper material and the other section is of an aluminium material.

3. A microwave multiplexer substantially as herein described with reference to, and as illustrated in, the drawing accompanying the Provisional specification.