GB2178905A - Radio-frequency coupling - Google Patents

Abstract

An R.F. Coupler comprises four conductive cores 11 carrying insulating coatings 12 within a conductive sheath 10, the rest of the space within the sheath being filled with a dielectric material 14. The dielectric constants of the coatings 12 and material 14 are the same, the coatings 12 enabling simple manufacture while retaining constant spacings for the cores 11 around the axis of the sheath. In use a first opposed pair 11a, 11c of the cores constitutes the input and a second opposed pair 11b, 11d the output of the coupler. A circuit is described in which input power is divided by a first such coupler to a number of amplifiers, and their outputs re-combined by a second coupler. <IMAGE>

Description

SPECIFICATION Radio frequency coupling This invention relates two couplersforcoupling electrical lines and more particularly to a couplerforcoupling electrical lines at radio frequencies.

The theory of coupled lines is well known. The required impedance of either of the lines to be coupled is readily calculable for both odd and even modes of propagation.

where ZO is the characteristic line impedance ZOO is the odd mode impedance Zoe is the even mode impedance Kisthevoltagecoupling ratio and K=10C'20 where C is the coupling expressed in decibels.

However, when close coupling is required the odd mode impedance is made low; this can lead to severe constructional difficulties in existing coupling methods. For instance, using broadside-coupled lines between earth planes it is relatively simple to achieve adequate coupling, but terminating the lines to the restofthe electronic circuitry is difficult. Edge coupled lines using microstrip require close physical proximity for close coupling which is difficult to achieve and maintain.

An alternative to these, which is also easierto use in circuits, is a wireline coupler in which two wires to be coupled are contained within a common tube. Here again very close physical proximity is a requirement; this makes them very difficultto manufacture as they have to make use of a thin film wrapped around one ofthe wires as a spacerto regulate the amount of coupling.

The present invention is defined in the appended claims to which reference should now be made.

A coupler embodying the present invention may be designed so asto be used in all applications in which conventional 3dB radio frequency couplers are employed.

It will of course be appreciated that prior proposals have been madeforscreened multicore and particularly four-core cables for normal low-frequency use. However in such arrangements the nature of the dielectric within the screen and the characteristic impedance is of no importance and furthermore the conductors are used independently of each other and indeed coupling between them is undesirable.

Coupling arrangements using screened multicore lines have been proposed in British Patents 1,165,202 and 1,322,202, but here the coupling is purely capacitativefrom the input to the output ring and there is no coupling wire-to-wire. The connections all give in-phase outputs/inputs. The 'birdcage' arrangement used is solely to enable the transmission lines to share a common screen, and to terminate in a power-sharing resistive platetermination. British Patent 981,739 describes in Figure 4the use of four square-section con- ductors in a square screen with an air dielectric. In this arrangement only the background wave is used asthe forward wave ends up in a terminating resistor.Only in-phase signals from the input conductors are available at the output conductors.

A specific embodiment ofthe invention will now be described byway of example with reference to the accompanying drawings in which: Figure lisa cross-section through a coupler according to the present invention, Figure2 is a schematic circuit diagram incorporating the coupler shown in Figure 1, Figure 3 is a graph illustrating the power-sharing between components coupled by the circuit of Figure 2, and Figure 4 is a schematic circuit diagram, equivalent to Figure 2 but coupled using the prior art.

Referring to Figure 1 ofthe drawings, the present invention in this embodiment consists of an electrically conductive cylindrical sheath 10, contained within which is a group offoursimilarelectricallyconductive cylindrical wires 11 a, 11 b, 1 Ic,lid extending along the length ofthe sheath 10. Each wire 11 is coated with a dielectric material 12 such as polytetrafluoroethylene (P.T.F.E.) or polythene.Thewires 11 are radiallyspaced about the axis 13 ofthe sheath 10 in the shape of a square and supported in that shape by the outersurfaceof the adjacent coatings 12. Thus the distance between the outer surfaces of two adjacent wires 11 is twice the thickness ofthe dielectric coating 12.The ratio ofthe outside diameter D2 of each coating 12 to the diameter D1 ofeach wire 11, i.e D2/D1, may be approximated by the equation:

where E is the dielectric constant of the dielectric material 1 2, Z,, is as previously defined, the ratio ofthe sheath diameterto the spacing between the wires, i.e. D3/D2, is 7:1, and the voltage coupling ratio is such that Z0=2Z00.

The required thickness ofthe dielectric material 12 is made such that it may extruded or wrapped onto a conductorwire 11.

The space enclosed by the sheath 10 which is not taken up by the coated wires 11 is filled by a dielectric material 14, ofthe same dielectric constant as that coating the wires 11, and preferably the same material, i.e.

P.T.F.E. or polythene. During this filling operation the bundle of insulated conductors are held togetherwith adjacent coatings 12 touching each other. This makes it particularlyeasyto holdthewires 11 atthecorrect spacings while completely filling the space within the sheath with dielectric of uniform dielectric constant.

That is,the spacing between the conductors is determined by the dielectric thickness (including tolerance) which is not too difficult to achieve using conventional wire-making techniques.

The sheath 10 may be formed oftape, braid or solid metal and has an inner diameter D3 which is determined from the equation:

where Zae is as previously defined, and the effective diameter ofthe inner conductor wires 11 is < D2.

Using thisconstructionthe coupler can be employed to combinefour input signals orsplitfouroutput signals with a minimum of circuit complexity.

In use the input group ofterminals consists of two non-adjacent core members (e.g. 11 a, 11 c) at one end of the cable with respect to the screen 10 and the other two core members (11 b, lid) again with respecttothe screen at the second end. The outputs are the remaining terminal wires, again with respect to the screen.

In the embodiment of Figure 1 the dielectric within the sheath 10, namely the dielectrics 12 and 14, should be as uniform as possible otherwise the directivity of the coupler may be adversely affected. It will also be appreciated that the dimensions must be such as to comply with normal transmission line constraints. In particular, the dimensions must be such as to give the appropriate odd and even mode impedanceswhen opposed pairs of wires 11 are connected together.

In a modification ofthis embodiment both the dielectric materials 12 and 1 4 are air; the wires 11 being of a rigid construction and supported in the square configuration by spacers located at intervals along the coupler.

In Figure 2the coupler is employed to combinefouroctave bandwidth amplifiers 15 using two lengths of coupler 16 and 20, and the mannerof connection ofthe coupler is illustrated. The octave of operation ofthe amplifier is centred around a frequencyf0 given by the formula:

where, c= velocity of light L = length of coupler = =dielectric constant ofthe dielectric material.

The centre frequency could be anywhere in the range of radio frequencies although, due to size con siderations, there are probably better methods at frequencies below30 MHz.

The first coupled lines are formed by connecting ends 17 and 18 of two radially opposite wires 11 a, 11 cto form inputs with respecttothe earthed outer sheath 10. The ends 19 and 20 ofthe remaining two wires 11 b, 11 dare similarly connected and led to earth through a matching impedanceZO. Each wire 11 then supplies a proportion ofthe available power fro the input 21 to the amplifiers 15.The output of each amplifier is connected with a corresponding wire 11 of a second section of coupler 20. This coupler 20 has ends 22 and 23, which are connected to ground through the matching impedance Z,, and ends 24 and 25 which are connected togetherto form a single output26which carriesthe amplified signalsfrom thefour coupled amplifiers outputs. To avoid confusion, the dielectric coatings 12 are coded by making those coatings 12 of mutually coupled wires 11, i.e. those opposite one another across the axis 13 ofthe coupler, the same colour.

It has been found that such couplers enable better power sharing amongst amplifiers ofthis bandwidth when connected in this way. This is shown in the graph of Figure3 in which the power sharing in one coupler over an octave bandwidth distributes the powerto the amplifiers by relative overloading of one amplifierat the band edges, as illustrated by curve 27, and relative overloading of the second amplifier at the band centre, as illustrated by curve 28. An equivalent conventional method would use a series of six '3dB couplers' 29 as illustrated in Figure 4,to combinethe amplifiers 30.

It should be noted that in the couplers illustrated in Figures 1 and 2 the forward and backward waves are equally important and provide in-phase and quadrature outputs/inputs.

Itisthus possible to couple lines using a coupler which is both easierto manufacture than previous forms, having more suitable dimensions, and is more convenientto use in electronic circuits and the like.

Claims (10)

1. A method of coupling electrical components which process electrical signals transmitted at radio frequencies, the method comprising providing a coupler comprising an outer cylindrical, electrically conductive sheath, a group of four electrically conductive core members extending along the inside of the sheath and equally radially spaced from each other about the axis of the sheath, and dielectric material occupying the remaining space within the sheath, and applying input electrical signals to an opposed pair ofthe core members and selecting outputsignalsfrom the other opposed pairofthe core members.

2. Acouplerforcoupling electrical components operating at radio frequencies, the coupler comprising an outer cylindrical, electrically conductive sheath, a group offour electrically conductive core members extending along the inside of the sheath and equally radially spaced from each other aboutthe axis ofthe sheath, and dielectric material having a substantially uniform dielectric constant occupying the space within the sheath around the core members.

3. A coupler as claimed in claim 2, wherein the dielectric material comprises a first cylindrical dielectric material on each core member and second dielectric material filling the remaining space within the sheath.

4. A coupler as claimed in claim 3, wherein the distance between the outer surfaces of two adjacent conductive members is twice the thickness ofthe first dielectric coating.

5. A coupler as claimed in claim 3, wherein the distance between the outer surfaces of two adjacent conductive members is twice the thickness of the first dielectric material, and the outside diameter D2 ofthe first dielectric material is defined substantially bythefollowing equation:

where is the dielectric constant ofthe first dielectric material ZOO is the odd-mode impedance ofthe coupler D1 is the outside diameter of each core member, and the inside diameterD3 ofthe sheath is substantially defined by the following equation:

where Zoe is the even-mode impedance of the coupler.

6. A coupler as claimed in any of claims 2 to 5, wherein the conductive core members are colourcoded according to their connection.

7. A coupler as claimed in any of claims 2to 6, wherein first ends of a pair of core members ofafirstlength of coupler, oppositely spaced across the axis of the sheath, are connected with earth through a terminating impedance, the remaining first ends being connected togetherto form a common input; amplifiers are each connected bytheir inputs with a second end ofa corresponding member ofthefirst length of coupler; a second length of coupler is connected to receive the output of the amplifier, each first end of the core mem bers ofthe second length of coupler being connected with a corresponding amplifier output; and a pairof second ends of the second length of coupler, oppositely spaced acrossthe axis of the sheath, are connected with earth through a terminating impedance, the remaining ends being connected together to form a common output.

8. A method of electrically coupling electrical components which process electrical signals transmitted at radio frequencies, the method comprising: supplying the signal to be processed to each first end of a pair of electrically conductive core members of a group offoursuch members which are parallel to and equally radially spaced aboutthe axis of an earthed sheath enclosing the members; splitting the signal between the said pair of members and the remaining pair of members in the group by earthing the first ends ofthe remaining pair of members through a terminating impedance; connecting the second end of each memberofthe groupto an inputofa corresponding component;; supplying the first ends of each of a further group of four electrically conductive members, also parallel to and equally radially spaced about the axis of an earthed sheath enclosing the further group of members with an output of a corresponding component; and combining the processed signals by earthing the second ends of a pairofthefurthergroup of members through a terminating impedance and connecting the second ends of the remaining members togetherto form a single processed output.

9. A method of making a couplerforcoupling electric components operating at radiofrequencies,the method comprising the steps of providing four electrically conductive core members of similarcross-section each having a coating of a first dielectric material thereon, placing the four members side by side so as to lie at the corners of a square with the coatings of adjacent core members touching each other, and placing a conductive sheath around the group of core members and filling the rest ofthe space within the sheath with a second dielectric material which has substantially the same dielectric constant as the first dielectric material to provide a uniform dielectric constant within the sheath around the core members.

10. A coupler substantially as herein described with reference to the accompanying drawings.