Application Date: May 5, 1941.
Complete Left: June 5, 1942.
Complete Accepted: Oct. 3, 1946.
Improvements in or relating to High Frequency Electric Circuit Arrangements
We, EDWARD CECIL CORK, of 50A, Kenilworth Road, Ealing, London, W.5, and ALAN DOWER BLUMLEIN, of 37, The Ridings, Ealing, London, W.5, both British Subjects, do hereby declare the nature of this invention to be as follows:-
The present invention relates to electric circuit arrangements particularly for use at high frequencies.
It is sometimes desired to employ a balancing system, for example, for comparing two impedances or for neutralising unwanted couplings, and such systems give rise to difficulties at high frequencies due to the appreciable impedance introduced by stray capacities.
It is the object of the present invention to overcome these difficulties.
According to the present invention, these difficulties are overcome by arranging the circuit in such manner that said undesired stray capacities are substantially short-circuited so that said capacities have little effect upon the balance.
One form of the invention will now be described by way of example with reference to the accompanying drawings, in which:-
Figure 1 shows a general circuit arrangement according to the invention, and
Figure 2 shows an adjustable balancing reactance suitable for use in the arrangement shown in Figure 1.
Referring to Figure 1, it will be seen that the arrangement comprises a coaxial feeder with an internal conductor 1 and a surrounding screen 2 which divides at the point 3 into portions 4 and 5 forming a U-shaped structure, the internal conductor 1 continuing within the portion 4 and emerging at the upper end thereof to make contact with the upper end of portion 5 at the point 6. Surrounding these elements is the conducting cylinder 7, and the lower end of said cylinder 7 is closed by a conducting annulus 8 which connects it to the screen 2. The upper end of said cylinder 7 divides into the two side tubes 9 and 10 within which are arranged conductors 11 and 12 respectively connected to the upper ends of the portions 4 and 5 of the screen 2 respectively. The length of the cylinder 7 is arranged to be such that said cylinder forms with the screen 2 a resonant half-wave line at the operating frequency. The portions 4 and 5 of the screen are arranged to be a quarter of a wavelength long at the operating frequency.
This arrangement may be used to compare two impedances 13 and 14 by connecting the two impedances as shown between the outer ends of conductor 11 and screen 9 and between the outer end of conductor 12 and screen 10. A source of high frequency oscillations 15 is connected between the conductor 1 and screen 2 as shown and a voltage detector 16, such as a diode, is connected between the common point 3 of the portions 4, 5 of the screen 2 and an adjacent point on the screen 7 as shown.
It will be seen that in this arrangement high frequency voltages are applied in opposite phases to the impedances 13 and 14 and if these impedances are equal the voltage setup across the voltage detector 16 will ideally be zero. Since the screen 7 and the screen 2 constitute a half wavelength line short circuited at the lower end by the annulus 8, the impedances between the upper ends of the portions 4, 5 of the screen 2 and the upper end of the screen 7 is substantially zero so that stray capacities across these points have substantially no effect upon the balance. Further, since the portions 4, 5 of the screen 2 effectively constitute a quarter wave line short circuited at its lower end 3, the impedance between the conductors 11 and 12 via said portions is high and high voltage may be conveniently set up between said conductors. Also due to the short circuit by the annulus 8, the impedance between the point 3 and the screen 7 is high, thus enabling a high impedance voltage operated detector to be used conveniently and preventing the balance from being affected by earth connections on the apparatus external to the screen 7. The annulus 8 is preferably earthed.
One of the impedances 13 or 14 may be adjustable so as to bring the arrangement into balance to measure an unknown impedance and the adjustable impedance may conveniently take the form shown in Figure 2. This adjustable impedance consists essentially of a half wavelength line terminated at each end by a resistance other than its characteristic impedance to provide a variable resistance and a line of adjustable length to provide a variable reactance, both of which are independently variable. Referring to Figure 2, the arrangement consist in the cruciform structure of cylindrical pipes 11, 21, 31 and 41, pipes 11 and 21 being coaxial and having their axis at right angles to the axis of pipes 31 and 41. A conductor 51 is supported along the axis of pipes 11 and 21, and a conductor 61 along the axis of pipes 31 and 41, conductors 51 and 61 being connected together at their point of intersection 71. Two conducting pistons 81 and 91 are arranged to slide within pipes 11 and 21 and are spaced apart by a half wavelength at the operating frequency by insulating rods 101 and 111. Each of the pistons 81, 91 make contact at their peripheries with the internal surfaces of pipes 11 and 21, but are insulated from the central conductor 51. A resistance, which is not equal to the characteristic impedance of the line formed by the conductor 51 and the tubes 11 and 21, is connected between each of the pistons 81, 91 and the conductor 51. These resistances are preferably provided by the resistance between the inner and outer conductors of a length of cable 121, 131 respectively having a characteristic impedance equal to the desired resistance and having an attenuation sufficient to prevent reflection from their open ends. Thus, as the pistons 81, 91 are moved together along the axis of the pipes 11 and 21, the resistance at the point 71 changes over a wide range. A further piston 141 is arranged to slide within the pipe 31 and to short circuit the inner conductor 61 to the pipe 31 so as to provide a variable reactance at the point 71. The impedance at the point 71 may thus be adjusted independently as to its resistance and its reactance by separate adjustment of the position of the pistons 81, 91 and the piston 141. The pipe 41 is arranged to be half a wavelength long so that the impedance at the point 71 is reproduced between the inner conductor 61 and the pipe 41 at the outer end thereof, and if applied to the arrangement shown in Figure 1, this pipe 41 and conductor 61 would be attached to screen 9 and conductor 11, or to screen 10 and conductor 12, shown in said figure.
If desired, the pistons 81, 91 may be fixed within the pipes 11, 21 and the pipes 31, 41 and conductor 61 moved laterally in slots provided in the pipes 11, 21 so as to slide the point of intersection 71 of the conductors 51 and 61 along conductor 61 and so vary the resistance component of the impedance set up at point 71.
It will be understood that, in the arrangement shown in Figure 1, the generator 15 and indicator 16 may be interchanged. Further, the portion 5 of the screen 2 may be omitted providing that other means, such as quarter wave elements connected to the screen 2, are provided to isolate the portion 4 of the screen from earth so that the conductor 11 connected to the upper end of the screen 2 and the conductor 12 connected to the inner conductor 1 will oscillate in voltage with respect to the screen 7 in opposite phases. The screen 7 may be any number of half wavelengths long, or, alternatively, may be an odd number of quarter wavelengths long if its lower and is joined to the screen 2 over a high impedance, the purpose of said screen 7 being to create a low impedance between said screen and the upper ends of screen 2 and conductor 1 so as effectively to provide a low impedance in shunt with stray capacities, the effect of which it is desired to eliminate.
The arrangement shown in Figure 1 may be conveniently applied to the neutralisation of coupling between electrical circuits. For example, in the detection of aircraft it is frequency necessary to radiate short pulses of high frequency oscillations from an aerial and to receive on the same aerial the oscillations reflected back by aircraft. As the pulses have to be radiated at high power and as the receiver has to be very sensitive, difficulty may arise due to the coupling between the transmitter and receiver via the aerial system which causes high voltages to be set up in the receiver during the transmission of pulses. This difficulty can be largely overcome by utilising the arrangement of Figure 1, the impedance 13 being the aerial, the impedance 14 an impedance electrically identical with that of the aerial, the generator 15 being the transmitter and the receiver being connected in place of the detector 16. With this arrangement substantially no voltage is set up across the receiver by the transmitter, but the receiver is nevertheless responsive to signals received by the aerial because these signals are not neutralised by signals from impedance 14, which, of course, does not receive signals. Alternatively, the receiver may be connected in place of the oscillator 15 and the transmitter in place of the detector 16.
The features hereinbefore described may be used separately or in combination.
Dated this 5th day of May, 1941.
F. W. CACKETT,
Chartered Patent Agent.
Improvements in or relating to High Frequency Electric Circuit Arrangements
We, EDWARD CECIL CORK, of 50A, Kenilworth Road, Ealing, London, W.5, and ALAN DOWER BLUMLEIN, of 37, The Ridings, Ealing, London, W.5, both British Subjects, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-
The present invention relates to electric circuit arrangements for use at high frequencies.
It is sometimes desired to measure or compare impedances at high frequencies and conventional bridge circuit arrangements such as are normally used at lower frequencies are found to be unsuitable, more particularly due to the fact that at high frequencies radiation tends to occur and also undesired couplings take place between different portions of the circuit, thus giving rise to instability and errors in measurement.
These difficulties may be overcome by suitable screening, but such screening is frequently found to decrease the efficiency of the arrangement.
It is therefore an object of the present invention to provide electric circuit arrangements suitable for the measurement or comparison of impedances at high frequencies which undesirable effects due to radiation or couplings are avoided without loss of efficiency.
According to the present invention there is provided an electric circuit arrangement for use at high frequencies comprising coaxial feeder having an internal conductor and a surrounding screen, a further conductor branching from said screen at a point intermediate its ends and extending therefrom substantially parallel to said screen to a point adjacent one end thereof so that the impedance at the operating frequency between said en of said screen and the end of said further conductor is high, an connection between said internal conductor and a point on said further conductor, a sheath substantially enclosing said feeder and said further conductor and connected to the other end of said screen, a source of high frequency oscillations and a detector of high frequency oscillations connected respectively between said internal conductor and said screen and between said sheath and said branch point or a point on said screen intermediate said branch point and said other end thereof at which the impedance is relative high, or vice versa, the arrangement being such that if an impedance is connected between said end of said screen and said sheath and a further impedance is connected between said end of said conductor and said sheath and electrical bridge network is formed.
Said impedance or said further impedance preferably comprises a coaxial line having a central conductor and a surrounding screen substantially one quarter of a wavelength long at the operating frequency and effectively terminated at its end by resistances which are inverse about the characteristic impedance of said line, and may also comprise a variable reactance in the form of a coaxial line having a movable short-circuiting piston adapted to control the value of said reactance.
In order that the invention may be more clearly understood and readily carried into effect, it will now be described in greater detail by way of example with reference to Figure 1 of the drawings accompanying the Provisional Specification and also the accompanying drawing which shows a variable reactance suitable for use in the arrangement of said Figure 1.
Referring now to Figure 1 of the drawing accompanying the Provisional Specification, it will be seen that the arrangement comprises a coaxial feeder having an internal conductor 1 and a surrounding screen 2 which branches at point 3 into portions 4 and 5 form a U-shaped structure, the internal conductor 1 continuing within the portion 4 and emerging therefrom at the upper end thereof to make contact with the upper end of portion 5 at the point 6. Alternatively, said internal conductor may emerge at a point intermediate the point 3 and the upper end of port 4 and make contact with the portion 5 at a point intermediate points 3 and 6.
A cylindrical sheath 7 surrounds the coaxial feeder referred to and is closed at its lower end by a conducting annulus 8 which is joint to the screen 2 as shown. The upper end of said sheath 7 divides into the two side tubes 9 and 10 forming together with conductors 11 and 12 connected to the upper end of portions 4 and 5 of the screen 2 respectively coaxial feeders to the outer ends of which impedances 13, 14 which are to be compared or balanced are respectively connected as shown. A source of high frequency oscillations 15 is connected between the conductor 1 and the screen 2 outside the sheath 7 as shown and a high frequency detector 16, such as a diode, is effectively connected between the point 3 and an adjacent point on the sheath 7 as shown. Although the detector 16 is shown inside the sheath 7 it may, if desired, be arranged externally of the sheath 7 and connected to the point 3 and the screen 7 over a coaxial feeder.
The lengths of the portions 4, 5 are chosen so that at the frequency of oscillations provided by the source 15 a high impedance is presented between their upper ends. Thus, each of these portions 4, 5 may be arranged to be approximately a quarter of a wavelength long at the operating frequency. With this arrangement, high frequency voltages of large amplitude and opposite phase are developed in operation at the upper ends of portions 4, 5, so that the portions 4, 5 operate as high impedance bridge ratio arms.
Further, the distance between the point 3 and the points at which the annulus 8 is connected to the screen 2 is so chosen that the screen 2, annulus 8 and the sheath 7 form a concentric line short circuited at one of its end sand presenting a high impedance at point 3. The distance between the point 3 and the point at which the annulus 8 is connected to the screen 2 may be for example a quarter wavelength. Thus, the sheath 7 and the annulus 8 do not introduce any inconveniently low impedance in shunt with the detector 16. It will be appreciated that, if desired, the distance between the point 3 and the points at which the annulus 8 is connected to the screen 2 may be longer than a quarter of a wavelength at the operating frequency and the detector 16 may be connected to an intermediate point of suitably high impedance so that, as before, the introduction of the sheath 7 and the annulus 8 do not give rise to any undesirably low shunt impedance.
It will be appreciated that the arrangement described constitutes a high frequency bridge circuit arrangement which is suitable for the measurement of high frequency impedance. Thus, for example, if the impedance 13 is an unknown impedance, its value may be determined by adjusting the impedance 14 until the detector 16 gives a minimum indication. As the apparatus is completely screen, no undesirable radiation takes place and errors in measurement which might otherwise be caused by such radiation are avoided.
The positions of the source of high frequency oscillations 15 and the detector of high frequency oscillations 16 may if desired by reversed.
Any form of adjustable impedance which does not radiate may be use3d as impedance 14 in the arrangement of Figure 1 above referred to, and one such impedance is shown in the drawing accompanying the present Specification.
Referring now the drawing accompanying the present Specification, the impedance comprises a variable resistance element in the form of a terminated coaxial line and a variable reactance element also in the form of a terminated coaxial line.
The variable resistance consists of the coaxial line comprising the cylindrical conductor 21 having an internal conductor 22 supported centrally within it. The ends of conductor 22 are secured to conducting pistons 23, 24 which are slidable within the conductor 21, but the conductor 22 is insulated from said pistons 23, 24. The ends of said conductor 22 are connected to said pistons 23, 24 by means of resistances 25, 26 respectively which may either be small resistances of the grid leak type or may be lengths of coaxial cable of sufficiently high attenuation to avoid reflections and therefore acting as non-inductive resistances of value equal to their characteristic impedance.
The variable reactance comprises a further cylindrical conductor 27 branching out of the wall of conductor 21 and having a central conductor 28 supported within it. A conducting piston 29 is arranged to slide on said conductor 28 within said conductor 27 so as to form a connection of low impedance between said conductors 27 and 28. The conductor 28 is slidably connected to the conductor 22.
A cylindrical conductor 30 and central conductor 31 are also preferably provided for effecting connection to the common point of conductors 22 and 28. Thus, the conductor 30 preferably branches out of the wall of conductor 21 at a point opposite to the point at which the conductor 27 branches therefrom so that conductors 27 and 30 are coaxial and conductor 31 is an extension of conductor 28. The variable impedance can then be conveniently connected to the arrangement shown in Figure 1 of the drawings accompanying the Provisional Specification by connecting conductors 30 and 31 to the conductors 10 and 12 of Figure 1 respectively.
It can be shown that if the length of conductor 22 is such that it forms with conductor 21 a coaxial line which is substantially a half wavelength long at the operating frequency and if the terminating resistances 25 and 26 are equal and have the value R different from the characteristic impedance Z of conductors 21 and 22 considered as a coaxial line, then the impedance at any point in the conductor 22 is always resistive and has a value equal toEQUAT. HERE
where q is the electrical length between one of the pistons 23, 24 and said point. Thus, by sliding the conductor 22 and the pistons 23, 24 fixed thereto inside the conductor 21, the resistive impedance at the point of connection between conductors 22 and 28 will vary from EQUAT. HERE according to the positions of the pistons 23, 24. It will therefore be appreciated that in order to obtain a wide variation of resistance R should differ considerably from Z.
Also, by sliding the piston 29 within the tube 27, the reactive impedance at said point of connection can be varied. Thus, the reactance and the resistance between the conductors 30and 31 can be varied independently at will.
If desired, the pistons 23, 24 may be fixed within the conductor 21 and the conductors 27, 30, together with the conductors 28, 31 may be arranged to slide along the conductors 21 and 22 so that the common point of the conductors 28 and 31 successively makes contact with different points on the conductor 22 and thereby causes the resistive component of the impedance across conductors 31, 31 to be varied. It will also be appreciated that the conductors 27 and 30 need not be coaxial and need not be arranged to be normal to the conductor 21 as shown. For example the conductor 27 may be cranked so as to have its axis parallel to the axis of conductor 22 if desired.
Further, although the conductor 22 has been stated to be preferably a half wavelength long it may if desired be only a quarter of a wavelength long providing that the terminating resistances are arranged to be inverse about the characteristic impedance of the conductors 21 and 22 considered as a coaxial line. Thus, if the terminating resistance at one end of the conductor 22 has a value R, the terminating resistance at the other end must have a value EQUAT. HERE. It will therefore be appreciated that the variable resistance is fundamentally constituted by a coaxial line a quarter of a wavelength long and terminated by means of resistances R and EQUAT. HERE which resistances may if desired by constituted by further lengths of coaxial line suitably terminated. The half wavelength case which has been describ3ed with reference to the accompanying drawing may thus be regarded as a quarter of a wavelength line terminated at one end by a resistance R and terminated at its other end by a resistance EQUAT. HERE in the form of a further quarter wavelength of line in turn terminated by a resistance R.
It will be understood that if the terminating resistances such as 26 and 26 have some small reactance component, this reactance component can be taken up by suitably adjusting the length of conductor 22 so that the terminations effectively become pure resistances.
In using the variable impedance as the adjustable impedance 14 in the arrangement previously described with reference to Figure 1 of the drawing accompanying the Provisional Specification, the lengths of the conductors 30 and 31 are conveniently chosen so that the impedance between the upper end of the portion 4 in the Figure 1 referred to and the impedance 13 is equal to the impedance between the upper end of the portion 5 in said Figure 1 and the common point of conductors 22 and 28 in the accompanying drawing, so that when the bridge is balanced the impedance 13 will be equal to the impedance to which said variable impedance is adjusted.
Apart from its use as a high frequency bridge for the measurement of impedance at high frequencies, the invention may also have other applications. For example, an aerial may be adjusted to match a feeder of some predetermined impedance by coupling one end of the feeder to the aerial and the other end of the feeder to the conductor 11 and screen 9 in Figure 1 previously referred to in place of the impedance 13. The impedance 14 required to balance the bridge may then be noted and the aerial adjustment modified until the bridge balances with a purely resistive impedance 14, thus showing the absence of reflection from the aerial.
The arrangement may also be employed to neutralise coupling between high frequency circuits. For example, in the detection of aircraft, short pulses of high frequency oscillations may be radiated at intervals and the reflections of such pulses from aircraft received upon the same aerial as that used for the transmission of the pulses. As the pulses have to be radiated at high power and as the receiver has to be very sensitive, difficulty may arise due to the coupling between transmitter and receiver via the aerial system causing high voltages to be set up in the receiver during the transmission of the pulses. This difficulty can largely be overcome by utilising the arrangement of Figure 1. Thus, the generator 15 may be the transmitter, the detector 16 the receiver, the impedance 13 the aerial and the impedance 14 and impedance equal to that of the aerial. As the arrangement forms a balanced bridge substantially no voltage from the transmitter 15 will be set up across the receiver 16. So far as high frequency oscillation received by the aerial 13 are concerned, however, the bridge is not balanced since similar oscillations are not developed in impedance 14, so that the oscillations received in the aerial 13 will be fed to the receiver 16.
Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-
Dated this 5th day of June, 1942.
F. W. CACKETT,
Chartered Patent Agent.
Leamington Spa: Printed for His Majestys Stationery Office, by the Courier Press. - 1946. Published at The Patent Office, 25, Southampton Buildings, London, W.C.2, from which copies, price 1s. 0d. each (inland) 1s. 1d. (abroad) may be obtained.