US2518665A

US2518665A - Connector for high-frequency transmission lines and the like

Info

Publication number: US2518665A
Application number: US521726A
Authority: US
Inventors: Collard John
Original assignee: EMI Ltd
Current assignee: EMI Ltd; Electrical and Musical Industries Ltd
Priority date: 1942-08-22
Filing date: 1944-02-09
Publication date: 1950-08-15
Anticipated expiration: 1967-08-15

Description

Aug. 15, 1950 2,518,665

J. COLLARD CONNECTOR FOR HIGH FREQUENCY TRANSMISSION LINES AND THE LIKE Filed Feb. 8, 1944 INVENTOR. John Collard A TTOB/VE'K Patented Aug. 15, 1956 UNITED STATES PATENT OFFICE CONNECTOR FOR HIGH-FREQUENCY TRANSMISSION LINES AND THE LIKE Britain Application February 9, 1944, Serial No. 521,726 In Great Britain August 22, 1942 Section 1, Public Law 690, August 8, 1946 Patent expires August 22, 1962 4 Claims.

This invention relates to connectors for high frequency transmission lines and other high frequency apparatus.

High frequency transmission lines are usually in the form of cables comprising an outer tubular conductor and an inner conductor disposed centrally within the outer conductor, such form of cable being usually referred to as a coaxial cable. It is customary to connect two lengths of such cable together or to connect a cable to high frequency apparatus or to connect two pieces of high frequency apparatus together by means of various types of connectors. The term connector herein employed is intended to cover a device comprising essentially two separate parts each part comprising an inner conductor and an outer conductor insulated therefrom whereby each part can be connected to a coaxial cable so as to form a continuation of the inner and outer conductors of said cable, the two parts being arranged to be connected together as by the provision of plug and socket portions, bayonet portions, screw-=threaded portions or the like, which permit disconnection without the neces sity of disconnecting the cable from either part Of the connector, the two parts of the connector when in engagement with one another serving to connect electrically the inner and outer conductors of one cable to the inner and outer conductors respectively of the other cable. The term connector is also intended to cover a construction of connector which is formed for connecting two pieces of high frequency apparatus together or for connecting a cable to a piece of high frequency apparatus, the outer conductor of the connector in such a case being suitably formed for connection to the apparatus, for example, the outer conductor may be formed by connection to the screening box of the apparatus.

When the cables or the apparatus are designed for operating on wavelengths of a few centimetres it is found that the connectors introduce irregularities due to the inductance and/or capacity of the connectors which become of increasing eiiect as the wavelength of the energy it is desired to transmit becomes smaller. At such centimetre wavelengths the connector must be considered as forming a transmission line having a characteristic impedance which may differ from that of the cables to be connected and therefore introduces into the circuit irregularities of inductance and/or capacity. At longer wavelengths it is possible to reduce the size of the connector so that the irregularities produced are not appreciable, but at centimetre wave- 2 lengths this step is physically impossible, with the result that reflection occurs of the high frequency energy passing through the connectors.

In the specification of copending patent application Serial No. 487,166, filed May 15, 1943, by Edward C. Cork, now Patent No. 2,490,622, issued December 6, 1949, it has been suggested to overcome the above-mentioned difficulties by making the length of the connector equal to a half wavelength of the energy to be transmitted through the connector. Such a connector serves to transmit high frequency energy without reflection but in cases where the wavelength of the high frequency energy is of the order of 3 centimeters, a connector half a wavelength long becomes inconvenient from mechanical considerations. It is, however, possible to make the connector a multiple of half wavelengths long, but while such a construction permits the connector to be made from the point of view of mechanical construction, it is found that if the wavelength departs from the wavelength for which the connector is designed reflections occur which are of course undesirable.

It is often necessary to measure the voltage across a cable and this measurement can be con- Veniently effected at the connector by disconnecting the connector and inserting between the two portions of the connector a further portion of coaxial line, for example, of a length equal to one and a half wavelengths of the high frequency energy. This additional portion of line will be referred to as a test connector and may be provided with a probe which is capacity-coupled to the centre conductor of the test connector or may be provided with some other form of coupling means. The test connector, together with the two portions of the connector, is thus two whole wavelengths long and providing the probe is disposed at a distance which is one wavelength from either end, the voltage at the probe will be equal to the voltage across either cable where it is attached to the connector, and the test connector in this case will also not introduce irregularities into the circuit. Here again, however, if the Wavelength of the energy should depart from the proper value errors will also be introduced.

The object of the present invention is to provide an improved connector which can be made longer than a half wavelength and with a view to avoiding or reducing the above-mentioned difiiculties.

According to the present invention a circuit arrangement is provided comprising a high frequency transmission line or apparatus coupled to another high frequenc transmission line or apparatus each having an impedance T by means oi a connector comprising two portions each having an impedance Z and each having a length sub stantially equal to an effective quarter of a wavelength of the high frequency energy transmitted through the circuit and an intermediate portion having an impedance X whichis's'ubstantially equal to Z /T whereby said high frequency energy can be transmitted through said ctor sub.- stantially without reflection. the provision oi the intermediate portion it will be appreciated that the connector can be made longer than a half wavelength and it is found that'a' connector constructed in this manner can .beconveniently manufactured and, moreover, it. is found that while the frequency response is substantially. the same as that of a connector a half wavelength long, the connector according to the present invention is, nevertheless, an improvement insofar ajs'its frequency response is concernedif, as'in the case oi. wavelengths of the order of" 3' centimetres it isv necessary to make the previously pro posed connector more than one-half wavelength long. It is also found, as will be herein-after more fully referred to, that the probe for eiiecting vol age measurements at the connector can be inserted into the intermediate portion and willnot be found. to introduce such a large error in the voltage'measurement such as. would be the case if the wavelength of the energy changed when emplaying a test connector of the form referred to above;

According to another feature of the invention a connecto is provided comprising two portions each having s'ubstantially the same impedance and; substantially the. same electrical length and an intermediate. portion having a difierent impedance, saidttwo portions and said intermediate portion each having a substantially uniform inrpedance along its length. Y

"In order that thesaid invention may be clearly understood .andreadily carried into effect it now be more fully described with reference to the accompanying drawing which illustrates a connectcr constructed: according to the invention.

As shown in the drawing, the connector comprises two separate parts i and 2 which: are formed in any suitable manner as to enable them to be-separated as by the. provision of'plug and socketportions, bayonet portions screw-threaded portions or the" like. The actual mechanical'construction of the connector is immaterial so far as the. inventionis concerned. The two parts I- and 2. are shown connecting together twolengths oih-igh. frequency cable 3 of the coaxial type, the cable comprisingan outer conductor 6 and an inner conductor '5. The manner of connecting thev calole'to the parts i and 2' of the connector is also immaterial so far as the present invention is concerned. The two. parts of the connector are providedwith inner conductors 6 and-'1" which serve to connect together'the inner conductors 5 of the. cable and with outer conductors '8 and 9" whichserve to connect togethe the outer conductors4 of the cablei Inaccor'dance with the invention the connector comprises two portions in. and H each of a length efiectively equal to a quarter of the wavelengthof the high frequency energy to'be transmitted and an intermediate a length equal to a quarter. of a, wavelength. of the high frequency ener y, said portions may if While desired have a length equal to an odd integral multiple of quarter wavelengths. Conveniently, the joint between the two portions I and 2 is made at the centre of the portion l2. The impedance of the portion !2 is chosen so that it provides as transformed by the portions [0 and II a matched termination for either of the lengths of the cable 3. If the impedance of the portion is X and the impedance of the portions l0 and Il is Z then it can be shown that the connector can connect together a cable having an impedance equal to Z /X without introducing irregularities into the circuit which are such as to cause reflection of the high frequency energy. For example, if the cable 3 has an impedance T of 75 ohms and the portions 1 or i l have an impedance Z of 60 ohms, then the impedance of the portion [2 is made equal to Z /T which, in the present case, is equal to' 48 ohms. With such a construction the cable 3 forms the correct termination for the portion [2 so that energy can be transmitted through the connector substantially without reflection. Providing the impedance of the portion i2. is chosen to satisfy the above relationshipits length can be determined solely from mechanical considerations. It is found that, with a connector constructed in this manner, the deviation of impedance with variation of frequency is substantially the same as a connector equal in length to a half wavelength. The overall length of the connector according to the invention will be longer than a half Wavelength, and, as stated above, it can be made ofa length suitable to meet mechanical requirements. Compared with a con-- nector made in accordance with the prior pro-- posedand made equal to two wavelengths long for mechanical reasons; a connector according to the present inventionshows an improvement of ap proximately fourtimes for thesame deviation of wavelength.

It is preferred to fi-llthe space between.- the inner and outer conductors ofthe connector with a suitable insulating medium, such as th material known by the trade name Polystyrene. Since the dielectric constant of this material is different from that of air it will, of course, be necessary totake this into consideration in determining the actual lengths of the parts of the connector as distinct from their electrical lengths. In this specification therefore where reference is made to the lengths of the various partsthis is to be understood as'm-eaning their electrical lengths unless the context otherwiseimplies their physical length. The appropriate impedance can be imparted to; the portions of theconnector by suitablychoosing the diameters of the portions taking into consideration the insulating material employecr in the construction of theconnector. Itis desirable that theimpedances of the portions I0; It and I2 should be uniform throughout their length.

A'further advantage of the present invention is that the conductor can be constructed so that voltage measurements can be made by the insertion of a probe through the outer conductor off the connector and: projecting into capacitative relationship with the inner conductor; Providingthatthe wavelength of'the energy transmittedi throughthe connector does';n ot depart from the wavelength for which the connector is designed, then the probe; can beinsertedat any point along the length ofthe portion; since this-portion inoperation has nostanding wave. However; assoon as the wavelength departs from itscorrect value a smallstanding wave will be produced which will cause slight errors in the voltage measurement. It can, however, be shown that these errors are a minimum over a frequency range if the probe is inserted at a position a quarter wavelength away from the junction between the portions I0 and i2 and I2 and H. For example, where a test connector is employed in the previously proposed connector making the overall length of the connector and test connector equal to two wavelengths an error in voltage measurement when the wavelength departs from the correct value of about 18 percent may be encountered, whereas when employing the connector according to the invention for measuring the voltage with the probe placed a quarter wavelength away from the junction between the two portions as referred to above, an error about only 3.9 percent will be encountered for the same deviation in wavelength.

What is claimed is:

1. A circuit arrangement comprising a high frequency two-conductor, coaxial transmission line or apparatus coupled to another high frequency two-conductor, coaxial transmission line or apparatus each having an impedance T by means of a connector comprising two portions each having an impedance Z and each having a length substantially equal to an effective quarter of a wavelength of the high frequency energy transmitted through the circuit and an intermediate portion having an impedance X which is substantially equal to Z /T whereby said high frequency energy can be transmitted through said connector substantially without reflection, the said intermediate portion having a length substantially greater than one wavelength, said intermediate portion being arranged to be separable at substantially its midpoint.

2. A connector for coupling two high frequency, two-conductor, coaxial transmission lines together and comprising two portions each integrally connected to one of said transmission lines and having substantially the same impedance and substantially the same electrical length and an intermediate portion having a different impedance, said two portions and said intermediate portion each having a substantially uniform impedance along its length, the said intermediate portion having a length substantially greater than one wavelength and said intermediate portion being arranged to be separable at substantially its midpoint.

3. A connector for coupling two high frequency, two-conductor, coaxial transmission lines together and comprising two portions each integrally connected to one of said transmission lines and having substantially the same impedance and substantially the same electrical length and an intermediate portion having a different impedance, said two portions and said intermediate portion each having a substantially uniform impedance along its length, said intermediate portion having a length substantially greater than one wave length and being arranged to be separable at substantially its midpoint, each of said two portions having a length substantially equal to an effective quarter of a wavelength at the operating frequency.

4. A connector for coupling two high frequency, two-conductor, coaxial transmission lines to gether and comprising two portions each integrally connected to one of said transmission lines and having substantially the same impedance and substantially the same electrical length and an intermediate portion having a diiferent impedance, said two portions and said intermediate portion each having a substantially uniform impedance along its length, said intermediate portion being arranged to be separable at substantially its midpoint, each of said two portions having a length substantially equal to an effective quarter of a wavelength at the operating frequency, the impedance of said intermediate portion being so chosen with respect to that of each of said two portions that the operation of said device is substantially unafiected by the length of said intermediate portion.

JOHN COLLARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,702,305 Lewis Feb. 19, 1929 1,921,117 Darbord Aug. 8, 1933 2,036,381 Zinn Apr. '7, 1936 2,187,014 Buschbeck et a1. Jan. 16, 1940 2,207,690 Cork et a1. July 9, 1940 2,305,456 Okabe Dec. 15, 1942 2,364,526 Hansell Dec. 5, 1944 2,432,094 Fox Dec. 9, 1947