US2462443A

US2462443A - Transmission line system

Info

Publication number: US2462443A
Application number: US639606A
Authority: US
Inventors: Robert S Wehner
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1946-01-07
Filing date: 1946-01-07
Publication date: 1949-02-22
Anticipated expiration: 1966-02-22

Description

Iatented Feb. 22, 1949 TRANSMISSION LINE SYSTEM Robert S. Wehner, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application January 7, 1946, Serial No. 639,606

4 Claims.

The present invention relates to transmission line systems and, more particularly, to such systems for feeding a single antenna at two widely spaced frequencies.

- An object of the present invention i the provision of a novel system for feeding and matching an antenna to a transmission line over two widely spaced frequency bands.

- Another object of the present invention is the provision of a system as aforesaid which permits the use of any desired type of matching section in the branches of the circuit.

A further object of the present invention is the provision of a single small antenna for use on two widelt spaced frequency bands.

Still a further object of the present invention is the elimination of the necessity for using a plurality of antennas on aircraft.

The foregoing objects and others which may appear from the following detailed description are attained by providing a reentrant branched transmission line circuit from a transmitter capable of operating on a pair of different frequencies to a single antenna. The branches of the transmission line are so arranged that energy of one frequency passes entirely through one branch of the system while energy at the other frequency passes through the other branch. Thus, each branch may contain its own individual impedance matching circuits for matching the antenna to the feed line from the transmitter.

The present invention will be more fully understood by reference to the following description which is accompanied by a drawing in which reference number It denotes an antenna which is to be fed by means of transmission line TL with one of two frequencies A1 and m from a transmitter (not shown) Antenna Iii, while illustrated as a two-frequency stub antenna of the type described in more detail in my copending application #636,932, filed December 22, 1945, need not necessarily be of this type. A simple stub antenna may be used if desired.

The following description will be generally predicated on the assumption that a simple stub antenna is used. Any one interested in a further elucidation of the principles of operation of the two-frequency stub antenna is referred to the more complete description appearing in the copending application referred to above.

The transmission line TL contains a pair of reentrant branches TL1 and TL2 connected in parallel relationship between points a and d. The inner conductor 29 splits into conductors 2| and 22 at point a and then again comes together in a single conductor at point d. Branch TL1 contains a short-circuited series line section l5, having a pair of parallel inner conductor l6 and I1, each a quarter wave long at the frequency A2 which is the frequency adapted to be passed 2 through the other branch of the network. Conductors It and I! are connected together and to the outer shell I8 of the line section [5 at one end and connected in series with the inner conductor 2| of transmission line TL at their other ends. Conductor 21 then passes through an impedance matching section MM designed to match the impedance looking out of the matching section IE to the impedance at the input of a second series line section 25 at the frequency M. The second short-circuited series line section 25., likewise having short-circuited quarter wave inner conductors i6 and I! and an outer casing I8 identical in dimensions and arrangement to the short-circuited series line section l5, follows the matching section MM. From the second series section the transmission line TLi again enters the main transmission line TL at point d and proceeds to the antenna 10. Branch TL: has the same reference numerals as applied to branch TLi but primed in order to indicate that the construction is generally similar but the actual dimensions are different. That is, the short-circuited series line sections l5 and 25 are designed to present high impedance at frequencies which are adapted to be passed by the line TLl. The

matching section MM is designed to match the impedance looking out of short-circuited line section l5 into the short-circuited series of line section 25.

In considering a signal of Wavelength A1 approaching the junction at (a) from the direction of the transmitter, the signal will take the right-hand branch at (a) into transmission line section TL1 for the following reasons: I

Looking into TLZ from (a) is seen a very high impedance due to line section l5, which, being shorted at its far end and one quarter wavelength long at A1, presents a very high impedance in the direction from (a) to Hi. I

Looking into the right-hand branch TLz is seen a matched impedance of the system since the matching section MM is designed to match the impedance of the entire system beyond the matching section to the system consisting of the line section IS in series with the transmission line TL leading from the transmitter. The transmitter is of course assumed to be matched to the main transmission line TL.

Therefore, all or almost all of the current entering the junction (a) at the wavelength M takes the right-hand branch passing in turn through the line section l5, through the matching section MM, through the line section 25, through the junction (d) at the base of the antenna and into the antenna l0. At the junction (41) almost all of the current passes into the antenna since there is a very high impedance look- 3 ing in the direction from (d) to the line section 25', this line section being identical with line section l" and, presenting a very high-impedance to energyat a"wavelength A1 thereby preventing circulating current in the branch circuit.

In similar manner it is evident that at thewavelength )\2 all or almost all of the: current will a go through transmission line TLz to the antenna I0, the line sections [5 and 25 'beingshorted quarter wave lines at this wavelength andcon actanceatfrequencies above resonance. At the wavelengthm the antenna is fed through branch:

The line section TLzwhich at thiswavelength is less than a quarter wavelength long introducesan inductive reactance in series with the antenna-at point (d). This inductance tends to cancel 'thecapacitive reactance of the antenna; Thus,-it is" easier to match the system between matching section MM and antenna ID to the transmissionline TL through-the matching section MM; Also matching section MM is effective-overa broaderband of frequencies than would-otherwise be-possible; Conversely for short wavelength- A2 at 1 which through the lefthand branch TLz, the line section 25 which at this wavelength is more than a quarter wavelength long introduces'a capaci-- tive reactance in series with-theantenna. This capacitance tends-to cancel the inductive reactance of-the antenna making the system easier to matchby meansof matching circuit Mk2, makingthe matchingcircuiteffect-ive over a broader While I haveillustrateda particular embodi ment of the present invention, it should be clearly understood-that it is not limited thereto since many, modifications may be made in the several elementsemployedand in their arrangement and it is therefore contemplated to cover any such modifications asfallwithi'n-the spirit and scope of theinvention.

What is claimed is:

1. 'Anantenna feed line system for. operation at ,two frequencies includinga branched'reentranttransmission line from, a source of energy at; said; frequencies to a single antenna each branch of said transmission line. including a first and a,secondshortecircuited series line: section, the line, sections inieachbranch presenting. a high" impedance to, a. different.- one, -.of1saidi. fre-v quencies, a series matching. sectioninterposedv between said series linesections in each. branch; said matching sections serving. to match the impedance of the first of said line sections to the the antenna is fed.

4 impedance of the second of said line sections and said antenna at the one of said two frequencies to which the linesections in said; branch do not present" a high'impedance:

2. An antenna feed line system for operation at two frequencies including a branched reentr-ant transmission line from a source of energy at said frequencies to a single antenna, each branch of said-transmission line including an innerconductorand outer shell and each branch of said transmission line including a first and a secondshort-ci-rcuited series line section, the line sections in each branch presenting a high impedance to a different one of said frequencies, a series matching section interposed in each branch between said seriesxline sections, said matching sections each serving to :match the ima; pedance of the firstof said line sectionsatoithe. impedance of the second of said linezsectionsz andLsaid antenna at the other-ofsaidtwdfrequencies; said series line sections each includingla pair of parallel conductors within an outer. Shel-1;

said conductors each having.allengthequalvto an odd multiple including unity of a quarter wavelength-at'the frequency to whichiitisito present. a high impedance, said conductors being connected together and to said shellat one pair of ad jacent ends, the other endsbeingconnected-in series with-the inner conductor of its branch.-

3 An antenna feed line system for operationat twofrequencies including a branched reentranttransmission line from a source of energy at said frequencies to a single-antenna, each branch of said transmission line including means presenting a high impedance to a different one of said frequencies, each'branch also including a series matching section, each of said matching sect-ions serving-to match the impedances in each 1 branch 'for the frequency which it is to pass.

4.--In an antenna feed line system for'operation at two'frequencies including "a branched re-- entrant transmissionline from a source of energy at said frequencies to asingle' antenna, said transmissionline including an outer shell andinner conductor and each-branch of said transmission-line including impedance elements presenting a'highj impedance to one of said frequencies andhaving a minimum effect on" the other "of said frequencies, said impedance elements', including apair 'of parallel conductors within an outer'shell, said conductors'each hav-'" ing a length equal to an odd multiple including unity of' a quarter wavelengthat the frequency to' which itis to present a high impedance," said conductorsbeing connected together. and to said shell-at one pair of adjacent'ends, the other ends being connected in'series' with the inner conductor of its branch.

ROBERTS. W'EHNERT REFERENCES CITED The following references are of record-inthe le. of this patent:

UNITED STATES PATENTS Number Name Date 2,128,400 Carter Aug. 30,1938 2,190,131.: Alford Feb. 13, 1940 2,212,214, Smith Aug. 20, 1940. 2,318,237 Lin-denblad May 4, 1943 2,341,408 Lindenblad Feb.-8,- 1944=