US2417290A

US2417290A - Antenna system

Info

Publication number: US2417290A
Application number: US476820A
Authority: US
Inventors: George H Brown
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1943-02-23
Filing date: 1943-02-23
Publication date: 1947-03-11
Anticipated expiration: 1964-03-11

Description

l March 1947. G, H, BRWN 2,417,290

ANTENNA SYSTEM f Filed Feb. 25, 19g?,

/ Gttorneg Patented Mar. 1l, 1947 ANTENNA SYSTEM George H'. Brown, Princeton, N. J., assignor to Radio Corporation of America, a corporation cf Delaware Application February 23, 19.43, Serial No.. 416.820

This invention rela-tes to antennas, and more 'particularly to structuresV for radiating energy with high efficiency over a broad band ci ultra high frequencies.

It is the principal object of this invention to provide an improved dipole antenna structure having a broad resonance characteristic.

Another object is to provide an improved dipole antenna which is mechanically strong, of light" weight, and simple in construction.

A further object is to provide an improved dipole antenna which may be coupled to a standard concentric transmission line without requiring transformers or complex networks.

These and other objects will become apparent 1 to those skilled in the art upon consideration of the following description with reference to the Fig. 1.

Referring to Fig. l, a pair of parallel tubular supportingmembers I and 3 are fastened by welding, for example, to a base plate 5. Tubular radiator elements 1 and 9 are similarly secured to the supports I and 3 at the upper ends, as shown more clearly in Fig. 2. The members I, 3

and

elements

1, 5 may be made of thin-walled y steel tubing or the like. Metal bushings Il and I3 are inserted at the upper ends of the supports I and 3. Coaxial lines l5 and I1 extend through the supports I .and 3 respectively and the bushings II and I3. The outer conductors of the lines are connected to the bushings. Insulatng caps I9 and 2l are provided for the bushings H and I3, and

conductors

23 and 25 respectively are supported by and extend through the Caps.. The inner Conductors of the lines I5 and l1 are connected to the

conductors

23 and 25. The

conductors

23, 25 are connected together through a snorting bar 21.

The dimensions of the various parts in terms of the wavelength, A, at the center of the band over which the system is tocperate, are indicated in Fig. 1. The supports I and 3 and the radiator elements 1 and l'i are each substantially one quarter wavelength long. The line l5, from the terminal 23 to the junction point 29, is electrically 5 Claims. (Cl. 25u-33) one half wavelength longer than the line I1. The

physical difference in length between the lines I5 and I1 will be somewhat less if the velocity of propagation of energy through the lines is less than that in space. A line 3l is connected to the junction 29 and leads to a source of energy or radio device, not shown. The base plate 5 may be grounded, as by connection to a. metallic sup'- porting framework or to a reiiector, and the outer conductors o f the lines I5, I1 and 3l are also grounded at any convenient points.

The operation of the above described' structure is substantially as follows:

Radio frequency voltage is applied between the inner and outer conductors of the lines I5 and I1 through the line 3l. At the instant the inner conductor of the line I1 is positive at the point 33 at the base 5, the inner conductor of the line l5 is negative at the point 35, `because the length of the line I5 between the

points

29 and 35 is 180 electrical degrees longer than that of the lineI1 between the points 29 and 33. Since the ends of the inner conductors of the lines I5 and I1 are connected together by the shorting bar 21, cur-.- rent flows up through the inner conductor of the line I1, across the bar 21, and down through the inner conductor of the line I5. The above conditions are reversed with each R.F. cycle. The currents flowing in the inner conductors of the lines I5v and I1 induce opposite currents in the respective outer conductors. Thus the instan! taneous flowof current on the outer conductor ci the line l5 is upward when that on the line I1 is downward. The supports I and 3 constitute a parallel line, short circuited by the base 5, and thus present a high impedance between the points 31 and 3S, at the ends ofthe outer conductors of the lines I5 and I1. The voltage between these points is applied to the dipole 1, 9.

The .dipole is equivalent to a series resonant circuit comprising capacitance, inductance, and resistance. Near the resonant frequency, the et, fective resistance between the inner ends of the radiator elements is 36 ohms. The parallel line formed by the supports I and 3 is equivalent to a parallel resonant circuit. With the dimensions indicated, the resonant frequency of the dipole is slightly lower than that of the parallel line, so that at a frequency at which the dipole exhibits, for example, 20 ohms inductive reactance, the line has suiiicient capacitive reactance to resonate the dipole. At this frequency the line and the dipole together act like a parallel resonant circuit connected between the points 31 and 39 with the re- 3 sistance in the inductive arm, and present a purely resistive load of R2 X5l l c Z= i which in this case is about 50 ohms, and is substantially equal to the surge impedance of the coaxial lines l and Il. At other frequencies, the reactance of the dipole is still opposite'in sign to that of the parallel line, since as the frequency is increased the series resonant circuit becomesV more inductive and the parallel resonant circuitV becomes more capacitive, and vice versa. Thus' the combined impedance of the radiator and the support is a relatively constant resistance over a.

wide frequency band, Vproviding uniformly high efficiency of radiation. Y i

If the lines I5 and Il match the dipole impedance, as described above, the impedance looking toward the antenna from the junction point -29 -is' one-halfA the line impedance, or in the example given above, 25 ohms. To prevent re- Yilection at this point, the impedance looking into the line 3| Vmust also be 25 ohms. If a single dipole is to be used, a 25 ohm line may be employed, or impedance matching means may be Aprovided on the line 3 I.

Referring to Fig. 4, a pair of dipoles like that shown in Fig. 1 may be connected together through quarter wave lines lll and 43 to a main line 45. With this arrangement all of the lines may have the same characteristic impedance and no further impedance matching means is required. If the impedance looking towards the antennas from the

junction points

29 and 29 is 25 ohms, the impedance looking into each of the quarter wave lines 4l and 43 from this junction with the line 45 is:

2 2 n Z in= zal' or =l00 ohms The impedance of the two lines in parallel is then 50 ohms, which matches the line 45.

Referring to Fig. 3, the curve lll represents the standing wave ratio in a line feeding the antenna system of Fig. 4, as a function of frequency. This ratio is a measure of the impedance match between the antenna and the line. Thus, if the ratio is 1, the impedances are perfectly matched, and substantially all of the energy fed into the line is applied to the antenna. Fig. 3 shows that this ratio remains above 'l0 precent over the range 'of 156 megacycles to 188 megacycles, for an i4 Y tice, the radiator elements are connected to the outer line conductors and the inner line conductors are connected to each other at the center of the dipole. nected together one quarter wavelength from the ends of the lines, and function asa short cirouited quarter wave parallel line shunted across the dipole. Currents flowing through the inner conductors induce currents in the outer conductors, providing coupling to the dipole. The quarter wave line provides loading Vfor the dipole to broaden the resonance and maintain substantially constant and resistive impedance over a Y wide frequency band.

` I claim as my invention:

1. A dipole antenna comprising a pair of parallel tubular supporting members, radiator eley ments secured in end-to-end relationship at the ends of said supporting members, and coaxial transmission lines extending through said supports with their outer conductors connected to said supports and their inner conductors connected together near the adjacent ends of said. radiator elements. k

2. A dipole antenna comprising two radiator elements disposed colinearly in end-to-end relationship, two concentric lines with the ends of their outer conductors connected respectively to said radiator elements and the ends of their inner conductors connected together, and a connection between the outer conductors of said lines at a point substantially one quarter wavelength from said ends.

3. A dipole antenna comprising a pair of parallel tubular supporting members substantially one quarter wavelength long, radiator elements each substantially one quarter wavelength long connected in end-to-end relationship at the upper ends of said supporting members, a plate ofv conductive material connected at the lower ends of said supporting members, and coaxial transmission lines extending through said supports with their outer conductors connected to said supports and the ends of their inner conductors connected togetherwsaidV transmission lines dif` fering in length by substantially electrical degrees.

4. An antenna array comprising at least one pair of dipoles, each of said dipoles including tubular supporting members, coaxial transmission lines extending through said supporting members with ends of their outer conductors connected to said supports and the ends of their inner conductors connected together, said lines differing in length by substantially one half wavelength and being connected together to a branch line of substantially one quarter wavelength, the two branch lines for each pair of dipoles being connected together to a common main line. n

5. The invention as set forth in claim 4 wherein said tubular supporting members are substantially one quarter wavelength long and are connected together at their ends remote from the radiator elements of said dipoles. Y

GEORGE H. BROWN.

The outer conductors are con-Y