US2633532A - Helically slotted cylindrical antenna - Google Patents

Description

W. SICHAK.

HELICALLY SLOTTED CYLINDRICAL ANTENNA March 31, 1953 Filed Feb. 6/1948 YINVENTOR. WILLIAM SICHAK ATTORNEY 7 o emwzumi Us MOMS. bk

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Patented Mar. 31, 1953 HELICALLY sLo'r' TED: CYLINDRICAL TENNA William Sichak, Lyndhurst, N. J assignor to International Standard Electric Corporation, New York, N. Y., a corporation of. Delaware Application February 6, 1948, Serial No. 6,616

The present invention relates to radio antennae and, more particularly, to antennae used for the transmission of high frequency energy. 1

Generally, any discontinuity in a conductor carrying high frequency alternating current will 4 Claims.- (Cl. 25033.63)

i one wavelength, reinforcement will occur. Also,

radiate some power, and antennae radiating from slots which extend'across current lines are known. In a single-slot antenna, however, the power radiated is limited by the width of the conductor, and it is therefore customary to provide a' plurality of slots from which power may be abstracted at discrete points.

An object of the present invention is to provide an antenna in which the power radiated is not limited by the width of the conductor, yet is abstracted from the conductor in 'a continuous fashion.

Another object of the invention is to provide an antenna of the character described which is of simple design and convenient to manufacture.

. A further object of the invention is to provide an antenna of the character described which has a circular pattern of radiation in a plane transverse to the radiating conductor and in which the direction of the beam and the plane of polarization are readily determinable.

Still another object of the invention is to provide an antenna of thecharacter described in which the power abstracted at different points along the conductor is substantially the same.

In order to attain the above objects andothers which will becomeapparent as the description proceeds, a tubular conductor is'pr'ovided with a helical slot the pitch of which is determined by the wavelength to be radiated. The tubular conductor may be a wave guide or the outer conductor of a coaxial transmission line. The pitch of the helix is preferably an integral number of wavelengths; if more than one wavelength, a plurality of helical slots may be interleaved in the fashion of a multiple-thread screw.

The invention will be better understood from the following description, taken in conjunction with the accompanying drawing in which:

Fig. 1 illustrates one embodiment of the invention;

Figs. 2. 3 and 4 are different modifications oi the structure shown in Fig. 1.

In Fig. 1 there is shown a. tubular conductor l, rising in a vertical direction and being suitably energized from a source of high frequency energy, as, for instance, the wave guide 2. A helical slot 3 is milled into the conductor I, making two complete turns around the latter. The energy radiated by each turn will add vectorially so that, if the pitch of the helix is at least roughly equal to since the slot extends over an angle of 720, the radiation pattern in a horizontal plane will be circular; in a vertical plane the pattern may have several lobes and will be dependent upon the number of turns.

In the example given in Fig. 1, the length of pitch has been chosen slightly greater than a wavelength A, with the result that two co-phas'al points such as A and B will be relatively displaced by a small angle. The line M, interconnecting the points A and B, indicates the direction of thewave front through these two points, and thus the beam will propagate itself in a direction N, normal to M, making an angle 0 with the horizontal. It will be seen, therefore, that the direction of the beam depends on the ratio between the pitch and the wavelengh and will be horizonal if the length of pitch equals A or a multiple thereof. 1

Another factor to be considered is the plane of polarization of the radiated beam. This plane of polarization will be normal to the slot and, for a point'C, has been indicated at P in Fig. 1, enclosing an angle with the horizontal. The cotangent of this angle is given as the ratio between the pitch of the helix and the circumference of the conductor l and, therefore, the angle!) will be large if the wavelength is small relative to the conductor diameter, resulting in a small horizon tal component 'Pr and a large vertical component Py. If this is undesirable, the angle 0 and, thereby, the vertical component of the radiated field may be reduced by making the pitch of the helix equal to severalwavelengths, such an anrangement having been illustrated in Fig. 2.

The antenna shown in Fig. 2 comprises a tubu-- lar conductor I formed with three helical slots 3, 4, and 5, the axial distance between slots being one wavelength and the pitch of each helix being, consequently, 37\. While Fig. 2 has been drawn to a smaller scale than Fig. 1, it will be noted that the proportion between the wavelength A and the diameter of conductor I has been left substantially unchanged. With the arrangement of Fig. 2, however, the plane of polarization P encloses a much smaller angle 0 with the horizontal, resulting in a large horizontal component Pat and a small vertical component Py'.

For maximum efliciency, as well as for the purpose of obtaining a truly circular pattern of radiation, it is desirable that the slot radiate the same amount of power per unit length along the entire active surface of the antenna. This may be achieved in two ways: The coupling of the slot may be increased uniformly so as to compensate for the power radiated along its length, or

the coupling may be held constant and special means may be provided for maintaining a uniform power density along the slot by supplyin added power to the conductor progressively over its length. The first case is illustrated in Fig. 3 where atubularconductor B -.-i s-.shown asgradually increasing; in; thickness, starting; at -its.-- point of coupling to a source of high frequency energy, as, for instance, the wave guide 1. The second case is illustrated in Fig. 4 wherein the tubular conductor I forms the outer conductor -of-a coaxial line having an inner conductor 8. The inner conductor 8 is tapered so as to decre the characteristic impedance of theg'linai idirecticn away from the source of energy, shown-asauco-s axial line 9. A suitable termination, indicated at ID, may be provided betweenthe'outer and-"inner conductors I and 8 of the line.

It may be noted that;.any reflection occurring at ;a particular pointalong the slotwillbeefiec- .ti-vely -canceled byaan, equal. reflection occiu'ring at another point a quarter wavelength removed,

such cancellation occurring point by oint if the ,slot extends over, an integral. number of. half wavelengths. Thus, with propenadiustment of ".Phe. power distribution along the. radiating portion of the antenna, a good impedance match will the ob a ne .It-.will be further. understood, that the slots},

4. and. 5. need not Joe .air. aps but may represent armdiscontinuity in the. conductonfilledwit dielectric materialwhichiatter may. advantageously take the ,formof a solid insulatonwhere it is desired, .to increasethe, mechanical rigidity of the structure.

.uniiormlrlr mc .taen c m r si aatubu a lQQ l QFO 'L-Aneans. .ior .a pl in -hiahi au mi anor xic sa c n c radi a nt-Qs anat e ci-lfi-idcm n w inarr r sd: witha l a one. helical isl'ot .extendingirom said one-end toward. theo ndharl ns a p tc whi e ua .subst ntial r a mtes l n mbe o w velens h bti he saidh z t re uenprins de 'dsqn ucto and mean .p e iyel e sscs ate .wit isaidisle ted conductor for compensating for power lost by radiation along the way from said one toward said other end.

2. An antenna according to claim 1 wherein said compensating means comprises an inner conductprs coaxial with-.-said;.tubular conductor, saidinner-conductor increasing in diameter from said one toward said other end.

3. A radio antenna comprising a tubular conductor and means for applying high frequency energytosaidconductor adjacent one end thereor; said conductor being formed with at least one Pin... gral number of w aye lengths of the said high frequencyinside'said conductor, said conductor havingathickness that increases gradually from said t;-=e gtend ing. heiically around the conductor m saidsone end toward the other and havaspitchxwhich equals substantially an inteone end to the other end in such a manner that the coupling. of. the slot .increases, with, growing distances. from said. one. end whereby the loss of power occurring alongthe slot will, be. substantially compensated.

K 4. -.A radio, antenna adapted .to radiatecnergy unif ormly; from .end to .end comprisinga tubular conductor, means .for applying. high} frequency .energytosaid. conductor adjacent-one end thereof,-,said..conductor being; provided with at least one helical slot extending .from .saidoneend toward theother and having apitch which equals substantially an integral number g t-wavelengths tth saidh sh re e i a c n ct and means in said antenna forcon pensating for power lost. by, radiation along the .way from; ,said

.on towar 'sa b he sad- .ZR F B NCE ITE .1 The following references; are .of-record zinc the 1fi 1e of. this patent:

"11 UNHYED j TATE EAT NTS Numb Nam Dat 2,129,712 Southworth, sept. 13, i938 swam Gutls e 2%.1 asses iaie a c rsp 4-19. FQREIGWPAIENIKS Numb qi ntr Qat am r- 7"--- -'1 n7:

FiGTHERf-REEERENCES i'riarrqmqs'a es 0 i l'i APr

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