US3136996A - Omnirange beacon antenna - Google Patents

Description

June 9, 1964 E. G. PARKER 3,136,996

OMNIRANGE BEACON ANTENNA Filed Oct. l5, 1960 4 Sheets-Sheet 1 LOW F/QEQUEA/C Y BAA/D l o F, Fa F/efqvaEA/cy 98o Mc nes/wc RF @EN F/ 5 ATTo/eA/EY June 9, 1964 E. G. PARKER 3,136,996

OMNIRANGE BEACON ANTENNA Filed Oct. 13, 1960 4 Sheecs-Sheecl 2 INVENTOR.

NVQ/VM@ ATTORNEY June 9, 1964 Filed Oct. 13, 1960 E. G. PARKER OMNIRANGE BEACON ANTENNA 4 Sheets-Sheet 5 INV EN TOR.

' ATTORNEY June 9, 1964 E. G. PARKER 3,136,996

OMNIRANGE BEACON ANTENNA Filed Oct. 15, 1960 4 Sheets-Sheet 4 q/l /l Ilm /l/l/l V41/ lll INVEN TOR. EKA/E57' G. PAR/ 67? BY yA/VL ATTORNEY United States Patent O 3,136,996 OMNIRANGE BEACUN ANTENNA Ernest G. Parker, Morristown, NJ., assigner to International Telephone and Teiegraph Corporation, Nutley, NJ., a corporation of Maryland Filed st. 113, 196i), Ser. No. 62,437 22 Claims. (Ci. 343-106) My invention relates to parasitic elements for use with antennas for creating a radiation pattern of the antenna. More specifically, the invention relates to means for selectively controlling the parasitic elements.

In the past, a number of different techniques have been used to create an antenna which produces a space radiation pattern which rotates through space according to a predetermined pattern. In many cases, it has been found desirable to create a space radiation pattern which varies systematically with the azimuth from the antenna as measured from a reference direction. Antennas of the type used to produce the radiation pattern for the well known Tacan navigation systems have been notably successful. Mechanical rotation of an array of parasitically excited elements about a central exciting source has proven to be one of the simplest and most satisfactory methods for producing a space pattern required for navigational aids such as Tacan. In the past, many such antennas have utilized a single set of parasitic elements. This has led to certain operating difficulties due to the fact that operation is often required over two relatively Widely separated discrete frequency bands. One set of parasitic elements which may be satisfactory in one of the frequency bands does not operate efficiently in the other frequency band. The prior art has attempted to remedy this situation by providing two sets of parasites, one for use at each frequency band for example. This technique has been only partially successful. At either frequency band the presence of the parasites which are intended to Work at the other frequency band has led to considerable distortion in the radiation pattern. Thus, multiple sets of parasites interfere in the frequency bands where they were not intended to be used.

In other prior art antenna systems which are entirely stationary, it is often desirable yto be able to change the shape of the antenna pattern at will for various purposes, such as beam shaping or signalling. The prior art methods have often involved clumsy expedients, such as mechanical linkages and motions, to relocate the parasitic elements and in some case necessitating reassembly .by hand so that the antenna may not be used at all times. The present invention is intended to overcome these difficulties and to provide other beneficial results as well.

It is 'another object of this invention to provide an antenna system which will operate with extremely low distortion over two discrete widely separated frequency bands.

f provide impedance elements which are coupled to a parasitic element to control the parasitic element.

It is still another feature of the present invention to provide an antenna with several distinct groups of para- 3,I36,996 Patented June 9, 1964 sitic elements and several distinct groups of impedance elements, one impedance element connected to each of the parasitic elements. By changing the frequency of operation of the central source, the impedance elements connected to the parasitic elements can be made to couple in orcouple out Various members of the parasitic element groups.

It is another feature of my invention to provide a plurality of parasitic elements eachphaving an associated impedance determining unit so arranged that the parasitic elements may be readily rotated at high speed without complications from the necessity for directly controlling the impedance terminating devices.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which:

FIG.,l illustrates the nature of the frequency spectrum for which the present invention is particularly suited;

FIG. 2 is a perspective view of one embodiment of my invention;

FIG. 3 is a perspective View of another embodiment of the invention; i

FIG. 4 is a plan View from the top of the embodiment of FIG. 3;

FIG. 5 is a side elevation View of FIG. 4, taken along lines 5 5.

FIG. 6 is a plan View of a third embodiment of the invention;

FIG. 7 is a side elevational view of the embodiment of FIG. 6, taken along lines 7-'7.

Referring now to FIG. 1, there is shown a plot of a two-band frequency spectrum, that is, there are two frequency bands of interest-a low frequency band centered about some frequency such as F1 and alhigh frequency band separated by a certain frequency spectrum from the low frequency F1. The high frequency band is centered about the frequency F2. The present invention is particularly suited for providing parasitic elements which can be made to operate over one frequency band but which will not interfere at all with the operation of the antenna over other different frequency bands. The present invention is also applicable over more than two frequency bands, such as 3, 4, 5 frequency bands, etc. as will be apparent from the explanation below. In the case of Tacan-type antennas, the lower frequency band centered about frequency F l has approximately a 6 percent bandwidth. The lower frequency F1 is approximately 980 megacycles per second. The high frequencyv band also has a bandwith of 'approximately 6 percent and is centered about a high frequency F2 of approximately 11857megacycles per second. The two frequency bands, measuring from center of the band to center of the band, that is, from frequency FlV to frequency F2, are separated by approximately an 18 percent change in frequency. There is thus a frequency band of approximately l2 percent between the two bands which is used for other purposes and in the case of Tacan, the l2 percent frequency spectrum is used for distance measuring equipment Vbut it makes no difference what particular use is made of the frequencies between the useful bands. The lower limit of the low frequency band, and the upper limit of the high frequency band are separated` by an approximately 25l percent change in frequency. Itis more useful and convenient to express the separations of the frequency bands in percentage because this is what is useful in the calculation of the appropriate type of impedance element to cooperate with the parasites. It should be understood that our method is not limited to the particular example we show in the specification but may be used to provide controllable parasites which are useful over any two separated frequency bands or even over two discrete tric structure 5. The dielectric support 5, could be aV sheet ora pedestal of dielectric material.V High-band parasite 3 consists of two equal portions, 3A and 3B. The lengthof the parasite 3A is shown as and this is also the length of the parasite 3B; The two portions of the high-band parasite 3A and 3B are co-l support member 5. The length of either member 4A or line 6 is connected to one portion 3A of the high-band parasite 3 at 7 as shown. The other conductor 6B of the transmission line 6 Vis' coupled to the other portion 3B lof the high-band parasite 3 at terminal 8 as shown. The transmission line 6 is terminated ina short circuit 9 which connects the one conductor 6A to the other conductorV 6B. The length of the transmission line 6 is shown as L2. Transmission line V6 lies in the plane of the suptral radiator 2. The central radiator 2 then radiates the energy outward toward the parasitic elements 3 and 4. It should be understood that the particular frequency' used may be located at anyVV discrete frequency within the lower frequency band centered about Fl for example. It need not be exactly at the frequency Fl as long as it isfwithin the calculated limits of the lower frequency band. The energy from the central radiator'Z strikes both parasites 3 and 4. However, only one of the two parasites will reradiate the energy and modify the Y radiation pattern. The other parasiterwill have no effect. The low-band parasiter4 is responsiveto the radiation from the central radiator 2 at the low band of fre-y quencies and the low-band parasite 4 will reradiate this energy. Thelength LP is appropriately chosen te bean efficient radiator of energy at the low frequency band. The length LPr' should be approximately one-*half Wavelength.V However, due to fringing effects and end effects, an actualpractical4 length of V.V633 wavelength is used for LP for the'low frequency band centered aboutFl;V

For yoperation in the low frequency band, the length L1' of the transmission 10 has been appropriately chosen so that at the 10W band frequencies, the transmission line 10 appears to be, asiseenfrom the terminals 11 and 12, a

short circuit. For example,'it iswell'known vthat a onehalf wavelength transmission line which is shorted appears to be a short circuit as viewed from its terminals. Thus,

the length L1 of the transmission line 10 is appropriately chosen in terms of the wavelengths of the low frequency bands centered about F1 to appear to be a short circuit as seen at the terminals .11'V and 12. The short circuit provided by the transmission line 10 effectively connects together the terminals 11 and y12 of the parasitic element 4. Thus, theqparasitic elements 4A` and 4B act togetherV asl one effective parasitic element Vof length LP and operating within the low frequency band. The parasitic'elements 3 and 4 may be made of either dielectric material or conductive material. When the parasitic element 4 is effectively reradiating it will produce a peak'in the radia` port member 5 and is located on aradial line running from the central radiator 2'v through the high-band parasite 3. The high-band parasite 3 is located a radial distance R1 from the central radiator 2 as shown. Another transmission line10 is connected to the low-band parasite 4. One conductor 10A of the transmission line 10 is connected to one portion 4A of the 10W-band parasite 4 at terminal 11. The other conductor 10B of the transmission line 10 is connected to the other yportion 4B of the low-band parasite 4 at terminal 12. Tlheilength of the transmission line 10 is shown as L1. Transmission line 10 is terminatedi-n a short circuit 13 which connects theV one conductor 10A to the other conductor 10B. The lowerband parasite 4 is located a radial distance R2 from the central radiator 2. Transmission lines 6 and 10 maybe axial cable. The transmission line 10 is also located on v a radial straight line from the central radiator 2 passing through the low-band parasite 4. `Radio frequency energy is provided to the .central radiator 2 by means ofthe coaxial cable 14 from the source `of RF energy 15. The source of RF energy 15 is capable of providing RF radiation at either of two discrete frequency bands centered about two frequencies F1, ,the center of low frequency band, and F2,vthe center of the high frequency band. r

' In operation, energy within'a frequency band centered about Fl' is provided by the source 15 through the coaxial cable 14 via switching means 1V4arto the cention pattern whenthe oper-ation is within the' low frequency band.V To understand how operation is accomplished over two frequency bands, we examine the behavior of the high-bandparasite 3 duringV operation Vof the antenna the lowvfrequency bands centered about Fl. Thehigh-band parasite 3 willbe producing virtually'no radiationvduring operation within 'the low Y frequency band. To understand why this is so, note that the transmission line 6 of lengthV L2 will not act as an Y lineV 6 appear to be a short circuit atV terminals 7 and-3 when viewed from terminals 7K', 8 for Voperation within thehigh frequency band centered above frequency F2.

-Butfor operation within -the low frequency bandythe transmission'line 6 is not the appropriate length iny terms of wavelengths ofthe high frequency band to provide an effective short circuit. HAtthe low frequencyband, the Aimpedance looking in terminals 7 and 8 appearsto beV quite'highj andthe shortcircuit19'at they otherend ofthe transmission line 6ldoes not provide an `effective RF, short j circuitasfseen at the terminalsj and 8.v Hence,- the high-(1 Y,

band parasite SisA- split Vinto two separate distinct portions,

3A Vand3B,rwhich are separated by a relatively high impedancefbetweeri` terminals 7 and '8, Thel high-band parasite 3 cannot act as an efficient one-half wavelength;

radiator -Iand virtually no radiationtakes-place frornthe high-bandfparasite' 3for radiation within theelow fre'- quency band.4A The high-bandnparasite 3 contributes vir- 'l highfrequency` band centered about the highfrequency y -the n` situationzisVV just reversed.l I V'l-`l1e source of tually no elect to Vreradiation and has very little eect Vupon the' space pattern ofthe antenna-lfm' Yoperation' withinthe low frequency band. j t n A v r- A. However,.foroperation of the antenna 1 within .the

energy provides the central radiator 2 with a frequency Within the high frequency band centered about F2, Highband parasite 3 is now the primary reradiator of energy and the low-band parasite 4 has virtually no effect upon the radiation pattern when operation is within the high frequency band. This is because transmission line 6 for operation at the high frequency band now has the appropriate length in terms of wavelengths to provide an effective short circuit as viewed from the terminals 7 and 8 so that the parasitic element 3 is an effective single coni' tinuous radiator. The high-band parasite 3 appears effectively as one complete unit because the relatively low impedance between terminals 7 and 8 effectively joins the Y two portions 3A and 3B into one continuous radiator of the appropriate length near one-half Wavelength. For practical purposes mentioned before, the actual length of LP will normally be .633 wavelength as calculated at the high frequency band. Likewise, for operation Within the high frequency band, the low-band parasite 4 appears to be two isolated separately distinct portions of radiator 4A and 4B, separated by a relatively high impedance seen at terminals 11 and 12. The impedance seen at terminals 11 and 12 is no longer a short but is instead a high impedance because the length L1 of the delay line 10 is no longer correct to provide an effective short circuit at the wavelength within the high frequency bands.

Thus, it can be seen that for operation at either frequency bands, only one of the two parasites 3 or 4 is effective at any one time for shaping the radiation pattern of the antenna. In effect, the transmission lines 6 or 10 remove their respective parasites when the operation is outside of the frequency band for which the parasite was intended. All that is necessary to eliminate the effect of the particular parasite is to operate the energy source 15 at a frequency outside of the frequency band for which the parasite and the delay of the transmission lines 6 or 10 was designed. An example will show that the method and technique illustrated by the embodiment of FIG. 2, can be used wherever an antenna is to operate over a multiplicity of frequency bands and it is desired to have an appropriate radiation pattern for each particular frequency. In general,

where A is the wavelength corresponding to the frequency F and V is equal to the velocity of propagation of the medium which is used. In'the case of electromagnetic radiation, V is equal to the speed of light and is approximately 3.0 l() meters per second.

V (2) M- lowpband (3) xa-Y- hh b d -F2 1g an Suppose we wish to design an antenna with two sets of parasites to operate over two discrete frequency bands, onel frequency band being centered about the frequency Fl. For convenience let us call this the low frequency band. Thus, the wavelength of radiation at the low band is Al and the wavelength of radiation at the high frequency band is A2. We can also terminate the particular transmission line in either a short circuit or an open circuit. A short circuit termination Was illustrated in FIG. 2 and for the purposes of discussion we will use a short circuit termination also. It is well known that if at some particular Vpoint along a transmission line the impedance at that point appears to be a short circuit, one quarter of a wavelength away the impedance will appear to be a maximum. Then one-half wavelength away from the original point, the impedance appears to be a minimum again and so on with maxima and minima of impedance alternating down the length of the transmission line. It is then clear what must be accomplished when the operation changes from within the frequency band near F l to a frequency within the band near F2. `The particular percentage change in frequency between the two frequen-l cy yhands must result in an equivalent 90 phase shift in the transmission line, that is, the effective length of the transmission line must go through approximately a one-quarter wavelength change or, in other words, a 90 phase change in effective length of the transmission line. This will cause the transmission line to look like a short circuit as used from its own terminals Vat its own frequency band. But when operation is changed to the other frequency band, the impedance at the terminals of the transmission line will appear to be very high due to the fact that in terms of the new frequency of operation at the other frequency band the effective length of the transmission line has been changed because of the change in wavelength by the change in frequency.

We can write this general relationship as Equation 4.

where Ao is the required phase change for switching from one frequency band to the other and AF represents the percentage change in frequency of the two bands as compared to one of them. From the discussion immediately above, it can be seen that Arp must be equal to 90 electrical degrees, that is, the effect of one quarter of a Wavelength change in length of transmission line between the two bands. AF in general will be determined bythe particular requirements of the communication system with which the antenna has to work. In the case of the Tacan type of equipment, the approximate calculations can be performed as follows. Let Fl be equal to 980 megacycles. Let F2 be equal to 1185 megacycles. This is illust'rated in FIG. l.

AF is equal to ll minus980 divided by 1185:

Ll is the required length of the transmission line for use at the low frequency band. Thus, AF equals .18 or an 18 percent change in frequency when going from one band to the other. For convenience, all the calculations will be performed based upon the higher frequency of 1185 as the reference. Now using the value of AF of 18 percent and the value Aq of electrical degrees, we ,can calculate L1 in terms of electrical degrees at the frequency of Fl the low frequency band.

M (6) L1-AF---l8 degrees (6A) L1=499 degrees The length of Ll is equal to 499 electrical degrees. To calculate this in terms of Wavelength, we simply divide by 360 degrees which represents one wavelength at the frequency F l.

So the choice of the nearest half-wavelength line must be made. It may be convenient tol choose one-half wavelength as the length of the transmission line because a shorter length of transmission line will be more suitable for physical mounting and packaging arrangements. However, the advantageous length that'must be chosen is 1.5 wavelengths because as the calculation shown by Equation 6 indicates, at least l.388 wavelengths are needed to produce the appropriate amount of change in electrical degrees per unit change in frequency. lIt. will be understood that although as far as producing a minimum impedance is concerned, a one-half Wavelength line and Wavelength or 1.5

a 1.5 wavelength line are equivalent, the, two lines are not equivalent in the total amount of phase delay or phase change which they produce because the electrical phase change is'cumulative and continuous down the length of the line so that a 1.5 wavelength line produces three times as much total delay or 3 times as much total'phase change as'I a one-half wavelength line does. Therefore, L1 will be chosen as 1.5 wavelengths at the lower band frequency. To actually calculate this length, we perform the followlng:

V (7) Y L1 1.5 \1 1.5

3X 108 meters (2B) masoxioaps (7A) L1=1.5 30.6 om.

k1 the wavelength at the low frequency band is simply V/Fl or 30.6 centimeters.

(7B) L1=45.9 em.

' for transmission line 10. (7A) Vshows that L1 :1.5 30.6

centimeters or 45.9 centimeters'for the length of Ll.

. AThe length L2 for the transmission line 6 of high frequency band parasite 3 may now be also readily calculated. It can be seen that the percentage change in frequency will be again 18 percent as shown by Equation 5 because the change in frequency is the same whether moving from the lowY band to the high band or from the high band back tothe low band. Again for the 90 phase change, a transmission line of length l.5)\2 is required; The actual physical length of this line is diferent from the length of Ll because a wavelength is physically diierent at the higher frequency band. To cal-y culate the length of line 6 rst calculate X2 from Equation 3 and Equation 3B 3X 108 meters 11.85 108 c.p.s.

A2 is equal to 25.3 centimeters. Hence, (8) L2=l.5 }\2=l.5 25.3 centimeters L2 is equal to 37.95 centimeters. tion, we can take the two ratios of L1 45.9 (9) '-nzg-l-LSI In Equation 10 which is Another embodiment ofthe invent1on is shown in FIG.

3. The above method of calculation can be used for any two separated frequency bands-to calculate theV appropriate length for transmission lines to accomplish the irnpedance matching of the parasites when the frequency is changed. FIG. 3 shows an antenna 16 with a central radiating unit 17. The antenna ofFIG. 3 is constructed, however, to provide for mechanical rotation of the parasites aboutthe central radiating unit 17. kThe central radiating unitf17 is composed, of two inner stationary radiating mein- bersV 18 and 19. The two members 18 and 19 may be Vin the shape of hollow cylindrical cups. The

two stationary radiating members 18 and 19 are separatf ed by anfnsulating element 20. RF energyV isprovided transmission lines which are yet to be described form a i 8 to the central radiating unit 17 through the'coaxial cable `21. The outer conductor 22 is connected to the lower radiating cup- A19. Therinner conductor 23 is connected to theupperrradiating cup 18 by being passed through a suitable' hole'through the center of the insulating member 20. Thus, the central radiating unit 17 acts somewhat like a `dipole and provides anY omnidirectional pattern of. RF energy-torbe radiated by the antenna. vMounted for rotation about ,theV central radiating'members 18 `and 19 is a dielectric cylinderr24. A supporting sheet or plate of Y Y The outer cylinders 26 and 27 are concentric with therdi-` electric` cylinder 24;` and` the cylinders 24, 26 and 27 and the dielectric support plate 25 Yrotateras a complete unit together. A` suitable bearing 28, gears 29 and a motor 30 provide power to rotate the antenna assembly. A fundamental parasite 31 made of a strip of conductor-or a vblock of dielectric'is mounted on` or'fwithin lthe Vinner cylinder 24/and`this provides the modulation of the funda; mental frequency fora bearing facility antenna,. suchas Tacanpl [i A-lrighk frequency band parasite 32 ismounted on or within the" middle cylinder V26 arid a low frequencyband parasite 33 is mounted within or lon the outer cylinder 27.` A top plan view of the antenna of FIG. 3 is shown in FIG. 4.and a side elevational view of the same antenna is shown in FIG. 5. The middle cylinder 26 islocated at a radius VR3* from the central radiator 17. The outer cylinder 21. is located at a radius R4 fromthe central radiator 17. The group of Vtwo parasites Vsuch Vas 32 and 33 with the associated cylinder walls and associated functional group'and as seen in FIG. 4, there are actually nine' such groups equally spaced on the antenna. There is an 'angular` spacing of 40 between the'groups of two parasites. The other groups of parasites are each gen-` erally designated as elements 34A, 34, 35, 36, 37, 38,

39, 40 and 41. The groups areidentical in construction and vfunction and a description of one group will suice To check the calcula- Y to make clear the operation of the antenna. The high frequency parasite 32 has two equal coaxial portions,

32A and 32B located in vertical alignment as shown. The length of the parasitic element 32B is given'as and the two halves ofthe parasiticelements'SZA and`32B are of equal length. L5 is chosen` to be an appropriate length to provide an eilicient dipole radiator and reradiator `at the high frequency band. The calculations will be explained below. Likewise, the low-band parasiteY 33 has two equal lengthcoaxial portions 33A and 33B vertically aligned as shown. n The length 'of the parasitic element 33A or 33B is A l L6 Y and the length L6 is chosen to be an eicient dipolere-KV radiator at the low frequency band.

Attached to the high frequency parasite 32 isa trans-V missionV line 42. The top conductor 43.0f transmission line 42v is connected to one end of the parasitic element 32A at terminal 44. The other conductor 45 of the transmission. line 42 is connected to the parasitic ele.- rnent 32B at the Vterminal 46. The two conductors 43 and 45 of the transmission line 42.*are spaced -apartby the dielectric Vsupport plate 2S. In FIG. 5, the'vertical dimensions of the transmission line 42 have been somewhat exaggerated for clarity of illustration. The trans# mission line 42 'might Vtypically consist of a thin, printed, Ystrip of conductor on -the surfaceof the plate 25. However, a two-wire transmission line may be used or a con-` ductor embedded inpapblock of dielectric material may be used at 43 with equal facility. The parasitic elements 32 and 33 have been shown embedded within the walls of the dielectric cylinders 26 and 27 for secure physical support during rotation. However, the parasites 32 and 33 could be mounted on the surface of the cylinders. The length of the transmission line 42 is L4. The length L4 of the transmission line 42 for high-band parasite 32 is calculated in the same manner as the length L2 of the high-band transmission line 6 for the parasite 3 as shown in FIG. 2 and explained above. For a 20 percent change in frequency in going from the high frequency band to the low frequency band, the length L4 will typically be 1.5 wavelengths at the high frequency wavelength. Again, the transmission line 42 acting in cooperation with the parasitic element 32 provides a frequency responsive device which effectively uncouples the parasite 32 from the central radiator at the low frequency band and the transmission line 42 effectively allows the parasite 32 to be coupled to the central radiator 17 for operation within the high frequency band. This operation is as was explained in conjunction with FIG. 3.

The low frequency band parasite 33 has coupled to it another transmission line 47. The transmission line 47 is formed from a top conductor 4S and a lower conductor 49. The two conductors 48 and 49 as before are mounted on each side of the support plate 25. However, the construction of the transmission line 47 oifers a number of unique advantages. The one conductor 4S of the transmission line 47 is formed in four parts, 43A, 48B, 48C and 43D. The portion 43C is connected to the parasitic element 33A at terminal 50. The conductor 43C then branches into a cross connecting piece 4SD. The connecting portion 48D connects together two straight parallel conductor portions 48A and 48B. 48A is located parallel to the transmission line 42 and on one side of it. Likewise, the conductor 48B is located on the other side of the transmission line 42 and parallel to it. Portions 48A and 48B of the transmission line 47 pass through ythe wall of the middle cylinder 26 by suitable apertures therethrough. This is indicated at FIGS. 4 and 5. For clarity, cylinder 26 has been omitted from FIG. 3. The lower conductor 49 of the transmission line 47 likewise is composed of four portions, 49A, 49B, 49C, 49D. The respective portions A, B, C, D, of the conductor 49 lie directly under and on the opposite side of the dielectric plate 25. The respective portions of conductor 49 correspond and are similarly located tothe portions A, B, C, D, of the top conductor 4S. Thus, the transmission Vline 47 is parallel to and brackets transmission line 42.

This type of construction has several important advantages. First, there is no possibility of RFinterference between the two transmission lines 42 and 47 because the two equal portions 43A and 48B of the transmission line 47 which bracket the transmission line 42 are parallel and equidistant from the transmission line 42 and all portions of the transmission line 47 carry the same current. Fluxy from the portion 48A of the transmission line 47 vwill cancel flux due to the portion 48B of the transmission line 47, and there will be no possibility of electromagnetic coupling with the transmission line 42, This allows the high-band and low- band parasites 32 and 33 to be located in line with each other along the same radial line as shown in FIGS. 3 and 4. Locating the parasites 32 and 33 in radial alignment has the advantage that only one set'of reference disks (not shown) are required to be used with such an antenna. @ne particular group, for example group 34 is picked as the reference group. Each time the group 34 passes a certain reference direction, for example north, the antenna is required to send out a reference signal. This is usually accomplished by having a reference disk with metal slugs or other devices mounted on it. This reference disk rotates in iixed relationship with the antenna assembly 24, 2S, 26, 27. Thus, when the two sets of parasites are radially aligned, operation at the two frequency bands may be accomlt) plished with the use of only one reference disk to create the reference pulses since the azimuth relationship of the two sets of parasites will be the same.

The length of the transmission line- 47 is shown as L3 and L3 is chosen to provide a low impedance match for low-band parasite 33 for operation in the low band. The transmission line 47 will appear as a large impedance for operation at the high frequency band. Thus, transmission line 47 acts as a frequency sensitive device which alternately couples and uncouples the parasite 33 to the central radiator 17 depending upon which frequency band is used. The parasitic element 33B is connected to the transmission line conductor 49C at terminal 51.

The total length L3 of transmission line 48 (or 47) can be written as where the subscripts refer to the four branches of the transmission line 48. The branches LA and are in series, and branches LB and The two portions are in series.

and

LD (Lets and v l LD (Liri-** in series with portion LC only lowers the magnitude of the maximum impedance of the line and does not change the phase shift of the line., v

Hence L3 is calculated asv explained before in connection with length L3 of FIG. 2. The portions LA, LB, (LA=LB), LC and LD can be chosen for convenience as long as the total length given by Equation 11 or 12 for L3 is satisiied. Thus for a particularvalue of L3 the lengths LB, LA, LC and LD can be chosen from a large range of combinations determined by the physical size and the operating frequency, etc. of the particular antenna in question.

It will be obvious to those skilled in the art that any convenient number of groups of parasitic elements could have been used. Thus, there might have been one group as was shown in the stationary antenna of FIG. 3 or there might be 2, 3, 4, etc., or any number of parasitic elements conveniently arranged in bearing on the antenna. Likewise, the antenna of FIGS. 3, 4 and 5 need not be rotated if such rotation is not required for the particular type of radiation pattern which it is desired to create. But the provision of two sets of parasites allows operation to occur over two distinct frequency bands Without any further adjustments and without stopping the operation of the antenna. It was pointed out that the high frequency band elements are located at a particular radius R3 from. the central radiator; that the low-band parasites, such as 33, at a radius ofA R4 from the centralk radiator` 17. The radii R3 and R4 are of critical importance in providing an eicient modulation of the 'radiation pattern. In my United States Patent No. 2,928,087 issued March 8, 1960, I have shown in some detail the mathematical basis for the choice of the radii R3 and R4 and the effect of this radii on the elevational pattern. FIGS. 5 and 7 in particular emphasize the importance of a proper selection of the radius R3 or R4. A,method of calculating the radius R3 and R4 does not form any part of the present invention. However, -the present invention is novel in that by allowing the use of two separate sets of parasites, one for each frequency band, the two sets of parasites may each be located at different radii as shown so that the radius of each group of parasites may be picked at the particular optimum physical radius which is desirable for operation at that particular frequency band corresponding to'the band of that particular set of parasites. Optimum modulation of the radiation pattern can be achieved at both frequency bands by the use of two sets of parasites which can be coupled in and coupled out as Y shown by the present invention.

The length L5 or the length L6 is chosen so thatl the particular parasite will be an efficient radiator at its own wavelength. Thus, L5 would be chosen to be approximately one-half Wavelength at the high frequency band. In other words, L5 will be .5 times A2 in wavelength. However, as was indicated, because of practical considerations of fringing, usually a length of .633 of a Wavelength will be used. L6 is likewise made .633 of a wavelength at the low frequency band so that L6 is equal to .633 times kl. A continuous half Wavelength type radiator is a very efficient reradiator of transmitted energy. However, when the parasite is split into twov equal portions which are each approximately a quarter of a wavelength long or .31 wavelength long then the impedance ofV the two separate portions is quite high and they do not form an effective radiator. This is the reason why the change in impedance of the transmission line can act as a means for effectively coupling in and out the parasites with the two different frequency bands. The use of the transmission line effectively makes the undesired group of parasites disappear at the undesired frequency band.

FIG. 6 and FIG. 7 show another type of construction which maybe used utilizing the principle of the present Y invention. Corresponding parts are numbered with cor-l There are provided a set of low frequency band parasitic elements 52 and a set of high frequency band parasites shown as 53. The parasites 52 and 53 may be made of conductors or of dielectric material. As before, a set of central portion of the low-band parasites 52. Likewise,

' there is a high frequency transmission line y55 coupled to the central portion of each'of the high-band parasites 53. The low frequency transmission line 54 has a length L1 calculated in theV same manner as the example shown in Y connection with FIG;V 2. The high'frequency transmission linev 55 has alength L2 calculated in the same mannerasthe length L2 shown in connection'with the'examp-le A,of FIG. 2.3 Thef transmission line for the low-band para-.

vides for fundamental modulation of the radiation pattern. GQ i low frequency transmission lines 54 are connected to the 65 site 54 passes through the wall of the cylinder 26 by means of a suitable slot or aperture. As before, the highfrequency parasites 53 are effective in changing the modulation pattern of the antenna only for operation at the high frequencies and the transmissionl line 55 effectively causes the high band parasites to stop radiating for operation in the low frequency bands. Likewise, the low frequency parasites 52are effective only through operation in the low frequency band and the transmission lines 54 cause parasite 52 to become an ineffective radiatorfor operation at the high frequency bands so that low frequency parasites 52 have little or no effect on the modulation pattern for operation at the high frequency band.

However, the antenna of FIGS. 6 and 7 has considerable advantages in the construction shown.- First, a dilferent number of high parasites may be used compared to the low frequency parasites. For example, nine low frequency parasites, such as 52,y may be used but any'ditferent number high frequency parasites-53 may be, used. This. is because transmission lines 55 and 54 of this antenna are not interrelated as Wasthe case in FIG. 3 and FIG. 4.' operation at onefrequency band, the other set for operation at the other frequency band, thev high Vfrequency band for example will be modulated by the 'number of parasites in the high frequency band. For example,.nine

is shown. However, iffor example six high frequency parasites 53 were used for operation at the high frequency band, modulation will be a sixth harmonic modulation rather Vthan a ninth harmonic modulation which would occur at the low frequency band. Thus, as before, trans mission lines 54 and 53 Will remove the effect of the other set of parasites foroperation during either frequency band. Thus, the antenna of FIGS. 6 and 7 provides a tern of the antenna' if a ditferentgnumber of parasites is used at 4the two frequencyv bands.

While I haverdescribed above'the principles'of my invention in connection with specific apparatus, it'is to be clearly understoodthat this description is made by way of illumination and not as a limitationto the scope of thereof and in 1. An omnirange beacon antenna comprising a centralV 'radiating element, a first group of parasitic reradiators adapted to operate at a first frequency, a first group ofV yparasitic reradiators, adapted to operate at a secondv `frequency, a secondV group of transmission lines, each of saidV second group of transmission lines coupled to-each parasitic reradiator of said secondgrouplof parasitic reradiators, each of said first groupof transmission lines having a length different from each of saidfsecond group of-transmission lines, Asaid centrale-radiating element-transmitting electromagnetic radiation at either said first'frequency or said second frequency as desired, whereby said rsrt groupV of reradiators Will affect the radiation Vpattern ofvsa'id central radiating element'at said rst frequency and said second'group of reradiators will alect the said radiation pattern at said second frequency.v i

2. An omnirangebeacon antenna according to Yclaimwl` i LAwherein each of said parasitic reradiators consists' of a first portion-and a second portion whose lengths are'equal to eachother, the lengthgof the respectivel portions of. k each o f said parasitic reradiators of said first group being different from the length of the'portions of each parasitic reradiator of said, second group,each of said transmission If two sets of parasites are used, one set for 13 lines having a short circuit at its end remote from said parasitic reradiators.

3. An omnirange beacon antenna according to claim 2 further comprising means for supporting said transmission lines and said parasitic reradiators, and means disposing said transmission lines and said parasitic reradiators for rotation about said central radiating element.

4. An antenna according to claim 3 wherein the number of parasitic reradiators in said first group is different from the number of parasitic reradiators in said second group.

5. An omnirange beacon antenna according to claim 4 wherein said first portion and said second portion of each said parasitic reradiator are aligned vertically on an axis with each other, each of said first group of parasitic elements being disposed'at a first radius from said central radiating element, each parasitic reradiator of said second group of parasitic reradiators being disposed at a second radius from said central radiating element.

6. An omnirange beacon antenna according to claim 5 wherein the length of each of said first group of transmission lines is substantially 1.5 wavelengths at said first frequency, the length of each of said transmission lines of said second group of transmission lines is substantially 1.5 wavelengths at said second frequency, the combined length of the two portions each of said parasitic reradiators of said rst group is substantially .633 Wavelength at said first frequency and the combined lengths of the two portions of each parasitic reradiator of said second group is substantially .633 wavelength at said second frequency.

7. Apparatus for controlling modulation for use in an antenna having a central radiating element, comprising a plurality of parasitic elements, a plurality of impedance elements, each one of said plurality of impedance elements being coupled to a corresponding onev of said plurality of parasitic elements, each of said impedance elements being responsive to a predetermined frequency transmitted by said central radiating element whereby said parasitic elements reradiate energy only at said predetermined frequency.

8. An antenna comprising a central radiating element,

a first parasitic reradiator adapted to operate at a first frequency,

a first transmission line connected to said first reradiator,

a second parasitic reradiator adapted to operate at a second frequency,

a second transmission line connected to said second reradiator, the length of said first transmission line being different from the length of said second transmission line,

the length of said first parasitic reradiator` being different from the length of said second parasitic reradiator, Y Y

said central radiating element vtransmitting electromagnetic radiation at either said first frequency or said second frequency as desired, whereby said first reradiator will affect the radiation pattern of said central radiating element at said first frequency only and said second reradiator will affect the said radiation pattern at said second frequency only.

9. An antenna comprising a central radiating element,

a first parasitic reradiator adapted to operate at a first frequency,

a first transmission line connected to said first parasitic reradiator,

a second parasitic reradiator adapted to operate at a second frequency,

a second transmission line connected to said second parasitic reradiator said second transmission line consisting of two conductors disposed parallel to each other,

each of said two conductors having two parallel pori4 Y tions of equal length, one portion disposed parallel to and on each side of said first transmission line, said first transmission line having a length different from said second transmission line,

said central radiating element transmitting electromagnetic radiation at either said first frequency or said second frequency as desired,

whereby said first reradiator will affect the radiation pattern of said central element at said first frequency and said second reradiator will affect the said radiation pattern at said second frequency.

10. An omnirange beacon antenna transmitting radio frequency energy at a first frequency and at a second frequency as desired comprising a first parasitic reradiator, adapted to operate at said first frequency, the length of said first reradiator being proportional to the Wavelength at said first frequency,

a first transmission line connected to said first reradiator, a second parasitic reradiator adapted toy operate at said second frequency, the length of said second reradiator being proportional tothe wavelength at said second frequency,

a second transmission line coupled to said second reradiator, said first transmission line having a length different from said second transmission line,

whereby said first reradiator will affect the radiation pattern of said central radiating element only at said first frequency and said second reradiator will affect the said radiation pattern only .at said second frequency.

ll. An antenna according to claim 10 wherein both said first and said second reradiators are disposed along the same line passing through said central radiating element. f

l2. An antenna according to claim l0 wherein said second transmission line consists of two conductors disposed parallel to each other, each of said two conductors having two parallel portions of equal length, one portion disposed parallel to and on each side of said first transmission line. i

13. An antenna comprising a central radiator,

a first parasitic reradiator tuned to a first frequency,

a second parasitic reradiator tuned to a second frequency,

impedance elements connected to said parasitic reradia tors and` responsive to said first and lsaidsecond frequencies so that when said central radiator transmits radio frequency energy at said first frequency, said impedance elements act to cause said first reradiator to reradiate a maximum of said energy and said impedance elements act to cause said second reradiator to radiate substantially no energy.

14. An antenna comprising a central radiator,

a first parasitic reradiator tuned to a first frequency,

a .second parasitic reradiator tuned to a second frequency,

impedance elements connected to said parasitic'reradiators and responsive to said first and said second frequencies so that when said central radiator transmits radio frequency energy at said first frequency said impedance elements act to couple said first reradiator and to uncouple said second reradiator.

l5. An antenna comprising a central radiator,

a first parasitic reradiator adapted to operate at a first frequency,

a second parasitic reradiator adapted to operate at a second frequency,

first impedance element connected to said first parasitic reradiator and responsive to said first and second frequencies, A I

second impedance element connected to said second parasitic reradiator and responsive to said first and second frequencies so that when said central radiator transmits radio frequency energy at said first frequency, said first impedance element acts to cause 16. An antenna transmititng radio frequency energy j comprising Y' a central radiator transmitting radio frequency energy at two predetermined frequencies, v Y a parasitic reradiator adapted to operate at a first of said predetermined frequencies, a rst transmission line connected to said first parasitic reradiator and responsive to both said predetermined frequencies, .A

a second parasitic reradiator adaptedtofoperate at a` secondone of said predetermined frequencies,

a second transmission line responsive to both said predetermined frequenciesconnecte'd to saidrsecond parasitic radiator, said rst transmission line having a length different fromV said second transmission line so that said first reradiator radiates a maximum of energy at said first frequency and said second reradiator radiates substantially no nergy at said first frequency and so that said second reradiator radiates a maximum of energy at said second frequency and said first reradiator radiates substantially no energy at said second frequency. Y v

17. An antenna according to claim 16 wherein said first reradiator is located at a radial distance Vfrom said central radiator which is different from that of Vsaid second reradiator.

18. An antenna according to claim 17 wherein said first reradiator and said second reradiator are disposed on the same radial line passing through said central radiator.

19. An antenna comprising'a central radiator,

a first parasitic reradiator tuned to a first frequency,

a secondv parasitic reradiator tuned to a secondfrequency, l

a first transmission line connected to said first reradiator and responsive to said rst and said second frequencies, Y Y

a second transmission line connected to said second reradiator and responsive to said first and said second frequencies, d a l so that when said central radiator transmits radio frequency energyl at said first frequency said first transmission line actsto cause'said first reradiator to radiate a maximum of said energy and saidsecond transmission line acts to cause said second reradiator to radiate substantially no energy,

and so that when said central radiatortransmitsvradio said rst transmission line acts to cause said first y a t reradiator tto radiate substantially no energy.

20. VAn antenna according to clairn 19 wherein said second transmission line consists of two conductors dis,-VL

posed parallel to each other each of said two conductors having two parallel portions of equal length one portion disposed parallel to and on each side of said first transmission line. Y Y 21. An antenna according to claim 20 wherein the length of said first reradiator is vdifferent from the length of said second reradiator. v .Y Y

22. An antenna transmitting radio frequency energy comprising a central radiator transmitting radio frequency energy at two predetermined frequencies as desired, f, v a first parasitic reradiator having a first portion and a second portion, v Y

said Y,reradiator to said second portion of said reradiator, said transmission line being responsive to a first frequency Aof said two predetermined Vfrequencies, said transmission line being short circuited r at one end,'so that said first transmission line presents a low impedance connected betwen said first portion and second portion of said first reradiator at said 'first frequency and whereby said first transmissionY Y second frequency of said two predetermined fre-.v quencies, said second transmission linebeing shortV circuited at one end so that said second transmission line presents a low impedance connected between said first portion and said second portion of saidv second reradiator at said second frequency and whereby said second transmission line presents a Vrelatively high impedanceconnected betwen said first portion and said second portion of said second reradiator at said first frequency whereby the amount'of energy reradiated by said reradiators is controlled by said two predetermined frequencies.

first transmission line coupling said firstV portion of Y

Claims (1)

1. 7. APPARATUS FOR CONTROLLING MODULATION FOR USE IN AN ANTENNA HAVING A CENTRAL RADIATING ELEMENT, COMPRISING A PLURALITY OF PARASTIC ELEMENTS, A PLURALITY OF IMPEDANCE ELEMENTS, EACH ONE OF SAID PLURALITY OF IMPEDANCE ELEMENTS BEING COUPLED TO A CORRESPONDING ONE OF SAID PLURALITY OF PARASTIC ELEMENTS, EACH OF SAID IMPEDANCE ELEMENTS BEING RESPONSIVE TO A PREDETERMINED FREQUENCY TRANSMITTED BY SAID CENTRAL RADIATING ELEMENT WHEREBY SAID

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