US2967300A

US2967300A - Multiple band antenna

Info

Publication number: US2967300A
Application number: US698076A
Authority: US
Inventors: Leo C Haughawout
Original assignee: LA Young Spring and Wire Corp
Current assignee: LA Young Spring and Wire Corp
Priority date: 1957-11-22
Filing date: 1957-11-22
Publication date: 1961-01-03
Anticipated expiration: 1978-01-03

Description

Jan. 3, 1961 L. c. HAUGHAWOUT 2,967,300

' MULTIPLE BAND ANTENNA Filed NOV. 22, 1957 2 Sheets-Sheet 1 INVENTOR. [0 6. bflW/VAWMT L 1 INSULATION .5/

Jan. 3, 1961 c. HAUGHAWOUT 2,967,300

MULTIPLE BAND ANTENNA Filed Nov. 22, 1957 2 Sheets-Sheet 2 INSULATION 34 WM QM MULTIPLE BAND ANTENNA Leo C. Haughawout, Los Angeles, Calif., assignor to L. A. Young Spring and Wire Corporation, Gonset Division, a corporation of Michigan Filed Nov. 22, 1957, Ser. No. 698,076

13 Claims. (Cl. 343-750 This application relates to antennas and more particularly to a new and improved antenna for radiating and receiving'radio signals having frequencies within different frequency bands.

In the transmission and reception of radio signals, it is well known to employ an antenna which is a half wave in length for the frequency of the radio wave being transmitted or received. Ordinarily, a half wave antenna is divided into two quarter wave sections which extend outwardly from a center point at which the antenna is driven from a transmission line. By feeding a half wave antenna at its center, advantage is taken of the fact that the current standing wave is a maximum at the center of a half wave antenna while the voltage standing wave is at a minimum. Thus, by center feeding a half wave antenna, the transmission line is connected to a point of relatively low impedance and extremes of voltage do not appear on the transmission line.

Where a transmitting station is arranged to transmit and receive radio signals having frequencies within several different bands, a problem arises in the use of a single half wave antenna in that the efficiency of the antenna deteriorates for signal frequencies other than a multiple of the half wave length of the antenna. In addition, the standing waves at the center of the antenna may be such as to establish unwanted conditions of high voltage on the transmission line.

Accordingly, for many types of radio transmission, a center fed half wave antenna may be an effective and desirable radiator and receiver of electrical energy for only one of the several frequencies employed. In spite of this disadvantage, center fed half wave antennas have come into common usage for amateur radio stations in which a number of parasitic elements may be combined with a dipole radiating element to achieve a high degree of directivity and to concentrate the transmission of radio signals so that the effective power of the transmitter is increased.

Although attempts have been made to construct an antenna including radiating and parasitic elements capable of efficient operation with signals having frequencies falling within several separate frequency bands, each of the known solutions is lacking in some desirab'e characteristic. One known antenna attempts to overcome the problem of multiple band transmission by mounting several dipole radiators of various lengths on a single antenna mast with a number of parasitic elements which are arranged to cooperate with the radiating element which is energized. However, the inclusion of a separate half wave radiator for each of the bands in which energy is to be transmitted unduly increases the weight of the antenna and the presence of the idle radiators frequently affects the operation of an adjacent energized radiator. In addition, it is generally necessary to energize each of the radiators separately from a separate transmission line or to provide some form of switching means at the antenna.

Another attempt to create a multiple band antenna utilizing half wave dipoles includes parallel resonant circuits to isolate various portions of the dipole antenna along its length. The parallel resonant circuits are tuned to present a high impedance to the passage of waves within an upper frequency band, thereby electrically shortening the antenna for signal frequencies within the band. For lower frequencies, the parallel resonant circuits pass the waves to an additional antenna section which radiates along with the first antenna section for transmission of lower frequency signals. By inclusion of several parallel resonant circuits tuned to different frequencies, the antenna may be arranged to radiate energy in several different frequency bands. A description of this type of antenna may be found in the Radio Engineers Handbook by F. E. Terman, 1st edition, at page 854.

A multiple band antenna including parallel resonant circuits comprising inductance coils and condensers suffers from the disadvantage that separate portions of the radiating elements must be electrically insulated one from the other with the parallel resonant circuits connecting the two. This leads to a structural deficiency in the antenna since the electrically conductive sections must be joined by insulating sections which are structurally weak. An additional structural defect arise from the fact that the inductance coils and condensers of the parallel resonant circuits may be physically large and relatively heavy. Both the weight and size of the parallel resonant circuits require a stronger supporting structure to keep the antenna from being damaged by wind and weather.

Apart from the structural disadvantages mentioned above, the inclusion of inductance coils and condensers in the parallel resonant circuits of a multiple band an tenna leads to corrosion and deterioration problems at the points of electrical connection which do not ordinarily exist in a well designed dipole antenna array.

Accordingly, it is one object of the present invention to provide a new and improved multiple band antenna.

It is another object of the present invention to provide a new and improved multiple band antenna including means for causing predetermined portions of the antenna to radiate in separate frequency bands.

It is an additional object of 'the present invention to provide a new and improved multiple band antenna capable of efliciently transmitting and receiving radio signals having frequencies within several frequency bands.

Briefly, in accordance with the invention, an antenna is provided including a plurality of coaxial elements of graduated length which are electrically connected at one end only, along with at least one tuning sleeve which is arranged to telescope between two of the elements. The tuning sleeve functions to isolate a portion of the antenna from signals having frequencies within a predetermined frequency band so that the antenna is capabe of operating with a high degree of efliciency in response to radio signals having frequencies within different frequency bands.

In a particular embodiment, an antenna is provided including a half wave dipole radiator compris ng two like sections, each including three coaxial elements of graduated length. Each of the like sections includes two tuning sleeves, each of which telescopes between two of the radiating elements. In addition to the radiator, the antenna may include parasitic elements which are spaced from the radiator to increase the directivity of the antenna. The parasitic elements may also inc ude coaxial elements and tuning sleeves. When the tuning sleeves are properly adjusted, the particular embodiment of the antenna described above is capable of efliciently operating to transmit or receive radio signals having frequencies within three separate bands.

A better understanding of the invention may be had from a reading of the following detailed description and an inspection of the drawings, in which:

Fig. 1 is a top view of a multiple band antenna in accordance with the invention;

Fig. 2 is a side view of the antenna of Pig. 1;

Fig. '3 is an enlarged view of one section of the radiator of Fig. 1, partially broken away; and

Fig. 4 comprises three diagrammatic representations of the section of the radiator of Fig. 3, illustrating the modes of operation of the antenna for different signal frequencies.

'In Figs. 1, 2 and 3 there is shown a multiple'band antenna array which is adapted to transmit and receive radio energy having frequencies within several frequency bands in accordance with the invention. The antenna array includes a support beam 1 comprising a piece of metal tubing which is fastened in a horizontal position by a bracket 2 mounted on the top of a conventional antenna mast 3 (Fig. 2). If it is desired to rotate theantenna, a conventional remotely controlled antenna rotater may be mounted between the bracket 2 and the antenna mast 3.

Supported by the beam 1 are the working portions of the antenna which in Figs. 1 and 2 comprise a radiator 5 and four parasitic elements 6, 7, 8 and 9. Each of the parasitic elements 6, 7, 8 and 9 and the radiator 5 is divided into two like sections extending outwardly and transversely of the beam 1. The separate sections of the radiator 5 are mounted on the top of stand-01f insulators on'the bracket 10 so as to electrically insulate the radiator sections from each other and'from the support beam 1. In like fashion, the separate sections of the outer parasitic elements 6 and 9 are also mounted on stand-off insulators attached to the brackets 11 and 12.

The need for insulating the sections of the radiator 5 and parasitic elements arises due to the fact that the radiator 5 and outer parasitic elements 6 and 9 are electrically lengthened by the

hairpin extensions

13, 14 and 15. Accordingly, the electrical center of the radiator 5 and the outer parasitic elements 6 and 9 occurs at the mid-point of the curved portion of the hairpin extension. In contrast, the electrical center of the inner parasitic elements 7 and 8 occurs at the ends of the separate sections which are mounted directly on the brackets 16 and 17 which are electrically connected to 'the'support beam 1. 1n the case of the outer parasitic elements 6 and 9, the electrical centers are electrically connected to the beam 1 from the mid-point of the hairpin extensions 14 and by means of the connectors 18 and 19.

In the case of the radiator 5, the electrical center is not connected to the beam 1 since this is the point at which the antenna is fed from a transmission line. Accordingly, the hairpin extension 13 is separated in two parts and connected to terminals on an insulated terminal board 20 to which may be connected a conventional transmission line (not shown).

The section of the radiator 5 illustrated in the upper portion of Fig. 1 includes an upper frequency band radiating element 21, a middle frequency band radiating element 22 and a lower frequency band radiating element 23. In like fashion, the section of the radiator 5 shown in the lower portion of Fig. 1 includes an upper frequency band radiating element 24, a middle frequency band radiating element 25 and a lower frequency band radiating element 26.

The radiating elements of each section comprise graduated pieces of tubing which are mounted coaxially and are electrically connected at the inner end only. The length of each piece of tubing may be equal to approximately a quarter wave length for a particular frequency within one of the several bands. Each half of thehair- .pin extension 13 is electrically connected to the inner ends of all of the coaxial radiating elements of oneof the sections of the radiator 5.

Between the

coaxial radiating elements

21, 22 and .23

of the upper illustrated section of the radiator 5 are disposed two tuning

sleeves

27 and 28 which telescope into and between adjacent radiating

elements

21, 22 and 23. Thus, one tuning sleeve 27 is electrically connected at its outer end only to the middle frequency band radiating element 22 and is adapted to telescope between the middle frequency band radiating element 22 and the upper frequency band radiating element 21. The other tuning sleeve 28 is electrically connected at its outer end only to the lower frequency band radiating element 23 and is adapted to telescope between the lower frequency band radiating element 23 and the middle fre quency band radiating element 22.

In an identical fashion, the lower illustrated section of the radiator 5 includes a tuning sleeve 29 between the upper and middle frequency band radiating elements 24 and 25 and a tuning sleeve 30 between the middle and lower frequency band radiating elements 25 and 26.

As illustrated in Fig. 3, the correct coaxial spacing between the radiating elements and tuning sleeves may be preserved by insulating rings. Thus, the insulating spacer rings 31 and 32 hold the radiating

elements

21, 22 and 23 in position and the insulating rings 33 and 34 hold the tuning sleeves in proper spaced relationship with respect to the

adjacent radiating elements

21, 22 and 23.

In operation, it has been found that when the tuning .

sleeves

27, 28, 29 audit are properly adjusted, the ra diator 5 is capable of efficiently functioning as a half wave dipole antenna for three distinct frequency bands. By means of standing wave tests it has been established that theisolation between the various radiating elements is exceptionally good with the tuning sleeves functioning in cooperation with the radiating elements to present a high impedance path to the flow of energy at the end of each of the radiating elements. Accordingly, when the antenna is energized from a transmission line connected to the hairpin extension 13, standing waves are established on the antenna producing relatively high voltage and low current adjacent the ends of a selected pair of the radiating

elements

21, 22, 23, 24, 25 and 26, depending upon the frequency of the signal with which the radiator 5 is energized.

The parasitic element 6 may be spaced from the radiator 5 and cut to an appropriate length to function as .a. reflector. Its construction may be similar to the construction of the radiator 5, but since it is adapted to cooperate with the radiator 5 for only the low and middle frequency bands, each half section of the parasitic ele ment 6 need only include two coaxial elements and a single tuning sleeve.

Thus, the section shown in the upper portion of Fig. 1 includes the elements 35 and 36 and the tuning sleeve 37, while the section shown in the lower portion of Fig. 1 includes the elements 38 and 39 and the tuning sleeve 40. The inner ends of the elements 35 and 36 of one section are electrically connected to one end of the hairpin extension 14 while the inner ends of the elements 38 and 39 of the other section are electrically connected to the opposite end of the hairpin extension 14.

Since the parasitic element 6 is not driven and operates from energy radiated from the radiator 5, the hairpin extension 14 provides a continuous electrical connection which may be grounded to the beam 1 by means of the connector 18 as noted above.

The parasitic element 9 may be spaced from the radiator 5 and cut to a proper length to function as a director. Construction of the parasitic element 9 is similar to that of the parasitic element 6. Accordingly, the section of the parasitic element 9 shown in the upper portion of Fig. 1 includes a pair of

coaxial elements

41 and 42 between which is located a telescoping tuning sleeve 43, while the section of the parasitic element 9 shown in the lower portion of Fig. 1 includes a pair of

coaxial elements

44 and 45 along with the telescoping tuning sleeve 46. The inner .endsof the

elements

41, 42, 44 and 45 are electrically connected to a hairpin extension which may be grounded to the beam 1 by means of the connector 19 as noted above.

In operation, the tuning

sleeves

37, 40, 43 and 46 of the parasitic elements 6 and 9 are adjusted to isolate the

elements

36, 39, 42 and 45 from the

elements

35, 38, 41 and 44 for signals within a middle frequency band.

Due to the high impedance appearing at the ends of the

elements

35, 38, 47 and 44 for signal frequencies within the middle frequency band, when the radiator 5 is energized with signals within the middle frequency band, the elements 35 and 38 along with the hairpin extension 14 function in the fashion of a conventional reflector element, while the

elements

41 and 44 along with the hairpin extension 15 function as an efiective director element.

While it is possible to arrange the parasitic elements 6 and 9 to function as effective reflectors and directors for more than two separate frequency hands, it has been found desirable to include additional auxiliary parasitic elements to cooperate with the radiator 5 for the upper frequency band.

Accordingly, two separate parasitic elements 7 and 8 are provided including the

sections

47, 48, 49 and 50. By properly spacing the parasitic elements 7 and 8 from the radiator 5 and cutting the

elements

47, 48, 49 and 50 to the proper lengths, the parasitic element 7 may be caused to function as a reflector and the parasitic element 8 as a director for the upper frequency band.

Although the antenna of the invention has been subjected to rigorous tests which have established its workability beyond doubt, the theory of operation is not completely understood. However, it is believed that the tuning sleeves cooperate with the coaxial elements to produce an effective high impedance parallel resonant circuit by combining the inductive and capacitive reactances existing between the coaxial elements and the tuning sleeves. One possible theory of operation of the antenna is based upon the well known fact that a section of transmission I line shorted at one end and having a length less than one quarter wave length long for the electrical wave applied to the transmission line exhibits inductive reactance at its open end.

Referring to Fig. 3, there is shown one section of the radiator 5 of the antenna of Fig. 1. Since each radiating element of the radiator 5 corresponds to a quarter wave length for a particular frequency and hence is resonant for that frequency, and since all of the radiating

elements

21, 22 and 23 are connected by a common connection to the hairpin 13, adjacent ones of the radiating elements form coaxial transmission lines shorted at one end which present a high impedance at the open end. Therefore, a high impedance may be expected to appear at the open end of the adjacent pair of radiating elements when the radiator is energized with the particular frequency.

In addition to the above described high impedance afforded by the radiating elements, the tuning sleeves also form transmission lines in combination with adjacent radiating elements which are shorted at one end and less than one quarter wave length long, thereby providing an additional amount of inductive reactance. Also, the tuning sleeves possess electrical capacity with respect to the adjacent radiating elements. It is believed that the combination of inductive and capacitive reactances described above cooperate to form parallel resonant circuits of adjustable frequency at the ends of the radiating elements which present a high impedance to the passage of signals having a frequency equal to the resonant frequency of the parallel resonant circuits.

The tuning sleeves may be moved in or out to vary the reactances and tune the antenna to the various frequency bands to be used. Although the length of each of the radiating elements will ordinarily be approximately equal to a quarter wave length for a frequency within each of the selected hands, it has been found that the effective radiating length of the radiating elements may be changed somewhat by adjusting the position of the tuning sleeves, thereby giving the antennaza versatility not enjoyed by other types of multiple band antennas.

Fig. 4a illustrates diagrammatically the operation of the radiating element of Fig. 3 when energized with upper frequency band signals. With the exception of the schematic circuit diagram symbols for the inductances and capacitances, the same reference characters have been used in Fig. 4 to identify the various portions of the radiator as were used in Fig. 3.

When the radiator is energized with upper frequency band energy, Fig. 4a illustrates that a high impedance is presented to the flow of upper frequency band energy at the end of the radiating element 21 by virtue of an inductance 51 appearing at the open end of the transmission line formed by the radiating

elements

21 and 22, an inductance 52 formed at the end of the transmission line formed by the tuning sleeve 27 and the radiating element 22 and a capacitance 53 provided by the tuning sleeve 27 in cooperation with the radiating element 21.

Although substantially no energy flows past the high impedance point at the end of the radiating element 21 for frequencies in the upper frequency band, the second tuning sleeve 28 in combination with the middle and lower

band radiating elements

22 and 23 presents only a capacitive reactance 54 in the manner of a parallel resonant circuit at a frequency higher than resonance.

Fig. 4b illustrates the operation of the radiator of Fig. 3 when energized with middle frequency band signals. The parameters of the tuning sleeve 27 are such that a relatively low impedance path exhibiting only an inductive reactance 55 is presented to the flow of middle frequency band energy. However, an inductive reactance 56 appears at the end of the transmission line formed between the middle and lower

band radiating elements

22 and 23, an inductive reactance 57 appears at the end of the tuning sleeve 28, and a capacitance 58 is formed by the tuning sleeve 28 and the middle band radiating element 22, thereby establishing a high impedance which effectively isolates the lower band radiating element 23 from the remainder of the antenna. Accordingly, the antenna is capable of operating efiiciently as a half wave dipole radiator for middle frequency band signals.

Fig. 4c illustrates the mode of operation of the radiator when energized with lower frequency band signals. Where the radiator of Fig. 3 is energized with signals in the lower frequency band, neither the tuning sleeve 27 nor the tuning sleeve 28 functions to provide a high impedance to the flow of energy. Only small inductive reactances 59 and 60 may appear at the ends of the upper and middle

radiating band elements

21 and 22. Therefore, the electrical length of the radiator is at a maximum with the entire length of the radiator being energized.

In operation, the antenna of the invention may be energized within each of three distinct frequency bands with the antenna behaving substantially identically to individual antenna arrays which are specifically tailored to length and designed for use in each of the several bands. Accordingly, use of the antenna of Figs. 1 and 2 is particularly advantageous where an amateur radio station contemplates the transmission of radio signals within the ten meter, the fifteen meter and the twenty meter amateur bands since the antenna is capable of automatically transmitting radio signals on any of the three bands without switching and with a degree of efficiency and directivity not found in other known types of multiple band antennas.

In addition, since the tuning between the elements of the antenna is accomplished by means of tuning sleeves, the structural rigidity of the antenna is enhanced since no auxiliary traps in the form of parallel resonant circuits need be supported by the antenna elements. Thus, in simplicity of construction, in ease of adjustment, and in -7 strength, the multiple band antenna of the present invention affords an exemplary devicerfor transmitting radio energy. in multiple frequency bands.

What is claimed is:

1. A multiple band antenna including a plurality of coaxially related radiating elements of graduated length, each of said radiating elements being adapted to radiate signalshaving different frequencies, and at least one coaxial tuning sleeve arranged to telescope between the radiating elements for isolating the signal frequencies radiated by one radiating element from the adjacent element.

2. A multiple band antenna having at least one radiator comprising two separate like sections which are centrally driven, each of said sections comprising a plurality of quarter wave radiating elements which are coaxially disposed and electrically connected at one end only, and tuning means disposed between the radiating elements of each'of the two sections for electrically isolating signal frequencies radiated by one radiating element from an adjacent radiating element.

3. A multiplerband antenna having at least one radiator comprising two separate like sections which are centrally driven, each of said sections comprising a plurality of quarter wave radiating elements which are coaxially disposed and electrically connected at one end only, and at least one tuning sleeveicoaxially disposed between the radiating elements of each of the two sections for electrically isolating signal frequencies radiated by one radiating elementfrom anadjacent radiating element.

4. An antenna in accordance with claim 3 including at least one parasitic element which is spaced from the radiator and which includes at least one tuning sleeve for electrically isolating portions of the parasitic element, whereby the parasitic element is capable of cooperating with at leasttwo of the plurality of radiating elements of the radiator for increasing the directional characteristic of the antenna.

'5. An antenna .in accordance with claim 3 including a first parasitic element spaced'from the radiator as a director, a second parasitic element spaced from the radiator as a reflector, each of said parasitic elements including coaxial elements and tuning sleeves for electrically isolating portions of the coaxial elements whereby the parasitic elements cooperate with at least two of the radiating elements of the radiator for increasing the directional characteristic of the anenna.

6. A multiple band antenna including the combination of a first pair of radiating elements arranged as an open ended coaxial transmission line which is less than one quarter wavelength long for signal frequencies within a firsttband of frequencies, and a tuning sleeve arranged to telescope into the open end of said transmission line for confining signal frequencies within a predetermined band of frequencies to one ofthe radiating elements whereby the antenna is electrically shortened for transmission of signal having frequencies within said predetermined band.

7. A multiple band antenna including the combination of a plurality of coaxial elements which are electrically connected at one end only, at least one of said coaxial elements being adapted to cooperate with an adjacent inner coaxial element to form an open ended transmission line which is resonant for signal frequencies within a predetermined band, and at least one tuning sleeve which is adapted to be positioned between the coaxial elements of the transmissionrline for confining signals having frequencies within the predetermined band to a portion of the antenna whereby the antenna is effectively shortened for signals having frequencies within the first predetermined band.

8. Armultiple .band antenna including the combination of a first radiating element having an electrical length for radiating lower frequency signals, a second radiating element in the'form of a tube which is coaxially disposed with respect to the first radiating element for radiating middle frequency signals, a third radiating element in the form of a tube which is coaxially disposed with respect to the second radiating element for radiating upper frequency signals, a first tuning sleeve between the first and secondrradiating elements for tuning the antenna to radiate the middle frequency signals, and a second tuning sleeve between the second and third radiating elements for tuning the antenna to radiate upper frequency signals.

9. A multiple band antenna including the combination of a first radiating element, a first tuning sleeve electrically connected to the first radiating element at one end only, a second radiating element, a second tuning sleeve electrically connected to the second radiating element at one end only, a third radiating element, said first, second and third radiating elements and said first and second tuning sleeves being telescoped together with a tuning sleeve being disposed between each of the radiating elements, and said tuning sleeves being adapted to cooperate with said radiating elements whereby a separate band of signal frequencies may be radiated by each of the first, second and third radiating elements.

10. A multiple band antenna including the combination of a plurality of coaxial elements of graduated lengths which are electrically connected at one end, at least two of the elements being arranged in the manner of an open ended coaxial transmission line having a wave length corresponding to a quarter wave length for signal frequencies within a first band, at least two of the elements being arranged in the manner of an open ended coaxial transmission line having a length equal to a quarter wave length for signal frequencies within a second band of frequencies lower than the first band of frequencies, a first tuning sleeve arranged to telescope within the coaxial elements of the first transmission line to confine signals within the first band of frequencies to a portion of the antenna which is adapted to radiate signals within the first band, and a second tuning sleeve arranged to telescope within the coaxial elements of the second transmission line to confine signals within the second band to a portion of the antenna which is adapted to radiate signals within the second hand.

11. A multiple band antenna including the combination of a first radiating element having an electrical length for radiating energy having frequencies within a first band of frequencies, a second radiating element in the form of a concentric tube which surrounds a portion of the first radiating element having an electrical length for radiating energy having frequencies within a second hand of frequencies higher than the first band of frequencies, a third radiating element in the form of a concentric tube surrounding a portion of the second radiating element having an electrical length for radiating frequencies within a third band of frequencies higher than the second band of frequencies, said first, second and third radiating elements being electrically connected at one end only, a first tuning sleeve electrically connected totthe first radiating element at one end only and adapted to be telescoped into the second radiating element to establish a high impedance between said first and second radiating elements for energy within the second hand of frequencies, and a second tuning sleeve electrically connected at one end only to the second radiating element and adapted to be telescoped into the third radiating element to establish a high impedance between the second and third radiating elements for energy within the third band of frequencies.

12. A multiple band antenna including the combination of a first dipole radiating element having an electrical length equal to one half the wave length of a first signal frequency, a second dipole radiating element in the form of a coaxial tube surrounding the first radiating element having an electrical length equal to one half the wave length of a second signal frequency higher than said first signal frequency, a third dipole radiating element in the form of a coaxial tube surrounding the second radiating element having an electrical length equal to one half the wave length of a third signal frequency higher than said second signal frequency, a first pair of tuning sleeves arranged to telescope between the ends of the first radiating element and the second radiating element, each of said first pair of tuning sleeves being electrically connected at one end only to the first radiating element, a second pair of tuning sleevesarranged to telescope between the ends of the second radiating element and the third radiating element, each of said second pair of tuning sleeves being electrically connected to the second radiating element at one end only, and means electrically connecting the first, second and third radiating elements to a pair of common center connections whereby signals applied to the center connections of any of the three signal frequencies are radiated by the antenna functioning as a substantially half wave antenna tuned to the signal frequency being radiated.

13. An antenna in accordance with claim 12 including at least one parasitic element spaced apart from the radiating elements, said parasitic element including at least two coaxial tubes and a pair of tuning sleeves, said 10 tuning sleeves being arranged to telescope between the coaxial tubes whereby the parasitic element is electrically shortened for predetermined ones of said signal frequencies to cooperate with the radiating elements for increasing the directional characteristic of the antenna.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Textbook, Networks, Lines and Fields, Ryder, New York, Prentice-Hall, 2nd ed. 1955, pp. 80-88.