US2421593A - Coaxial half-wave microwave antenna - Google Patents

Description

June 3, 1947 N. BISHOP CIOAXIAL HALF WAVE MICROWAVE ANTENNA Filed April 6, 1945 Fig.1.

HIGH FREQUENCY; souncs 5 Patented June 3, 1947 COAXIAL HALF-WA AN TE Nathaniel Bishop, Fairfield, Conn,

General Electric Company,

New York VE MICROWAVE NNA assignor to a corporation of Application April 6, 1943, Serial No. 482,045

8 Claims.

My invention relates to ultra high frequency communication systems and in particular to an antenna system for centimeter waves. It is an object of my invention to provide an improved half-wave vertical antenna, in which the cross sectional dimension of the radiator element is of a practical size in view of the wavelength of the signal radiated.

My invention relates more particularly to an antenna for transmitting a rotating beam of signal in which the half-wave vertical radiating element remains stationary and the use of rotating joints in the coaxial transmission lines supplying signals to the antenna is avoided.

A further object of my invention is to provide an improved rotating high frequency antenna in which standing waves on the transmission lines connected therewith are reduced to a minimum intensity.

Another object of my invention is to provide an improved high frequency antenna employing a rotating reflecting element in which the radiating element of the antenna is provided with an unobstructed view of the reflecting element.

The features of my invention which I believe to be novel are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which Fig. 1 shows an antenna, partly in section, constructed in accordance with my invention and Fig. 2 shows an antenna system, partly in section, embodying the antenna of Fig. 1.

Referring to Fig. l, I have shown a vertical half-wave antenna having a radiating element I supplied with very high frequency signals, such as centimeter waves, from a high frequency source H by means of a concentric transmission line l2 having an inner conductor l3 and a tubular outer conductor l4. Interposed between the radiating element In and the transmission line I 2 is an impedance matching transformer comprising a section of transmission line having an im'ier conductor constituted by an extension l5 of the radiating element l0 and a tubular outer conductor [6. The inner conductor of the impedance matching transformer is connected to the inner conductor [3 of the line I2 and tubular outer conductor [6 is connected to the outer conductor I I. In order that the section of transmission line comprising the conductors I5 and I6 maybe utilized to match the impedance of 55 the base-fed radiator l0 to the transmission line l2, the length of this section is made equal to a quarter wavelength at the frequency of the signals supplied by the high frequency source H.

In order to reduce the magnitude of currents flowing from ground to the radiating element I0 over the outer conductors i4 and [6, a tubular conductive member I! is concentrically placed about outer conductor l6 and is coextensive therewith to form therewith another section of a concentric transmission line having a length equal to a quarter wavelength at the operating frequency of the system and which functions as a quarter-wave choke for ground or outside line currents flowing in the aforesaid path. This second quarter wavelength section of transmission line formed by conductors l6 and I1 is shortcircuited at its end remote from the radiating element by means of a cross conductor l8. Physically, the conductors l6 and [1, together with the conductive member l8, may be formed as a, unit and provided with a neck portion H! to encircle outer conductor I4 and which may be joined, as by soldering or welding, to this outer conductor.

Mechanical support for the radiating element It] and its extension [5 with respect to the conductor I6 is provided by a plug 20 of suitable insulating material, such \aspolystyrene. The plug 20 surrounds the base portion of extension I5 of the radiating element and has a portion extending within the transmission line 12 to support inner conductor [3 coaxially with respect to outer conductor l4.

In order to provide means for protecting the radiating element In and the impedance matching section of the antenna from adverse weather conditions, an insulating shield in the form of a cylinder 21 of a suitable material, such as polystyrene, may be placed over the radiating element and may have its lower portion in screw-threaded engagement with the outer surface of tubular conductor ll. An insulating insert 22 within the shield 2| may likewise be provided to furnish mechanical support for the top portion of radiator II].

By way of example, one antenna system for transmitting a microwave si nal, which has been found to give highly satisfactory results, has a transmission line whose surge impedance is ohms and a, radiating element whose diameter is inch and whose end impedance is approximately 250 ohms. In providing a surge impedance for the quarter-wave impedance matching transformer which is the geometric mean of the waves are substantially eliminated from the outer surface of the antenna support, 1. e., the transmission line l2. In this way, unwanted radiation from the supporting member is minimized and the vertical doublet appears as an antenna in free space.

Referring to Fig. 2 of the drawing, I have shown an antenna system for transmitting a rotating beam of signal comprising the radiating element Ii], again supplied with suitable high frequency signals over a transmission line l2, and in which parabolic reflector 25 and back reflector 26 are mounted for rotation about the antenna l0 and transmission line 2. The parabolic reflector 25 is provided with a vertical supporting member 21 and a horizontal supporting member 28. Formed integrally with the horizontal supporting member 28 is a bearing 29 which is journaled on the outer conductor of the transmission line l2. The hearing 29 likewise has an outer bearing surface 32 rotatably positioned within a cooperating bearing surface within vertical supporting member 39. The stationary supporting member 30 is held in an upright position by means of a plurality of supporting rods 3!. Bearing member 29 may be rotated with respect to the stationary support 38 and the concentric transmission line l2 by any suitable means, such as the gear 35 secured thereto and the driving pinion 36. In this manner, the parabolic reflector 25 rotates about the vertical line l2 and the radiating element Ii] located substantially at the focal point of reflector 25. The back reflector 26, which is of a relatively small size compared to the size of reflector 25, is preferably a section of a cylindrical surface and is supported by a rod 33 rigidly attached to the horizontal supporting member 28. In order to provide additional support for rod 33, the member 34 is afiixed to the upper portion of rod 33 and is provided with a surface (not shown) journaled on the outer surface of transmission line 12 to maintain a constant spacing between element It and reflector 28 as the latter is rotated about the former.

By the system of Fig. 2, the radiating element I0 is provided practically with an unobstructed view of the surface of parabolic reflector 25. Since the quarter-wave choke l'l isolates the radiating element from ground, this element appears essentially as a free-space doublet or halfwave radiator at the focal point of the reflector 25. Since, also, in my construction the reflecting element is rotated about the radiating element, the rotating beam of signal is obtained without the use of rotating joints in the trans-mission line supplying signals to the radiating element. Furthermore, by using a radiating element having a length equal to a half wave at the operating frequency of the system and using an impedance -matching transformer to connect the radiating element to the transmission line, the cross sectional dimension of the radiating element is kept to a practical size in view of the wavelengths of the signals being radiated, while standing waves are substantially eliminated from the transmission line. Since the cross sectional dimension of the radiating element is held down to a practical value and is not too large a portion of a wavelength of the radiated signals, the described radiating element functions as a highly efficient radiator, especially when used at the focal point of a parabolic reflector. In such an antenna structure, moreover, the Q or quality is held to a reasonably low value and undesirable sharpness of the antenna system is avoided. An antenna constructed in accordance with my invention,

therefore, provides a symmetrical vertical halfwave antenna structure of reasonable dimension which is quite efficient for microwave signals and is adapted for use in conjunction with coaxial transmission lines.

While I have shown a particular embodiment of my invention as used in the transmission of radio signals, it is apparent that the invention may also be used in the reception of radio waves. It will be understood, therefore, that I do not wish to be limited to the particular embodiment shown since various modifications may be made, and I contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, an antenna, a transmission line connected therewith to supply signals thereto, an impedance matching transformer interposed between said line and said antenna comprising a section of concentric transmission line having a length equal to a quarter wavelength at the frequency of said signals and an impedance intermediate the impedance of said antenna and the impedance of said transmission line, and means to give a free-space effect to said antenna comprising a tubular member concentrically surrounding the outer conductor of said section, said tubular member and said outer conductor being conductively connected at their extremities remote from said antenna.

2. In an antenna for a centimeter wave signal, the combination of, a vertical radiating element having a length equal to a half wavelength of said wave, a vertical transmission line connected therewith to supply said signal thereto, an impedance matching transformer interposed between said line and said element comprising a section of concentric transmission line havin a tubular outer conductor and an inner conductor connected to said element, said section having a length equal to a quarter wavelength of said wave and an impedance intermediate the impedance of said antenna and the impedance of said transmission line, and a tubular conductive member surrounding said tubular conductor and coextensive therewith, said member and said tubular conductor being connected together at their ends remote from said element.

3. The combination, in a rotating antenna system for microwaves, of, a vertical radiating element having a. length equal to a half wavelength at the frequency of said waves, a vertical concentric transmission line for supplying said waves to said element, the inner conductor of said line being connected to said element and the outer conductor of said line being enlarged at its upper end for a distance equal to a quarter wavelength at the frequency of said waves to provide means for matching the impedances of said line and said element, means connected to said transmission line to impede the flow of currents along the outer surface of said outer conductor to said radiator, a reflector, and means to support said reflector in spaced relation to said element, said last means including means whereby said reflector may be rotated about said element.

4. The combination, in a rotating antenna system for microwaves, of, a vertical radiator having a length equal to a half wavelength at the frequency of said waves, a substantially rigid vertical transmission line for supplying said waves to said radiator, said line having an inner conductor connected to said radiator and a concentric outer conductor, said outer conductor having a section of increased diameter at its upper end for a distance equal to a quarter wavelength at the frequency of said waves to provide means for matching the impedances of said line and said radiator, means to prevent the transmission of currents along the outer surface of said outer conductor to said radiator comprising a tubular member surroundin said section and coextensive therewith, said section and said member being conductively connected to their ends remote from said radiator, a pair of reflecting elements, and means to support said reflecting elements on opposite sides of said radiator and in spaced relation therewith, said last means including means whereby said reflecting elements may be rotated about said radiator.

5. The combination, in an antenna system for a microwave signal, of a vertical radiating element having a length equal to a half wavelength at the frequency of said signal, a parabolic reflector in spaced relation with said element, means including a vertical concentric transmission line to supply signals to said element and to support said element substantially at the focal point of said parabolic reflector, means connected to said concentric transmission line to impede the flow of currents between said element and ground, a relatively small reflector spaced in close proximity to said element on the side thereof opposite said parabolic reflector, and means to rotate said reflectors about said element.

6. The combination, in an antenna system for a microwave signal, of a vertical radiating element having a length equal to a half wavelength at the frequency of said signal, a parabolic reflector in spaced relation with said element, means including a vertical concentric transmission line to supply signals to said element and to support said element substantially at the focal point of said parabolic reflector, a relatively small reflector spaced in close proximity to said element on the side thereof opposite said parabolic reflector, means to impede the flow of currents between said element and ground, said last means comprising a tubular member surrounding said transmission line adjacent its-point of connection to said element, said tubular member having one of its ends connected to said transmission line, and means to rotate said reflectors about said element.

7. In combination, an antenna, a transmission line connected therewith to supply signals thereto, an impedance matching transformer interposed between said line and said antenna comprising a section of concentric transmission line having a length equal to a quarter wave length at the frequency of said signals and an impedance intermediate the impedance of said antenna and the impedance of said transmission line, means to impede the flow of currents between said antenna and ground comprising a tubular member concentrically surrounding the outer conductor of said section, said tubular member,

having one of its ends said outer conductor.

8. An antenna having a length equal to a half wave length of the wave at which it operates, a concentric transmission line having an inner conductor connected to one end of said antenna and an outer conductor, the portion of the outer conductor surrounding the last quarter wave length of said inner conductor adjacent said antenna, being proportioned to match the impedance of said line to the impedance of said antenna, and a further concentric conductor about said portion of said outer conductor throughout a major part of the length of said portion and insulated therefrom throughout its length and connected to said outer conductor at one of its ends.

conductively connected to NATHANIEL BISHOP.

REFERENCES CITED The following references are of record inthe file of this patent:

UNITED STATES PATENTS Ponte Jan. 7, 1936

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