US2637533A

US2637533A - Multi-v fm antenna

Info

Publication number: US2637533A
Application number: US117628A
Authority: US
Inventors: Marvel W Scheldorf
Original assignee: Andrew LLC
Current assignee: Commscope Technologies LLC
Priority date: 1949-09-24
Filing date: 1949-09-24
Publication date: 1953-05-05
Anticipated expiration: 1970-05-05

Description

y 1953 M. w. SCHELDOFF MUL'FI-V FM ANTENNA I5 Sheets-Sheet 1 Filed Sept. 24, 1949 [Lit M y'5, 1953 M. w. SCHELDORF 2,637,533

MULTI-V FM AN 'IiENNA Filed Sept. 24, 1949 3 Sheets-Sheet 2 4% wad/ Patented May 5, 19 53 MULTI-V FM ANTENNA Marvel W. Scheldorf, Palos Heights, 111., assignor to Andrew Corporation, Chicago, 111., a corporation of Illinois Application September 24, 1949, Serial No. 117,628

7 Claims. (01. 25033.53)

The present invention relates to a radio transmitting antenna and more particularly, to a transmitting antenna designed for operation in the FM range of 88 to 108 megacycles.

Many broadcasting companies have obtained permits to transmit in the FM range in addition to the present AM frequency. It would be highly desirable to locate the FM transmitter at the AM transmitter site and to use the AM antenna tower for the FM antenna.

In making provision for a simple, lightweight and inexpensiv FM transmitting antenna, it would be desirable to provide an arrangement whereby the antenna elements might be readily attached to the sides or corners of existing AM antenna towers or to a relatively short mast to be mountedat the top of an existing AM antenna tower. Such requirements necessitate an asymmetric construction which however, should provide in a horizontal plane a substantially circular radiation pattern.

In accordance with the present invention, a relatively simple, lightweight and inexpensive transmitting antenna for operation in the FM range is obtained by providing a plurality of bays, each of which has a shape corresponding generally to a truncated V. In an antenna of this type however, it is desirable to provide a driving impedance having a substantial value of the order of 50 ohms so as to minimize the matching problem in joining the antenna radiators to conventional transmission lines.

In order to realize further economies in the construction of an FM antenna, it is possible to provide radiating elements which are adjustable. Generally, however, such adjustable arrangements result in changes in both the horizontal radiation or field intensity pattern and variations in antenna impedance. Some of the pattern variations are not sufficiently severe but what they can be tolerated. However, impedance variations and changes require additional adjustable elements such as a multiplicity of matching elements or tuneable stubs. In accordance with the present invention however, it has been found possible to tune an antenna for resonance for any desired frequency of transmission without any substantial change in the antenna impedance value. A single matching device has been found sufficient to transform the impedance at the junction point of the antenna structure and the transmission line in spite of an inherent characteristic whereby the driving impedance of the antenna varies over the frequency range. By means of a matching structure, it has been possible to obtain an impedance at the junction point which averages 51.5 ohms over the FM frequency range.

Since it is customary in FM transmitters to employ a plurality of bays, the present invention contemplates the use of as little as two bays, each having a conservative power rating of 5 kw.

It is therefore, an object of the present invention to provide a simple, lightweight and inexpensive transmitting antenna designed for operation in the FM range of 88 to 108 megacycles.

It is a still further object of the present invention to provide an FM transmitting antenna having a substantially constant field intensity pattern throughout the FM range without utilizing a multiplicity of tuning and matching devices.

It is a still further object to provide a multiple bay antenna having a relatively simple antenna feed system.

Still another object of the present invention is to provide a simple assymmetric FM antenna having a substantially circular horizontal field intensity pattern.

A still Iurtner object of the present invention is to provide an FM antenna arrangement to permm insulation resistance checks without disconnectlng the antenna radiating elements.

Still another object of the present invention is to pIOVlQe a simple FM antenna which may be attached to exlstlng antenna tower structures.

A still further OlJJeCt of the present invention is to provide an FM antenna design suitable for mass production whereoy a single design may be ad usted or pre-tunea at the Iactory for any frequency of operatlon.

Still another object of the present invention is to provide an lmproved BM antenna structure whlcn may employ a plurality of bays and which has a standing wave ratio ranging lrom 1.2 to 1.4 in the range of as to luzi meg'acycles.

Other and further objects or the present inventlon subsequently W111 become apparent from reference to the lollowlng description taken in COnJHIlCDiOH with the drawings. wherein:

Figure 1 is a perspective view of a mounted Flvl antenna embodying the principles or" the present invention;

bigure z 1s a eetailed view partially in cross section of the upper antenna bay;

b Figure 3 1s a bottom view or the upper antenna Figure 4 is a detailed view partially in cross sec non of the lower antenna bay;

Figure 5 is a graphical representation of the horizontal held intensity pattern;

Figure 6 i a schematic showing or the electrical circuit of the antenna; and

Figure 7 is a schematic showing of a modified electrical circuit for the antenna.

Most existing antenna structures which are to serve for the transmission of both FM and AM radiations are adapted to support an additional member such as an I or H beam ll shown in Figure 1. The support mast H preferably is a steel H beam so oriented that the well on one side forms a protective enclosure for a transmission line !2 whereas the opposite side or well of the H beam serves as enclosure for a power conductor l3 shown in Figure 3. The H beam H carries at its top a beacon light HZ of any suitable type which meets the requirements of the Federal Communications Commission regulations. In order to service the light and the antenna radiators, the H beam is provided with a plurality of steps it; which preferably are placed in those portions or the walls of the'well containing the power conductor l3.

For the purpose of illustrating the present invention there has been shown in Figure l, a transmitting antenna embodying an upper bay IE and a lower The upper bay i5 preferably is carried by a support plate iii which is secured in position by a plurality of straps l9 extending across the dances of the H beam H. The strap construction, subsequently to be described, permits vertical movement of the upper bay iii to allow for the difierence in expansion of the copper transmission line i3 and the steel H beam H. Such an arrangement also permits the mast ii to bend without unduly straining or stressing the transmission line [2.

The lower bay ii is also mounted on a supporting plate 2i which may be retained in position by a strap or'if desired the plate 2! may be rigidly connected to the H beam 1 i.

Each bay consists of two radiating elements arranged in the form of a truncated V. Each element is preferably formed of a telescopic structure which is most conveniently formed of a plurality of tubes. The larger tube has a length which is selected for the highest frequency operation of an antenna of this type within the I nected to the outer conductor of the coaxial transmission line 52. The outer extremity of this larger tube of one of the radiators is also connected by aconductive element to the outer conductor of the coaxial transmission line l2. The other larger tube of the radiating elements is connected by a conductive strap to the inner conductor of the transmission line [2.

Certain details of the antenna structure will become more apparent by reference to the detailed figures. In Figure 2, there is shown the upper bay structure which is supported by a mounting plate It. The mounting plate 18 carries the stud 23 which is retained in position by a plurality of bolt or cap screws 2 The stud 23 supports the outer tube or primary radiating element 25. This tube '25 has a length which is determined by the highest frequency radiation in an antenna of this type within the FM frequency range. The outer extremity of the tube 25 is connected to a conductive strip or power lead 26 which terminates at a binding post 21 carried by an insulator 28. The tube 25 carries an extension tube 29 arranged to telescope relative to the tube 25 so that the overall effective length of the radiating element provides a tuned 4 radiator at the transmitting frequency. The outer extremity of the tube 29 may be sealed by a cap or other closure member 3 l.

The insulator 28 shown partially in cross section carried by a plate 32 retained in position by suitable cap screws 33 which pass into threaded openings in the mounting plate 8. The mounting plate i8 is provided with a suitable opening for receiving a junction box 3d which may be secured in position by any suitable manner including silver soldering. In order to provide a weather-proof and gas tight seal an O ring is interposed between the mounting member 32 and the junction box 34.

From the cross sectional View of the insulator 28 it will be seen that the insulator has a hollow portion which carries a central conductor stem 36 arranged to receive a banana plug The banana plug 37 is a bifurcated cylindrical portion of a conductor which is secured to a conductor 3d. The conductor 39 is insulated by a plurality of beaded insulators l! and held in spaced coaxial relation to the inner conductor 42 or the coaxial line 12. The inner conductor 42 of the transmission line section 53 is mounted on spaced insulator beads 3 carried within the outer conductor d5. The conductive member 39 extends within the inner conductor 32 of the transmission line section it? a sufficient distance to form a capacitor of the proper value. The outer conductor 15 terminates in a mounting flange Eli which is coupled by suitable bolts 4'! to a cooperating mounting flange carried by the other section of the transmission line H2. The series capacitor interposed between the inner conductor c2 and the transmission line and the primary radiating element 25 serves to produce several advantages. Since one element of each bay is insulated by a capacitor it is possible to make resistance checks without disconnecting the radiating elements. The series capacitor also provides positive reactance elements in the direct driving impedance so that the antenna element is operated at a frequency several megacycles above resonance. Operation at a frequency above resonance stabilizes the horizontal field intensity pattern.

By reference to Figure 3 it will be seen that the primary radiating element :38 i provided with a telescopic portion 419 which has an end cap 5|. The outer extremity of the larger conductor 48 is connected by a conductive strip 52 to the mounting plate l8 by means of a cap screw 53. From this it becomes apparent that each of the fixed extremities of the radiating elements is connected to the mounting element is which is in electrical conductivity with the outer conductor 55 of the coaxial transmission line section 43.

The details of the construction of the lower bay are apparent from Figure 4. The mounting plate 2| carries a stud 54% held in position by a suitable cap screw 55. The stud 54 carries the larger conductor or tube 56 which supports a telescopic portion 51 having its end sealed by a cap 58. The outer extremity of the primary conductor 56 of the radiating element is connected by a conductive strip 59 to the binding post 61 carried by the insulator 62. The insulator 62 is supported by a plate 63 secured in position by suitable cap screw 84 which engages threaded holes in the mounting plate 2!. The mounting plate 2| adjacent the insulator 62 is provided with a suitable opening for receiving a portion of the junction box 65 which is retained in position by suitable fastening means as by silver between the junction box 65 and the mounting or support plate 63 of the insulator 62 to provide a gas tight seal.

lhe insulator 62 which has a hollow portion is provided with a central conductor 61 having a hollow extremity for receiving the banana plug end 68 of a conductive conductor 69. The conductive conductor Bil extends through an insulated bushing H which is fitted into an aperture H in the inner conductor 13 of the coaxial transmission line section. The conductive conductor $9 is connected to a conductor rod M which is supported in coaxial relation to the inner conductor 13 by means of a plurality of insulating beads 15. The conductive rod 14 together with the inner conductor 13 of the coaxial transmission line forms a capacitor which thereby insulates the radiating element from the transmission line so that resistance measurement may be made without disconnecting the antenna elements.

The inner conductor 13 is mounted in coaxial relation with respect to the outer conductor 16 by means of a plurality of insulating beads or supports Tl. Because of the junction box 65 the outer conductor has a portion cut away to receive this junction box. The junction box at is provided with suitable grooves which are filled with silver solder 18 so as to provide a gas tight connection between the junction box and the several portions of the outer conductor Hi. The outer conductor 16 at either end is provided with mounting flanges F9 for connection to cooperating sections of the coaxial transmission line. The upper and lower bays are spaced one Wave length apart and hence sufficient coaxial conductor is employed to provide this spacing with the frequency at which the antenna is to operate.

In order to match the impedance of the an tenna to the impedance of the coaxial conductor which interconnects the lower bay with the transmitter, the inner conductor 13 is provided with a sleeve 8|. The sleeve 8! is a quarter wave length long. The impedance of the present antenna construction is high as compared to other constructions since this construction involves the folded dipole principle. The use of this principle minimizes unbalanced currents. Furthermore, the higher impedance makes it possible to feed only one half of the system with energy. Hence, as previously described, one of the radiating elements has one end at an intermediate point thereon connected to the outer conductor of the transmission line. It will also be noted that in antennas of this type, where the effective operating length is about one quarter wave length, the highest voltage appears at the outer extremity or the open end. Therefore, it is desirable to keep these open ends as far apart as possible for all frequency values. Due to the truncated v construction, these desirable requirements are met and there has been provided an excellent safety factor. Thus, any tendency toward flash over is minimized.

In the development of the present antenna, it was found that the present construction avoids some of the disadvantages occuring in assymmetrical antenna structures. It has been found that the present antenna provides a power gain of 1.6 to 1.8 and a favorable standing wave ratio of 1.2 to 1.4 throughout the frequency range of 88 to 108 megacycles.

Referring to Figure 5 of the drawing, there is shown a typical horizontal field intensity pattern by the curve A in relation to the physical orientation of the radiating elements. Measured radiation patterns show that the greatest deviation from circularity at any frequency is 1.5 decibels.

may be made without disconnecting the radia- 'Ihese resistance measurements tion elements. test the insulation resistance of the feed line.

An antenna constructed in accordance with the present invention is tuned at the factory by adjusting the telescopic members radiating elements of each bay. 'Ihese members are then soldered in place to hold the tuning. If, at any time, the frequency of operation of the transmitter is to be changed, the antenna may be readily tuned by unsoldering the telescopic members, ad usting them accordingly and resoldering them in position.

To permit movement of the base Id of Figures 1, 2 and 3 relative to the H beam ll, the base it is held in position by two pairs of straps i9. Each strap terminates in an apertured portion bent at right angles to the body thereof so that suitable bolts or cap screws can be inserted through the apertures. at one end of each strap engages the base it. At the other end of each strap the bolt or cat screw engages an anchor stud plate 22 which is welded at right angles to the flange of the H beam II. For convenience in manufacture and installation, it is preferable to have each bay supported in this manner, and this construction is of definite advantage where more than two bays are installed.

While a prelerred arrangement for single or multiple bay antennas has been described and shown in Figures '1 to 6, another arrangement may be employed. This other arrangement is shown schematically in Figure 7 and may be of advantage where an economical single bay antenna is desired. In such case the construction shown in Figures 2 and 3 would be modified by eliminating the conductors 26 and 52, and by mounting the radiating element #3, 19 on an insulator so that the end of the tube 48 adjacent the insulator would be connected to the binding post 21.

While for the purpose of illustrating and d..- scribing the present invention a particular embodiment has been illustrated in the drawings, it is to be understood that the invention is not to be limited thereby, since such variations in the construction and in the instrumentalities employed are contemplated as may be commensurate with the spirit and scope of the invention as defined in the accompanying claims.

What I desire to protect by United States Letters Patent is claimed as follows:

1. An asymmetric antenna system having a generally circular horizontal radiation pattern comprising a single transmission line connected to a plurality of bays, each bay comprising a pair of generally horizontal conductors arranged at an angle to each other to form a truncated V,

in the The bolt or cap screw ,7. the fixed end of. eachof. said". conductors. connected to one of said conductors. of said. transmission line, means connecting the other conductor of said transmission line to a. point intermediate the ends. of onezoi the other horizontal conductors, and a. capacitor interposed between said transmission line and said secondmentioned conductor of each. bay.

2. An asymmetric antenna system. hating av generally circular horizontal radiation pattern. comprising a single coeaxial transmission=v line. connected to. a plurality of. radiating bays, each bay comprisinga pair of. telescopic membersarranged at an angle to each other in a generally horizontal plane so as to form a; truncated. v, the fixed endof each of said conductors being connected to an outer conductor of said coaxial transmission line, means interconnecting the in-v ner conductor of said transmission line with a point intermediate the ends of one of said horizontal conductors. of each bay, a capacitor interposed between said inner conductor of said transmission line, and said latter mentioned conductor of each bay, andan impedance match-- ing section connected immediately beneath the lower main bay.

3. An antenna consisting of a plurality of bays connected to a. coaxial transmission line, each bay comprising a pair of generally horizontal linearly extending radiating elements mounted onaba'se to form atruncated V, one-of said elements being connected to theouter conductor oi ati'ansmission line and the other element being connected through a capacitor tothe inner 6011- ductor of the transmission line, said bays being spaced a wavelength apart, an H beam for supporting said bases, and means connecting said bases to said H beam for limited vertical movement relative thereto.

4. An antenna comprising a coaxial transmission line. a pair of generally horizontal radiat ing conductors mounted" upon a base, said conductors being arranged at an acute angle to each other to form a truncated V, thefixed end of one of said conductors being c-onnected to the outer conductor of said transmission line, and means connecting the inner conductor of trans mission line to saidother horizontal conductor.

5. An antenna comprising" a coaxial transmis-- sion line, a pair ofgenerally horizontal radiating conductors mounted upon a base, each of said. conductors being formed of two" telescopic members for adjustment-in accordancewith the frequency to be transmitted, said conductors being arranged at an angle to each other to form a-truncated V, the fixed end of one ofsaid-condoctors being connected to the outer conductor of said transmission line, and means connecting the: inner conductor of said transmission line to a point, intermediate the ends of said other horizontal conductor.

6. An antenna comprising a coaxial transmission line, a pair of generally horizontal radiating conductors mountedon a base, each of said conductors being formed of a fixed member and a movable member arranged telescopically for adjustment in accordance with the frequency to be transmitted, said conductors being arranged at an angle to each other to form a truncated V, the fixed end of one of said conductors being connected to the outer conductor of said transmission. line and the inner conductor of said transmission line being connected to a point intermediate the ends of said other horizontal'conductor adjacent theouter end of. the-fixed mem-- ber thereof.

7. An asymmetric antenna. having a. generally circular horizontal; radiation pattern compris ing a coaxial transmission line connected to a. plurality of bays, each bay comprising a pair ofv linearly extendinghorizontal radiating conductors arranged at an angle to each other to form a truncated v, each of said conductors being formed of two telescopic members for adjustment in accordance with the frequency to be transmitted, the fixed end of. each of said conductors being connected to the outer conductor of said transmission line, and capacitor means connecting the inner conductor of said transmission line to said other horizontal conductor intermediate the ends thereof adjacent the outer end of the fixed supporting section of said telescopic member.

MARVEL W. SCI-IELDORF.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,911,234 Meyer May 30, 1933 2,111,743 Blumlein Mar. 22, 1938 2,131,108 Lindenblad Sept. 27, 1938 2,175,363 Roberts Oct. 10, 1939 2,258,406 Carter Oct. 7, 1941 2,259,623 Fener Oct. 21, 1941 2,267,951 Roosenstein Dec. 30, 19.41 2,272,608. Hoffman Feb. 10, 1942 2,324,462 Leeds July 13, 1943 2,420,967 Moore May 20, 1947' 2,467,962 Caraway Apr. 19, 1949: 2,480,154 Masters Aug. 30, 1949 2,514,020 Wehner July 4,1950 2,516,500 Alford July 25,1950] OTHER REFERENCES Radio-Electronics, August 1949, page 68'.