US3482249A - Broadband tower antenna system - Google Patents

Description

Dec. 2, 1969 J. H. MULLANEY BROADBAND TOWER ANTENNA. SYSTEM 3 Sheets-Sheet 1 Filed Aug. 29, 1966 HGia FIGBD INVENTOR JOHN H. MULLANEY BY 4 ,c9'd Mi,

ATTORNEYS Dec. 2, 1969 J. H. MULLANEY BROADBAND TOWER ANTENNA SYSTEM 3 Sheets-Sheet 3 Filed m 29, 1966 INVENTOR JOHN H. MULLANEY FREQ-(KHZ) BY 0757 a za.

ATTORNEYS Dec. 2, 1969 J. H. MULLANEY 3, 4

BROADBAND TOWER ANTENNA SYSTEM Filed Aug. 29, 1966 3 Sheets-Sheet 5 FIGIO INVENTOR JOHN H. MULLANEY BY wfl a ATTORNEYS United States Patent 3,482,249 BROADBAND TOWER ANTENNA SYSTEM John H. Mullaney, Potomac, Md., assignor to Multronics,

Inc., Rockville, Md., a corporation of Maryland Filed Aug. 29, 1966, Ser. No. 575,613 Int. Cl. H01q 9/00 U.S. Cl. 343750 9 Claims ABSTRACT OF THE DISCLOSURE A top-loaded, series-fed, low frequency antenna system having a vertical radiator which is electrically short (20) in comparison to its operating wavelength, mounted on a base insulator and wherein the top loading comprises a plurality of staggered tuned drop wires terminated in the ground plane by respective adjustable capacitors.

This invention relates to broadband radio antenna systems and more particularly to a transmitting antenna system of the vertical tower type which has a relatively short length in comparison to the desired wavelength of operation.

When operating at low radio frequencies, electrically short vertical radiators are normally utilized due to the fact that quarter wavelength transmitting antennas for such frequencies become physically impractical and extremely expensive. For example, at an operating frequency of 100 kilocycles per second (kl-12.), a wavelength is 9,843 feet and therefore a quarter wavelength transmitting antenna would have a height of approximately 2500 feet above the ground. It is the practice, therefore, to employ antennas having electrical lengths in the order of 5 to 20, where 360 is defined as 1 wavelength. Acoordingnly, it is common practice to use antennas that have physical heights varying from 300 to 800 feet when operating at 100 kHz. A technique for overcoming the limitations of such electrically short radiators is the use of top-loading which effectively increases the electrical height of the antenna while leaving its physical height unchanged.

Top-loading can take the form of a large plate, either square or circular in shape, mounted on top of the antenna or it can take the form of an umbrella. Umbrella top-loading comprises the use of the three or more guy wires as loading elements. It is not uncommon to utilize as many as 12 guy wires as loading elements. These guy wires are electrically connected to the top of the tower and their length is fixed by the insertion of guy insulators at desired points in the guy wires. Adjusting the length of the loading wires and the angle at which they extend from the tower will control the radiation resistance and the reactance of the antenna which in turn directly affects the efliciency and the dynamic bandwidth of the antenna systern. Although the dynamic bandwidth of the antenna system is materially increased from that of the same antenna without top-loading, the bandwidth is not controllable, that is, a given bandwidth will be obtained for a given length and angle of top-loading guy wires in relation to the height of the tower. The only way in which the bandwidth can be increased therefore is to physically increase the height of the tower and/or top-loading or inserting a loss resistor in series with the antenna feedpoint in order to broadband the antenna. The latter expedient, however, results in reduced efficiency. One type of antenna system which discloses a means to control the bandwidth and efficiency for a short length (height) vertical antenna is disclosed in U.S. Ser. No. 502,852, now Patent No. 3,386,098, entitled Electrically Short Tower Antenna With Controlled Base Impedance, filed Oct. 23,

3,482,249 Patented Dec. 2, 1969 C&

1965, in the name of John H. Mullaney and assigned to the assignee of the present invention.

It is an object of the present invention therefore to provide an improved short length antenna system in which the radiation efiiciency, bandwidth and driving point impedance are controllable;

It is another object of the present invention to provide a series-fed, top-loaded antenna system suitable for wide band, low frequency communications;

It is yet another object of the present invention to provide an improved monopole antenna system utilizing multiple tuning techniques for increasing the efiiciency and bandwidth of electrically short vertical antennas.

Briefly, the subject invention contemplates top-loading a series-fed monopole antenna having a vertical radiating tower which is mounted on a base insulator with the toploading comprising a plurality of top-loading conductors connected in a predetermined configuration to the top of the antenna and terminated at their distal extremities by means of variable capacitors which are coupled to ground so that the top-loading conductors may be stagger tuned. In addition, the present invention contemplates utilizing a fat monopole antenna having its base insulated from ground comprising a vertical tower and one or more fold conductors located adjacent the tower, running substantially parallel thereto, and being coupled not only at the top of the tower but also at its base. It is also desirable, in certain instances, that a second fold conductor be located adjacent the antenna having one end connected to the top of the antenna with the other end terminated in a variable capacitor which is connected to ground.

Other objects and advantages will become apparent after a study of the following specification when read in connection with the drawings wherein like reference numerals represent like components, and

FIGURE 1a and FIGURE lb is a schematic diagram and equivalent circuit, respectively, of a series-fed, toploaded monopole antenna typical of known prior art antenna systems;

FIGURE 2 is a perspective view of a first embodiment of the subject invention;

FIGURES 3a and 3b disclose a schematic diagram and equivalent circuit diagram, respectively, of the embodiment shown in FIGURE 1;

FIGURE 4 is a schematic diagram of a second or simplified embodiment of the subject invention;

FIGURE 5 is a schematic diagram of a third embodiment of the subject invention;

FIGURE 6 is a schematic diagram of a fourth embodiment of the subject invention;

FIGURE 7 is a schematic diagram of a fifth embodiment of the subject invention;

FIGURE 8 is a graphical illustration of the electrical characteristics exhibited by selected embodiments of the subject invention; and

FIGURES 9 and 10 are illustrative of Smith chart representations of the electrical characteristics of the selected embodiments.

Referring now more particularly to the drawings, FIGURE 1a discloses in schematic form what is considered to be known prior art apparatus. It is comprised of a vertical radiator or tower 12 mounted on a base insulator 14 which has one side or terminal returned to a point of reference potential 16 hereinafter referred to as ground. A feed point 18 is coupled to the opposite side of the base insulator 14 which is common to the base of the tower 12. A radio transmitter source, not shown, is adapted to be coupled to the feed point 18. This configuration is known to those skilled in the art as a seriesfed antenna. At the top of the tower 12 is a plurality of guy wires 20 extending outwardly therefrom at a predetermined angle and terminated in guy insulators 22. The

top-loading of the antenna system thus established is referred to as umbrella top-loading. Electrically, the antenna system shown in FIGURE 1a can be represented as a capacitor 24 in series with a resistance 26 as shown in FIGURE lb where R is the radiation resistance exhibited by the antenna and the capacitor 24 is representative of the distributed capacity of the tower and top-loading elements.

In such an antenna, the resistance value R is characteristically very low while the capacitive reactance X is very large. The Q of an antenna (a figure of merit indicative of the sharpness of the resonance curve) is approximately equal to the base reactance (X) divided by the radiation resistance (R). An antenna of the type shown schematically in FIGURE la therefore has relatively narrow bandwidth and is extremely selective. The effect of the top-loading is to increase the radiation resistance while decreasing the reactance, i.e., increasing the capacitance of the structure. This lowers the Q which in turn increases the bandwidth.

It should be noted for purpose of explanation that the bandwidth is generally defined as the frequency difference between the upper and lower frequency points where 50% of the applied power is delivered to the antenna. Another way of expressing it is by saying the bandwidth is commonly considered to be the frequency band within which the power is equal to or greater than one-half the power radiated at resonance. The bandwidth of an antenna depends upon its input impedance and the rate with which its reactance and resistance changes with frequency. There are two types of bandwidths to be considered. One is the static bandwidth which is the antenna reactance divided by two times the antenna radiation resistance and is the bandwidth which would be obtained if the antenna system had no losses. The other is the loaded or dynamic bandwidth which is the net bandwidth after consideration is given to total antenna system losses and the reactance used to resonate the antenna. The loaded or dynamic bandwidth considerations can only be obtained by considering the coupling components used to resonate the antenna. For example, considering the antenna shown schematically in FIGURE 1a, to resonate this antenna, it would be necessary to cancel out the capacitive reactance. This requires an inductive reactance equal in magnitude to the capactive reactance coupled to the circuit. Furthermore, the bandwidth is not controllable, that is, a given bandwidth will be obtained for a given length of top-loading, angle of top-loading guy wire, and height of the radiator.

Considering the present invention in detail, attention is called to FIGURE 2 which discloses a first embodiment of the invention and comprises a vertical radiator 12 in the form of a radiating tower mounted on and insulated from ground by means of a base insulator 14. A radio transmitter 28 is coupled across the base insulator 14 by means of the feed point 18 which is coupled to the common connection between base of the tower 12 and the insulator 14. The transmitter 28 then acts to feed the vertical radiator 12 at its base 15. A fold conductor 30, hereinafter referred to as a told, is located adjacent the vertical radiator 12 such that it is situated substantially parallel thereto being held from the tower by means of stand-off insulators 32 a small distance, for example, from three to five feet. One end of the fold 30 is attached or connected to the top of the radiator tower 12 at the terminal 34 while the opposite end of the fold is directly connected to the base 15 of the tower 12 which is also common to the insulator 14 and feed point 18. It would appear that such a connection would act to place ashort circuit across the vertical radiating tower 12 but, in effect, it acts to increase the effective diameter of the vertical radiator so as to provide what might be termed a fat monopole antenna. Three top-loading conductors 36 are connected to the top of the tower by means of the terminal 34 and are equally spaced in azimuth about the vertical axis of the tower such that they extend outwardly at a predetermined angle therefrom towards respective masts 38 and guy wire insulators 40. The top loading conductors 36 are not terminated at the insulators 40 as in prior art systems but extend downwardly substantially parallel to the masts 38 in the form of drop wires 36'. These drop wires 36' are also spaced away from their respective masts 38 by means of stand-off insulators 42. The distal ends of the drop wires 36' are terminated in variable capacitors 44.

What is accomplished by the present invention is to transform an antenna system which has an inherently high Q such as shown in FIGURE 1a into an antenna system which has a low Q. Utilizing the configuration as shown in FIGURE 2 wherein a fat monopole antenna is achieved, the ratio of the length to the diameter (L/D) of the antenna is decreased because in effect the fold acts to effectively enlarge the diameter of the vertical radiator. Keeping in mind the relationship that Q=X/R, the characteristic of the fat monopole is one which has both a lower radiating resistance and a lower reactance; however, the decrease in resistance is not as significant as the decrease in reactance; therefore, the Q of the circuit is decreased. The effect of the top-loading elements effectively increases the electrical length or height of the vertical radiator which effectively increases the radiation resistance while reducing the reactance X even more, thus effectively lowering the Q of the circuit even further. As noted earlier, the effect of top-loading such as shown in FIGURE la is to add capacity to the top of the structure. In the subject invention, the capacitance as eX- hibited by the top-loading is further enhanced and made controllable by the efiect of the combination of the drop wires 36 and the variable capacitors 44, respectively. By means of the capacitors 44 the top-loading can be controlled at will. By selectively tuning the top-loading elements by means of capacitors 44, the radiation efficiency, the bandwidth, and the driving point impedance are controlled so as to optimize the desired operating characteristics of the antenna system. When desirable, the tuning can be obtained by means of the dro wire 36 alone, i.e., varying the length and spacing from the mast of each drop wire 36. It is contemplated that the preferable method of tuning the top-loading is to stagger tune the top-loading elements in order to provide a symmetrical bandwidth characteristic. Although stagger tuning is desirable, it is not absolutely necessary. These characteristics will be discussed in greater detail subequently with reference to FIGURES 8, 9 and 10.

FIGURE 3a is a schematic diagram of the antenna system of the embodiment shown in FIGURE 2. The height H of the vertical radiating structure or tower 12 is in the order of 520 (360 being a full wavelength). The fold 30 is connected both to the base and the top of the vertical radiator 12 and the antenna system is series-fed by means of the feed point 18 coupled to the common connection between the base insulator 14 and the base 15. The top-loading elements are schematically represented by three variable capacitors 46 which is meant to include the combined capacity of each top-loading condoctor 36, the drop wire 36' and the variable capacitor 44. Although three top-loading elements are shown, any number may be utilized when desirable. Increasing the number of top-loading elements merely decreases reactance while increasing the radiation resistance.

The equivalent electrical circuit of the embodiment shown in FIGURE 2 is shown in FIGURE 3b wherein the base insulator 14 is represented by a capacitance 48 while the resistance, inductance and capacitance of the vertical radiator or tower 12 is represented by reference characters 50, 52 and 54, respectively. The resistance and inductance of the top-loading elements, which is almost negligible, is represented by the resistance 56 and inductance 58. Connected in series therewith is the combined variable capacitance 46.

Basically, what the equivalent circuit shown in FIG- URE 3 illustrates is a network which can be selectively tuned to resonance by proper adjustment of the value of the capacitance 46 which is essentially determined by the value of the variable capacitors 44 shown in FIG- URE 2. As it is well known to those skilled in the art, the point at which maximum power is transmitted to the antenna system is at the point of resonance wherein the inductive reactance is cancelled by the capacitive reactance so that the entire circuit substantially acts as a lossy resistor which radiates electrical energy.

Referring now to FIGURE 4, there is illustrated a second embodiment of the subject invention which discloses the simplest configuration contemplated and schematically shows a series-fed monopole antenna which has its base insulated from ground. It is comprised of a vertical radiator or tower 12 devoid of a fold such as shown in FIG- URE 2 being fed froma transmitter, not shown, by means of feed point 18 which is coupled to the common connection between the base of the radiating antenna and the base insulator 14. The configuration is top-loadedby three folds, such as shown in FIGURE 2, having their distal ends terminated in variable capacitors, such as capacitors 44, with the combined capacities being illustrated schematically as the variable capacitors 46.

FIGURE 5 illustrates schematically a third embodiment of the subject invention which is similar to the embodiment shown in FIGURES 2 and 311. It is similar in all respects but additionally includes a variable inductance 50 coupled across the base insulator 14 to ground. The variable inductance is coupled to the common connection between the base of the vertical radiator 12 and the base insulator 14 which is common to the feed point 18 and the fold 30. The purpose of the variable inductance 50 is to provide still further control of the bandwidth and driving point impedance of the antenna system. The addition of the inductance 50 has an inherent limitation in that it introduces additional losses in the system which will in a slight manner reduce the radiation efiiciency; however, it is often desirable to sacrifice efliciency for further bandwidth control.

The embodiments of the present invention shown schematically in FIGURES 6 and 7 additionally include another fold 52 which is located adjacent the tower radiator 12 in a manner similar to the fold 30 shown in FIGURE 2. The additional fold 52, moreover, runs substantially parallel to the tower 12 and is connected to the radiator at the upper end thereof at terminal 34 but is terminated at its other or distal end in a variable capacitor 54 returned to ground.

FIGURE 6 illustrates a series-fed monopole antenna with adjustable top-loading elements and includes the additional fold S2 terminated in capacitor 54. With respect to FIGURE 7, there is disclosed a fat monopole antenna system which is series-fed comprising the vertical radiator 12 and a fold 30 coupled across the tower in addition to the second fold 52 which is connected in series with the capacitor 54 from the top of the radiator to ground. The purpose of the additional fold 52 and the adjustable capacitor 54 is to provide still further control. By adjusting the three termination capacitances 46 by means of the adjustable capacitors 44 forming a part thereof and adjusting the fold capacitor 54 a more precise bandwidth is obtained while at the same time providing an exact driving point resistance at the desired frequency of operation.

The antenna configurations disclosed by the subject invention have approximately an omnidirectional radiation pattern in the horizontal plane; however, they can be directionalized to have a minimum-to-maximum radiation pattern of approximately 6 db by introduction of stagger turning of the variable capacitors as mentioned above; thatis, by selectively adjusting the values of capacity in the folds, both the top-loading folds and the second folds when considering the embodiments shown in FIGURES 6 and 7.

While the subject invention has been described as a means of obtaining relatively wide bandwidth which is controllable as well as controlled driving point impedance and radiation efficiency, it is of interest to note the comparison of the band-width characteristics of the subject invention with that of the known prior art series-fed monopole antennas having top-loading as shown in FIG- URE 1a. In this regard, attention is directed to FIG- URES 8, 9 and 10. First, FIGURE 8 is a plot of the relative forward power vs. frequency for selected configurations taken from test data obtained in making comparative measurements of electrical characteristics at an operating frequency of 50 kHz. The curve a is representative of the characteristics of the series-fed monopole antenna with top-loading which is typical of the prior art. This curve indicates that such antenna system has relatively narrow bandwidth and high selectivity. Curve b is a characteristic curve of the antenna systems embodied by the subject disclosure in FIGURES 3a, 4, and 5. Noting that the bandwidth is the frequency difference where the radiation power falls off to 0.5, it can be seen that the bandwidth for the subject invention is approximately 20 kHz. whereas the prior art apparatus has a bandwidth of approximately 1 kHz. Curve 0 is illustrative of bandwidth characteristic which can be obtained utilizing the embodiments shown in FIGURES 6 and 7 whereby the bandwidth can be increased to 35 kHz.

FIGURES 8 and 9 are further illustrative of the dynamic bandwidth characteristics of the above mentioned configurations when plotted on a Smith chart which is a coordinate system of two orthogonal families of circles, corresponding to constant standing wave ratio and to constant electrical length, respectively, when superposed upon a rectangular coordinate system in which relative reactances are plotted as ordinates against relative resistance as abscissas. It can also be shown that when an antenna has a VSWR (voltage standing wave ratio) of 5.83:1, the available power delivered to the antenna is one-half (0.5). This being the case, when the impedance versus the frequency of an antenna is plotted on a Smith chart, the useful bandwidth is readily determined by considering the portion of the curve that falls within a 5.83:1 VSWR circle.

The experimental data acquired in actual tests made has been plotted on the Smith charts shown in FIG- URES 9 and 10 (FIGURES 1, 2, and 4 of disclosure) and more precisely indicate the bandwidth characteristics shown in FIGURE 8.

Referring now to FIGURE 9, curve a illustrates the 5.83 :1 VSWR circle. Curve b depicts the impedance characteristic of the prior art system shown in FIGURE 1a. It will be observed that at the frequency of 50 kHz. the impedance is purely resistive and therefore resonant. The curve is semicircular in shape and intersects the 5.83:1 VSWR circle in the vicinity of the frequency 49.41 and 50.27 kHz. This indicates that the system includes reactive components on each side of 50 kHz. and having but one point of resonance. Furthermore, the bandwidth is less than 2 kHz. wide, considering the intersection of the 5.83:1 VSWR circle. Curve c on the other hand is characteristic of the impedance of the subject invention and embodiments shown in FIGURES 3a, 4 and 5. Again it is resistive at 50 kHz.; however, the curve doubles back on itself and intersects the VSWR circle at 43.32 and 64.17 kHz. It has two other resonant points where the curve crosses the abscissa and exhibits a bandwidth of more than 20 kHz. This provides an improvement unobtainable with the prior art embodiment shown in FIGURE 1.

With respect to the Smith chart shown in FIGURE 10 curve d is indicative of the embodiments shown in FIG- URES 6 and 7 which exhibit a still wider bandwidth inasmuch as the curve folds back upon itself twice intersecting the 5.83:1 VSWR circle (curve a) at approximately 44.96 and 79.72 kHz. A bandwidth of approximately 35 kHz. is obtained.

The bandwidths obtainable by means of the present invention are far superior to those measured on a simple top-loaded, series-fed antenna of the same physical height. Inasmuch as the bandwidth also is a function of the L/ C ratio (the inherent inductance of the antenna structure and its top-loading wires plus that introduced by the guy termination and/or fold capacity to ground), it necessarily follows that the subject invention is an appreciable improvement inasmuch as there are no tunable or adjustable reactance elements in the prior art top-loaded structure for controlling bandwidth.

The present invention moreover provides the following advantages: It permits the heretofore very low resistance of the antenna to be adjusted to a value of 50 ohms or higher for ease of coupling; it increases the dynamic bandwidth for a given L/ C ratio to a controlled bandwidth suitable for wide band communications; it permits the drive point current to be lower for higher power inasmuch as the drive point resistance has been raised to a value, for example, 50 ohms, where heretofore it has been in the order of 2 ohms or less.

What has been described in the present invention is an improvement in series-fed, monopole antennas for obtaining a reasonable drive point resistance and bandwidth for electrically short vertical antennas. In addition, the present invention provides a means for using high power with low feed point current and voltage.

While there has been shown and described what is at present considered to be the preferred embodiments of the invention, other modifications will readily occur to those skilled in the art. It is not desired therefore that the invention be limited to the specific arrangements shown and described, but it is to be understood that all equivalents, alterations and modifications within the spirit and scope of the present invention are herein meant to be included.

I claim as my invention:

1. An antenna system in which the radiation efiiciency, bandwidth, and driving point impedance are controllable, comprising in combination; antenna base insulator means; a series-fed, top-loaded moopole antenna mounted on said antenna base insulator means and having a vertical radiator which is of a relatively short length (height) in comparison to the Wavelength of operation; a feed point coupled to the common connection of said base insulator means and said vertical radaitor; a plurality of top-loading conductors commonly coupled to the upper end of said vertical radiator and extending outwardly and towards the ground plane; a series reactance connected from the distal end of each of said plurality of top-loading conductors to ground for tuning said antenna system so as to obtain relatively wide bandwidth, high radiation efficiency and matched driving point impedance; and a fold mounted adjacent said vertical radiator in a substantially parallel configuration incluing means for coupling said fold to said upper end and said common connection of said base insulator means and said vertical radiator.

2. An antenna system as set forth in claim 1 and additionally including an inductance coupled across said base insulator means so as to have one terminal coupled to said feed point and the opposite terminal coupled to ground.

3. The antenna system as defined by claim 1 wherein the length of the vertical radiator is of the order of 5 20 and said series reactance comprises a capacitance of a selected value whereby said top-loading conductors are staggered tuned.

4. The invention as define-d by claim 1 and additionally including a series capacitive reactance connected from the distal end of each of said plurality of top-loading conductors to ground.

5. The invention as defined by claim 4 wherein said series capacitive reactance is of a selected value for staggered tuning said top-loading conductors.

6. The antenna system as claimed by claim 4 and additionally including a second fold mounted adjacent said vertical radiator in a substantially parallel configuration therewith and including means for coupling one end of said fold to said upper end of said vertical radiator and a series capacitance coupled to the opposite end of said second fold, with means terminating said capacitance to ground.

7. An antenna system in which the radiation efiiciency, bandwidth, and driving point impedance are controllable, comprising in combination; antenna base insulator means; a series-fed, top-loaded monopole antenna mounted on said antenna base insulator means and having a vertical radiator which is of a relatively short length (height) in comparison to the wavelength of operation; a feed point coupled to the common connection of said base insulator means and said vertical radiator; a plurality of top-loading conductors commonly coupled to the upper end of said vertical radiator and extending outwardly and towards the ground plane; a series reactance connected from the distal end of each of said plurality of top-loading conductors to ground for tuning said antenna system so as to obtain relatively wide bandwidth, high radiation efficiency and matched driving point impedance; and fold mounted adjacent said vertical radiator and running substantially parallel thereto and including means for coupling one end of said fold to said upper end of said vertical radiator and means for coupling the distal end of said fold to a capacitive reactance which has one terminal connected to ground.

8. An antenna system as set forth in claim 7 in which the length of said vertical radaitor is of the order of 5 20 and said series reactance comprises a capacitance of a selected value whereby said top-loading conductors are staggered tuned.

9. An antenna system in which the radiation efficiency, bandwidth, and driving point impedance are controllable, comprising in combination; antenna base insulator means; a series-fed, top-loaded monopole antenna mounted on said antenna base insulator means and having a vertical radiator which is of a relatively short length (height) in comparison to the wavelength of operation; a feed point coupled to the common connection of said base insulator means and said vertical radiator; a plurality of top-loading conductors commonly coupled to the upper end of said vertical radiator and extending outwardly and towards the ground plane; a series reactance connected from the distal end of each of said plurality of top-loading conductors to ground for tuning said antenna system so as to obtain relatively wide bandwidth, high radiation efiiciency and matched driving point impedance; 2. fold conductor located adjacent and substantially parallel to said vertical radiator and having one end thereof connected to the upper end of said vertical radiator, an adjustable capacitive reactance connected to the opposite end of said fold conductor and being terminated at ground; and wherein said series reactance connected from the distal end of each of said plurality of top-loading conductors comprises an adjustable capacitive reactance for selectively staggered tuning said antenna system.

References Cited UNITED STATES PATENTS 2,048,726 7/1936 Bohm 343-854 X 2,283,618 5/1942 Wilmotte 343-850 X 2,998,604 8/ 1961 Seeley 343-874 X ELI LIEBERMAN, Primary Examiner US. Cl. X.R.

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