US3249945A - Tunable whip antenna with combined loading coil and shock spring - Google Patents

Description

R. F. LEWIS TUNABLE WHIP ANTENNA WITH COMBINED LOADING May 3, 1966 3,249,945

COIL AND SHOCK SPRING 2 Sheets-Sheet 1 Filed July 5, 1962 INVENTOR. POBERTE LEW/s ATTORNEYS .and radiating element.

United States Patent Robert F. Lewis, Lincroft, N.J., assignor to Prodelin,

Inc., Hightstown, N.J., a corporation of New Jersey Filed July 5, 1962, Ser. No. 207,746 Claims. (Cl. .343749) My invention relates to antennas, and more particularly, to a highly sensitive mobile whip antenna and to a novel mounting arrangement for such antenna on an automobile. I

In mobile radio communication, the system is designed to operate at a predetermined frequency within a narrow frequency band. Two-way mobile systems generally use low power transmitters, and it is therefore desirable to optimize the sensitivity of the whip antenna.

In regard to the antenna mounting arrangements, whip antennas have utilized an insulator base usually porcelain to mechanically support the extending radiating element on the vehicle frame. The antenna is electrically coupled to the vehicle transmitter/receiver by a shielded lead-in cable in which the inner conductor is connected to the radiating element and'the shield is grounded to the vehicle frame.

When using the conventional type of whip antenna, foreign matter such as dirt, dust, water, films and the like tends to collect on the surface of the insulator base which over a period of time, reduces the antenna efiiciency. For example, the foreign matter causes stray low resistance paths to be established over the surface of the insulator between the radiator and the vehicle frame and occasionally causing direct shorting. The foreign matter also accumulates electrostatic charges which cause noise to deteriorate further the performance of the antenna.

The whip antenna is conventionally resiliently mounted by a coil spring attached between the base insulator When the radiating element hits an obstruction, the spring yields but the shock and zvibration are transferred to the insulator. Further, when the vehicle is subjected to severe vibration by riding over bumpy roads over an extended period of time, the base insulator often suffers breakdown caused by cracking or chipping. When the .whip is used on military vehicles which often travel over rugged terrain and under adverse weather conditions, the aforementioned problems are severely aggravated.

Accordingly, it is an object of this invent-ion to provide a whip antenna, adaptable for mounting on a vehicle, and which eliminates the use of an insulator base.

Another object of this invention is topr-c'vide a whip antenna mounted on a finite ground plane, through an electrical coil spring in series therewith, to provide an essentially resistive input impedance, and which resistance remains essentially constant over prolonged period of usage.

A further object of this invention is to provide a coil spring mounting for the whip antenna which acts as a series tuning coil; the coil being provided with an adjustable tap, the position of which determines the impedance presented to the lead-in cable.

Still another object of this invention is to provide a whip antenna having no base insulator and in which shocks and vibrations applied to the whip are transmitted directly to the vehicle frame.

Still another object of this invention is to provide a whip antenna having improved gain characteristics.

Yet another object oft-his invention is to provide a I "ice may be tuned to a preselected frequency without physically cutting the antenna.

Another object is to provide an efficient, reliable and rugged whip antenna.

Briefly, my invention comprises a whip element connected in series with a coil spring which is electrically connected to the vehicle frame. The bottom of the coil spring is secured to a metal base or block which is attached to the vehicle frame. The metal base or block is provided with an opening through which the inner conductor of the lead-in cable may pass. The length of the antenna is selected to exhibit a capacitive reactance. The coil spring is provided with a movable tap, electrically connected to the'lead-in, to cancel the capacitive reactance and to provide a matched impedance to the antenna.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawing, wherein:

FIGURE 1 is a partially exploded and a partial vertical sectional view through the center of the whip antenna showing the internal construction thereof;

FIGURE 2 is a sectional view showing the coaxial cable prepared for assembly; and

FIGURES 3 and 4 are various plots of VSWR-frequency for different mounting positions.

Referring now to FIGURE 1, there is shown a whip antena mounted on a vehicle 2 or finite ground plane. The radiating element '1 is firmly connected to the upper end of a coil spring 3 which provides the necessary fiexibility in mounting.

In accordance with a first aspect of the invention, a conductive base mount block 6 is used instead of the insulative base. threaded and screwed into a tapped hole in the block and locked thereto by a lock nut 5. The metal block is a strong and rugged member which supports the relatively large whip and transfers shocks directly to the vehicle frame. As will be seen, the metal block is in direct electrical and mechanical contact with the vehicle. frame 2. Connected to the base of the metal block is a conductive extension or stud 7 having an external thread 8 at its lower end and having an internal opening 9. A similar opening 10 extends centrally through the metal block. The vehicle frame is provided with an opening to receive the threaded extension 8 of the stud 7.

A tuning clip 11, shaped for slidable movement, substantially encircles a turn of the coil spring and is clamped to a turn at a selected point by tightening the bolt and nut 12. The clip may be easily moved by loosening the nut and applying asliding force. A flexible wire 14 connects the tuning clip to a terminal 13 insulatively mounted on the top surface of the base 6.

The coil spring is provided with a sufficient number of turns to as to cancel the capacitive reactance of the antenna, as will be described shortly.

A circumferential recess or groove 15 is formed in the outer surface of the base 6 and a boot 16 having corrugated or bellow-like sides surrounds the entire coil spring and the top surface of the conductfng block. When the antenna is subjected to shocks or vibrations, the boot and spring are shaped to flex, bend or twist. It is apparent that the antenna mounted on an automobile will be in a constant state of lateral movement relative to the base, thereby causing the boot to flex and inhibit the formation of a continuous stray path such as one of ice or snow.

The boot is a cup-shaped member preferably formed of an elastomeric material such as rubber, and preferably a low loss rubber but its resilience is achieve-d by the cor- The other end ofthe coil spring isrugated side Walls 17. It has a central aperture 41 in the top surface through which the whip passes. The lower side wall has an inwardly extending peripheral bead 40 which is snap fitted below the shoulder of the recess 15. The antenna fits tightly in the aperture 41 so as to prevent foreign matter from entering the interior of the boot and cause undesired stray impedance paths.

The whip antenna is mounted on the vehicle frame in conventional fashion. Preferably, tapered washers 2t) and 21 are mounted on opposite sides of the vehicle frame. There washers may be adjusted to position the antenna vertically on the vehicle. A rubber gasket 22 is tight fitted on the stud 7 and is positioned against the vehicle to serve as a seal. Lockwashers 23 and 24 employed in the conventional manner and jam nut 25 clamps the whip to the vehicle frame.

The cable is prepared conventionally by stripping off sections of the jacket, braiding and insulator respectively leaving parts of each exposed. The exposed braiding 3t) fits into the hollow of a clamp nut 26 (FEGURES 1 and 2). The tapered end of a clamp collar 27 also embraces the braiding as the nut 26 is tightened on the collar. The inner conductor 32 is slipped through insulator 28 which protects it from contacting the conductive block 6 and extension 7. The inner conductor 32 makes direct contact with the conductive element of terminal 13. The insulator along with the entire cable assembly is assembled into the whip. Locking nut 35 secures the entire assembly.

The novel whip antenna operates preferably in the frequency range of 144l64 or 144174 megacycles/second although by means of the series tuning coil higher selectivity and performance are obtained at narrow frequency bands within the specified range. It is possibie to obtain a VSWR (voltage standing Wave ratio) remarkably close to 1 by tuning out the capacitive reactive component of the antenna and carefully selecting a feed point on the series coil to match the impedance of the lead-in cable.

My antenna is cut to a length such that the base impedance is preferably capacitive and I have found that a rod length of A; waveiength (approximately 59" maximum) is satisfactory. I obtain considerable variation in VSWR by adjusting the tap point on the coil as shown in FIGURE 3.

FIGURE 3 shows the VSWR over a range of frequencies for preselected tap points of 0; 1; 1.5; 2.0; 2.5; 3.0; 3.25 and 3.50 turns. Of course, tap points intermediate the selected tap points yield better results for certain frequencies. Thus, for a frequency of approximately 151.5 me. a tap point at the end of the second turn would provide a substantially matching impedance with a 50 ohm lead-in cable.

The vehicle frame upon which the whip antenna is mounted acts as a finite ground plane. erably mounted centrally on the vehicle frame with respect to surrounding metal to yield better impedance characteristics and pattern configuration than one which is edge mounted on a vehicle. FIGURES 3 and 4 illustrate the results achieved with two different whip mountings. In FIGURE 3, the whip was mounted centrally while in FIGURE 4, the whip was mounted on the fender. It is seen that the VSWR increased over the upper range of frequencies in FIGURE 4. it can also be shown that the radiation patterns in the horizontal and vertical planes are more distorted when the Whip antenna is not centrally mounted.

The presence of a finite ground plane makes it ex- The Whip is preftil Cir

tremely difiicult to calculate the input impedance of a It is seen that the invention provides a simple means for tuning out the capacitive reactance to match the input impedance to that of the coaxial lead-in thereby producing improved standing wave ratios. For proper tuning it is only necessary to slide the boot up the whip enough to expose the coil and tuning clip and adjust the clip for peak radiation. Thus physical cutting of the whip in the field to obtain proper match to the transmission line is no longer required. When properly tuned, my antenna possess approximately a 2.5 db gain over a A Wavelength antenna in the horizontal plane when mounted to the same vehicle and in the same place While also eliminating the base insulators which frequently develop shock failures. The series tuning coil provides mechanical flexibility and is exceptionally rugged because all stresses are transferred to the vehicle via metal and not an insulator.

While the foregoing description sets forth the principles of the invention in connection with specific apparatus, it is to be understood that the description is made only by way of example and not as a limitation of the scope of the invention as set forth in the objects thereof and in the accompanying claims.

I claim:

1. A whip antenna for use on a vehicle frame comprising an extending radiating element,

rigid conductive means mounted on said frame in electrical contact therewith, said frame forming a finite ground plane for said antenna,

means for securely connecting said rigid conductive means to said frame so as tobe in both mechanical and electrical contact therewith,

said conductive means having an aperture extending therethrough to receive a lead-in conductor,

resilient helical means having a plurality of turns of a relatively stiif wire to resiliently support said radiating element and having a predetermined inductance,

one end of said helical means being rigidly attached to said radiating element andthe other end being rigidly and conductively secured to said conductive means,

means to couple said helical means to said lead-in conductor comprising movable conductive tap means coupled to said helical means,

and a protective boot surrounding said helical means to prevent the formation of stray electrical paths by foreign matter on said helical means and on said lead-in conductor,

' said tap means being movable along the turns of said helical means and providing a secure electrical connection thereto, whereby said conductive means transfers shock and vibrational forces to the vehicle frame and said coil serves the dual function of a resilient spring mount and an electrical inductance.

2. A whip antenna for use on a vehicle frame comprising an extending radiating element,

a conductive base mounted on said frame in substantially rigid and conductive relationship, said frame forming a finite ground plane for said antenna,

said conductive base having an aperture extending therethrough to receive a lead-in conductor,

resilient coil means having a plurality of turns of a relatively stiff wire to resiliently support said radiating element and having a predetermined inductance,

one end of said coil means being rigidly attached to said radiating element and the other end being rigidly and conductively secured to said conductive base,

means to couple said coil means to said lead-in conductor comprising movable conductive tap means coupled to said coil means,

said tap means being movable along the turns of said coil means and providing a secure electrical connection thereto,

said radiating element having a length so that the reactance at the base thereof is capacitive,

said coil means having an inductance value to compensate for said capacitive reactance,

a protective covering surrounding said coil means,

and a lead-in cable having a predetermined impedance,

said tap being positioned to obtain matching with said impedance, whereby said base transfers shock and vibrational forces to the vehicle frame and said coil serves the dual function of a resilient spring mount and an electrical inductance.

3. A whip antenna for use on a vehicle frame comprising an extending radiating element,

a conductive base adapted to be mounted to said frame in electrical contact therewith, said frame forming a finite ground plane for said antenna,

said conductive base having an aperture extending therethrough to receive a lead-in conductor,

resilient coil means having a plurality of turns of a relatively stiff wire to resiliently support said radiating element and having a predetermined inductance,

one end of said coil means being connected to said radiating element and the other end being rigidly and conductively secured to said conductive base,

means to couple said coil means to said lead-in conductor comprising movable conductive tap means coupled to said coil means,

said tap means being movable along the turns of said coil means to provide a variable inductive reactance,

means to secure the one wire of said lead-in conductor conductively to said base and to secure the other wire conductively to said tap means,

and a resilient protective cover surrounding said coil means and extending from said base to said radiating element,

the range of said inductive reactances being such as to compensate for capacitive reactance presented by said radiating element, whereby said base transfers shock and vibrational forces to the vehicle frame and said coil serves the dual function of a resilient spring mount and an electrical inductance.

4. A vehicular antenna system comprising a conductive vehicle frame, an extending radiating element,

a conductive base mounted directly on said frame, said frame forming a finite ground plane for said radiating element,

said conductive base having an aperture extending therethrough to receive a lead-in conductor,

resilient helical coil means having a plurality of turns of a relatively stiff wire to resiliently support said radiating element and having a predetermined inductance connected in series with said radiating element,

one end of said helical means being rigidly attached to said radiating element and the other end being rigidly and conductively secured to said conductive base, a protective resilient boot surrounding said helical means to prevent the formation of stray electrical paths by foreign matter on said helical means and on said lead-in conductor,

and means to couple said helical means to said lead-in conductor comprising a movable conductive tap means coupling selected turns to said lead-in conductor to match said lead-in conductor, whereby said base transfers shock and vibrational forces to the vehicle frame and said coil serves the dual function of a resilient spring mount and an electrical inductance.

5. A whip antenna for use on a metal vehicle frame comprising a radiating element,

a conductive base adapted to be mounted on said frame in electrical contact therewith, said frame forming a finite ground plane for said antenna,

a lead-in conductor extending through an opening in said base,

a coil having a predetermined number of turns of relatively stiff wire,

one end of said coil being connected to said radiating element and the other end being conductively secured to said base and a movable tap coupling selected turns of said coil to said lead-in conductor, whereby said base transfers shock and vibrational forces to the vehicle frame and said coil serves the dual function of a resilient spring mount and an electric inductance,

and a .slidable protective boot means covering said helical means and being anchored to said base, the sides of said boot being flexible, whereby said sides may follow the expansion of said helical means.

References Cited by the Examiner UNITED STATES PATENTS 2,498,350 2/1960 Walsh 343-900 HERMAN KARL SAALBA'CH, Primary Examiner.

Claims (1)

1. 2. A WHIP ANTENNA FOR USE ON A VEHICLE FRAME COMPRISING AN EXTENDING RADIATING ELEMENT, A CONDUCTIVE BASE MOUNTED ON SAID FRAME IN SUBSTANTIALLY RIGID AND CONDUCTIVE RELATIONSHIP, SAID FRAME FORMING A FINITE GROUND PLANE FOR SAID ANTENNA, SAID CONDUCTIVE BASE HAVING AN APERTURE EXTENDING THERETHROUGH TO RECEIVE A LEAD-IN CONDUCTOR, RESILIENT COIL MEANS HAVING A PLURALITY OF TURNS OF A RELATIVELY STIFF WIRE TO RESILIENTLY SUPPORT SAID RADIATING ELEMENT AND HAVING A PREDETERMINED INDUCTANCE, ONE END OF SAID COIL MEANS BEING RIGIDLY ATTACHED TO SAID RADIATING ELEMENT AND THE OTHER END BEING RIGIDLY AND CONDUCTIVELY SECURED TO SAID CONDUCTIVE BASE, MEANS TO COUPLE SAID COIL MEANS TO SAID LEAD-IN CONDUCTOR COMPRISING MOVABLE CONDUCTIVE TAP MEANS COUPLED TO SAID COIL MEANS,

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