US2218083A - Antenna system - Google Patents

Description

Oct. 15, 1940. w CARLSQN ET AL 2,218,083

ANTENNA SYSTEM Original Filed June 13, 1936 I 1 I I Gttorncg Patented Oct. 15, 1940 PATENT OFFICE ANTENNA SYSTEM WendellL. Carlson and William H. Conron, Haddonfield, N. J., assignor to Radio Corporation 7 of America, a corporationof Delaware Application June 13, 1936, Serial No. 85,022

. Renewed February 5, .1940

. I 10 Claims.

Our. invention relates to antennas and antenna coupling circuits. More specifically our invention relates to an antenna system which has low losses and is; especially eifective on mobile vehicles.

One of the problems which arises in a radio installation on an automobile, aircraftor the like, is the antenna system. Generally the physical dimensions of the antenna must be-limited. In some instances the antenna may be eifective as a collector of radio frequency energy, but the ratio of ignition noise to the received signal is unfavorable. In other installations, the noise to signal ratio is favorable but the antenna is an insufficient collector of the desired signals. These difficulties are especially troublesome in an automobile with a metal top.

This invention contemplates an antenna of short length but of suitable effective height for the reception of radio energy. Oneobject of our invention is found in a very effective antenna coupling system. Another object isto greatly reduce the losses in the means coupling the antenna to the'radio receiver. An additional object is to increase the ratio of signal pickup to noise pickup.

The antenna and coupling system of our invention is schematically illustrated in Figure 1,,

member I is connected to aninductor 5 which is,

grounded, and to a series capacitor 1'. The series capacitor is connected to an inductor 9 one terminal of which is grounded. The. inductors 5, 9 are connected in opposed magnetic relation; 1. e., the sense of direction of the windings is reversed. The two inductors 5, 9 are coupled to a third inductor H which may be grounded. The ungrounded terminal of the third inductor is connected to a wire l2 which is covered by a grounded shield l3.

ends. The terminal of the wire l2 remote'from the third or coupling'inductor II is connected in series to theprincipal inductor l5 of a tunable circuit I1. The tunable circuit iszcomprised of inductors ll; l5-and aivariable capacitor. IS. The.

The shield may be either single or double and grounded at one end or at both tunable circuit I1 is connected to the input of a radio receiver 21, which is not shown in detail as any tuned radio frequency or superheterodyne receiver may be used. I

The operation of the circuit is essentially as follows: The value of the inductance of the inductor 5 is chosen so that the circuit comprising inductor 5 and antenna capacity I will resonate preferably below the lowest frequency range of the tunable circuit IT. The series capacitor 1 is preferably about half the capacity of the antenna. The antenna capacity, and the series capacitor 1 are connected across the inductor 9 and resonate this circuit (I, l, 9) to a frequency preferably above the highest frequency of the tunable circuit H.

The third inductor II is composed of a relatively few turns, which may be turns common to either of the other inductors 5, 9. These few turns are shunted by the capacity of the shielded conductor 13. This combination H, l2, I3 is resonant at a frequency which is substantially higher than the highest frequency of response of the tunable circuit I! and therefore has a negligible detuning effect on the tunable circuit I1. Furthermore, since the inductor H is a small portion of the total inductance of the tunable circuit H, the losses across the coupling inductor II have a relatively small effect on the complete circuit 11.

The antenna circuits I, 5 and I, 'l, 9 being tuned beyond the extremes of the tunable circuit, and arranged in opposed inductive relationship, provide an antenna coupling system whose transfer or gain is substantially uniform at all frequencies within the tunable range. With the conventional automotive antenna installation, a long shielded lead-in is employed to prevent ignition pickup. Such lead-in represents a capacity, '75-150 micromicrofarads, which generally shunts the antenna, causes substantial losses, and lowers the voltage because of shunting capacity effects. The present antenna network (5, l, 9, II), is located within a grounded shielded container 23 which is preferably positioned as near the base of the antenna as possible. Because of the proximity of the antenna network to the antenna, the lead-in shunt capacity is extremely low and has a negligible eifect. It is important to minimize the lead-in capacity when using antennas having small effective capacities, which in the present arrangement may be of the order of ten micromicrofarads. Since the series capacitor 1 is made small, having about half the capacity of'the antenna, itsshuntingi effect is substantially minimized. In this arrangement the conductor l2 from the antenna circuit to the tunable circuit is shielded but this shielding has a negligible effect on the antenna network.

In the automobile antennas of the prior art, the ground, or metal top is adjacent the antenna over a relatively large part of its length and offers a correspondingly large common path for the disturbing ignition currents which are invariably induced in the metal body of the automobile. The usual grounding of the antenna circuit at the receiver also causes the return currents to flow back to the antenna through an appreciable length of chassis framework. Disturbing noise currents are impressed, along with the desired signal currents, on the radio receiver thereby establishing unfavorable signal to noise ratios. In the present antenna system the ground return through the primaries 5 and 9 is very short. Therefore the effective capacity currents from the antenna only flow through the ground, or metal top, in a region immediately adjacent the base of the antenna. This greatly limits the portion of the top which provides a common path for signal currents and disturbing ignition currents, thereby improving the signal to noise ratio.

One application of our invention is illustrated in Figure 2 in which a portion of the metal roof adjacent the windshield of an automobile is represented by reference numeral 25. A split bushing 21, 29 is fastened to the top 25. A hollow shaft 3| passes through the bushing. On one end of the shaft, and suitably insulated therefrom by bushings 33, 35, is fastened a rod-like antenna member 31.

A means for retaining the antenna member in vertical or horizontal position is indicated by the spring 4|, balls 43, and ball sockets (not shown). The spring is fixedly held, with respect to the shaft, by the collar 45.

The antenna member 31 is connected by a flexible lead 51 which extends, through the hollow shaft, to the pair of antenna circuits (see Figure l). The antenna circuits are located within the shielded container 23 which is positioned adjacent the base of the antenna. The balance of the system is substantially as described in connection with Figure 1. By way of example, the antenna member may be a streamlined rod about onequarter inch in diameter and from one foot to two feet long for reception of the present day broadcast band.

The foregoing application of our invention illustrates an antenna which is suitable for automobiles. With slight modifications the antenna may be adapted to aircraft, railway trains, mobile tanks, and the like. It should be understood that a plurality of antennas may be used. Likewise a pair of antennas may be connected; one to one inductor 5 and the other to the other inductor 9.

We claim as our invention:

1. An antenna system for a mobile vehicle including in combination, an antenna member mounted substantially perpendicular to the adjacent surface of said vehicle; a circuit tunable over a band of frequencies; a pair of resonant circuits connected in parallel between said antenna and the surface of said Vehicle at a point adjacent the base of said antenna, one of said circuits resonant to frequencies above the highest frequency in said band of frequencies and the other of said circuits resonant to frequencies below the lowest frequency in said band of frequencies; said circuits including a pair of inductors arranged in opposed magnetic relation; low

capacity means connecting said antenna member to said pair of circuits; an inductor coupled to said pair of inductors and serially included in said tunable circuit.

2. A noise reducing radio receiving system for a vehicle having a metal body in which an ignition system induces disturbing currents, including in combination a substantially vertical antenna whose associated ground represents a relatively small area of said body; a radio receiver including a circuit tunable over a range of radio frequencies, a pair of circuits connected in parallel, positioned and grounded adjacent said antenna and fixedly tuned one to a frequency above and one below said range; a low capacity connection from said antenna to said circuit; means mutually coupled to said pair of circuits, and means connecting said mutual coupling means to said tunable circuit.

3. In an interference reducing antenna system, the combination of a low capacity antenna mounted substantially perpendicular to an adjacent grounded metal surface, a tunable receiver input circuit, a pair of resonant circuits connected in parallel between said antenna and the portion of said grounded surface adjacent said antenna, the first of said resonant circuits being resonated by its distributed capacity and the antenna capacity to a frequency below the lowest frequency covered by said receiver input circuit, the second of said resonant circuits being resonated by a series connected capacitor to a frequency above the highest frequency covered by said receiver input circuit, and an inductor coupled to said resonant circuits and serially included in said receiver input circuit whereby the shorting effect of a shielded lead on a low capacity antenna is prevented, and interference pick-up is reduced due to the substantial elimination of the common path in said metal surface for interference and signal currents.

4. In an interference reducing antenna system. for use on a vehicle having a metal body in which disturbing noise currents are induced, the combination including an antenna arranged to have a minimum capacity to said metal body, a tunable input circuit located in a radio receiver at a substantial distance from said antenna, and means coupling said antenna and said input circuit comprising a pair of shielded resonant circuits positioned in close proximity to said antenna and connected in parallel between said antenna and a point on said metal body adjacent the base of said antenna, and a low impedance inductor coupled to said resonant circuits and included in the circuit of said tunable input circuit by means of a shielded conductor.

5. In combination, a receiving circuit tunable over a frequency range, an antenna remotely positioned with respect to said receiving circuit, a low impedance line coupling said antenna to said circuit, a coupling transformer located adjacent said antenna and including a primary and a secondary winding, said secondary winding being included in series in said tunable circuit and said line.

6. An antenna system for a radio receiving circuit tunable over a band of frequencies, comprising an antenna, a low impedance shielded line coupling said antenna to said circuit for transferring signals thereto, a coupling unit located adjacent said antenna and including a primary inductor, said line including a secondary inductor as an element of said coupling unit, and

said tunable receiving circuit including an inductor connected in series with said secondary inductor.

7. An antenna system for a radio receiving system tunable over a band of frequencies, comprising an antenna, a low impedance transmission line connected between said antenna and said receiving circuit, a pair of resonant circuits connected between said antenna and said line, said circuits being tuned to widely separated frequencies, and said line including an inductor coupled to said resonant circuits and serially included in said tunable receiving circuit.

8. The invention as set forth in claim 5 wherein said secondary winding is a relatively small portion of the total inductance of said tunable receiving circuit with a consequently small loss effect upon said circuit.

9. The invention as set forth in claim 5 wherein said antenna, operated on a moving vehicle having a metal body, has an efiective capacity of the order of ten micromicrofarads to adjacent areas of said body acting as ground, and a negligibly small lead-in capacity associated therewith.

10. The invention as set forth in claim 9 wherein said line resonates with said secondary inductor to a frequency substantially above the highest frequency of response of said tunable receiving circuit with a consequent negligible detuning effect thereon.

WENDELL L. CARLSON. WILLIAM H. CONRON.

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