Rod Antenna, Particularly for Mobile FM Signal Transducing Applications

The present invention relates to a rod antenna, and more particularly to a rod antenna for mobile applications, for example for use with an automobile radio or the like, and especially adapted for receiving FM signals although, of course, the antenna can be used equally for radiation of signals in a band for which it is designed.

Rod antennae which are made of insulating material, for example fiberglass or the like, with a wire wound thereon, are known--see U.S. Pat. No. 2,938,210. The wire is wound over essentially the entire length of the rod. The end of the winding at the upper end of the rod, that is, remote from its socket which is adapted to be secured to a vehicle, for example, is carried through a hole within the rod and electrically connected to a sleeve connector, such as a coaxial connector, fitting against the lower end of the rod. The rod as well as the winding are covered by an insulating material, for example of plastic or the like.

Rod antennae of this type intended for use in the frequency modulation (FM) band should have a length of about 1 meter. For mobile use, as an automobile radio antenna, for example, it is desired to have as short as possible an antenna, for example a length of only about 40 cm. Reducing the antenna length to that extent causes a loss in signal, however, which in the FM band may be between about 14 to 20 dB with respect to a simple rod antenna of about 95 cm length, that is, approaching 1 m.

It is an object to provide an antenna which can have, physically, a short length but which, nevertheless has good signal gain and signal reception characteristics.

Briefly, an insulating rod is provided which is wound with a first winding, wound, spirally progressing, in a first direction, along a longitudinal portion of the rod; a second winding, and wound, spirally progressing, on an adjacent second portion of the rod, is applied over the rod, the direction of winding of the second winding being opposite to that of the first winding. The two windings are connected at adjacent ends, that is, along an intermediate point of the rod. The sum of the length of the the rod covered by wires forming the winding i.e. the first and the second portions should be about 1/8 of the median wave length of the signal band of operation of the antenna.

The antenna has the advantage that, in comparison to the frequency spectra and the directional diagram of a simple rod antenna of standard length, the transduced signal will be approximately of the same signal strength. It is possible, usually, to eliminate a preamplifier which previously was used with a foreshortened antenna, so that the additional distortion or noise introduced by the preamplifier can be eliminated.

In accordance with a feature of the invention, the extended length of the first and second winding, each, should be approximately 1/4 of the median wave length of the frequency band. The antenna, then, will provide for optimal reception.

FIG. 1 is a side view of the rod antenna wound on a conical insulating rod with a bifilar winding;

FIG. 2 is a longitudinal sectional view of an antenna surrounded by a shrunk-on tube, and illustrating a connecting terminal; and

FIG. 3 is a part-sectional view of the antenna in which the lower end of the insulating rod is connected to an electrical terminal by an elastic intermediate element.

The rod antenna is made of an insulating longitudinal rod 10 which tapers upwardly. A typical material is fiberglass. The length l of the rod which corresponds to almost the entire length thereof is subdivided into two portions a and b, on which respective windings 12 and 11 are wound, spirally progressing, in opposite directions of winding. The windings 11, 12 are formed by bifilar winding of a single wire, and adhered to the fiberglass rod by an adhesive applied thereover, a lacquer, paint or the like. The length of the wires 11, 12 wound over the respective portions a and b, each, are about the same, so that the covered portions a and b will be different due to the reduction in cross section of the rod as it approaches its upper end.

The length l of the rod antenna corresponds approximately to 1/8 of the median wave length. For the entertainment FM band, this means about 40 cm. The wire of the windings 11, 12 each has a straight line extent of about 1/4 of the median wave length.

The lower winding portion 12 terminates from the lower end 13 of the rod by a distance c. The lower end 13 is not covered with winding and is provided to permit attachment of an insulating fitting 14 (FIG. 2) thereto with an inner, conductive sleeve-like extension 15. The free end 16 of the lower winding 12 is electrically connected with the conductive sleeve 15. The sleeve 15 is connected in well known manner to a connecting pin 22. The upper end of the other winding 11 is left unconnected and merely mechanically adhered to the insulating rod 10. The windings 11, 12 and the remaining portion of the insulating rod 10, as well as the attachment sleeve 15 of the connecting element 14, are commonly surrounded by an elastic protective coating 17. A suitable element is a heat-shrinkable plastic tube, which can be slipped over the rod and the sleeve 15 and then exposed to a suitable temperature, whereupon it will shrink to provide a tight fit. Heat shrink tubing is commercially available.

The connecting element 14 is used to secure the rod antenna to a connecting fitting--not shown--and standard in the industry, for example attached to the body of an automotive vehicle, for further connection to a cable and then to an automobile radio receiver.

In some installations, it is desired to provide for the ability of the rod to deflect substantially from an essentially vertical position, for example in order to permit the rod to bend when attached to a vehicle which is run through an automatic automobile washing and scrubbing station. Antennae which can deflect substantially are less subject to damage. The embodiment of FIG. 3 illustrates a connecting element which has a highly resilient intermediate portion 20. The resilient intermediate portion 20 includes a spiral spring 21, one end of which is connected to the pin-like end 22' at the free end of the terminal portion 14, and the other end of which is held in a pin-like upwardly extending projection 23 of a further connecting element 24. The spring 21 thus forms a resilient mechanical connection for the rod and additionally functions as a ground loading coil. Coils connected to an end of a whip or rod antenna are well known. The function of such a coil element as a ground loading coil is inherent in the structure. The connecting element 14, spring 21, and the further connecting element 24 are surrounded by a highly elastic cover sleeve, for example by a polyvinylchloride (PVC) sleeve, applied thereover, for example, by a friction or press or shrink fit.

The connecting element 24 has a connection pin 26 which is free from the elastic sleeve 25 and which forms the electrical connection to the antenna. A protective cap 27 is applied to the tip end of the antenna.

Various changes and modifications may be made within the scope of the inventive concept.