US 2748386 A
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May 29, 1956 w. J. PoLYDoRol-F ANTENNA SYSTEMS Filed Dec. 4, 1951 VENTOR .N1 h@ l I I lll l I l l u I1Ii|||| IV i l I Il |ll| Illll l I1 I Juli J- N illu w LAA v Arlt ANTENNA SYSTEMS Wladimir I. Polydorolf, Kensington, Md.
Application December 4, 1951, Serial No. 259,77)
Claims. (Cl. 343-787) This invention relates to antennas for radio apparatus either for radiating or for receiving electromagnetic energy. The invention is principally directed to antennas having dimensions related to the frequency of the energy to be transmitted or received.
An object of the invention is to provide improved antennas and a principal object is to decrease the size of antennas without reduction in its performance.-
The invention is based on the fact that wavelength of electromagnetic energy of a given frequency is reduced in the vicinity of ferromagnetic material from the wavelength in vacuo. According to Maxwells equation, the velocity of propagation of the energy,
where a and k are respectively the permeability and permittivity of the medium in which the energy is travelling. As the wavelength AEVO/f where f is the frequency, the wavelength of energy of a given frequency is decreased, owing to decreased velocity, when it strikes a medium of higher permeability.
Thus, a radio antenna according to the invention is made in part of ferromagnetic material to shorten in the vicinity of receiving or transmitting conducting element or elements, the wavelength of the radio waves to be received or transmitted. In this way, the dimensions of an antenna can be reduced since the wavelength in the vicinity of the antenna is reduced. For example, when the arms of a dipole antenna are covered in ferromagnetic material, the length of the antenna can be almost halved from the value normally required since the wavelength in the vicinity of the dipole is reduced proportionally.
The invention is applicable both to open and closed types of antennas particularly when the antennas of the latter type are considerably elongated.
The invention will be more readily understood by way ofexample from the following description of examples of antennas in accordance therewith, reference being made to the accompanying drawings, in which:
Figure l is a side view of a magnetically loaded dipole antenna,
Figure 2 is a side View, partly in section, of a magnetically loaded closed type antenna, and
Figure 3 is a sectional view of a modified form of the antenna of Figure 2 adapted for tuning.
Figure l illustrates a so called Hertzian Dipole of the linear type commonly employed for the reception and transmission of horizontally or vertically polarised high frequency radiation. Such a dipole operates most elliciently when its length is equal to an odd multiple of half wavelength of the radiations received or transmitted. Usually two arms each a quarter wavelength long are used, slightly spaced apart at their inner ends. The dipole is usually oriented in accordance with the polarization of the radiated energy; thus placed in a horizontal plane it is particularly suited for reception of horizontally polarized waves.
The theoretical optimum dimension of the dipole can United States Patent O 2,748,386 Patented May 29, 1956 ICC be easily verified by actual experiments; if the length of the dipole is varied the maximum signal occurs when the total length of the dipole is equal to the half wavelength ofthe incoming signal. Thus for a frequency of megacycles per second the required total length will be 7.7 feet to produce a maximum signal. The theoretical resistance of such a dipole is 73.2 ohms and this resistance sometimes referred to as the radiation resistance, is a measure of the efficiency of an antenna both as a radiator and as' a collector. With suitable high frequency bridge type instruments it can be measured fairly accurately.
The effective length of an antenna is reduced by the location on or about the conductors of the dipole a magnetic material. Conveniently, the material is a ferromagnetic material having such high frequency characteristics that the magnetic losses introduced can be tolerated.
i in Figure l a large number of beads 1 of high frequency ferromagnetic material are threaded onto the arms 2 of the dipole so that the arms are substantially covered. Each bead is in the form of a cylindrical tube about in diameter and has an axial bore 3 slightly larger than the diameter of the conducting arms 2 of the dipole preferably made of compressed carbonyl iron powder. For the same frequency of 60 mcs. the total length of the dipole is reduced to approximately half; to tune the antenna for maximum efficiency the branches are approximately 25 inches long. Further reduction in length results if the diameter of the beads 1 is increased so that the ferromagnetic material is better utilized. In this latter case it is possible to increase considerably the thickness of the arms 2 thus reducing the magnetic material by making the beads 1 thin-walled.
Actual measurements of the inductance of dipole arms 2 surrounded by the magnetic material 1 reveal a net gain in inductance of 4 times, thereby corroborating the theoretical results obtained from the above Maxwells formula in this application.
Further notatable gains can be realized with the employment of new high permeability magnetic materials, known as ferrites, which, however, are more suitable for lower frequency applications. Thus a vertical antenna can be considerably improved by loading its antenna wire with beads made of ferritematerials, the antenna showused may produce a very high inductance gain (effective height) while the characteristic impedance is greatly increased, thus facilitating the matching to the associate apparatus.
These magnetic ferrites since they possess high permeabilities depending on the frequencies at which they are used may produce a very high inductance grain (effective permeability) especially if utilized in closed type antennas. ln an elongated solenoid, the effective permeability nerf is related to the permeability ,u of the material in the following manner:
where d is demagnetization coefficient depending on the length-to-diameter-ratio (L/D) of the cylindrical core. Coefficient d, as calculated by Thompson may vary from 0.02 for L/D=8 to 0.0004 for L/D=l00. In case of other than round cross-sectional cores the effective D may be calculated from the area of the circle which is equivalent to the cross-sectional area of said other shaped cores. Thus, since ,u in the case of a ferrite is 500, for the two values of L/D given above, aeff=45 and 420 respectively. Therefore in elongated closed type antennas employing high permeability ferrites the ratio of length to diameter must be as great as possible and in any case not less than 8 in order to utilize the benefits of the high effective permeability.
Figure 2 shows a new construction of such an elongated antenna in which a plurality of elongated beads 1 are strung together on a supporting conducting rod 4 to form a cylinder` of great length around which an insulated wire 5 forms a Vcoil which substantially covers the entire length of the core beads 1. As will be clear from this figure, the coil of insulated wire is characterized by the turns thereof being spaced one from the other. Experience shows that an antenna of this form possesses an extremely high ability to pick up the signal or effective height, which ability depends on nerf, the number of turns which is made as large as possible, and on the axial length of the antenna; by increasing the number of turns and the axial length of volume of space from which to pick up the radiations is increased. Such an antenna may be termed a coil antenna. This vterm is used to distinguish the structures of the present invention from the usual loop antenna. In the latter the electromotive force is usually derived from the center of the loop, while with the antenna of the present invention the entire space occupied by the coil antenna contributes to the sensitivity thereof. In a practical form such an antenna may have its diameter from 1/2" to 2' and even 4" with the length correspondingr to from 8 to l0() diameters. The hollowing of the core-beads saves a considerable amount of magnetic material without materially affecting the performance. It is evident that in very small size antennas the hollow cores may be produced as single pieces which can be made by extrusion of ferrites during their pre-fabrication.
As shown in Figure 3, lthe small antennas described above may be modified to have further advantages. A thin-walled tubing 6 made from ferrite of lower-permeability is wound with a number of turns 5a of insulated wire to correspond to an inductance which with its own so-called distributed capacity or with an external capacitor is made resonant to a high frequency of a certain band of frequencies. A cylindrical core 7 of higher permeability magnetic material is slidably arranged inside the tubing to increase thereby the effective permeability and to lower the frequency at which the circuit will resonate. The extreme in` position of the slidable core 7 thus corresponds to the lowest frequency of the band of frequency and by varying the position of the core 7 the coil-antenna may be directly tuned. The movement of the core 7 may be synchronized with the movement of other tuning members in a radio apparatus, as well as with the tuning indicator by coupling the core 7 to the other tuning members by means of the coupling 8, for example.
It is possible to reverse the action of the self-.tuned antenna of Figure 3 by employing a fixed cylindrical mag-` netic core in the centre ofthe winding 5a and to slide a ferrite tube to fill the gap between the core and the windmg.
If it is desired to cover a frequency range of which the highest and lowest frequencies are in the ratio of 3:1 for example, the total possible inductance variation of the tuned antenna must be the square of that ratio or 9:1. This can be accomplished by the employment of thinwalled tubing (or central stationary core member) producing an effective permeability of say 4. Then dimensions and permeability of the movable core is so chosen as to produce an increase of effective permeability of nine times so thatl the ultimate 10W frequency permeability will become 4 9=36.
The above described magnetically loaded antenna possesses high sensitivity in spite of the greatly reduced dimensions. In particular the tunable antenna of Figure 3 exhibits the advantage in that when receiving low frequency (longer wavelength) signals the eiciency does not drop but is amply compensated by the increase of eftective permeability.
What I claim is:
l. A coil antenna comprising an elongated ferromagnetic element having its length to effective diameter ratio not less than about 8 and not more than about 100 and said element having an effective permeability of not less than about 4 and not more than about 420, an axially associated conductive member, and a conductive winding surrounding but insulated from said ferromagnetic element the turns of said winding being spaced between themselves and spread over substantially the entire length of said element thereby materially to increase the electromagnetic radiation pick-up properties of said antenna in the space occupied by said antenna.
2. A coil antenna as defined in claim l wherein said elongated ferromagnetic element is in the form of a cylinder.
3. A coil antenna as defined in claim l wherein said ferromagnetic element is a hollow cylinder.
4. A coil antenna as dened in claim 3 wherein the conductive winding is connected in series with said axial conductive element.
5. A coil antenna as defined in claim 3 wherein lsaid hollow cylindrical element is comprised of a plurality of elongated beads.
6. A coil antenna as defined in claim 1 wherein said conductive winding is insulated from said ferromagnetic element by a hollow tube.
7. A coil antenna as dened in claim 6 wherein said ferromagnetic element is movable with respect to said winding.
8. A coil antenna as defined in claim l wherein said ferromagnetic element is made of high-permeability ferrite material.
9. A coil antenna as defined in claim 1 wherein said ferromagnetic element is made of compressed carbonyl iron powder.
10. A coil antenna as defined in claim 6 wherein said hollow tube is made of high-permeability ferrite material.
References Cited in the iile of this patent UNITED STATES PATENTS 1,710,085 Cooper Apr. 23, 1929 2,311,364 Buschbeck et al. Feb. 16, 1943 2,335,969 Schaper Dec. 7, 1943 2,438,680 Polydoroff Mar. 30, 1948 FOREIGN PATENTS 430,548 Great Britain June 20, 1935 592,763 Great Britain Sept. 29, 1947 641,575 France Apr. 2l, 1928 OTHER REFERENCES Polydorolf et al.: Effective Permeability of High Frequncy Iron Cores, Radio, Nov. 1945, pages 38-41 an 70.