US2184940A - Aerial mounting - Google Patents

Description

Dec. 26, 1939. Q CORK 2,184,940

AERIAL MOUNTING Filed July 19, 1938 :f /8 i 25 e5; 2 m L 4 iffy/5 26 ,3 27* &

INVENTORS I EDWARD CECIL CORK AND BY 7fL/Offl L405 PAWSEY m ATTORNEY.

Patented Dec. 26, 1939 UNITED STATES AERIAL MOUNTING Edward Cecil Cork, London, and Joseph Lade Pawsey, Hillingdon, England,.assignors to Electric & Musical Industries Limited, Hayes,,Mid-' dlesex, England, a company of Great Britain Application July 19, 1938, Serial No. 219,972 In Great Britain July 27, 1937 Claims.

The present invention relates to mountings for aerials and is concerned with a method of attenuating waves propagated along the outside of a structure such as a metal mast used for sup- 5 porting an aerial.

In general, it is found that the presence of a metal mast on which a receivin-g aerial is supported causes energy to be conveyed from a source of interference near the base of the mast to the aerial located at the top of the mast and so tends to increase the amount of interference picked up by the aerial. The effect of this additional interference may be considered as a distortion of the directional diagram of the aerial due tothe presence of the mast in which currents are induced. Another efiect produced by the mast or similar metal structure adjacent the aerial arises from the varying distribution of currents therein which may take place intermittently owing to the swaying of the mast in the wind. The movement of the mast or other structure gives rise to interrupted currents creating noise or crackling in the receiver connected with the aerial. c

In a particular case in which a receiving aerial was placed on the same mast as a transmitting aerial, about thirty feet above the latter, the transmitting aerial was found to cause large currents to flow in the mast which induce in the receiving aerial large potentials which are normally filtered out by means of frequency selective filters. If, however, the currents set up in the mast by the transmitter are interrupted due to intermittent contacts arising from the motion of the mast, transients are set up which contain components of the frequency to be received on the receiving aerial. The unwanted voltages may be induced into the receiving aerial by transmission as a wave up the mast and from thence by transference to the aerial along the outside of the feeder and by re-radiation to the aerial.

In British Patent No. 438,506 and in the specification of our co-pending British application No. 17,436/37, methods of reducing the attenuation of waves along the outside of a feeder are described consisting in introducing auxiliary quarter wave length conductors which are applied to the outer sheath of the feeder and produce attenuation for currents flowing along the outside of the shell which would otherwise exert a disturbing influence on the aerial. Such methods are applicable to any metal structure in the neighborhood of the aerial provided that the diameter of the cross section of the structure is small compared with a wave length.

The present invention is concerned with the problem of attenuating waves along the outside of a structure in which the diameter of the cross section is large compared with a wave length, for example a-mast.

In a typical case, the mast may consist of a number of main longitudinal members with a number of cross bracings which serve to section.- alize the structure. I The base of such a mast may comprise four legs from each of which a main member and subsidiary members diverge. In general, the cross section of the mast will taper though this may not be truein all cases, and the top section may be considered to be efliectively a uniform cylinder which is, even at its highest point, still of cross section large with respect to a wave length.

According to the present invention the interference produced by a metal or similar electrically conductive structure located in the vicinity of an aerial is reduced by providing low impedance paths to'earth at a number of suitable points on the structure so increasing the attenuation of the wave propagated along the structure. These low impedance paths may consist of half wave conductors contacted at the centre or alternatively quarter wave length bars may be used extending outwards from the mast or other structure or bent as described, for example, in British Patent No; 438,506, refered to above; These half or quarter wave conductors would be attached in rings to the principal girders of the mast, which constitute in general the principal current carrying members. Preferably, attachments are made at the junctions of'radiating members of the mast;

:A more complete understanding of the present invention will be had by reference to the following detailed description which is accompanied by a drawing in which Figure 1 shows diagrammatically a typical form of mast treated in accordance with the invention, while Figures 2 and 3 show alternative methods of treating the top of a supporting mast in accordance with the invention. The mast shown in Figure 1 comprises base or foundation members Ill, usually of concrete from which extend the inclined main supporting members I! which are braced by struts l2 and connected in their upper regions by horizontal members l3.

The upper part of the mast It comprises a system of vertical members I5 linked by horizontal members I6.

Suitable galleries or crows-nests maybe provided at I! and I8, and the mast may carry a system of aerials (not shown) arranged around the upper part M. The mast may be constructed of steel.

It will be seenthat the part M of the mast may be considered to be a conducting cylinder of very low impedance and in consequence it is necessary to provide a correspondingly lower impedance for effectively earthing the mast. This low impedance may be obtained by providing rings of radial half wave elements 20 connected at their middle to the outside of the mast, at the top and bottom respectively of the section, each of the rings forming what may be termed a low impedance skirt. If desired, the rings may be attached at suitable intervals, preferably of quarter wave length, up the mast. In addition, centre connected half-wave length .bars may be provided at the bottom of the mast, as indicated at 2|, attached to the junctions of main girders of the mast.

It has been found that a skirt of quarter wave or centre connected half-wave length bars at the top of a steel structure, is most effective and that the system of radiating bars may be replaced by a disc of wire netting of similar dimensions laid across the top of the mast.

Two possible methods of treating the top of a mast in accordance with the invention are illustrated diagrammatically in Figures 2 and 3 of the accompanying drawing. In the arrangement in Figure 2 of the drawing the top of the mast I4 is shown as being surrounded by a cylinder 22 of quarter wave length bars which may extend outwardly from the handrail or edge 23 of the top of the mast and then be bent downwardly to form a skirt 24. The top of the mast is suitably covered with a wire netting or similar shield 25. The receiving aerial 26 which is shielded by the structure described is represented as a dipole carried on a suitable support or feeder 21 which extends axially above the mast. In the modified arrangement shown in Figure 3 the top of the mast 23 has been extended by providing an annular system of quarter wave length conductors 28 each bent through two right angles as shown, or of substantially inverted U shape, toprovide a handrail.

The application of the invention is not limited to the cases described or shown, but may be extended to include the more effective earthing of large masses of metal and the attenuation of waves along large extended electrically conductive structures.

We claim:

1. In combination with an antenna supporting mast having a transverse dimension which is large compared to the operating wave length of said antenna, said mast being formed of a number of main longitudinal conductive members and a number of cross-bracing conductive members of means providing a low impedance path to earth connected at junctions of said longitudinal mem bers and cross-bracing members.

2. A conductive supporting mast for a transmitting antenna formed of the number of main longitudinal members and a number of crossbracing members, said mast also carrying a short wave receiving antenna, said mast having a transverse dimension which is large compared to the operating wave length of said receiving antenna and means for preventing disturbances in said receiving. antenna due to variations in conductivity in the joints between said structural members comprising a plurality of conductors each having a length equal to a quarter of the length of the operating wave of said receiving antenna and connected at one end to said mast at said joints.

3. A conductive supporting mast for a transmitting antenna formed of the number of main longitudinal members and a number of crossbracing members, said mast also carrying a short wave receiving antenna, said mast having a transverse dimension which is large compared to the operating wave length of said receiving antenna and means for preventing disturbances in said receiving antenna due to variations in conductivity in the joints between said structural members comprising a plurality of conductors each having a length equal to half the length of the operating wave connected at their midpoints to said mast at said joints.

4.. In combination, a receiving antenna and a transmitting antenna, an electrically conductive structure in the vicinity of said antennas, said structure containing a number of portions of intermittently varying conductivity and means for eliminating disturbances in said receiving antenna due to said intermittent variations in conductivity comprising a plurality of conductors each having a length equal to a quarter of the length of the operating wave of said antennas and connected at one end to said structure at said points of intermittent varying conductivity.

5. In combinatioma receiving antenna and a transmitting antenna, an electrically conductive structure in the vicinity of said antennas, said structure containing a number of portions of intermittently varying conductivity and means for eliminating disturbances in said receiving an tenna due to said intermittent variatons in conductivity comprising a plurality of conductors each having a length equal to half the length of the operating wave of said antennas and connected at their midpoints to said structure at said points of intermittently varying conductivity.

EDWARD CECIL CORK. JOSEPH LADE P AWSEY.

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