US2397261A - Antenna system - Google Patents

Description

March 26, 1946. L. J'. HEAToN-ARMSTRQNG 2,397,261

ANTENNA SYSTEM Filed Jan. 8, 1943 F/GS.

I I. I

Patented Mar. 26, i946 ANTENNA SYSTEM Louis John Heaton-Armstrong, London W. C. 2, England, assignor to Standard Telephones and Cables Limited, London, England, a British ml) any Application January 8, 1943, Serial No. 471,703 In Great Britain January 12, 1942 4 Claims. (Cl. Z50-11) The present invention relates to antenna systems for blind approach arrangements for vehicles, for instance aircraft or ships, and of the kind in which the radiation distribution is oscillated about the approach path by means of auxiliary antennae adapted to be rendered effective as reilectors alternately so as to produce characteristic signals on each side of said approach path and to create along said path a zone or beam of equi-signal strength or continuous dash. The edges of this approach path, zone or beam of equi-signal strength are determined by the points where an indication of the characteristic signals first becomes apparent, and the width of the beam depends upon the inclination of the radiation diagrams to each other at the point oi intersection. Such a well known system comprises a central directly fed antenna and two auxiliary parasitically excited antennae placed one on each side of said central antenna.

When the auxiliary antennae are keyed, that is are alternatively made to function as reflector, three neld distributions are produced, namely, that for the energised antenna operating alone and two field distributions when the auxiliary `antennae function respectively as reflector. The

field strength of all three distributions must be equal in the equi-signal zone otherwise keying clicks will be heard in a receiving apparatus in that zone which is at right angles to the line joining the antennae. It is desirable for practical reasons in the landing of aircraft, for example that this beam or zone be made narrow.

The object of the present invention is to provide arrangements by which a narrower beam may be obtained than is possible with a single antenna and two reectors. The antenna system according to the present invention comprises a plurality of antennae directly fed with exciting current and arranged so as to produce in the direction of the reflectors a field of greater strength than along said approach path and two auxiliary antennae arranged in alignment at right angles to said approach path one on each side thereof.

In the simplest and preferred embodiment of the invention, two antennae are fed with inphase current and the two reflectors are similar and placed at equal distances from the fed antennae.

The invention will be better understood from the following description taken in conjunction with the accompanying drawing in which:

Fig. 1 shows diagrammatically a side view of the known arrangement of antennae;

Fig. 2 is a plan view of Fig. 1;

Fig. 3 is a plan view showing the simplest and preferred arrangement of antennae according to the present invention;

Fig. 4 shows the radiation distribution in the system of Fig. 1 and Fig. 5 is a Vvector diagram referred to in the description.

Referring to the drawing, Fig. l shows a wellknown system in which 0 is an antenna directly excited by the transmitter, I and 2 are parasitically excited reflectors spaced at a distance d from il. The reflectors are opened at the centre by means of relays, onlyV one reflector being connected at a time and there is a period while the relays are changing over when neither relay is closed. When the reflectors are keyed there are three field patterns produced, the iirst L, Fig. 4, for the antenna El and reiiector I operating, the second N for the antenna 0 alone, and the third M for antenna IJ and reector 2. The field strength from all three patterns must be equal on the equi-signal line AA which is the line at right angles to that joining the antennae. The patterns L, M and N must all pass through the same point on AA in order to eliminate keying clicks, and L and M must pass through the same point on AA to produce equal strength signals on AA.

The equations for the system .will now be considered Let Vo=Voltage applied to antenna Il Io=Current in antenna Il I1=Current in reflector I Zoo=Self impedance of antenna 0 Zoi==Mutual impedance between antenna 0 and reiiector I Zn=Self impedance of reiiector I R11=Resistance of reflector I 0m=Angle of mutual impedance between antenna Il and reflector I TJZL 1020. .1,- Zu Zn ne.. T) Fig. 5 shows the currents vectorially. It is assumed that the antenna is fed from a varies with the value of d, having a minimum. l Y.

value for a particular value of d, other factors remaining constant. V

In order to reduce this minimum beam width,

the magnitude of the vector I1 should be increased.

This may be done in the following way, according to one embodiment of the present invention, reference being directed to Fig. 3.

Instead of one antenna 0, as in F'ig. 1, two antennae 01 and 02 are used in alignment with the equi-signal line and spaced apart a distance di and fed with in-phase exciting currents. The resultant eld radiated from 0i and 02 along the equi-signal line will with'suitable distance di be' less than that radiated towards the reiiectors I and 2. Hence the ratio of the field radiated from the reflector to that radiated from the antennae '01 and 02 on the equi-signal line may be made any Value required by varying di, assuming d2 is xed.

It is found that a minimum beam width occurs for a specific value of the ratio Field from antenna 01 and 02 Field from reflector 1 and also for a specic value of di.

Since the ratio can also be varied byvarying e d1, it follows that the beam width can be adjusted to the minimum or other desired width by adjustment of the distance betweenV the two excited antennae 01 and 02. It is also found that there is a minimum beam width for a specificv distance do, that is the distance between a reflector and one of the fed antennae or di.V Thus for any given distance d1, the distance dz can be adjusted to give a minimum width beam.

Thus the distances d1 and d2 may be adjusted so that key clicks are eliminated and to give a minimum beam width. In one practical example a beam width of approximately 1.3`5 can be` obtained with a spacing do=.3757\ where x is the working wavelength. d1 is then approximately .362. and d2 is .66N The loss of forward radiation is approximately 4.4 db as compared withv rthe system shown yin Fig. 1. The minimum beam width with the system shown in Fig. 1 when 11:35h is approximately 2.4".

It will be understood by those skilled in the art that if the distances between the antennae and transmitting' antennae.

reectors are maintained constant adjustments of the point of intersection of the radiation distributions and the slope of the distribution diagrams at the point of intersection may be made by adjusting the electrical length of tuning of the reectors.

Furthermore whilstthe embodiment described has only two fed antennae it will be understood that any other number may be used and fed with suitably phased currents.

What is claimed is:

1. A directive antenna system for an aircraft approach system comprising a pair of reflector antennae arranged one on each side of a transmitting antenna structure and in alignment at right angles to the approach path, means for directly feeding said transmitting antenna structure with exciting current, means for alternately keying said reiiector antennae, said transmitting antenna structure comprising a plurality of antennae arranged to vproduce a field ofN greater strength in the directionof lthe reflector'antennae than along the lapproach path, the spacing of the reflector antennae from the transmitting antenmeans for directlyexciting said transmitting ari-- tennae with high frequency currents in the same phase, a pair of reflector antennae arranged one on each side of the said transmittingantennae and with a lineA connecting ythem at'right angles to the approach path','the spacing of the reflector antennae from the transmitting antennae Abeing adjusted so as to produceca minimum radiation beam width, and meansl for alternately keying said'reector antennae. 3;An antenna system as claimed'in claim"2 wherein the reiiector antennae are similar to each other and placed at equal distances from the 4.' An antenna system as claimedv in claim 2 wherein'v the distance between the two said transmitting antennae kis so adjustedthatthe iield strength at a `point alongj-the'approach path due to these antennae alone is' equal' to the eld strength at the Vsame point due .to the combined fields of the transmitting antennae anda reflector antenna.`

Louis JOHN naA'ronfaaMs'raorG.

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