US2489290A - Direction finder - Google Patents

Description

Nov. I29, 1949 L. J. HEAToNARMsYTRoNG 2,489,290

Filed Nov. 15, 1945 i DIRECTION FINDER 2 Shee'ts-Sheet l sam/wm ro @EL-wf@ www@ d20 S/V 9 /A/ A OUT Zo BWM@ ATTO R N EY Nov. 29, 1949 L. J. HEAToN-ARMSTRONG 2,489,290

DIRECTION FINDER 2 Sheets-Sheet 2 Filed Nov. l5. 1945 INVENTOR OU/5 f HEHTON-HRMS/PO/VG ATTORNEY Patented Nov. 29, 1949 DIRECTION FINDER ApplicationV November 15, 1945, Serial No. 628,925

in Great Britain November 28, 1944 2 Ciaims. (Cl. 343-121) The present invention relates to radio direction nders and more particularly to antenna systems therefor comprising one or more pairs of spaced antennae, the antennae of a pair being connected by transmission lines of equal lengths and like characteristics to a radio receiver or other device for obtaining the desired indication and anondirectional antenna also coupled to said radio receiver or other device-by a transmission line.

It is essen-tiai, in such systems-thatV the signal waves at the outputs-oi the transmission lines at the receiver-should always bear a constant phase relationship. When however, the operating frequency ischanged, the phase relationship ofthe signal waves at` the outputs of the transmission lines is'changed on account of the variation with frequency of the diiierence in electrical lengths of the transmission lines coupled respectively tothe antennaecf the pairs and to the non-directional or sense antenna.

An object of this invention is to provide a direction nder antenna system of the type speciied for operation over a wide bandof'frequencies and this object is attained in accordance with the present invention by inserting in at least the transmission lines conveying the signal waves from the pairs tc the receiver or'other device or in the transmission line conveying the signal from the non-directional antenna to the receiver or other device, means which automatically compensate tor the divergence from a desired ccnstant phase relationship between the signal waves at: the outputs of the transmission lines at the receiver or other device.

In one embodiment of theinvention to be described in detaii hereinafter a phase advance network which advances the phase of the signal passed therethrough inversely as the frequency is connected in series in the transmission line from the ncn-directional antenna. Usually the transn-Lission lines used for couplingthe antennae to the receiver are the same except in physical network; then comprises the inversion y the same characteristic, namely frequency versus phase-change, as a transmission i 3 whose ieneth is the difference in lengths ben the respective iines connecting the pairs i nonmoir the transmission lines oi the pairs irectional antenna Substantially in outputs and non-L ",iine or 1r-netwcrk which has snb- 1 .otional antenna to the receiver the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 shows diagrammatically a direction find.- er antenna system of the type specied and a receiver to which the signal waves from the an.- tenna systems are applied;

Fig. 2 is a plan view ofthe system shown. in Fig. 1;

Fig. 3 is a vector diagram used in the description;

Figs. 4 and 5 show suitable forms` of phaseretard orV advance networks respectively;

Fig. 6 shows various graphical curves-usedv in the description.

Fig. 1 shows the known antenna arrangement for a direction finder. It comprises two dipole pairs I, 2 and 3, 4 arranged on the corners of a square of diagonal, for example not exceeding, .28 the operating wave length and a non-directional antenna 5 located at the centre of the system. The pairs are connected by half-wavelength transmission lines t and l' which are each crossed over at one end. The fifth antenna 5 is, as is well known, used for sense determination and provides a source of signal whose amplitude and phase are independent of the direction of arrival ofthe signal provided the latter lies in thehori.- Zonta] plane. The dipole pairs give an output which is dependent on the direction of arrival ofy the signal. The phase relationships of the signal outputs from the antennae will be clear from Fig. 3. In this gure vector E5 is the E. M; F. induced in the aerial 5 and produces a Voltage Vs at its base. Vector E1 is the E. M. F. in antenna l which is inadvance of E5 for the direction of signal shown by arrow B, Fig. 2. E2, the E. M; F. in antenna 2 lags on Es. The difference vector between E1 and E2, i. e. between E1 and Erreversed,` i. e. E12 is always at right angles to E5 and is brought into phase at the receiver by the quarter wavelength lines 6. Vi is the voltage on the line e at mid-point 9, it lags behind E1 by 90 due to the quarter wavelength off line 6. E2 isthe vector of E. M. F. in antenna 2, it is reversed at E12 due to the cross over of the line 6 and, after 90 retardation in the line 6, it appears at point ii as V2. The resultant of V1 and V2 is Vawhich is-.inphase withVs. It will be noted that the-*resultant VR varies in magnitude and sense with the direction of arrival of the signal.

In one direction nder as described fully in British patent specication No. 490,940 and shown schematically in Fig. 1, the outputs at 9 and IG are fed into balanced modulators Il and l2, which have different modulating frequencies e. g. 5 kc. and 6 kc. applied from I8 and I9 respectively so that sidebancls foi kc. and foi kc. are produced at the outputs of the modulators. The signal from the central aerial of frequency fn is fed to an amplifier I3. The three outputs are then combined at I4 and fed to further apparatus and eventually to an indicator such as a cathode ray oscilloscope to obtain a direct reading of the bearing. It is `essential to Y have the inputs to the modulators l I, I2 in phase or anti-phase with that to the amplifier I3 from the central antenna.

When a wide band of frequencies is used it is no longer possible to keep the lines S and I between the antennae and their mid-points one quarter wavelength long at all frequencies and the resultant voltages from the pairs and the voltage from the centre antenna will no longer be inphase.

It is well known that a transmission line section may be represented by the vr-network I5 shown in Figure 4, where is the electrical length ofthe line in degrees, 360 being one wavelength A. This network or artificial line is designed to produce a phase lag oi between the outp-ut and input volta-ges, and this phase lag will be very Ynearly proportional to the frequency applied provided that 0 is less than about 120 for any ofthe frequencies considered, This is shown in Figure 6, curve a. The series inductance is y'Zn sin 9 and the shunt capacities are each -iZo/tan 0/2, where Zo is the characteristic impedance of the line. The phase lag occurring in a natural transmission line is shown in curve b. With a similar 1r-network in which the inductance is replaced by carpacity of the Same value and the capacities by inductances of the same value as shown at l in Fig. 5, which is the inversion of the network I5, the output voltage has a phase advance on the input voltage phase, and furthermore the angle e of the advance varies inversely as the applied frequency as shown in Fig. 5, curve c. In accordance with this invention a network such as I6, Fig. 5, is connected in series with the central antenna, as shown for example in the input of ampli- Iier II (Fig. 1) which is usually present at the central antenna. Then the quadrature phase shift between the voltages at the antennae I, 2,

3 and d and the voltage at the central antenna 5, and usually produced by the quarter wave lengths of lines t, l is now produced by the lines Ii or 'I and the network I6 and the signal from the central antenna is maintained in a substantially constant in-phase relationship to the combined signals from the respective pairs of antennae over a band of frequencies.

'will make this clear.

Suppose it is required to utilise a frequency band from -100 inc/s. (megacycles per second).

The transmission lines E and l, measured between the antennae and mid-points 9, I0, are made of length equal to 67 electrical degrees at 100 mc./s. Then from Fig. 6 at 30 mc./s. the length will be twenty degrees. The inverted network IE, Fig. 5, to be put in series with the central antenna will be made 19 electrical degrees at 100 mc./s. and wil be 72 degrees at 30 mc./s., see Fig. 6, curve c. The total phase shift due to the An example 4 line and the circuit in the central antenna will then be approximately over the whole frequency band as shown in Fig. 6, curve d.

While the simplest and most practical embodiment of the invention has been described other more complicated embodiments will occur to those skilled in the art, for example, a phase compensation could be obtained equally well by utilising a phase retard network in the line from the nondirectional antenna and phase advance networks in lines fro-m the dipole pairs.

What is claimed is:

l. A radio direction finder antenna system for use over a broad band of operational frequencies comprising two pairs of spaced antennae, the antennae of the pairs being located at the opposite ends of the respective diagonals of a square and coupled to each other by auxiliary transmission lines of equal lengths and characteristics, and having a cross over in one half, a non-directional antenna located at the centre of said square and like means for coupling the mid-points of said auxiliary transmission lines to a radio receiver for obtaining the desired indication, and in series with the said non-directional antenna a phase advance network consisting of an inverted equivalent artificial line 1r-network of one half of one of said auxiliary transmission lines.

2. A radio direction finder antenna system for use over a broad band of operational frequencies comprising two pairs of spaced antennae, the antennae of the pairs being located at the opposite ends of respective diagonals and being coupled to a radio receiver for obtaining the desired indication by transmission lines of equal lengths and characteristics, a non-directional antenna coupled to said radio receiver by a transmission line, and in the line from said non-directional antenna, a phase-advance network which advances the phase of a signal wave passed therethrough by amounts substantially inversely as the frequency of said signal wave and which together with the differences in phase delays produced in the first mentioned transmission lines and the transmission line from the non-directional antenna with increase in operational frequencies, maintains the output signal waves from said first mentioned transmission lines and said line from said non-directional antenna, substantially in constant phase relationship, said transmission lines all having the same electrical characteristics and said phase advance network comprising the inverted equivalent artiiicial line vr-network of the difference in lengths of said transmission lines.

LOUIS JOHN HEATON-ARMSTRONG.

REFERENCE S CIT ED The following references are of record in the rile of this patent:

UNITED STATES PATENTS Number Name Date 2,003,932 Greig June 4, 1935 2,107,633 Hoeven Feb. 8, 1938 2,257,594 Chireix Sept. 30, 1941 2,321,478 Freeman et al. June 8, 1943

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