Phase directional antenna array and phased ring combiner for radio direction finding

A novel Direction Finding system is described, using a small aperture circular array of vertical dipoles, the outputs of which are rotated in phase by angles corresponding to their angular location on the array circle. A phased ring combiner is used to add the dipole pair outputs, giving two resultant waveforms, one advanced in phase and the other retarded by the same angle .theta., where .theta. is the relative bearing angle of the incoming signal. The phasing elements are made independent of the signal frequency either by the use of all-pass networks or by prior conversion to a fixed intermediate frequency. DF processing can be performed by a digital meter which computes the phase difference between the two resultant waveforms. In addition, an orthogonal DF display is available through a .SIGMA./.DELTA. conversion.

BACKGROUND OF THE INVENTION 
This invention relates to direction finding systems. 
When designing HF direction finding (DF) systems for shipborne or mobile 
applications, the prime restriction is the antenna size which must be 
necessarily small as compared with the received wavelength. However, a 
narrow aperture DF system has certain inherent drawbacks: 
(1) Field strength sensitivity is poor, since only a small volume of the 
radiated space energy can be sampled. 
(2) Reflections from nearby conducting bodies, particularly those at 
resonance, produce local distortions of the electromagnetic field which 
may extend over a radius of several wavelengths. A narrow aperture antenna 
can only measure the average field vector over its own base. Hence bearing 
errors will be shown depending on the frequency and the relative positions 
of the re-radiating bodies with respect to the antenna. 
(3) Near-field components of the re-radiated field may be picked up by the 
antenna elements through direct electrostatic or electromagnetic coupling, 
the results combining with the far field effects of item (2) above. 
(4) In contrast to the local re-radiations with errors which are constant 
for a given site, there are time-variable errors due to ionospheric 
reflections. The combined field vectors of the ground wave and the 
sky-wave produce wavefront corrugations which may be drifting slowly 
across the base of the antenna. Bearing angle displayed by a narrow 
aperture system will show periodic oscillations about its mean value. 
(5) Polarization tilts of the electromagnetic wave array may also be a 
source of errors if the antenna elements are allowed to pick up the 
horizontal component of the field. 
ANTENNA CONSIDERATIONS 
Shipborne HF/DF antennas must be symmetrical around the axis of the mast. 
Traditionally, they have been constructed as crossed loops which provided 
adequate sensitivity, but suffered from errors due to horizontally 
polarized components of the field making it impractical to take bearings 
on skywaves, elevated targets and any other re-radiation effects causing 
tilts of the wavefront plane. Spaced-loop arrays, while immune to the 
polarization tilts, show a pronounced lack of sensitivity at the low end 
of the spectrum since their effective height varies proportionally with 
the square of frequency. 
Within the dimensional constraints of the antenna base this leaves a choice 
of pairs of vertical dipoles or Adcock elements which have the advantage 
of rejecting the horizontal component of the field and yet retain a 
sensitivity comparable to that of the crossed loops. Diametrically opposed 
pairs are preferred for balancing-out of mast-induced currents, the array 
consisting of 2N dipoles or N pairs equally spaced around the circle. N 
can be any number greater than 2. In practice the number of dipoles will 
be limited by spacing errors on one hand and by cost and inter-element 
coupling on the other. An eight-element array is a practical compromise 
for the shipborne LF/HF bands. 
In a Watson-Watt configuration, such an array acts as a sine/cosine 
resolver of the vertically polarized component of the field. The ensuing 
output has to be processed by a twin-channel receiver with closely matched 
amplitude and phase characteristics. Signal strength of the DF channels 
varies with the bearing angle for the two DF receiver channels, passing 
through a zero on one, while reaching a maximum on the other. For weak 
signals the signal to noise ratio is quite different for the two channels, 
making it harder to process the bearing angle from the amplitude ratio of 
the two channels. In addition, a sense signal is required to be amplified 
by a third, phase-matched, receiver channel further adding to the 
complexity and cost of the system. 
SUMMARY OF THE INVENTION 
The impact of limitations of the narrow aperture is minimized in the 
present invention which makes the best use of the available parameters in 
the critical areas, achieving what is believed to be a cost effective 
compromise. The guidelines leading to this design can be summarized as 
follows: 
(a) Construct an antenna array to utilize the maximum volume available. 
(b) Select antenna elements to respond to the vertical polarization 
component of the field while rejecting the horizontal component. 
(c) Collect all the available energy from all the antenna elements on a 
fulltime basis. 
(d) Process the data continuously to give an instantaneous bearing readout 
(minimum one cycle of the intermediate frequency). 
(e) Provide time integration, weighted or otherwise, to take care of the 
high noise and fading conditions. 
This description is limited to details of the novel elements or concepts. A 
brief mention is made of those parts of the DF system which follow 
conventional design. 
THE PHASE-DIRECTIONAL ANTENNA ARRAY 
In the present invention, the outputs of the successive dipole pairs are 
shifted in phase so as to add cumulatively at the output terminals of the 
phasing networks. All the field energy picked up by the antenna array is 
utilized, regardless of the direction of arrival of the signal. This type 
of array is phase-directional, translating the bearing angle into an 
electrical phase angle, positive or negative, displayed between the two 
outputs of the phasing network. The magnitude of the phase angle is 
measured to represent the bearing while sense is obtained from the 
relative sign of the phase. 
Thus, in accordance with a broad aspect of the invention, there is provided 
a radio direction finding system for determining the relative bearing 
angle .theta. of an incoming signal comprising a circular array of an even 
number of vertical dipoles arranged in diametrically opposed pairs equally 
spaced apart around the array, each dipole pair having an output feeding 
two branches, one branch electrically advancing the phase of a signal from 
each dipole pair by an amount equivalent to the angular displacement in 
one direction of each dipole pair with respect to a reference direction 
established by a reference dipole pair and the other branch retarding the 
phase of a signal from each dipole pair by an amount equivalent to said 
angular displacement, means for combining all the phase advanced signals 
to form a signal voltage V.sub.d which is retarded in phase by an angle 
.theta. corresponding to said relative bearing, means for combining all 
the phase retarded signals to form a signal voltage V.sub.s which is 
advanced in phase by said angle .theta., and means for measuring the phase 
difference (2.theta.) between V.sub.d and V.sub.s whereby .theta. is 
obtainable by dividing said phase difference by 2.

FIG. 1 shows an array of dipole pairs AE, BF, CG, etc. spaced by an angle 
.pi./N where N is an integer corresponding to the total number of dipole 
pairs. Let .theta. be the bearing angle of the field velocity vector with 
reference to the axis AE. Then the EMF induced in the pair AE is given by 
EQU V.sub.o =2V sin (A cos .theta.) sin .omega.t 
where 
V=Eh=peak EMF induced in each dipole 
A=.pi.d/.lambda.=angular phase shift of the incoming wave over half the 
array diameter 
.omega.=angular frequency of the incoming wave 
For the k-th dipole pair, rotated by the angle k.pi./N, the EMF becomes 
##EQU1## 
where k can have integral values from 0 to N-1. 
Now, let the output of each pair be shifted electrically by a phase angle 
of k.pi./N so that 
##EQU2## 
and the sum of all such phased outputs added together becomes 
##STR1## 
For a small aperture, a linear approximation may be taken and the 
expression simplifies to 
##EQU3## 
Supposing now the outputs of the dipole pairs are displaced electrically in 
the opposite direction, corresponding to a phase angle of -k.pi./N. Then 
the sum of such phased outputs for N dipole pairs around the circle 
becomes 
##EQU4## 
or, simplified for a small aperture 
EQU V.sub.d =NVA sin (.omega.t+.theta.) 
If both phase-shifted outputs are obtained simultaneously, a phase meter 
applied between V.sub.s and V.sub.d will show an angle of 2.theta. and the 
sense ambiguity can be resolved by considering the relative sign of the 
phase difference. 
PHASED RING COMBINER 
Each dipole pair of the phase-directional antenna array has to contribute 
to two outputs, V.sub.s and V.sub.d via appropriate phase shifting 
networks: one in advance and the other in retard with respect to the 
reference plane. This can be accomplished in a single network by cascading 
the phase-shifting elements with the dipole pairs feeding in at the 
consecutive junctions. 
For a given frequency of the incoming wave, the phasing network can take 
the form of a transmission line tapped at points corresponding to the 
required phase shifts. This arrangement is illustrated in FIG. 2 where 
four antenna pairs, A-E, B-F, C-G and D-H, spaced at 45.degree. around a 
circle, are connected to a transmission line ring 15 at intervals of 1/8th 
of a wavelength. The total length of the line 15 is one full wavelength 
and the line is terminated at either end in its characteristic impedance 
Zo. Signals from the individual dipole pairs are fed into the line 15 via 
identical amplifiers A which provide high output impedance so that 
reflections obtained along the line at each junction become negligible. 
The Antenna pair AE, corresponding to the reference plane is connected to 
the 180.degree. point on the line, equidistant from both outputs. A signal 
from the reference antenna reaches outputs V.sub.s and V.sub.d in-phase 
after a delay of 180.degree.. This is the reference phase. Signals from 
the next antenna pair (BF) will reach output V.sub.s after 225.degree. but 
output V.sub.d after only 135.degree.. Similarly signals from antenna pair 
CG will reach V.sub.s after 270.degree. and V.sub.d after 90.degree., etc. 
Thus, output V.sub.s combines all the antenna signals with phase delays 
corresponding to the physical shift angles of the respective antenna pairs 
while output V.sub.d combines the signals with the respective phase 
advances. 
Conditions of operation of the phase-directional antenna array are thereby 
satisfied. 
In a practical circuit the output line terminations Zo are made part of the 
phase measuring equipment. 
A narrow band of input frequencies can be accomodated in the above 
arrangement by replacing the transmission line sections with networks 
displaying constant phase over the frequency band in question. 
For wide band surveillance, the amplifiers A are preceded by identical 
frequency converters M (FIG. 3) supplied from a common variable local 
oscillator frequency and adjusted to be matched in phase. The frequency of 
the incoming signals is thus converted to a constant, intermediate 
frequency where the converter outputs can be combined in the fashion 
described above. 
Exact physical alignment of the antenna array is always difficult. It is 
convenient, therefore, to provide for an electrical adjustment of the 
reference or zero azimuth. FIG. 3 shows an intermediate frequency type 
Phased Ring Combiner fitted out with azimuth reference adjustment 
.phi..sub.1 and .phi..sub.2 ganged so that .phi..sub.1 +.phi..sub.2 
=constant, i.e., when .phi..sub.1 is increased, .phi..sub.2 is decreased 
by the same amount. The total phase shift around the ring means remains at 
360.degree. and the lengths of transmission line connecting to the 
networks .phi..sub.1 and .phi..sub.2 both have to be shortened by the 
maximal amount of swing of .phi..sub.1 or .phi..sub.2. 
Full .+-.180.degree. zero azimuth adjustment can be also provided by 
extending the ring perimeter to 2.lambda., and inserting two 0-180 
variable phase shifters, one before each terminal end of the ring. It will 
be noted that the ring perimeter must be an integral number of 
wavelengths. 
The theory of the small aperture phase-directional array and ring combiner 
applies also to a circular array of individual dipoles in place of the 
diametrical dipole pairs described above. However, there are practical 
reasons for preferring the paired elements arrangement: 
(i) There is a better assurance of balancing out horizontal components of 
the field before reaching the active elements of the circuit as by 
connecting dipole pairs to form Adcock antennas. 
(ii) The number of matched amplifier/mixer channels is reduced by half. 
(iii) The number of phasing sections is also reduced by half. 
DF PROCESSING AND DISPLAY 
Outputs V.sub.s and V.sub.d of the phased ring combiner have to be 
amplified in a matched pair of selective receivers, tuned to the ring 
frequency and provided with band pass filters to accept the wanted signal 
while eliminating external interference. Phase matching of the receiver 
channels is important because it directly influences the accuracy of the 
measured bearing. Either a channel commutation technique or a 
self-balancing network by sample injection may be employed to reduce the 
phase mismatch error. 
Alternatively, the voltages V.sub.s and V.sub.d may be used to modulate a 
single carrier, which can be then amplified in a conventional, single 
channel receiver and demodulated to recover the amplified components with 
the original phase shift. 
In either case, the amplified signals have to be applied to the input of a 
phase meter, which will measure the phase 2.theta. between them and divide 
it by 2 in order to obtain the DF bearing. Such a phase meter may 
conveniently be of digital type with a clock frequency designed to display 
the bearing angle to the desired accuracy. A time averaging control may be 
added to overcome the effects of fading. 
In addition to the digital bearing display, it may be convenient to present 
an analog, cathode ray tube display of the Watson-Watt type. For this 
purpose the amplified signals V.sub.s and V.sub.d are passed through a sum 
and difference network to give two orthogonal voltages V.sub.y and V.sub.x 
so that V.sub.y /V.sub.x =tan .theta.. Correct sense indication for the 
CRT display should be obtained from an omnidirectional voltage V.sub.o 
derived from the average of all the dipole element outputs. This can be 
applied directly to the grid of the cathode ray tube in order to blank out 
the undesirable half of the trace. Within the bandwidth constraints of the 
receiver, this analog display will show all the familiar patterns 
associated with the crossed Adcock DF system, giving the advantage of 
instantaneous visual recognition of noise background, modulation, 
interference and of multiple signals at closely spaced frequencies inside 
the band. 
Intelligence content of the signal may be derived by demodulating V.sub.s 
or V.sub.d, the amplitude of either being independent of the bearing angle 
.theta.. 
EXAMPLE OF A RADIO DF SYSTEM 
Application of a phase-directional array and ring combiner in a radio DF 
system can be illustrated typically in FIG. 4. 
In this example an eight element array 40 is shown connected to the phased 
ring combiner 41 which supplies outputs V.sub.s and V.sub.d at the first 
intermediate frequency to phase-matched twin channel receivers 42 and 43. 
The receiver outputs are fed in parallel to a phase counter 45 and to a 
sum and difference network 46. The phase counter 45 provides pulses for an 
LED type digital bearing display 50, while the sum and difference network 
46 carries out the conversion to a pair of orthogonal signals V.sub.y and 
V.sub.x for display on a cathode ray tube 52. Two additional outputs are 
taken from the receiver: from channel 3 (V.sub.o) to the Z-modulation of 
the CRT (sense signal), and from channel 1 (V.sub.d) to the detector 53, 
audio amplifier 54 and loud speaker 55. A common frequency synthesizer 60 
is used for supplying the last local oscillator frequency to the frequency 
converters of the ring combiner and the other local oscillator frequencies 
necessary for the twin-channel i.f. receiver. 
Other applications are conceivable, such as that using a single-channel 
receiver or any extensions of the system aimed at improved accuracy of the 
DF bearing measurement or at simplified surveillance facilities. 
APPLICATIONS AND ADVANTAGES 
The phase directional array is a narrow aperture system and as such is 
limited to applications where the array diameter is a fraction of the 
wavelength. Typical examples include LF through HF ranges for shipborne 
and for mobile DF. Axial symmetry of the array permits its location on a 
cylindrical mast which is essential for both of the above applications. 
The advantages of the present system in the above application can be 
divided as those due to the antenna array and due to the bearing processor 
and display. The phase-directional array offers the following points of 
merit: 
(i) rejection of the horizontal polarization component reduces errors due 
to sky-wave propagation, polarization tilts and reception with a high 
angle of incidence. 
(ii) independence of the sense information from the effect of mast 
currents. 
(iii) instantaneous response i.e. no scanning or time-sampling delays 
involved. 
(iv) field strength sensitivity comparable with that of a crossed-loop 
system. 
In addition, the processing and display system associated with the 
phase-directional array offers the simultaneous advantage of: 
(i) digital bearing accuracy is not dependent on the amplitude matching of 
the two DF receiver channels. Only accurate phase matching is essential 
(ii) digital bearing readout for display and transmission to a remote 
location 
(iii) analog cathode ray tube display for fast tuning in and recognition of 
nature of signal and the conditions of reception 
(iv) simultaneous display of bearings of two or more signals of different 
frequencies inside the selected i.f. band 
As a partial disadvantage may be counted the inability to resolve the 
direct rays from those locally reflected, resulting in certain residual 
errors to be corrected by calibration.