Direction finder

A direction finder having two directive loop antennas mutually orthogonally disposed and a non-directional vertical antenna. Due to the difference in line constant between the loop antenna channels and the vertical antenna channel, the voltage signals induced in one of the loop antennas can not be combined with the ones induced in the vertical antenna in a mutual phase relationship of the same or the opposite phase, which causes errors in measuring a correct bearing of a source of incoming signals. Such errors in bearing indication are eliminated by combining the voltage signals induced in one of the loop antennas and in the vertical antenna to obtain positive phase and negative phase combined voltage signals with 180.degree. out of phase to one another, and by generating driving signals for an indicator based on the rectified voltage signals from the vertical antenna and the rectified combined signals appropriately selected.

BACKGROUND OF THE INVENTION 
This invention relates to a direction finder including a directive X-axis 
antenna having a directional sensitivity pattern in the direction of the 
X-axis of Cartesian co-ordinates, a directive Y-axis antenna having a 
directional sensitivity pattern in the direction of the Y-axis thereof and 
a non-directional antenna. The direction finder separately combines 
voltage signals induced in the non-directional antenna and voltage signals 
induced in each of the directive antennas to produce combined voltage 
signals in association with each of the X-axis and Y-axis antennas, and 
respectively generates X-axis and Y-axis component signals by utilizing 
the combined voltage signals and the voltage signals induced in the 
non-directional antenna with these voltage signals being amplified and 
rectified, and indicates the direction determined by a vector addition of 
the X-axis and Y-axis component signals as a correct bearing of a source 
of incoming signals. More particularly, this invention relates to a 
direction finder of this type capable of precisely indicating a correct 
bearing of incoming signals. 
PRIOR ART STATEMENT 
Various kinds of direction finders having by way of example of the two loop 
antennas respectively having directional sensitivity patters in mutually 
orthogonal directions and a non-directional vertical antenna have, so far, 
been proposed. As one type of these direction finders, such a direction 
finder has been proposed that separately combines and rectifies the 
voltage signals induced in the non-directional antenna and the voltage 
signals induced in each of the loop antennas having directional 
sensitivity patterns in the X-axis and Y-axis directions, and produces the 
X-axis and Y-axis component signals necessary for indication by utilizing 
these rectified combined voltage signals, and indicates the direction 
determined by a vector addition of these component signals as a correct 
bearing of a source of incoming searched signals. This kind of direction 
finder has been disclosed, for example, in Japanese Patent Publication No. 
9722 of 1958 and Japanese Patent Publication No. 10728 of 1959. 
In this kind of direction finder, the voltage signals induced in the 
vertical antenna must be combined with the voltage signals induced in each 
of the loop antennas in phase relationship of exactly the same phase or 
180.degree. out of phase with each other, in order to precisely measure 
the bearing of a source of incoming searched signals. In actual cases, 
however, due to the difference in line constant between the loop antenna 
channels and the vertical antenna channel, the phase difference 
therebetween is inevitably produced and observed at the moment of the 
combination. This phase difference has caused errors in bearing 
indication, thus rendering the direction finder incapable of indicating a 
correct bearing of the source of the searched radio waves incident to the 
antennas. Moreover, such errors in indication differ depending on the 
bearing of the source of incoming searched signals, thus making it 
difficult to manufacture commercially usable direction finders.

SUMMARY OF THE INVENTION 
First, the technical problem to be solved by the present invention will be 
explained with reference to a prior art system. 
Referring to FIG. 1, loop antennas 1 and 2 are respectively disposed on 
mutually perpendicular X-axis and Y-axis of Cartesian co-ordinates. An 
antenna 3 is a non-directional vertical antenna. 
The signals induced in the directional loop antennas 1 and 2 are 
transmitted to a switch 4. The switch 4 comprises contact elements a, b, c 
and a moving connector m, and is driven by a switch actuator 5. The 
voltage signals induced in the loop antennas 1 and 2 are respectively 
connected to the contact elements a and b of the switch. 
The output signals derived by the moving connector m of the switch 4 are 
transmitted to a combining circuit 6. The combining circuit 6 functions to 
combine these derived signals with the voltage signals induced in the 
non-directional antenna 3 and fed thereto through a phase shifter 7. 
Thus, the combining circuit 6 combines the voltages induced in the loop 
antenna 1 and the non-directional antenna 3 with each other, during the 
period the moving connector m is connected to the contact element a of the 
switch 4. Similarly, it combines the voltages induced in the loop antenna 
2 and the non-directional antenna 3 with each other, during the period the 
moving connector m is connected to the contact element b. The combining 
circuit 6 produces output signals in response only to the voltages induced 
in the non-directional antenna 3 when the moving connector m is connected 
to the contact element c, since the moving connector m does not pass the 
output signals from any one of the loop antennas 1 and 2. It should be 
noted that the phase shifter 7 functions to shift the voltages induced in 
the non-directional antenna 3 to bring these voltages and the voltages 
induced to one of the loop antennas 1 and 2 in phase or 180.degree. out of 
phase with each other. 
The combined output signals from the combining circuit 6 are transmitted to 
a moving connector m' of a switch 4', after being amplified and rectified 
in a receiving unit 8. 
The switch 4' is driven by the switch actuator 5 to perform switching 
operation in synchronism with that of the switch 4, thereby selectively 
transmitting the rectified output signals from the receiving unit 8 to 
each corresponding one of storage circuits 9a, 9b and 9c. 
The storage circuit 9a operates to store the combined voltages obtained by 
combining the signal voltages induced in the loop antenna 1 and the 
nondirectional antenna 3 to one another. 
As illustrated in FIG. 2, when radio waves Esin wt come from a direction 
.theta. with respect to the X-axis, signals Ecos .theta. sin wt are 
induced in the X-axis loop antenna 1. These induced signals are combined 
with the voltage signals induced in the nondirectional antenna 3 to become 
EQU E(cos .theta.+1) sin wt 
These combined signals are rectified by the receiving unit 8 to result in 
signals E(cos .theta.+1) representing the amplitude variation of the 
combined voltage signals. The rectified voltage signals are stored in the 
storage circuit 9a. 
In the similar manner, the storage circuit 9b stores the signals E(sin 
.theta.1) which are obtained by combining the voltages induced in the loop 
antenna 2 and the nondirectional antenna 3 with each other and by 
detecting them. 
The storage circuit 9c stores the rectified voltage signals E, since it 
receives only the voltages induced in the nondirectional antenna 3. 
A bearing indicator 10 comprises a X-axis exciting coil L.sub.x, a Y-axis 
exciting coil L.sub.y, magnet M and a bearing pointer I. The magnet M is 
caused to point in the direction determined by a vector addition of the 
magnetic field created by the exciting coil L.sub.x and the one created by 
the exciting coil L.sub.y, and hence the bearing pointer is directed in 
the same direction. 
The voltage signals E(cos .theta.+1) stored in the storage circuit 9a are 
applied to the X-axis exciting coil L.sub.x, and the voltage signals E(sin 
.theta.+1) stored in the storage circuit 9b are applied to the Y-axis 
exciting coil L.sub.y, and the signals E stored in the storage circuit 9c 
are applied in common to the one end of each of the X-axis and Y-axis 
exciting coils. 
Consequently, the X-axis exciting coil L.sub.x is excited by the difference 
voltage signals E cos .theta. obtained by substracting E from the stored 
voltage E(cos .theta.+1), and the Y-axis exciting coil L.sub.y is excited 
by the difference signals E sin .theta. obtained by substracting E from 
the stored voltages E(sin .theta.+1), thereby pointing the magnet M in the 
direction .theta. determined by a vector addition of the two magnetic 
fields and thus making the bearing pointer point at the bearing .theta. of 
radio waves' incident on the antennas. 
The aforementioned apparatus indicates a bearing of a source of incoming 
radio waves by utilizing magnitude ratio of the voltage signals induced in 
the directional loop antenna 1 to the ones induced in the other loop 
antenna 2, with the loop antennas 1 and 2 being mutually perpendicularly 
disposed. In this apparatus in order to make the bearing indicated by the 
bearing indicator 10 to exactly coincide with the bearing of a source of 
incoming radio waves, the magnitude of the respective ones of the combined 
voltages produced by the combination circuit 6 must be varied in 
predetermined manners correspondingly with the change of the direction of 
incoming radio waves. In order to attain this object, the phase 
relationship between the voltage signals induced in each of the X-axis and 
Y-axis loop antennas and the ones induced in the non-directional antenna 3 
must always be maintained to be in phase or 180.degree. out of phase with 
each other when they are received and combined by the combining circuit 6. 
Actually, however, the phase difference between the two kinds of the 
signals is inevitably generated due to the difference in line constant 
between the loop antenna channels and the non-directional antenna channel, 
and observed when they are received and combined with each other by the 
combining circuit 6. Such a phase difference causes an error in a bearing 
indication. 
The mutual relationship between the phase difference and the errors in 
bearing indication will be explained as follows: 
Referring to FIG. 2, when the radio waves: 
as 
EQU E sin wt 
come from the direction O with respect to the X-axis, voltage signals: 
EQU E cos .theta. sin wt 
are induced in the X-axis loop antenna 1, and the voltage signals: 
EQU E sin .theta. sin wt 
are induced in the Y-axis loop antenna 2. 
The non-directional antenna 3 induces voltages having phase 90.degree. 
spaced with respect to that of the voltage signals induced in the loop 
antennas 1 and 2. The voltage signals from the antenna 3 are phase-shifted 
by the phase shifter 7 to establish phase coincidence with the voltage 
signals from one of the loop antenna channels. In actual cases, however, 
when these phase-shifted voltage signals are received by the combining 
circuit 6, the phase difference .delta. is measured with respect to the 
signals from the loop antenna channels, and hence the received signals are 
represented by 
EQU E sin (wt+.delta.). 
The combined signals E.sub.x obtained by combining the received signals 
with the signal voltages transmitted from the X-axis loop antenna 1 become 
##EQU1## 
Similarly, the combined signals E.sub.y obtained by combining said received 
signals with the signal voltages transmitted from the Y-axis loop antenna 
2 become 
##EQU2## 
FIG. 3 illustrates the magnitude variation of the combined voltage signals 
represented by the equations (1) and (2) depending on the bearing of a 
source of the radio waves incident on the antennas. 
FIG. 3A shows the magnitude variation of the combined signals E.sub.x 
represented by the equation (1) obtained by combining voltage signals 
induced in the X-axis loop antenna and the non-directional antenna 3. FIG. 
3B illustrates the magnitude variation of the combined signals represented 
by the equation (2), obtained by combining with each other the voltage 
signals induced in the Y-axis loop antenna and the non-directional antenna 
3. Referring to FIG. 3A and FIG. 3B, full line curves are the 
characteristic curves obtained by combining the reception signals 
transmitted from the loop antenna channels and from the non-directional 
antenna 3, with the two kinds of the signals being maintained in phase or 
180.degree. out of phase with each other, in other words with .delta. in 
the equations (1) and (2) being zero. Dotted curves are the characteristic 
curves obtained by combining the reception signals transmitted from the 
loop antenna channels and from the non-directional antenna with the phase 
difference therebetween being maintained at 15.degree. 
(.delta.=15.degree.). Dash line and dot-dash line curves are 
characteristic curves respectively with .delta. being 30.degree. and with 
.delta. being 60.degree.. 
As is apparent from FIG. 3A and FIG. 3B, with the phase difference .delta. 
being zero, the magnitude of the respective combined signals E.sub.x and 
E.sub.y respectively obtained in association with the X-axis loop antenna 
1 and the Y-axis loop antenna 2 respectively varies cosinewise and 
sinewise for all the possible directions of a source of the radio waves 
incident to the antennas. Accordingly, the bearing pointer I precisely 
indicates the bearing of incoming radio waves, since the direction 
determined by a vector addition of the magnetic fields created by the 
X-axis and Y-axis exciting coils L.sub.x and L.sub.y of the bearing 
indicator 10 also correspondingly varies with the change of the bearing of 
a source of the radio waves incident to the antennas. 
With the phase difference .delta., the magnitude of the combined signals 
E.sub.x in association with the X-axis loop antenna 1 varies also 
cosinewise with the change of the bearing of incoming radio waves within 
each of the azimuth ranges from 0.degree. to 90.degree. and from 
270.degree. to 360.degree., as illustrated in FIG. 3A. Whereas, within the 
azimuth range from 90.degree. to 270.degree., the magnitude of the 
combined voltage signals E.sub.x traces distorted cosine curves. 
Similarly, the magnitude of the combined voltage signals E.sub.y in 
association with the Y-axis loop antenna 2 varies in a sinusoidal way for 
azimuth angles from 0.degree. to 180.degree., but traces distorted sine 
curves for azimuth angles from 180.degree. to 360.degree.. 
Consequently, with the phase difference .delta., the direction determined 
by a vector addition of the magnetic fields created by the X-axis and 
Y-axis exciting coils L.sub.x and L.sub.y of the bearing indicator can not 
faithfully follow the change of the bearing of incoming radio waves 
incident to the antennas. This causes the bearing pointer I to indicate a 
different bearing than a true bearing of a source of radio waves incident 
to the antennas. 
Assuming now that radio waves come from a bearing of 150.degree., the 
magnitude ratio of the voltage signals E.sub.y applied to the Y-axis 
exciting coil L.sub.y to the ones E.sub.x applied to the X-axis exciting 
coil L.sub.x must be 
EQU (E.sub.y /E.sub.x)=-0.57735 
in order to make the bearing pointer I indicate the correct bearing of 
150.degree.. 
With the phase difference being 60.degree., however, the combined voltage 
signals E.sub.x 60 obtained by combining the voltage signals induced in 
the X-axis loop antenna 1 with the ones from the vertical antenna 3 
become, in accordance with the equation (1), 
EQU E.sub.x 60=0.940E 
Whereas, the combined voltage signals E.sub.y 60 obtained by combining the 
voltage signals induced in the Y-axis loop antenna 2 with the ones from 
the vertical antenna 3 become, in accordance with the equation (2), 
EQU E.sub.y 60=1.323E 
The X-axis and Y-axis exciting coils L.sub.x and L.sub.y are respectively 
excited by the difference signal voltages E.sub.x and E.sub.y respectively 
obtained by subtracting the signal voltages E originating in the vertical 
antenna 3 from the combined voltage signals E.sub.x 60 and E.sub.y 60. The 
exciting voltages E.sub.x for the X-axis exciting coil L.sub.x become 
EQU E.sub.x =0.940E-E=-0.06E 
The exciting voltages E.sub.y for the Y-axis exciting coil L.sub.y become 
EQU E.sub.y =1.323E-E=0.323E 
As a result, ratio of the magnitude of the voltages becomes 
##EQU3## 
thus resulting in an indicated bearing .theta.. 
##EQU4## 
As apparent, an indication error in this case becomes 49.5.degree., since 
the indicated bearing is 100.5.degree. against the correct bearing of 
150.degree.. 
An object of this invention is to provide a direction finder capable of 
indicating a true bearing of a source of searched signals, even in such a 
case that the line constant of the loop antenna channels is different from 
the one of the vertical antenna channel. 
A direction finder according to this invention includes a directive X-axis 
antenna having a directional sensitivity pattern in the direction of the 
X-axis of Cartesian co-ordinates and a directive Y-axis antenna having a 
directional sensitivity pattern in the direction of the Y-axis thereof and 
a non-directional antenna, and separately combines the voltage signals 
induced in each of the directive X-axis and Y-axis antennas and the 
voltage signals induced in the non-directional voltage signals to produce 
combined voltage signals in association with each of the directive X-axis 
and Y-axis antennas, and respectively generates X-axis and Y-axis 
component signals by utilizing the combined voltage signals and the 
voltage signals induced in the non-directional antenna, with these voltage 
signals being amplified and rectified, and indicates the direction 
determined by a vector addition of the X-axis and Y-axis component signals 
as a correct bearing of a source of the searched incoming signals incident 
to the antennas. More particularly, it combines and rectifies the voltage 
signals induced in the directive antennas and the ones induced in the 
non-directional antenna to separately obtain rectified positive phase and 
negative phase combined voltage signals with a phase relationship of 
180.degree. apart to one another for each of the directive X-axis and 
Y-axis antennas, and appropriately selecting the rectified positive phase 
combined voltage signals and the rectified negative phase combined voltage 
signals within an azimuth range wherein the magnitude of the rectified 
positive phase and negative phase combined voltage signals varies in 
distorted manners with respect to sine or cosine curves correspondingly 
with the change of the bearing of a source of incoming searched signals. 
Such an arrangement serves to achieve the required objects. 
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Hereinafter, each of the components which is given the same reference 
numerals as in FIG. 1 performs the same function. 
Referring to FIG. 4, the voltage signals induced in the X-axis loop antenna 
1 are supplied to the switch 4 from the secondary winding of a transformer 
Tx. The mid point of the secondary winding of the transformer Tx is 
grounded, and from both ends of the winding thereof voltage signals with 
the same magnitude and with their phase relationship being 180.degree. out 
of phase with each other are respectively transmitted to contact elements 
a and a. 
Similarly, signal voltages, originating in the Y-axis loop antenna 2 and 
with the same magnitude and with their phase being 180.degree. out of 
phase with each other, are respectively transmitted to contact elements b 
and b. 
The switch 4 comprises the contact elements a, a, b, b, c, and the moving 
connector m, and successively performs switching operation by using the 
moving connector m. The signals derived by the moving connector m are 
transmitted to the combining circuit 6, and are combined with the voltage 
signals induced in the vertical antenna 3 as the same manner in FIG. 1. 
The output signals from the combining circuit 6 are, after being amplified 
and rectified in the receiving unit 8, transmitted to the moving connector 
m' of the switch 4'. 
The switch 4' is driven by the switch actuator 5 to perform a switching 
operation in synchronism with that of the switch 4, thereby selectively 
transmitting the output signals from the receiving unit 8 to each of the 
corresponding contact elements a', a', b', b, and c'. The contact elements 
a' and a', of the switch 4' are respectively connected to storage circuits 
9a and 9a. The contact elements b' and b' of the switch are respectively 
connected to storage circuits 9b and 9b. The contact element c' is 
connected to the storage circuit 9c. 
Therefore, when the moving connector m' is connected to the contact element 
a', the storage circuit 9a stores the combined voltage signals in 
accordance with the equation (1), which result from a combination of the 
voltage signals induced in the X-axis loop antenna 1 and the vertical 
antenna 3. 
As a next step, at the moment the moving connector m' is connected to the 
contact element a', the voltage signals: 
EQU -E cos .theta. sin wt 
which are reverse in polarity as the ones induced in the X-axis loop 
antenna 1, are transmitted from the contact element a of the switch 4 to 
the combining circuit 6 and are combined therein with the voltages 
EQU E sin (wt+.delta.) 
induced in the vertical antenna 3. 
The resultant combined voltage signals E.sub.x become 
##EQU5## 
and these are stored in the storage circuit 9a. 
Next, when the moving connector m' is switched to the contact element b', 
the hold circuit 9b holds the combined voltage signals E.sub.y in 
accordance with the equation (2), which result from a combination of the 
voltage signals induced in the Y-axis loop antenna 2 and the vertical 
antenna 3. 
At the moment the moving connector m' is connected to the contact element 
b, the combining circuit 6 combines the voltage signals: 
EQU -E sin .theta. sin wt 
which are reverse in polarity as the ones induced in the Y-axis loop 
antenna 2, with the voltage signals: 
EQU E sin (wt+.delta.) 
transmitted from the vertical antenna 3. 
The resultant combined voltage signals E.sub.y become 
##EQU6## 
and are stored in the storage circuit 9b. 
Further, at the moment the moving connector is switched to the contact 
element c', the only signal voltages 
EQU E sin (wt+.delta.) 
from the vertical antenna 3 are transmitted to the storage circuit 9c and 
are maintained thereby. 
It should be noted that the rectified output signals from the receiving 
unit 8 are supplied to the respective storage circuits 9a, 9a, 9b, 9b and 
9c, and hence the voltage signals representing the magnitude of the 
combined signals obtained in accordance with each of the equations are 
maintained in corresponding one of the storage circuits. 
The voltages held in both of the storage circuits 9a and 9a are supplied to 
a selection circuit 11, and the voltages held in either one of the storage 
circuits are selectively connected to the X-axis coil L.sub.x of the 
bearing indicator 10. 
The selection circuit 11 comprises a selection switch 111, an inversion 
circuit 112 and comparison circuit 113. 
The voltages held in the storage circuit 9a are connected to one input 
terminal of the selection switch 111, and the voltages maintained in the 
hold circuit 9a are supplied to the other input terminal thereof after 
being inverted in polarity by the inversion circuit 112. The inversion 
circuit 112, with the voltages stored in the storage circuit 9c being 
supplied thereto, inverts in polarity the output voltage signals from the 
storage circuit 9a at a level of the magnitude of the stored voltages in 
the circuit 9c. 
As illustrated in FIG. 5, the inversion circuit 112 includes an operational 
amplifier OP with the output signals from the storage circuit 9a being 
supplied to the input terminal P.sub.1 and with the output signals from 
the storage circuit 9c being supplied to the input terminal P.sub.2, and 
at the output terminal thereof inverted voltage signals are produced. 
The comparison circuit 113 functions to compare magnitude of the voltages 
held in each of the storage circuits 9a and 9a with each other, and to 
control the selection switch 111 to pass the voltages held in the storage 
circuit 9a when the magnitude of the voltages stored in the storage 
circuit 9a is larger than that of the hold circuit 9a. Conversely, when 
the voltages maintained in the storage circuit 9a are larger than those of 
the other circuit 9a, it controls the selection switch 111 to pass the 
output voltages from the inversion circuit 112. 
The operations by the comparison circuit 113 can be performed by the 
comparison circuit comprising logic circuits. The comparison circuit 
compares the magnitude of the voltages maintained in each of the storage 
circuits 9a and 9a with each other, and produces a high level output 
signal when the magnitude of the voltages held in the storage circuit 9a 
is larger than that of the other circuit, thereby controlling the 
selection switch 111 to pass the voltages from the storage circuit 9a. 
Conversely, when the magnitude of the voltages stored in the storage 
circuit 9a is larger than that of the other circuit, it produces a low 
level output signal, thereby controlling the selection switch 111 to pass 
the output signals from the inversion circuit 112. 
With such a circuit configuration, a variation manner of the magnitude of 
the output voltage signals from the selection switch 111 depending on the 
bearing of a source of the radio waves incident to the antennas will be 
considered. 
First, as explained in the foregoing, the magnitude of the voltages in the 
storage circuit 9a varies in a manner as illustrated in FIG. 3A. The 
magnitude variation of the voltages in the storage circuit 9a varies in 
accordance with the equation (3) and is represented as in FIG. 6A'. The 
inversion circuit 112 reverses these characteristic curves of voltage, 
thereby producing the voltage signals represented by the characteristic 
curves in FIG. 6A'. 
Comparison between FIG. 3A and FIG. 6A' reveals that the magnitude of the 
voltages held in the storage circuit 9a (FIG. 3A) is larger than that of 
the other circuit for azimuth angles from 0.degree. to 90.degree. wherein 
the bearing of a source of incoming radio waves lies. Hence, within the 
azimuth range D.sub.1, the selection switch 111 passes the voltages stored 
in the storage circuit 9a (FIG. 3A). 
Within the azimuth ranges D.sub.2, D.sub.3 from 90.degree. to 270.degree., 
the magnitude of the voltages held in the storage circuit 9a (FIG. 6A') is 
larger than that of the other circuit. Hence, within these azimuth ranges 
D.sub.2 and D.sub.3, the output voltages (FIG. 6A') from the inversion 
circuit 112 are passed through the selection switch 111. 
Within the azimuth range D.sub.4 from 270.degree. to 360.degree., the 
magnitude of the voltages stored in the storage circuit 9a (FIG. 3A) 
becomes larger than that of the other circuit, and accordingly, the stored 
voltages (FIG. 3A) are passed through the selection switch 111. 
Consequently, the magnitude of the output voltages from the selection 
circuit 11 varies, depending on the incidence bearing of radio waves to 
the antennas, as illustrated in FIG. 8A''. This diagram clearly reveals 
that the magnitude of the output voltages from the selection circuit 11 
varies almost cosinewise even when the voltage signals originating in the 
X-axis loop antenna 1 and those originating in the vertical antenna 3 are 
not exactly in phase or 180.degree. out of phase with each other at the 
moment they are combined by the combining circuit 6. 
In the similar manner as the selection circuit 11 selecting the voltage 
signals stored in either the storage circuit 9a or the storage circuit 9a, 
the voltages held in the storage circuits 9b and 9b are supplied to 
selection circuit 11' and selectively passed thereby. 
The selection circuit 11' is constructed exactly in the same way as the 
selection circuit 11. The output signals from the storage circuit 9b are 
applied to selection switch 111', and the ones from the storage circuit 9b 
are transmitted to the selection switch 111' through an inversion circuit 
112'. 
FIG. 7B', illustrates the magnitude variation of the voltages stored in the 
storage circuit 9b, and FIG. 7B' shows the magnitude variation of the 
voltage signals inverted in polarity by the inversion circuit 112'. 
The waveform comparison between FIG. 3B and FIG. 7B' reveals that the 
voltage signals (FIG. 3B) stored in the storage circuit 9b are produced 
within the azimuth ranges D.sub.1 and D.sub.2 from 0.degree. to 
180.degree., and the output voltages B' from the inversion circuit 112' 
are produced within the azimuth ranges D.sub.3 and D.sub.4 from 
180.degree. to 360.degree.. Consequently, the selection switch 111' 
produces the voltages as represented by the characteristic curves of 
voltage in FIG. 8B". 
The output voltages (FIG. 8A" and FIG. 8B") from the selection circuits 11 
and 11' are respectively transmitted to the X-axis exciting coil L.sub.x 
and Y-axis exciting coil L.sub.y of the bearing indicator 10, and 
accordingly a bearing is indicated in the same way as in FIG. 1. As 
apparent from FIG. 8, the exciting voltages applied to one of the exciting 
coils L.sub.x and L.sub.y vary cosinewise or sine-wise correspondingly 
with the change of the bearing of a source of incoming radio waves. 
Therefore, the direction of the combined magnetic field created by the 
X-axis and Y-axis exciting coils L.sub.x, L.sub.y varies exactly 
correspondingly with the change of the incidence bearing of radio waves. 
This makes the bearing pointer I indicate a correct bearing of a source of 
incoming radio waves, as opposed to an incorrect bearing indicated by the 
prior art apparatus. 
As aforementioned, this invention generates positive phase and negative 
phase voltages from the voltage signals induced in each of the loop 
antennas, and combines the generated voltages and the voltage signals 
induced in the vertical antenna to respectively obtain the corresponding 
characteristic curves of voltage in FIG. 3, FIG. 6 and FIG. 7 representing 
the magnitude variation of voltage signals against the bearing of a source 
of incoming radio waves. Further, the invention derives the portions of 
each of the characteristic curves of voltage which vary sinewise or 
cosinewise to obtain the curves as illustrated in FIG. 8 and utilize them 
for bearing indication. Accordingly, a correct bearing indication is made 
precisely corresponding to the bearing of a source of incident radio waves 
for all azimuth angles from 0.degree. to 360.degree.. 
In the foregoing, an explanation has been made of an embodiment of the 
invention wherein the magnitude of the voltage signals is equal to that of 
the voltage signals induced in the non-directional vertical antenna 3. The 
invention also effectively works even when the magnitude of the voltage 
signals induced in each of the antennas is not equal to one another. 
Referring to FIG. 9 showing a vector diagram, a vector OP represents the 
reception signals caught by the vertical antenna 3, and a vector PQ 
represents the reception signals caught by the loop antennas 1 or 2. This 
is the case wherein the magnitude of the signals induced in an antenna is 
equal to that of the other signals. 
When the reception signals caught by the loop antennas 1 or 2 and the ones 
received by the vertical antenna 3 are combined in phase or 180.degree. 
out of phase with each other, the vector PQ lies on the vector OP or on an 
extended line thereof. Hence, the resultant added vector is given by 
joining the point O and a position on the line OQ determined by a varying 
vector PQ. This added vector corresponds to the characteristic curves with 
.delta.=0.degree. maintained in FIG. 3. 
When the voltage signals induced in the loop antennas 1 or 2 are combined 
with the voltage signals induced in the vertical antenna 3 with the phase 
difference .delta. existed therebetween, the vector PQ can be represented 
by a vector PQ.sub.1 obtained by turning it by an amount of .delta.. The 
reception signals caught by the loop antennas 1 or 2 can be represented by 
a varying vector decreasing from the vector PQ.sub.1 to a vector PQ.sub.2 
depending on the incident bearing of radio waves. Hence, the combined 
signals obtained by combining the voltage signals induced in one of the 
loop antennas and the vertical antenna can be represented by added vector 
resulting from an addition of the vector OP and either one of the vectors 
PQ.sub.1 or PQ.sub.2. Thus, the resultant added vector varies along the 
line Q.sub.1 Q.sub.2 from the vector OQ.sub.1 to the one OQ.sub.2. 
The magnitude variation of the vector in this case corresponds to the 
characteristic curves with .delta. being maintained as 15.degree. or 
30.degree. or 60.degree. as illustrated in FIG. 3. 
Referring to FIG. 9, with the magnitude variation of the vector PQ.sub.1, 
the resultant added vector changes from the vector OQ.sub.1 to the one OP. 
The added vector varies in a simple manner between the maximum vector 
OQ.sub.1 and the minimum one OP. Such a variation manner of magnitude of 
vector corresponds to the azimuth ranges D.sub.1 and D.sub.4 in FIG. 3A 
and to the ranges D.sub.1 and D.sub.2 in FIG. 3B. 
Next, with the magnitude variation of the vector PQ.sub.2, the added vector 
changes from the vector OP to the one OQ.sub.2 along the line PQ.sub.2. 
The minimum vector therebetween is given by a line OH perpendicular to the 
line PQ.sub.2. In this case, the added vector decreases from the vector OP 
to the minimum vector OH and then increases to the vector OQ.sub.2. Thus, 
when the vector PQ.sub.2 varies between the points P and Q.sub.2, the 
magnitude of the added vector does not vary in a simple manner as 
aforementioned, but varies in such a distorted way as firstly decreasing 
from the maximum magnitude of the voltage signals induced in the vertical 
3 to the minimum magnitude and then increasing. This mode of variation 
corresponds to the azimuth ranges D.sub.2 and D.sub.3 in FIG. 3B. 
The degree of the distorted variation increases with the magnitude of the 
voltage signals induced in the loop antennas 1 or 2. When the magnitude of 
the voltage signals induced in one of the loop antennas 1 and 2 is smaller 
than that of the vector PH, the added vector varies almost monotonously 
from the vector OP to the minimum vector OH. Consequently, within this 
range of vector variation, deviation from correct bearing indication can 
be maintained under a certain level. 
When the magnitude of the voltage signals induced in one of the loop 
antennas 1 and 2 become larger than that of the vector PH, a mode of the 
distorted variation as aforementioned appears. The degree of the distorted 
variation increases with the increase of the voltage signals induced in 
one of the loop antennas. In this case, as easily anticipated, deviation 
from correct bearing can be considerably decreased by the embodiment 
according to the invention as illustrated in FIG. 4. 
Although, the loop antennas 1 and 2 are used in FIG. 4, these are not the 
only antennas to be utilized and can be replaced by any kinds of antennas 
one of which having a directional sensitivity pattern in corresponding one 
of the X-axis and Y-axis directions. For example, four vertical antennas 
symmetrically disposed on mutually perpendicular X-axis and Y-axis with 
the crossing point at its center can also be used in place of two loop 
antennas. 
The voltage signals induced in the non-directional vertical antenna, can 
also be replaced by averaging in phase the voltage signals induced in each 
of the vertical antennas disposed on the X-axis and Y-axis. 
In FIG. 4, the voltage signals induced in one of the loop antennas 1 or 2 
are transformed by means of the transformers T.sub.x or T.sub.y to derive 
positive phase or negative phase voltages, and these respective voltages 
are separately combined with the voltage signals induced in the 
non-directional antenna 3 to generate positive phase and negative phase 
combined voltages. Instead of deriving positive phase and negative phase 
voltages from the voltage signals induced in one of the loop antennas 1 
and 2, it is also arranged in such a way that the voltage signals induced 
in the non-directional antenna 3 are switched to obtain positive phase and 
negative phase voltages therefrom and these voltages are then separately 
combined with the voltage signals in each of the loop antennas. In this 
case, a switch must be fitted with the non-directional antenna 3, and be 
switched from one position to the other one during the period the voltage 
signals induced in one of the loop antennas being transmitted, thereby 
producing positive phase and negative phase voltages. 
Although, in the embodiment according to the present invention as 
illustrated in FIG. 4, the comparison circuit 113 (113') compares the 
stored voltages in the storage circuit 9a (9b) with those in the storage 
circuit 9a (9b), this invention should not be limited to this circuit 
configuration. It may also be arranged such that the comparison circuit 
113 (113') compares the voltages V.sub.1 (V.sub.3) maintained in the 
storage circuit 9a (9b) with the voltages V.sub.o held in the storage 
circuit 9c wherein the voltage signals from the vertical antenna 3 being 
stored, and controls the selection switch 111 (111') to pass the positive 
phase combined voltages produced from the storage circuit 9a (9b) when the 
mutual magnitude relationship becomes V.sub.1 (V.sub.3)&gt;V.sub.o, and 
controls the selection switch to alternatively pass the inverted negative 
phase combined voltages from the storage circuit 9a (9b) when the mutual 
relationship is V.sub.1 (V.sub.3)&lt;V.sub.o. Similarly, it may also be 
arranged such that the comparison circuit 113 (113') compares the voltages 
V.sub.2 (V.sub.4) maintained in the storage circuit 9a (9b) with the 
voltages V.sub.o in the storage circuit 9c, and controls the selection 
switch 111 (111') to pass the positive phase combined voltages when the 
resultant mutual magnitude relationship is V.sub.2 (V.sub.4)&lt;V.sub.o, and 
to alternatively pass the inverted negative phase combined voltages when 
the resultant mutual relationship is V.sub.2 (V.sub.4)&gt;V.sub.o. 
INDUSTRIAL APPLICABILITY 
This invention can also be embodied in an apparatus for measuring the 
bearing of a source of sound waves. In this case, sound receptors each 
having a similar directional sensitivity pattern with that of the other 
one are used in replace of the directive antennas, and a non-directional 
receptor is used in replace of the non-directional antenna.