U.S. patent number 3,617,890 [Application Number 04/697,069] was granted by the patent office on 1971-11-02 for induction radio system for vehicles.
This patent grant is currently assigned to Sumitomo Electric Industries, Ltd.. Invention is credited to Noritaka Kurauchi, Kenichi Yoshida.
United States Patent |
3,617,890 |
Kurauchi , et al. |
November 2, 1971 |
INDUCTION RADIO SYSTEM FOR VEHICLES
Abstract
An induction-radio system with a low induced noise level and
unwanted radiation level for transmission of communication and
control signals between a vehicle following a track and wayside
station. The transmission line is positioned in parallel along the
track and consists of two conductors helically wound with an equal
and constant pitch and the antenna is mounted on the vehicle
adjacent the transmission line for continuous coupling therewith.
The antenna consists of at least one pair of adjacent elements
positioned to receive the carrier current in the transmission line
with a phase shift difference of an odd multiple of 90.degree. and
a phase combining means connected to the elements additively phase
combines the received outputs of the elements.
Inventors: |
Kurauchi; Noritaka (Suita,
JA), Yoshida; Kenichi (Sakai, JA) |
Assignee: |
Sumitomo Electric Industries,
Ltd. (Osaka, JA)
|
Family
ID: |
27563183 |
Appl.
No.: |
04/697,069 |
Filed: |
January 11, 1968 |
Foreign Application Priority Data
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|
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|
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Jan 12, 1967 [JA] |
|
|
42/2324 |
Feb 16, 1967 [JA] |
|
|
42/9981 |
Mar 20, 1967 [JA] |
|
|
42/17274 |
Mar 20, 1967 [JA] |
|
|
42/17276 |
Mar 20, 1967 [JA] |
|
|
42/17277 |
Mar 20, 1967 [JA] |
|
|
42/17278 |
Jun 18, 1967 [JA] |
|
|
42/38985 |
|
Current U.S.
Class: |
455/41.1; 246/8;
379/55.1; 455/523; 343/895; 340/936 |
Current CPC
Class: |
H04B
5/00 (20130101); B61L 3/225 (20130101) |
Current International
Class: |
B61L
3/22 (20060101); B61L 3/00 (20060101); H04B
5/00 (20060101); H04b 007/10 (); H01q 001/36 () |
Field of
Search: |
;179/82 ;191/10
;246/8,29,30,63,194 ;324/7B ;325/51,305,369,52,56 ;332/45
;340/22,31,32,38,47,258 ;343/711,713,717,895 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
1,129,191 |
|
May 1962 |
|
DT |
|
1,132,607 |
|
Jul 1962 |
|
DT |
|
720,114 |
|
Dec 1954 |
|
GB |
|
Other References
Proceedings of the IRE, Vol. 49, No. 7, July 1961, The Institute of
Radio Engineers, Inc. New York, New York, pp. 1222, 1223..
|
Primary Examiner: Safourek; Benedict V.
Claims
We claim:
1. An induction-radio system for vehicles following a track
comprising a transmission line positioned in parallel with a track
and consisting of two conductors helically wound with an equal and
constant pitch around a longitudinal axis and axially separated by
one-half of said pitch, a transmission signal source connected to
said transmission line to transmit an intelligence-modulated
carrier-frequency current thereon, antenna means mounted on a
vehicle following said track and adjacent said transmission line
for continuous coupling with the magnetic flux produced by the flow
of said carrier current in said transmission line, a receiver on
said vehicle connected to said antenna means, said antenna means
having at least one pair of elements positioned relative to each
other and said transmission line to receive said carrier current
respectively by electromagnetic induction coupling with a spatial
phase shift of one-fourth the transmission line winding pitch
whereby the receiving level of one of said elements is always
maximum when the receiving level of the other element is minimum,
and phase-combining means operable to phase shift the received
signal of one of said elements by 90.degree. over the signal band
of the carrier frequency and to add to it the received output of
the other element to obtain an output having a uniform
signal-receiving level when said vehicle is moving.
2. The induction-radio system of claim 1 characterized in that said
antenna elements in each pair are positioned in respective planes
which meet at right angles and are centered in a common plane
normal to the axis of said transmission line.
3. The induction-radio system of claim 1 wherein each of said
antenna elements is a loop coil.
4. The induction-radio system of claim 1, said antenna means
characterized by a plurality of pairs of said antenna elements,
each pair of said antenna elements being centrally spaced from each
other in the direction of said transmission line by an odd number
multiple of one-fourth of the helix pitch of said transmission
line, and with each pair positioned substantially the same relative
to said transmission line.
5. The induction-radio system of claim 1 characterized in that said
antenna means includes two spaced pair of said antenna elements
positioned along said transmission line with a center-to-center
spacing between pairs of an odd number multiple of one-half of the
helix pitch of said transmission line, said phase-combining means
operative to reverse the polarity of the output terminals of one of
said element pair and interconnecting it to the output terminals of
the other.
6. The induction-radio system of claim 1 wherein said antenna means
consists of four uniformly spaced conductors helically wound about
a longitudinal axis parallel with the axis of said transmission
line and with a pitch equal thereto, the opposed pairs of said four
antenna conductors being shunted at one end to provide said antenna
element pairs and said phase-combining means connected to the
terminal ends of the other end of said elements.
7. The induction-radio system of claim 6 characterized in that the
winding direction of said helical conductor of said antenna is
opposite to that of said transmission line.
8. An induction-radio system for vehicles following a track
comprising a transmission line positioned in parallel with a track
and consisting of n (n 3) uniformly spaced conductors helically
wound with a constant and equal pitch around the axis of said
transmission line, signal transmission means connected to said
transmission line and operable to transmit an
intelligence-modulated carrier-current signal on said transmission
line with a phase shift between adjacent conductors of 360/n
degrees, antenna means mounted on a vehicle following said track
and adjacent said transmission line for continuous coupling with
the magnetic flux produced by the flow of said carrier current in
said transmission line, and a receiver on said vehicle connected to
said antenna means.
9. The induction-radio system of claim 8 characterized in that said
antenna means is composed of two elements spaced apart in the
longitudinal direction of said transmission line by an odd number
multiple of one-half of the helical pitch of said transmission line
and interconnected to additively combine the received signal
outputs thereof.
Description
BACKGROUND OF THIS INVENTION
1. Field of the Invention
This invention relates to an induction radio system for vehicles or
trains using an improved transmission line and an antenna aboard
the vehicle.
2. Description of the Prior Art
The conventional induction radio transmission systems have been
constructed as shown in FIG. 1, 2, 3 and 4. These have the
shortcomings explained as follows:
Referring to FIG. 1, FIG. 2, Fig. 3 and FIG. 4, it is to be noted
that the A figures show the systems as seen in the direction of the
track and the B figures show a view of the system as seen in the
direction of e of the A figures.
First there is a known system, as shown in FIG. 1, wherein two
transmission wires 2 are installed in parallel to rails 1 at equal
heights from the ground and a loop antenna 3 coupling with the
electromagnetic fields created along the transmission wires is
provided aboard the train for sending and receiving signals. There
is also a known system that as shown in FIG. 2, wherein two
transmission wires 2 are installed in a vertical plane which
extends parallel with rails 1 and an antenna 3 for the transmission
wires 2 is provided on board the train.
FIG. 1 and 2 illustrate systems wherein the transmission line
consists of two parallel conducting wires installed along a track
and a loop antenna aboard the train couples with the magnetic field
of the line. These systems have high induced noise levels and high
unwanted radiation levels because of their construction.
The system shown in FIG. 3 is such that one transmission wire 2 is
installed in parallel to the rails 1 and an antenna is coupled with
the wire and mounted on board the train. The electromagnetic field
created between the one transmission wire 2 and the ground is
coupled with the antenna 3 for communication. The system as shown
in FIG. 3 has a further fatal shortcoming. That is to say, a part
of the transmitted electromagnetic wave energy propagates along the
ground, and hence realization of the uniformity in the transmission
line is very difficult.
The system shown in FIG. 4 is such that two transmission wires 2
extend parallel to the rails and are crossed in a plane which is
substantially parallel to the ground and an antenna for these
transmission wires is provided aboard a train.
The system described in FIG. 4 has the periodically arranged cross
points of two conducting wires to provide a transmission line which
will suppress induced and unwanted noises from outside radiation of
other communication systems to a low level. But this system cannot
use a high-frequency signal because the transmission frequency
characteristic of high-frequency waves is influenced by the
capacity existing at the cross points of the two transmission
wires, and the cross points of the transmission line cause
interruptions of the coupled signal between the transmission line
and an antenna aboard the train. This causes noises in the
communication signal.
In the case of the systems shown in FIG. 1, FIG. 2, FIG. 3 and FIG.
4, all the transmissions wires are installed in a parallel plane
near to the ground, so that foreign bodies find their way into the
space where the electromagnetic field is created and degrade the
transmission characteristics, while at the same time they create
radiation to the outside and interfere with other communication
systems. They are also found to be obstacles to the maintenance of
the railroad track. Also in the case of the systems shown in FIG.
1, FIG. 2, FIG. 3 and FIG. 4, it is impossible to repress entry of
outside noise into the antenna and power radiated outside from the
antenna, because the systems use a single antenna.
SUMMARY OF THE INVENTION
The induction-radio system of the present invention is provided for
vehicles following a track wherein a transmission line is
positioned in parallel along the track and consists of two
conductors helically wound with an equal and constant pitch around
the axis of the transmission line with the conductors
circumferentially separated by one-half of the aforesaid pitch. A
transmission source is connected to the line to transmit an
intelligence modulated carrier frequency current thereon. An
antenna is mounted on the vehicle adjacent the transmission line
for continuous coupling with the magnetic flux produced by the flow
of set carrier current in the transmission line. A receiver on the
vehicle is connected to receive the received output of the antenna
which has at least one pair of adjacent antenna elements positioned
to receive the carrier current respectively with a phase shift
difference of an odd multiple of 90.degree. therebetween, and a
phase combining means is connected to the elements to additively
phase combine the received outputs thereof to provide a received
antenna output for said receiver having a uniform signal receiving
level when the vehicle is moving or traveling.
The transmission line of the present invention makes it possible to
maintain a uniform space between two wires along the line axis as
they are wound on the cylindrical surface. Consequently, the
transmission characteristics at high frequencies (50 kHz. to 400
kHz.) do not degrade, so that a broad band transmission is
possible.
A further object of the invention is to provide an induction radio
system preventing noise from entering both the transmission line
and the antenna of the vehicular induction-radio system from
outside, eliminating unwanted radiation from both the transmission
line and the antenna which interferes with outside communication
systems.
A still further object of the invention is to provide an induction
radio system which prevents obstacles from entering the space
between the conducting wires where most of electromagnetic energy
is concentrated.
A still further object is to provide the induction-radio system
having an antenna aboard the train, this antenna being constructed
by helically winding conducting wires on a cylindrical surface, and
thereby eliminating unwanted radiation.
A still further object is to provide the induction radio system
having an improved phase combiner with uniform phase-shifting
characteristics in a wide frequency band. A still further object is
to provide an improved vehicle speed detector by using the
induction-radio system of this invention.
Further objects and advantages of the invention will become
apparent from the following description and claims, and from the
accompanying drawings, wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is a diagrammatic transverse-sectional view in elevation of
a two-wire induction radio-transmission system in the prior art for
railroad use.
FIG. 1b is a diagrammatic plan view of the system illustrated in
FIG. 1a.
FIG. 2a is a diagrammatic transverse-sectional view in elevation of
another two-wire induction radio-transmission system in the prior
art for railroad use.
FIG. 2b is is a diagrammatic view in side elevation of the system
illustrated in FIG. 2a as viewed in the E direction indicated in
FIG. 2a.
FIG. 3a is a diagrammatic transverse sectional view in elevation of
a single-wire induction radio-transmission system for railroad use
in the prior art.
FIG. 3b is a diagrammatic plan view of the system illustrated in
FIG. 3a.
FIG. 4a is a diagrammatic transverse-sectional view in elevation
illustrating a periodically reversed two-wire induction
radio-transmission system for railroad use as found in the prior
art.
FIG. 4b is a diagrammatic plan view of the system illustrated in
FIG. 4a.
FIG. 5 is a diagrammatic perspective view of one embodiment of the
two-wire induction radio-transmission system of the present
invention.
FIG. 6 is a transverse diagrammatic sectional view in front
elevation of the system shown in FIG. 5.
FIG. 7 is a diagrammatic transverse-sectional view in front
elevation showing a variation of the antenna structure arrangement
illustrated in FIG. 5.
FIG. 8 is a diagrammatic sectional view in front elevation
illustrating another embodiment of the antenna structure
incorporated in the system of the present invention.
FIG. 9 is a diagrammatic view in side elevation illustrating a
fourth antenna arrangement for the transmission system of the
present invention.
FIG. 10 is a diagrammatic view in side elevation illustrating still
another embodiment or arrangement of the antenna for the
transmission system of the present invention.
FIG. 11 is a schematic drawing illustrating the electrical
connections of the antenna elements illustrated in FIGS. 5 through
10.
FIGS. 12a, 12b, and 12c are diagrammatic views illustrating the
magnetic fields in a cross section of the transmission line of the
present invention as respectively seen at adjacent one-fourth-pitch
intervals therealong.
FIG. 13a is a diagrammatic perspective view illustrating a
different construction of the induction-radio system of the present
invention wherein more than two conductors comprise the
transmission line.
FIG. 13b is a diagrammatic sectional view in front elevation of the
system illustrated in FIG. 13 a.
FIG. 13c is a diagrammatic view in side elevation of the system
illustrated in FIG. 13a.
FIGS. 14a, 14b, 14c, 14d, and 14e are diagrammatic sketches
illustrating the magnetic fields at points (a-a'), (b-b'), (c-c'),
(d-d'), and (e-e') as shown in the structure of FIG. 13c
respectively.
FIG. 15 is a schematic diagram illustrating the connection of the
two antenna elements illustrated in FIG. 13a.
FIG. 16 is a schematic diagram illustrating the connection of a
transmission line of the present invention having n -conducting
wires and an n-phase converter.
FIG. 17a is a transverse sectional view illustrating one embodiment
of the transmission-line structure of the present invention.
FIG. 17b is a transverse sectional view of the transmission line of
the present invention illustrating another embodiment of the
structure thereof.
FIG. 17c is a perspective view of the two-wire-transmission system
of the present invention illustrating another embodiment of
manufacture of the transmission line of the present invention.
FIG. 17d is a perspective view illustrating a further embodiment of
the physical structure of the transmission line incorporated in the
system of the present invention wherein four conductors are
employed.
FIG. 17f is a perspective view illustrating another embodiment of
the transmission-line structure for a four-line transmission line
employed in the system of the present invention as illustrated in
FIG. 17d.
FIGS. 17g, 17h, 17i, and 17e are cross-sectional views of four
different four-wire transmission lines of the present invention
respectively illustrating different methods of providing the
helical transmission-line structure of the present invention.
FIG. 18 is a perspective view of one embodiment of the present
invention illustrating the actual installation of the transmission
line in relation to the vehicle tracks.
FIG. 19 is a diagrammatic perspective view illustrating another
embodiment of the antenna which may be employed in the two-wire
transmission-line system of the present invention.
FIG. 20 is a diagrammatic sketch and graph illustrating variations
in the axial direction of the total signal-receiving voltage
illustrating the function of the antenna as shown in FIG. 19.
FIG. 21 is a diagrammatic graphical sketch illustrating variations
in the axial direction of the coupling condition between the
two-wire transmission line of the present invention and an antenna
therefor.
FIGS. 22a, 22b, and 22c are cross-sectional views illustrating a
different embodiments respectively of the antenna structure shown
in FIG. 19.
FIGS. 23a, 23b, and 23c are schematic diagrams respectively
illustrating three different examples of circuits which may be
employed for shifting the phase in the antenna employed in the
induction-radio system of the present invention.
FIG. 24 is a schematic diagram illustrating a phase-shifting
circuit which may be employed in the system of the present
invention.
FIG. 25 is a diagrammatic perspective view illustrating an
induction-radio system capable of detecting the vehicular
speed.
FIG. 26 is a diagrammatic perspective view illustrating another
embodiment of the structure shown in FIG. 25.
FIG. 27, FIG. 28, FIG. 29, and FIG. 30 are respectively schematic
block diagrams illustrating embodiments of the speed detection
circuits used in conjunction with the vehicular speed detectors
shown in FIGS. 25 and 26.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
An example of an induction radio system for vehicles or trains
embodying the principles of the present invention is explained,
with reference to FIG. 5 and FIG. 6.
The transmission line is provided by helically winding at an equal
and constant pitch two conducting wires 5 and 6 axially separated
by one-half of the winding pitch in the direction of the
cylindrical axis on the surface of a cylinder 4 parallel to the
rails 1.
The antenna consists of two elements 7 and 8 are provided such that
when the detecting amplitude of one of them is the maximum, the
detecting amplitude of the other becomes the minimum and are
mounted aboard the vehicle moving on the rails 1 and along this
transmission line in such a manner that the planes of the antenna
elements cross each other at right angles at a point in a plan
normal to the transmission axis.
As shown in FIG. 11, the two antennas 7 and 8 are connected to the
composite circuit 10 via a circuit 9 which shifts by .pi./2 the
phase of the current induced in the antenna 8.
A transmission signal is sent to the two conducting wires helically
wound around the cylindrical surface 4. Induced voltages appear
respectively in the antennas 7 and 8 which couple with the magnetic
fields accompanying these conducting wires 5 and 6. The current
created by the induced voltage of the antenna element 8 is
phase-shifted by .pi./2 by the circuit 9 and then combined with the
current created by the induced voltage of the antenna element 7 via
the circuit 10 to obtain a receiving signal.
The relationship between the conducting wires 5 and 6 and the
magnetic fields is as shown in FIG. 12. FIG. 12A shows with arrows
the directions of a magnetic field at a point where the conducting
wires 5 and 6 are positioned vertically. At a point one-fourth of
the winding pitch further from that point, the conducting wires 5
and 6 are positioned horizontally as shown in FIG. 12B and the
directions of the magnetic field are as shown by the arrows. At a
point one-fourth pitch still further from this point, namely at a
point one-half pitch further from the point of FIG. 12A, the
conducting wires 5 and 6 are positioned vertically and the
directions of the magnetic field are as shown by the arrows.
That is to say, when a transmission signal is sent to two
conducting wires helically wound and separated from each other by
one-half of the winding pitch, the spatial distribution direction
of the magnetic field accompanying the transmission line is
reversed at points separated from each other by one-half of the
winding pitch in the direction of the transmission line.
As a result, the radiation fields leaking to the outside from the
transmission line which consists of two conducting wires 5 and 6
cancel each other at every one-half of the winding pitch and become
effectively zero. This results in the prevention of interference
with outside communication systems.
Conversely, electromagnetic fields invading from the outside induce
electric current of antiphase at every one-half winding pitch in
the transmission line and cancel each other, thus becoming
effectively zero. This results in the prevention of the invading
noise from outside into the transmission line. The two antenna
elements 7 and 8 provided aboard the vehicle moving on the rails 1
along the transmission line are so positioned that two planes in
which they lie are normal to each other at a point in a third plane
normal to the transmission axis, so that the antenna element 7
detects the magnetic field in the direction a and the antenna
element 8 detects the magnetic field in the direction b which is at
a right angle to a as shown in FIG. 6. Explained in other words,
the magnetic field intensity vectors of antennas 7 and 8 are
perpendicular to each other and in a common plane normal to the
transmission axis.
Thus, the signal receiving voltage V.sub.c combining the detection
voltage V.sub.a of the antenna element 7 and the detection voltage
V.sub.b of the antenna element 8 is obtained by the circuit shown
in FIG. 11. There is the following relationship between them.
V.sub.c .sup.2 = V.sub.a .sup.2 + V.sub.b .sup.2 (1)
In FIG. 6, the conducting wires 5 and 6 are positioned vertically,
so that the magnetic field is directed from right to left, the
detection amplitude of antenna element 8 being the maximum and that
of the antenna element 7 the minimum, or zero.
At a point one-fourth pitch further from the position of the
conducting wires 5 and 6 in FIG. 6, the conducting wires 5 and 6
are positioned horizontally as shown in FIG. 12B, so that the
direction of the magnetic field is upwards, the detection amplitude
of the antenna element 7 becoming the maximum and that of the
antenna element 8 zero, or the minimum.
As mentioned already, when two such antenna elements 7 and 8 are
positioned such the two planes including them or in which they lie
are normal to each other, the amplitude of induced current in one
of them is maximum while the amplitude of the other becomes minimum
when they are installed aboard the vehicle moving along the
transmission line and the signal-receiving voltage is obtained by
the phase-combining circuit as shown in FIG. 11 from the voltages
detected by these two antenna elements 7 and 8 respectively. In
consequence, a signal-receiving voltage which is uniform at all
times is obtained irrespective of the position of the vehicle and
its moving speed.
FIG. 7 and FIG. 8 show a system in which two antenna elements 7 and
8 of the antenna are positioned in planes normal to each other
which meet at a point in a plane normal to the transmission axis or
in other words which meet on a common line of intersection which is
parallel with the transmission line. In the case of FIG. 7, the
respective planes which include the antenna elements 7 and 8 are
made normal to the radical direction with respect to the center of
the transmission axis.
FIG. 9 shows a system in which two antenna elements 7 and 8 are
installed aboard the vehicle moving along the transmission line as
explained in FIG. 5 at two points P and Q which are one-fourth
winding pitch apart from each other on the transmission axis so
that when the detection amplitude of one of them is maximum, the
detection amplitude of the other becomes minimum.
The planes which include the antenna elements 7 and 8 are parallel
to each other, and they are provided on a straight line parallel to
the transmission axis.
In this case, the antenna elements 7 and 8 are distant from each
other by one-fourth of the winding pitch on the transmission axis,
so that the directions of magnetic fields at the two points P and Q
have a difference of 90.degree.. In consequence, when the detection
amplitude of the antenna element 7 is maximum, the detection
amplitude of the other antenna element 8 becomes minimum.
If the voltages detected by these two antenna elements 7 and 8
respectively are phase-combined using a circuit as shown in FIG.
11, a composite receiving signal voltage is obtained just as in the
case of FIG. 5 and FIG. 6.
FIG. 10 shows a system in which the transmission line is assembled
by helically winding two conducting wires 5 and 6 axially separated
from each other by one-half the winding pitch in the direction of
the cylindrical axis around the cylindrical surface 4 and two sets
of antenna elements 7 and 8 are provided at points R and S which
are spaced apart by one-half of the winding pitch on the
transmission axis such that when the detection amplitude of one of
them is maximum, the detection amplitude of the other is minimum
are provided aboard the vehicle moving along this transmission
line.
The antenna elements 7 and 7 and the antennas 8 and 8 provided at
two points along the transmission axis respectively are of mutually
parallel directions.
If the voltages detected by the two antenna elements 7 and 8
provided at point R respectively are phase-combined, a composite
receiving signal voltage is obtained as in the case of FIG. 5. If
the voltages detected by the two antenna elements 7 and 8 provided
at point S are phase-combined, a composite receiving signal voltage
is likewise obtained. Since point R and point S are distant from
each other by one-half of the winding pitch, the directions of the
magnetic fields at the two points R and S are in the opposite
direction to each other. If the receiving signal voltages combined
at point R and point S are added together in reverse direction
i.e., the polarity of one is reversed, therefore, it is possible to
obtain a double receiving signal voltage and also to suppress the
outside noise coupled with the antennas and the interference with
other communication systems by radiation from the antennas.
Since two such antenna elements 7 and 8 are provided such that when
the detection amplitude of one of them is maximum, the detection
amplitude of the other is minimum are provided aboard the vehicle
moving along the transmission line, and the receiving signal
voltage is obtained, by phase-combining the voltages detected by
these two antenna elements 7 and 8 respectively, a constant
receiving signal voltage is obtained irrespective of the position
of the vehicle and its moving speed.
The coupling level between the transmission line and the coupler
system can be kept uniform along the transmission line. The
transmission characteristic is uniform and stabilized because the
transmission line has a uniform distance between the two wires
which are wound on the cylindrical surface and a broadband
transmission can be made. It is also possible to avoid such
shortcomings as the instantaneous interruption at the cross point
of the wires of the transmission line as shown in FIG. 4 caused by
movement of the train and changes in coupling level.
In the case where as shown in FIG. 10, two coupled pairs of
antennas having elements 7 and 8 are provided at each of two points
R and S separated apart by an odd number multiple of one-half of
the winding pitch along the transmission axis, and the voltages
detected by the two antenna elements 7 and 8 at each of the two
points R and S are phase-combined and added together in reverse
directions, outside noises entering the system at the two points R
and S are cancelled as a result of the addition in reverse
directions, so that the entry of noise from outside is prevented
and at the same time radiations from the antennas to the outside
are also cancelled and interference with other communication
systems are prevented.
Other embodiments of the present invention will be explained in
detail, referring to the drawings. The embodiments which are
explained by FIG. 5-FIG. 10 have a transmission line which is made
of a pair of conducting wires opposed or equally spaced apart and
wound helically at a constant pitch on a cylindrical surface and
two antennas above on a train and the output signals of which are
phase-combined to get a uniform receiving signal.
The embodiments which will be explained now, however, have a
transmission line which is made of a plurality of conducting wires
(n wires, n 2) equally spaced apart and wound helically at a
constant pitch on a cylindrical surface, n wires being fed in such
a manner that the phase difference between a current in a wire and
that in its adjacent wire is 2.pi./n radians, and a single antenna,
as a rule, provided aboard a train.
FIG. 13A is a perspective view showing the construction of an
embodiment of this invention. FIG. 13B is a view from e.sub.1 of
FIG. A. FIG. 13C is a view from e.sub.2 of FIG. A. In FIG. 13, 1
denotes the rails, 4 the transmission line, 3.sub.1 and 3.sub.2 the
receiving antennas.
The transmission line according to this invention is characterized
in that four wires (generally, three or more wires) are cubically
twisted together. This condition is shown at 5, 6 and 5', 6' in
FIG. 13A, B and C. As a result of this arrangement, outside
broadcasting electric waves from all directions and outside noises
such as those generated by the commutator of the motor of the
train, trolley wires, switches, etc. cannot induce current in the
transmission line for the reason hereinafter mentioned, while at
the same time the power radiated to the outside from the
transmission line is cancelled and becomes zero, thus eliminating
interference with other communication systems.
FIG. 14A, B, C, D, and E show by a group of curves, including
arrows the distribution of magnetic field at a certain instant in
the sections (a-a'), (b-b'), (c-c'), (d-d'), (e-e') of FIG. 13C
respectively. The group of solid lines indicate the magnetic field
accompanied by electric current of a transmitted signal flowing in
5, and 6, and the group of dotted lines indicate the magnetic field
accompanied by electric current flowing in 5' and 6'. In FIG. 13,
the distance between the section (a-a') and section (e-e')
corresponds just to one-half of the pitch, and, as is clear from
FIG. 14A and E, the directions of spatial distribution of the
magnetic fields are opposite to each other. In consequence,
radiation fields of the transmitted signal leaking from the line
cancel each other ever one-half of the pitch and become effectively
zero. Conversely, electromagnetic fields from outside induce
current of the inverse phase every one-half of the pitch and also
become effectively zero.
In order to satisfy the requirement for uniformity of coupling
between the aforementioned transmission line and the antenna aboard
the train, it is necessary to carry out the phase shifting of
signals at the signal sending end or the signal receiving end. This
will be explained below.
The above-mentioned purpose can be attained by feeding two pairs of
conducting wires 5 and 6, 5' and 6' (Generally, three or more
conducting wires) constituting the transmission line with one and
the same signals, each of which has a phase difference of .pi./ 2
(generally, the quotient of 2.pi. divided by the number of
conducting wires). This is tantamount to applying 4-phase
alternating current to the four conducting wires one by one.
Referring to FIG. 14, if the magnetic fields for the pairs are
h.sub.1, h.sub.2 respectively, the voltage V induced in the antenna
3 is obtained by the formula given below.
V=K(H.sub.1 cos.theta. .+-.Jh.sub.2 sin.theta. ) (2)
The angle .theta. in the formula (2) is shown in FIG. 14 and is the
displacement angle of the magnetic field to the antenna direction
caused by the twist of the conducting wire. K is a constant which
is determined by the frequency of the signal current and the area
of the antenna. Since h.sub.1= h.sub.2 in the formula (--2), it
follows that
V =Kh.sub.1= Kh.sub.2 (3)
The formula (3) means a single antenna coupled with said
transmission line can effect the same uniform coupling as the
uniform coupling in the direction of the line effected by an
antenna coupled with the conventional two parallel wire
transmission line. Generally speaking, the same result as that
shown by the formula (3), namely the result that the signal
receiving level V =constant, can be obtained if the signals with
the same amplitude having a phase difference of 2 .pi./n between
adjacent two wires are applied to input terminals through a
single-phase n-phase converting circuit 12 as shown in FIG. 16,
namely, if n-phase alternating current is applied to the input
terminals.
3.sub.1 and 3.sub.2 in FIG. 13A, B, and C denote antennas aboard
the train installed at a distance of one-half pitch from each
other. As mentioned in the foregoing description of the
transmission line, the directions of spatial distribution of
magnetic fields along the transmission line are reversed just at
one-half of the pitch, so that the two antennas 3.sub.1 and 3.sub.2
are connected to combine the induced voltages of two antennas as
shown in FIG. 15. As a result of such a connection, the
signal-receiving power becomes double that of a single antenna.
Moreover, outside electric waves and noise directly entering the
antennas do not appear at all between output terminals 10 and 10'
because the magnitude and phase of the outside electric waves and
noise are both equal if the pitch is sufficiently small with
respect to the wave length.
It is also possible through the use of the apparatus of this
invention to prevent outside broadcasting electric waves from all
directions and outside noise caused by the commutator of the motor
of the train, pantagraphs, etc. from inducing current in the
transmission line and entering it. The transmission line is
provided by helically winding conducting wires on a cylindrical
surface at a constant pitch, so that the portion of the
electromagnetic field propagated along the transmission line which
has the greatest electromagnetic magnitude, namely the part where
the concentration of the transmission electromagnetic field is
greatest is in the space between the conducting wires. This
precludes the entry of any obstacles, and makes it possible to
prevent the disturbance of the transmission characteristics.
Since the transmission line is constructed by helically winding
conducting wires on a cylindrical surface, it is possible to make
the transmission line small and compact and mechanically strong. It
has a stabilized transmission characteristic and does not interfere
with the maintenance work for the railway tracks.
FIG. 17 shows embodiments of the transmission-line structure used
in the train induction-radio system. Fig. 18 shows embodiments of
the installation of the lines as shown in FIG. 17.
FIG. 17A shows an example of the transmission line. It is made by
helically winding two conducting wires (generally, n conducting
wires nl, 2, 3 ... ) separated apart one-half of the pitch
(generally 1 /n of the pitch) along the axis on a hollow circular
cylinder 11 made of an electric-insulating plastic which may be a
hollow elliptic cylinder.
Since a hollow cylinder 11 is provided in the transmission line 4
where electromagnetic field concentrates, this transmission line
has a stabilized characteristic. No obstacles enter there to
degrade the transmission characteristic and no rain or moisture can
enter there to increase transmission loss. Furthermore, as the
spacing between the two conducting wires 5 and 6 can be maintained
at a uniform value, the transmission characteristics at high
frequency are not degraded.
FIG. 17B shows a transmission line made by helically winding two
tape-form conducting wires 5 and 6 generally n tape-form conducting
wires n: 2, 3, .....) separated from each other one-half (generally
1/n) of the pitch along the axis of a hollow cylinder 11 made of
electric-insulating plastic and providing over them an
electric-insulating covering layer 12 of polyvinyl chloride,
polyethylene, etc.
The provision of a covering layer 12 protects the conducting wires
5 and 6 and at the same time helps the conducting wires 5 and 6
stick to the hollow cylinder 11, thereby preventing disorder in the
pitch and displacement of the conducting wires 5 and 6.
FIG. 17C shows another example of the transmission line. It
consists of conducting wires 5 and 6 (generally n wires) each
covered with an electric insulating sheath which are stranded
together.
FIG. 17D shows the simplest construction. It is constructed by
supporting four (generally three or more) conducting wires 5, 6, 5'
and 6' by means of supporters 13, such as porcelain insulators, in
such a manner that their spacing conforms to the design value. In
this case, the conducting wires will have the form of a zigzag
lines instead of continuous curves.
FIG. 17E shows a construction in which conductors 5, 6 and 5', 6'
are provided with a covering 15 of polyvinyl chloride or the like
in order to protect the conducting wires and also to help the
conducting wires stick to the plastic cylinder 14, making it
possible to preclude disorder in the pitch and displacement of the
conducting wires. FIG. 17F shows hanger parts 16 made as an
extension of the covering 15 in the construction of these
transmission lines, except those of C and D aforementioned, when
used as an aerial line.
FIG. 17G shows a construction obtained by arranging four pipes
(generally, n pipes: n 3) as shown in the drawing, further placing
four (generally n, n 3) conducting wires 5, 6 and 5', 6' as shown
in the drawing, twisting them together and wrapping them together
with polyethylene tape 18 or the like to keep them in place.
FIG. 17H shows a construction in which conducting wires 5, 6, 5',
and 6' are buried in the four ends of the cross-shaped plastic or
supports 1a (generally, a shape having n radial arms) and a
covering 15 is provided.
FIG. 17I shows a construction in which four (generally, n: n 3)
conducting wires 5, 6 and 5', 6' are placed equidistantly on a
plastic cylinder 20, filling the gaps with cords 21 of plastic or
the like, and providing them with a covering 22.
In the foregoing figures showing embodiments of the present
invention, the cylinder on which conducting wires are wound is
shown as having a circular cross section. However, the section of a
cylinder need not necessary be a circle but may be elliptical or
otherwise.
FIG. 18 shows an example of the transmission line installed. 1
denotes the rail, 4.sub.1, 4.sub.2, and 4.sub.3 the transmission
lines, and 23 the supports supporting these lines. The transmission
line 4.sub.1 is supported on high supports beside a rail track,
4.sub.2 is supported on low supports between the rails, and 4.sub.3
is suspended from a support besides a rail.
The ordinary loop antennas, loop antennas having ferrite core in
them, etc. are used as signal sending and receiving antennas
coupling with the magnetic field of the transmission lines as
explained.
The antenna element used in the induction-radio system of the
present invention are loop antennas, loop antennas having ferrite
cores in them, etc. as already mentioned. However, the antenna
embodiment described below is that of a type unique to the
induction-radio system of the present invention.
As shown in FIG. 19, the antennas for the train induction radio of
this invention have a construction in which two pairs of twisted
wires having the same pitch as the transmission line are assembled
and separated axially from each other by one-fourth pitch and their
terminals are short-circuited by short-circuit wires 32 and 33. The
opposed shunted pairs thus provide two separate antenna elements.
It is suitable that the length of the antenna, i.e. P.sub.O to
P.sub.4 in FIG. 20, is equal to the pitch multiplied by an integer.
As the length increases, the amount coupled is also increased. The
signal-receiving system using the antennas of this invention is one
in which the receiving signal voltages of the pairs constituting
the antenna are phase-combined by means of the phase-combining
circuit 30. 31 and 31' are terminals. These will be explained in
detail below.
Referring to FIG. 19, the receiving signal voltages of the antenna
pair or element 34, 34' vary sinusoidally as indicated by the curve
A in FIG. 24 depending on the position of the terminals 35, 35' of
the pair or element 34, 34'. On the other hand, the receiving
signal voltages of the pair or antenna element 36, 36' vary
sinusoidally as indicated by the curve B having a spatial phase
difference of .pi./2 from the curve A in FIG 20, depending on the
position of the terminals 37, 37' of the pair element 36, 36'. The
receiving signal voltage becomes maximum at the points where the
positional relationship between the conducting wires 34 and 34' or
36 and 36' constituting the antenna, and the conducting wires 5, 6
constituting the transmission line become as shown by the sections
a.sub.2 a.sub.4 or a.sub.1, a.sub.3, and a.sub.5, while the
receiving signal voltage becomes zero in the section a.sub.1,
a.sub.3, and a.sub.5 or a.sub.2 and a.sub.4 and respectively. If
such two receiving signal voltages having a phase difference of
.pi./2 are compounded by giving them temporal or an additional
phase difference of .pi./2, it is made possible to realize a
uniform receiving signal level independent of the position of the
antenna. The reason for this is as follows: If the receiving signal
voltage of the pairs 34, 34' is V.sub.A =A sin(2.pi./P )Z, and the
receiving signal voltage of the pair 36, 36' is V.sub.b =A
cos(2.pi./p )Z, then, by giving temporal phase difference of
.pi./2, the compound voltage V.sub.r is reduced to the following
equation:
V.sub.r =V.sub.a +jV.sub.B
=A sin(2.pi./P)Z.+-.JA cos(2.pi./P)Z
Hence,
V.sub.r .sup.2 = V.sub.A .sup.2 + V.sub.B .sup.2 =A sin.sup.2
(2.pi./P)Z+A.sup.2 cos.sup.2 (2.pi./P)Z=A.sup.2
Thus receiving signal voltage level can never be zero.
It is preferable that the direction of twist of the conducting
wires of the antenna is opposite to the direction of twist of the
wires of the transmission line. This will be explained below.
FIG. 21 shows the positional relationship along the transmission
axis between antenna element 36-36' and conductors 5-6 when the
rear end of element 37 and 37' of the antenna is in the position of
a.sub.1 in FIG. 20 with respect to the transmission line, and the
penetration of the magnetic field is generated by conductors 5-6
through element 36-36'. From the Figure it can be seen that
voltages induced in antenna element 36-36' are added together at
each point. That is to say, the polarity of the induced voltage is
of the same phase along the antenna element. This will not be the
case if the direction of twist of the antenna is the same as that
of the transmission line.
The receiving signal voltage level increases as the length of the
antenna increases. However, the length must be an integral multiple
of the pitch. If the length is an integral multiple of the pitch,
electromagnetic fields of reverse spatial directions are created
every one-half pitch when it is used as a signal sending antenna,
the reason being the same as that already mentioned in connection
with the transmission line. Thus the radiation power to outside
becomes zero. However, signals are effectively excited on the line
because of its twist structure with the same pitch as that of the
antenna and in the opposite-twisting direction to the antenna.
FIGS. 22 show examples of the antenna of this invention.
FIG. 22A shows an antenna made of a plastic pipe on which four
conducting wires 5, 6, 5' and 6' are helically wound in the same
direction and at the same pitch. In this Figure, the group of
broken lines having arrows show the spatial distribution of the
electric field, and the group of solid lines having arrows shows
the spatial distribution of the magnetic field. From this Figure,
it should be noted that the electromagnetic field concentrates in
the cylinder 14, so that the characteristics of the antenna become
stable, there being no degradation of properties due to entry of
obstacles and no degradation of properties due to rain or moisture,
41 denotes an example of installation fittings.
FIG. 22B shows an construction in which the conducting wires 5, 6,
5' and 6' of A are provided with a covering 42 made of polyvinyl
chloride or the like in order to protect the conducting wires and
aid in securing the conducting wires to the plastic cylinder 14,
which makes it possible to prevent disorder of the pitch and
displacement of the conducting wires.
FIG. 22C shows a construction in which the conducting wires 5, 6,
5' and 6' of B are replaced with foil tapes so as to make the
covering 42 thinner.
FIGS. 23 show examples of circuits for phase shifting.
FIG. 23A shows an example where a delay line is used; the change of
phase generally is proportionate to the length of the line. FIG.
23B and C show the circuits which utilize inductance and
capacity.
FIG. 24 shows another embodiment of a phase-shifting device. If
this device is used, it becomes possible to obtain a uniform
characteristic over a broad band.
This phase-shifting device is characterized in that a signal having
a constant phase shift is obtained by demodulating the signal which
is phase-shifted by a constant amount for a single side band
component which is produced by modulation at a very high frequency
compared with the frequency of the signal to be transmitted in the
line. In the induction-radio system, receiving electrical signals
by phase-compounding electrical signals, makes the phase
characteristic uniform over a broad frequency band and improves the
system characteristics.
In this case, as shown in FIG. 24, the electrical signal from the
power source 46 is subjected to single sideband modulation at high
frequency by a single-sideband modulation circuit 43 and then its
phase is shifted by a constant amount by a phase-shifting circuit
44, then detected by a demodulation circuit 45 to be sent to the
conducting wires 5 and 6 of the transmission line, and at the same
time the electrical signal from the power source 46 is sent to the
conducting wires 5' and 6' of the transmission line via the circuit
47 for making its amplitude equal to that of said phase-shifted
signal. The two electrical signals fed to wires 5 and 6 and wires
5' and 6' of the transmission line which have phases different by
.pi./2 are uniformly received by a single antenna.
When the electrical signal from the power source 46 is subjected to
single-sidebond modulation by the single-sideband modulation
circuit 43, the modulated signal will be as shown by the formula
below, if the signal of one frequency component in the signal band
is represented by se.sup.j t and the carrier wave by ti ce.sup.j t
:
ce.sup.j t +mse.sup.j( )t (4)
where c and s are constants and m the degree of modulation.
If the modulated signal represented by the formula (4) is given a
phase difference of .theta. by shifting the phase at a high
frequency in the phase-shift circuit 44, it becomes as shown
below.
ce.sup.j t.sub.+ j +mse.sup.j( .sub.+ )t.sub.+j (5) If this is
detected by the demodulation circuit 45 to take out the signal
component, then it becomes m.mu.se .sup.j .sub.+j This means that a
signal having a phase difference of .theta. from the output signal
of the source 46 is produced. Here .mu. is the ratio of change in
the amplitude of signal which takes place at the circuit of the
demodulation.
If .omega./.omega. in the formula (5), the frequency bandwidth used
in the phase-shift circuit 44 to give the phase difference of
.theta. is sufficiently small as compared with the carrier
frequency .omega..sub.o, so that the phase characteristic in the
signal band becomes uniform. According to the present invention,
detection is made after shifting the phase of a
single-sideband-modulated signal, so that at a high frequency a
detected signal having a phase difference of .theta. from the power
source 46 is obtained.
The foregoing refers to a system wherein electrical signals are
transmitted to the transmission line and received by an antenna
provided aboard a train moving along this transmission line.
Needless to say, the same process can be applied to a system
wherein electrical signals are fed from an antenna provided aboard
the train onto the transmission line for reception.
According to this invention, since an electrical signal fed to the
transmission line at a low-frequency band is once modulated by a
carrier wave of a high frequency and/or phase shifting, the
modulated wave is easily and uniformly made at high frequency, it
is therefore possible to ensure good system characteristics.
The induction-radio system of this invention may also be applied to
the detection of train speed and train location. The principle of
this system is hereinafter explained.
If the time oscillation term is added to the formula (2) to make
the complete expression, the received signal Sr will reduce to the
following equation: ##SPC1## where P is the twist pitch in meters
If z=vt (v: velocity of the train) is substituted, ##SPC2##
Hence, the amount of deviation of frequency is:
.DELTA.f=.+-. V/P (8)
The above formula with respect to the amount of deviation .DELTA.f
is applicable to both sending and receiving of signals.
This relates to a train speed-detecting system which is
characterized in that the speed of a train is obtained by measuring
of the frequency deviation .DELTA.f of the electrical signal
transmitted between the antenna and the transmission line which
deviates in proportion to the moving speed of the antenna aboard
the train.
The detection of the travelling speed of a train is necessary for
the automatic control of a train. For this purpose, the following
systems have heretofore been used.
The first of them is a system in which the number of revolutions in
a given unit of time or the shaft of wheels of the travelling train
is counted to the travelling speed. A second such system is shown
in FIG. 25 wherein signal sending elements 53 are periodically
provided at fixed intervals on the transmission line 52 installed
along the rails 1. An antenna 54 corresponding to them is provided
aboard the train. The number of signals received by the antenna 54
in unit time is counted to find out the speed of the train.
A third such system is as shown in FIG. 26 wherein two conducting
wires 52 installed in parallel to the rails 1 are periodically
crossed in a plane at fixed intervals. An antenna 54 is provided
aboard the train. An electrical signal is transmitted to the
conducting wires 52 and the number of interruptions in unit time of
the signal received by the antenna 54 at the crosses 55 of the
conducting wires 52 is counted to find out the speed of the
train.
In case of the first-mentioned system, an error takes place as a
result of wear on the wheels, so that it is necessary to make
correction for the error. In the case of the second-mentioned
system, if the antenna 54 should fail to receive a signal from a
signal sending element 53 for some reason or other, the speed
detected by the device is lower than the actual speed. This could
be a serious safety problem. In addition, this system will be
inevitable that it is interfered with by outside noise waves and
will interfere with outside communication systems. In the case of
the third-mentioned system, the construction is such that an
obstacle is likely to find its way into the transmission space
between the two conducting wires 52, causing not only interference
with outside communication systems but also a Moreover,
transmission loss. Moreover, it obstructs the maintenance work on
the railway tracks. Furthermore, the upper limit of the
transmission frequencies is restricted under the influence of
capacity existing in the neighborhood of the crosses of the two
conducting wires, so that the frequency band available for use is
narrow. Here also, the number of interruptions in a unit time is
counted. As in the case of the second-mentioned system, therefore,
the difficulty arises that the detected speed is less than the
actual speed if failure to detect interruption should take place
for some reason or other.
According to the present invention the above-mentioned shortcomings
of the speed detection systems heretofore in use are eliminated. It
makes it possible to measure the travelling speed of a train
without error. The speed detection system of this invention also
possesses many such features such as being free from interference
from outside noise waves, avoidance of interference with outside
communication systems, effecting a stabilized transmission, and
making the construction simple and strong and not obstructing to
the maintenance work being done on the railway tracks.
The frequency of the received signal deviates from the frequency of
the sending signal by .DELTA.f which is proportional to the
travelling speed Vm/sec. This deviation amount .DELTA.f may be
expressed as follows:
.DELTA.f=.+-.V/P
where p is the twist pitch in meters.
The sign of .DELTA.f is determined by the direction of travel of
the train, whether the wires are twisted left-hand or right-hand,
and the phase-shift direction of the phase-shift circuit as shown
in FIG. 11 circuit 9 is for advancing or delaying the phase of the
receiving voltage of one antenna with respect to that of the
receiving voltage of the other antenna.
Thus it is possible to find out the travelling speed by measuring
deviation amount .DELTA.f between the frequency of the sending
signal sent to the transmission line and the frequency of receiving
signal received by the antenna and converting this measured value
to the actual speed value.
An embodiment of this invention is shown in FIG. 27. A transmission
signal of a frequency fo is sent from the ground station 63 to the
transmission line 64. This transmission signal is received by the
antenna 65 which couples with magnetic fields accompanied with the
transmission line 64 and which is provided on the travelling
train.
The frequency of this received signal is fo .+-..DELTA.f, a
frequency deviation from the frequency fo of the transmission
signal by an amount .DELTA.f which is proportional to the speed of
the train.
A signal having the same frequency fo as the transmission signal is
generated by a local oscillator 66 and given to the mixer 67
together with the received signal and the difference .DELTA.f in
frequency is measured by the mixer. If this value is calibrated by
the pitch of the transmission line 64, the travelling speed v of
the train is obtained.
FIG. 28 shows another embodiment. Transmission signal of frequency
fo is sent from the ground station 63 to the transmission line
64.sub.1 of right-hand winding and the transmission line 64.sub.2
of left-hand winding. The signal is received by antennas 65.sub.1
65.sub.2 aboard the travelling train which couple with the magnetic
fields accompanying the transmission lines 64.sub.1 and 64.sub.2
respectively.
These received signals have deviations of frequency in opposite
directions because the directions of winding of the transmission
lines are opposite to each other, so that the frequencies of the
received signals are fo .+-..DELTA.f and fo.-+..DELTA.f
respectively.
These receiving signals are put in the mixer 67 and the frequency
deviation amount .DELTA.f is measured. Then the travelling speed of
the train will be determined.
FIG. 29 shows another embodiment. A transmission signal of a
frequency fo is sent from the ground station 63 to the transmission
line 64; it is received by two antennas 65.sub.1 having the phase
shift circuit which advances phase and at the same time by two
antennas 65.sub.2 having a phase-shift circuit which delays the
phase; the received signals or frequencies fo .+-..DELTA.f and fo
respectively are put in the mixer 67 to measure the deviation
amount .DELTA.f to determine directly the travelling speed of the
train. FIG. 30 shows still another embodiment. A transmission
signal of a frequency fo is sent from the ground station 63 to the
transmission line 64; the signal is received by the antenna 65
aboard the train; this received signal of a frequency fo
.+-..DELTA.f is amplified by a reflex amplifier 68 and then again
sent from the antenna 65 to the transmission line 64 and received
by the ground station. The frequency of the signal received by the
ground station is fo.+-.2.DELTA.f. This is put in a mixer 67
together with the transmission signal of the frequency fo to
measure the frequency deviation amount .DELTA.f, and then the
travelling speed of the train is obtained.
In the case of the speed-detecting systems of this invention, a
signal between the transmission line and the antenna is received
continuously, so that an interruption in the receiving signal due
to some cause or other does not affect the speed detection. This
invention makes it possible to prevent outside broadcasting waves,
noise generated by the commutator of the motor of the train,
pantagraph, etc. from inducing electric current in the transmission
line and antennas and thus to eliminate detection errors.
At the same time, electromagnetic fields radiated from the
transmission line to the outside cancel each other and become zero
every one-half of the pitch of the transmission line, so that it is
possible to preclude interference with outside communication
systems.
The transmission line is constructed by helically winding two
conducting wires separated apart by one-half of the pitch on a
cylindrical surface, so that the part of the electromagnetic field
propagated along the transmission line which has the greatest
electromagnetic intensity, i.e. the part where the concentration of
electromagnetic field is great, is the space between the two,
helically wound conducting wires. As a result, obstacles will not
enter this space, so that the transmission characteristics may be
kept stable and the detection error made small. It is possible to
keep the coupling of the transmission line and the antenna uniform
in the axial direction of the transmission line. Transmission
characteristics are stabilized and a broad band transmission is
possible because the spacing between conducting wires is kept
uniform. Such obstructions to transmission as the instantaneous
interruption due to the running of the train and other changes in
level can be avoided and the detection error is therefore made
small.
As the travelling speed is detected by measuring the deviation
amount of the frequency of the transmission signal, it is possible
to decrease noise and and to make errors small.
If the system of this invention is employed, the travelling speed
of the train can be detected by stations both on board the train
and on the ground simultaneously.
* * * * *