Noise suppression device

A noise suppression device for noisy transmission wires in a motor vehicle including a first low impedance element connected to an arbitrarily chosen point on the noisy wire and a second low impedance element connected at the nodal point of the standing wave of the noise current on the noisy wire.

BACKGROUND OF THE INVENTION 
1. Field of Invention 
This invention relates to noise suppression devices and more particularly 
to noise suppression devices utilized in motor vehicles. 
2. Description of the Prior Art 
In recent years automobiles have been equipped with various kinds of 
electronic systems such as FM radios, radiotransmitters, electrically 
controlled fuel injection devices, etc. Since the systems operate with 
signals in the high frequency range, they are vulnerable to adverse 
effects caused by high frequency noise currents steming from the ignition 
system, voltage regulator, and various other kinds of manual and automatic 
switches (horn relays, turn signals, etc.). These harmful effects, are, 
for instance, noise in the radio signal, malfunction or loss of function 
of the system, etc. Accordingly, it has become frequently necessary to 
provide some remedy to eliminate these harmful effects. 
In the prior art, it has become practice to suppress the noise by 
preventing noise generation itself by means of an anti-noise device such 
as a capacitor that bypasses high frequency noise currents at their 
sources or by avoiding any coupling of a wire that passes noise currents 
with another wire that is coupled to an electronic device which would be 
vulnerable to noise effects. The latter of noise prevention methods is 
effective when a high frequency noise current is being coupled from one 
wire to another. In this case the noise prevention is achieved by either 
increasing the distance between the two wires or by reducing this 
coupling, using, for example, a shielded wire. Previously used methods, 
however, have not proven fully effective and are also associated with 
drawbacks such as increased vehicle cost due to installation of additional 
special equipment and increased space requirements for the special 
equipment and redesign of the electrical wiring harness, etc.. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general object of the present invention to provide a 
noise suppression device for the wires in motor vehicles in which high 
frequency noise currents are present which suppresses the noise to an 
extremely low level. 
It is another object of the present invention to provide a noise 
suppression device which does not consist of any special equipment. 
It is yet another object of the present invention to provide a noise 
suppression device for motor vehicles which does not require any extra 
space in the automobile body. 
It is still another object of the present invention to provide a noise 
suppression device for motor vehicles which is low in cost. 
In keeping with the principle or principal concept of the present 
invention, the objects are accomplished with a unique noise suppression 
device comprising a first low impendance element connected to an 
arbitarily chosen point on a noisy wire in the motor vehicle thereby 
partially reducing the noise on the noisy wire and creating a standing 
wave on the noisy wire. A second low impendance element is then connected 
at a nodal point of the standing wave of the noise current on the noisy 
wire thereby substantially reducing the noise current on the noisy wire. 
In practice the low impendance element may consist of a length of wire 
with an open at one end and electrically coupled to the noisy wire at the 
other. Furthermore, the length of wire should have a length equal to 
approximately odd multiples of the wave length of the noise current 
divided by four.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 1, shown therein is the state of a noise current on a 
noisy wire such as a connecting wire to a defroster heater wire. The 
defroster heating element 10 is installed on the rear windshield of the 
vehicle and its connecting wire 12 extends to the battery via a switch 
located near the drivers seat of the vehicle. Therefore, the connecting 
wire 12 is apt to pick up noise from the ignition system, voltage 
regulator, etc. and is liable to become very noisy. Since this wire 12 
extends from the front section of the vehicle all the way to the rear 
section of the vehicle through a long distance, it tends to be 
electromagnetically coupled with other wires. It is well known that if the 
radio antenna is placed near the rear window, the noise current will be 
electromagnetically coupled to the antenna and generate noise in the 
radio. 
As shown in FIG. 1 the intensity of the noise current i of the noisy wire 
12 is substantially uniform over its entire length. This is the case where 
the impedance matching between the noisy wire 12 and the defroster heating 
wire 10 is favorable; i.e., the value of the characteristic impedance 
Z.sub.L of the defroster heaing wire 10 at its input is approximately 
equal to the value of the characteristic impedance Z.sub.O of the noisy 
wire 12. Furthermore, the noise current along the noisy wire 12 does not 
form a standing wave pattern in this case. Instead the current density is 
nearly constant as shown in FIG. 1. 
Referring to FIG. 2, the first noise current suppression wire 14 is 
connected at an arbitrary location to the noisy wire 12 to form the first 
low impedance branch path as shown in FIG. 2. The first noise suppression 
wire 14 may be a simple single conductor wire with an open at one end 
placed adjacent a conducting surface or the like of the vehicle body. It 
is desirable to set the length of the suppression wire 14 at (.lambda./4) 
+ (.lambda./2) .times. n (n = 0, 1, 2, . . . ) where .lambda. is the wave 
length of the high frequency noise current that transmits on the wire. For 
the case shown in FIG. 2, the distribution of the noise current i and the 
noisy wire 12 changes into the form shown by the broken line. The input 
impedance of the first noise suppression wire 14 is at an extremely low 
value of only a few ohms as seen from point 101 where the noise 
suppression wire 14 is connected, whereas the input impedance of the 
defrostive wire 10 seen from the same point is its characteristic 
impedance Z.sub.L which is a relatively high value, like a few hundred 
ohms. Consequently, a substantial portion of the high frequency noise 
current flows into first noise suppression wire 14. The noise current 
distribution in the noisy wire 12 is therefore determined by the placement 
of the first noise suppression wire 14. Since the impedance of wire 14 is 
lower than that of wire 12, an impedance mismatch occurs between wire 14 
and the noisy wire 12. Accordingly, as a result, a standing wave having a 
set pattern is formed on noisy wire 12 as shown in FIG. 2. 
Since the initial intensity of distribution of the noise current is 
approximately uniform with the noisy wire 12, the first noise suppression 
wire 14 can be connected at any arbitrarily selected point, such as an 
empty space in the vehicle body. 
It is clear from the preceding explanation that the standing wave pattern 
is established over noisy wire 12 by the coupling of the first noise 
suppression wire 14 to the noisy wire 12. Although, with this measure a 
certain decrease in the noise current in the noisy wire 12 is recognized, 
it is still at a level that will cause a serious detrimental effect. To 
suppress the noise even further, a second noise suppression wire 16 that 
forms a second low impedance branch is coupled to noisy wire 12 as shown 
in FIG. 3. 
In FIG. 3, the second noise suppression wire 16 of a length (.lambda./4) + 
(.lambda./2) .times. n (n = 0, 1, 2, . . . ) with an open end is coupled 
at point 102 which has become a node of the standing wave pattern on noisy 
wire 12. Since the impedance of noisy wire 12 at the junction point 101, 
as seen from junction point 102, is very high at the frequency of the 
noise current and since the impedance of the second suppression wire 16 is 
extremely low, substantially all of the high frequency noise current i 
carried by noisy wire 12 flows into noise suppression wire 16 and almost 
no noise current preceeds towards junction point 101. This phenomenon is 
not limited to exactly the frequency of the noise current as set by the 
length of the noise suppression wires 14 and 16, but also applies to the 
frequency range in the vicinity of the frequency of the noise current and 
accordingly enables the noise suppression in that frequency range. 
Incidentially, when the second noise suppression wire 16 is connected at 
junction point 102, the location of the node of the standing wave shifts 
to point 103. 
By locating the second noise suppression 16 as disclosed above, the 
transmission of the noise current into defroster heating wire 10 is 
greatly suppressed and accordingly the capture of noise by antenna for the 
radio is prevented. 
In practice, the lengths of the first and second noise suppression 14 and 
16 in FIG. 3, are set at (.lambda./4). (.lambda./4) is approximately equal 
to 70 centimeters for a frequency of the FM radio (85 MHz). To install the 
two noise suppression wires 14 and 16 on the vehicle, the noise 
suppression wires 14 and 16 are taped to the noisy wire 12 with plastic 
tape. Such a structure has the merit that the wire harness can be easily 
attached to various parts of the vehicle body and it is also possible to 
further enhance the effectiveness of the noise suppression as a whole due 
to phase cancellations of high frequency currents in those sections where 
noise suppression wires 14 and 16 are parallel to the noisy wire 12. 
Moreover, it is possible to increase the noise suppression even more by 
increasing the number of second noise suppression wires 16. FIG. 5 shows 
an embodiment wherein the noise suppression wires 14 and 16 are provided 
in parallel with the noisy wire 12. Provided in a rear window 22 is a 
defogger heating wire 10, one end of which is connected to a battery (not 
shown) via a wire 12, and the other end of which is earthed. The noise 
suppression wires 14 and 16 are connected onto said wire 12. These wires 
14 and 16 are adhesively attached to the wire 12 with plastic tape, or may 
be formed of electrically conductive materials coated on the insulating 
shield of the wire 12. Disposed in a rear window 22 is a radio antenna 26 
connected to a radio receiver, i.e., FM radio receiver. 
The embodiment disclosed herein has been described in relation to the noise 
contamination of an FM radio due to the defroster heating wire 10. 
However, the same will apply to noise problems due to high frequency noise 
currents on other wires. Furthermore, it is also possible to substitute 
other types of low impendance elements such as a capacitor for the noise 
suppression wires 14 and 16 and to connect the second low impendance 
element between a node of the standing wave and the vehicle body. 
FIG. 4 shows experimental results that demonstrate the effectiveness of the 
invention as a means of noise suppression in the frequency range of an FM 
radio. The abscissa in FIG. 4 is the distance (L) measured from the input 
end 100 of the defroster heating wire 10 along the noisy wire 12 in a 
direction away from the heating wire 10. The ordinante is the intensity 
(I) of the noise current. The initial noise current 200, as shown in FIG. 
4, is suppressed by about 10 db, line 201, by connecting the first noise 
suppression wire 14 to the junction point 101. The noise current is 
further suppressed another 10 ab as shown by line 202 when a second noise 
suppression wire 16 is coupled to the nodal point 102 of the standing wave 
pattern thus formed. Therefore, it has been confirmed that a noise 
suppression of approximately 20 db from the initial state can be achieved. 
According to the preceeding explanation, it has been shown that it is 
possible to reduce the intensity of the noise current to such a low level 
that it has no effect upon other wires. Furthermore, since the invention 
allows a free choice for the location of the first noise suppression wire 
14 and then fixes the second noise suppression wire 16 at a specific 
position relative to the first wire, it is possible to choose an optimum 
location for the first and second noise suppression wires 14 and 16 in the 
generally limited available space of a vehicle. Moreover, the invention 
has the following merits: 
1. It effectively suppresses noise without effecting the primary function 
of the wire; 
2. The structure is simple and inexpensive and resists damage due to 
vibration or impact; 
3. The frequency range in which it provides noise suppression can be 
readily changed by properly adjusting the length and location of the 
attachment of the noise suppression wires; and 
4. It is inexpensive. 
In all cases it is understood that the above described embodiment is merely 
illustrative of one of the many possible specific embodiments which 
represent the applications of the principle of the present invention. 
Furthermore, numerous and varied other arrangements can be readily devised 
in accordance with the principle of the present invention by those skilled 
in the art without departing from the spirit and scope of the invention.