US 3694754 A
Description (OCR text may contain errors)
United States Patent Baltzer  SUPPRESSION OF ELECTROSTATIC NOISE IN ANTENNA SYSTEMS  Inventor: Otto J. Baltur, Austin, Tex.
 Assignee: Trac'or, Inc., Austin, Tex.
 Filed: Dec. 28, 1970  App]. No.: 101,871
 US. Cl ..325/377, 250/199, 317/2 R, 343/701  Int. Cl. ..I'I04b 1/10  Field of Search ..343/701, 704, 885; 325/377, 325/380, 382, 387, 361, 473, 378, 379, 381, 387, 338, 340; 317/2 R, 2 F; 307/93, 94;
4 1 Sept. 26, 1972 Primary Examiner -Albert J. Mayer Attorney-Arnold, White & Durkee, Robert A. White,
Bill Durkee, Frank S. Vaden, lll, Louis T. Pirkey and John F. Lynch I ABSTRACT Antenna noise reduction apparatus and method, especially suitable for airborne service, including an antenna discharging circuit for periodically grounding the receiver antenna through a switching system at a 250/199 frequency substantially higher than the carrier frequency of the radio system with which the receiver operates.
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SWITCH CLOSED 0 r A P l SWITCH OPEN T/ME FIG. 1E Otto J. Baltzer INVENTOR Y lbuwi, Wide X 0M2 A T TORNE Y5 P'A'TENTEDsarzs I972 3.694. 754
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T----w 5 W 5 I n 1 RECEIVER l I -E 24 l PULSE I OENERATOR 7H NEGATIVE BIAS VOLTAGE PULSE REcE/VER 4 25 GENERATOR i J: POSITIVE BIAS VOLTAGE PULSE 7'0 GENERATOR REcE/V' ER Otto J. Baltzer INVENTOR BY FIG. 5 ma, WM & 17mm NEGATIVE BIAS VOLTA GE ATTORNEYS PATENTEBsms m2 3,594,754
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r RECEIVER NEGA r/ VE POSITIVE BIAS- BIAS VOLTA GE VOL TA GE NEGATIVE PULSE SIGNAL IL POSITIVE PULSE SIGNAL 60 52 K82 84 NEGATIVE ANHZZNNAO BIAS COUPLER VOL TA GE 86 RECEIVER 88 V v VULTS Otto J. Baltzer A UORNEYS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to the suppression of electrostatic noise interference in antenna systems and more specifically to an improved system for minimizing the effects of precipitation interference in antenna systems.
2. Description of the Prior Art Atmospheric or environmental noise often interfere greatly to reduce the reliability of radio navigation and/or communications. Noise or static problems can become particularly. accute in airborne receivers operated to receive CW or pulsed signals modulated on carrier waves at VLF and LF frequencies. Prior techniques for reducing certain types of atmospheric interference have included the use of shielded loop antennas in place of electric field antennas. Use of such loop antennas, although proven to be advantageous in some aircraft installations in reducing certain types of environmental noise, has increased the complexity and cost of the systems with which such antennas operate. For example, loop antennas are known to be less omnidirectional than electric field antennas. Compensation for this lack of uniform directivity response when such directivity is required for operation usually involves complex switching circuitry. Moreover, the use of loop antennas often increase the susceptibility of the receiver systems to certain types of aircraft electrical noise.
Another technique which has been employed successfully in reducing the effects of impulse type noise, such as caused by remote lightning discharges, is the use of noise blanking or limiter circuits within the receiver. Such circuits operate to switch disconnect the antenna input from the receiver when there is present a noise much larger than the information signals. As will be explained, precipitation static is essentially continuous and hence any noise blanker operating on precipitation static would blank off the receiver most of the time, thereby obliterating the information signals.
Yet another technique that hasbeen employed is the resistive coating of the antennas or the antenna housing to remove accumulated electrostatic charges. Such coating may be effective in draining away the triboelectric charging that would otherwise build up on insulated materials e.g., radomes antenna insulators, windows). Unfortunately, however, any coating, particularly on the antenna base insulator or housing, will tend to short out the information radio signals as well.
Significant noise reduction has been achieved, at least at HF frequencies, by the use of wicks and rod dischargers to control corona discharging. However, such devices are not as effective for LF and VLF frequencies and do not achieve as great a reduction as the present invention.
It is therefore a feature of the present invention to provide an improved technique for the reduction of the electrostatic noise interference in antenna systems for radio receivers.
It is another feature of the present invention to provide an improved antenna discharging switching system for a radio receiver that reduces the effects of precipitation static without appreciably degrading the receipt of the information signals.
It is still another feature of the present invention to provide an improved electric field antenna system that includes high speed switching means for minimizing the effects of precipitation static without causing appreciable loss in the receipt of information signals, particularly in an airborne VLF or LF navigational or communications receivers.
SUMMARY OF THE INVENTION A preferred embodiment of the present invention for a typical VLF radio receiver installed aboard an aircraft comprises an electric field (E-field) antenna connected to the receiver and of suitable dimension or other characteristics for receiving the operating carrier frequencies for the receiver and a switching means connected between the antenna and the ground plane for the antenna. The switching means includes one or more solid state switching devices, such as a field effect vtransistor, photosensitive transistor or switching diodes, and a pulse generator-connected to gate the solid state device or devices off and on at an overall cycle rate of 20 or more times the carrier frequency of the information signals. Switching in this manner effectively discharges the precipitation static before it can build up to high potential levels and at the same time has negligible effect on the receipt of the information signals.
BRIEF DESCRIPTION OF THE DRAWINGS So that the manner in which the above-recited features, advantages and objects of the invention, as well as others which will become apparent, are attained and can be understood in detail, more particular description of the invention briefly summarized above may be had by reference to the embodiments thereof which are illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted however, that the appended drawings illustrate only typical embodiments of the invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
DRAWINGS FIG. 1A-1E is a waveform diagram illustrating the operation of an antenna discharging switching system in accordance with the present invention.
FIG. 2 is a simplified block diagram illustrating the basic concept of the present invention. FIG. 3 is a simplified diagram of an embodiment of the present invention using a field effect transistor in the switching means.
FIG. 4 is a simplified block diagram of an embodiment of the present invention using a photosensitive field effect transistor in the switching means.
FIG. 5 is a simplified block diagram of an embodiment of the present invention using a photoelectric cell in combination with a field effect transistor in the switching means.
FIG. 6 is a simplified block diagram of an embodiment of the present invention using a dual pair of switching diodes in the switching means.
FIG. 7 is a schematic diagram of a preferred embodiment of the present invention.
3 DESCRIPTION OF PREFERRED EMBODIMENTS Severe radio signal interference is frequently encountered when aircraft fly through atmospheric or cloud formations containing ice crystals or other precipitation. Such interference; commonly called precipitation static, can be quite disruptive of the operation of radio navigation and communication equipment aboard the aircraft. Precipitation static may be generated by various mechanisms, each of which may involve the electrical charging of either the antenna itself or some aircraft element by the precipitation particle (e.g., ice crystals, dust, rain, sleet).
As an example, an aircraft flying through a cloud containing ice crystals may accumulate an extremely high electrical potential as the result of triboelectric charging (which occurs whenever two similar materials come in contact and separate). Under this condition, the potential increases until corona discharge occurs somewhere on the aircraft, each corona discharge resulting in a sudden change in the aircrafts potential and in the generation of electrical noise at the antenna terminals.
Precipitation noise can also be generated by streamer or spark discharges which occur across the dielectric surfaces (e.g., plastic radomes, cockpit windows, and other insulating materials) subject to triboelectric charging by precipitation particle impingement. Each streamer discharge generates a burst of interference energy.
Finally, the individual precipitation particles, if impinging either on the antenna or in its vicinity, can directly generate small electrical charges which appear as a form of precipitation static.
Precipitation static can be especially disrupting in the case of an Omega aircraft navigation system. Omega signals are relatively weak. Therefore, the occurrence of any precipitation static can seriously interfere with the navigational capability of the aircraft. Indeed, the precipitation static problems, unless satisfactorily solved, are believed by many to be a major limitation in the use of Omega hyperbolic navigation as a reliable aircraft navigation method.
In addition to understanding the nature of precipitation static it is also helpful to understand the antenna reception theory which establishes the operating principle for the present invention. There are two basic classes of antennas: electric field or E-field and magnetic field or H-field. The electric field antennas include all antennas that are sensitive to the electric field intensity of a radiated signal. Included in this type are whip antennas, long wire antennas, blade antennas and probe antennas.
Loop antennas, on the other hand, are representative of the l-I-field antennas. Such antennas are sensitive to the magnetic field component of a propagated electromagnetic radio signal.
And has been previously pointed out, experience has shown that the electrostatically shielded loop antennas are less susceptible to precipitation static than any simple electric field type. This is expected since much of the precipitation static interference involves electrostatic charging and discharging, with extremely large fluctuations in electric field strength with relatively small magnetic fields being produced in the process.
Although shielded loop antennas may be attractive from the viewpoint of precipitation static, they have other disadvantages. Loop antennas have a figure eight directivity pattern, with null responses at certain angles. Accordingly, to avoid the possibility of blind spots" in reception at these bearing angles, it is necessary that more than one loop being employed for a single receiver. Generally, a pair of crossed-looped antennas are orthogonally oriented or used in an attempt to avoid excessive null-response problems. This means, in the case of a hyperbolic navigation system such as Omega where reception from multiple transmitters is required for operation, that some switching mechanism must be provided to enable the use of the proper antenna for reception of signals from the various transmitters at the proper time. Alternatively, the output of the orthogonal-pair of loop antennas may be suitably combined, such as in electrical quadrature, to prevent there being nulls at any azimuthal angle.
In addition to the directivity problem, loop antennas generally possess low sensitivity and may be susceptible to interference produced by various electrical equipment aboard the aircraft (e.g., inverters, generators, battery chargers, solenoids.)
Since vastly different physical mechanisms are involved in the generation of antenna voltages in the case of precipitation static and true radio signals, recognition of the basic differences may be effectively used for signalnoise discrimination. The electrical fields produced by precipitation particle charging and discharging are essentially electrostatic. On the other hand, the electric field associated with a propagated radio signal is a radiation field with radio wave radio energy beam transferred through space at a velocity of 3 X 10 meter/second. For purposes of discussion, consider the case of an aircraft Omega navigation system operating in the 10 kiloHertz frequency region. The aircraft is preferably equipped with an E-field antenna, such as a flush-mounted capacitive plate type, with the aircraft structure acting as the ground plane for the antenna.
In the presence of an Omega radio signal field, there will be established an electromagnetic field surrounding the aircraft and causing a potential difference between the antenna and its ground plane. In the case of precipitation particle charging, there will be established an electrostatic field surrounding the aircraft and causing a potential difference between the antenna and its ground plane. As will be explained there is a fundamental difference between the establishment of these two fields that the present invention exploits to reduce the precipitation static efiect without appreciably degrading the radio signal field reception.
Now referring to the drawings and first to FIG. 2, a simplified block diagram of the basic operation is illustrated. A normally open switch 10 is connected across the terminals of antenna 12, thereby connecting the antennas through the switch to ground potential. Receiving system 14 is likewise connected between the antenna and the ground plane.
If we assume that switch 10 is momentarily closed, then opened, then closed, and so forth repeatedly at a very high repetition rate, it may be seen that any voltage present on the antenna is shorted to ground and not allowed to build up. For concreteness of discussion, it
may be assumed that the switching rate is in the neighborhood of l megal-Iertz. If the closed term of the switch is percent of a total switch cycle, this means that there is a switch closure for a period of 0.1 microsecond.
Now referring to FIG. 1, typical resultant antenna waveform voltages are illustrated, with and without antenna switching in the manner just described. Respectively, FIG. 1A is the antenna terminal voltage caused by the received radio signal; FIG. 1B is the noise voltage caused by precipitation static; Fig. 1C is the received radio signal voltage with switch discharging; and Fig. 1D is the noise voltage with switch discharging. Fig. 1E shows the corresponding open-close cycle assumed for the switch. Notice particularly that the switch is open an appreciably longer period of time within its cycle then the switch is closed.
Now referring to FIG. 1, and particularly FIGS. 1C and 1D, it is shown that the transient response of the antenna immediately after the switch returns to its normally open position is basically different in the separate cases of electromagnetic signal reception and of precipitation static. As shown in FIG. 1C, the recovery to the radiated energy of the electromagnetic signal is virtually instantaneous, with a time constant determined by the propagational velocity of the electromagnetic energy over some limited spatial region surrounding the antenna. The physical size of the antenna, rather than the aircraft dimension, is believed to determine the extent of this region.
Precipitation static, however, produces an entirely different build up of antenna voltage than the signal voltage. After each antenna switch closure cycle, the antenna voltage due to the effect of the surrounding electrostatic field is initially zero, as illustrated in FIG. 1C. Antenna noise voltage build up occurs only as a result of subsequent charge accumulation or corona discharge, either from the antenna itself or from the aircraft structure. The rate at which the aircrafts absolute potential builds up is determined by the total charging or discharging current and the aircraft capacitance. Similar potential build up will occur if precipitation particles impinge directly on the antenna.
The higher the repetition rate of the switch, within limits to be explained, the greater the noise reduction. The following table shows a significant reduction in interference as the switching rate, that is, closures per cycle of the signal carrier frequency, is increased. These values are approximate and assume a simplified model of noise spectrum for the precipitation static charging current.
Switching Rate Noise Reduction (per RF cycle) (db) 20 X 16 100 X 30 200 X 36 500 X 44 1000 X 50 With currently available solid state switching devices, such as field effect transistors and high speed diodes, repetition rates of 2 megaHertz can be readily achieved. Thus, with radio frequency carrier operation in the 10 kiloHertz region, an electrostatic noise reduction of 36 db is clearly feasible. Even greater improvements can be achieved by careful attention to switch and antenna design.
The ultimate theoretical limit will be reached only when the repetition rate is increased to the point where the propagation for recovery of the electromagnetic field near the antenna exceeds the time interval during the charging" cycle (i.e., during the time the switch is in the open position).
It is also desirable that the antenna system have sufficient bandwidth so that it can be quickly discharged. In some airborne Omega installations, for example, low pass RF filters have been used in the antenna lead-in so that the radiated energy from the aircrafts HF communication transmitting antenna would not burn out or overload the Omega receiver. Any such filter can limit the upper switching rate. Therefore, when the present invention is employed, it is desirable to avoid any such low pass RF filter. This may be done by relocating the Omega antenna so that it is shielded from the HF transmitting antenna.
It may be further observed that the antenna discharging switch should possess several characteristics. First, the discharging switch should possess low leakage and have low noise characteristics in the off state. Further, the switch should have very high conductance in the on state. The switch should have low capacitance both to achieve fast response and to avoid voltage loss at the antenna terminals. Finally, the switch should have the ability to withstand high voltage such as may be present during thunderstorms or regions of exceptionally severe electrical activity.
FIG. 3 shows a simplified block and schematic diagram for an antenna discharging switch system utilizing a field effect transistor as the switch element. As illustrated, an antenna 12 is connected to a VLF or LF receiver 20 through a low pass filter or choke circuit 22 for preventing undesirable R-F energy from the receiver. Antenna 12 is also connected to the drain electrode of field effect transistor 24, the source electrode being connected to ground potential. The gate electrode of the field effect transistor 24 is connected to a pulse generating means comprising pulse generator 26 for producing a control voltage for the gate electrode through capacitor 28. The output of the capacitor is connected to the gate electrode of transistor 24 and to a negative bias voltage through a resistor 30.
The gate electrode of the field effect transistor is normally held to a negative value by the bias voltage, thereby preventing the field effect transistor from conducting. Periodically, the pulse generator produces a positive voltage which causes the voltage at the gate electrode to become positive. This causes the field effect transistor to conduct and permit discharging of antenna 12 to ground potential in a manner previously described.
FIG. 4 illustrates an embodiment of the invention utilizing a photosensitive field effect transistor 34 in place of the regular field effect transistor shown in FIG. 3. In such case the antenna is again connected to the drain electrode of the photosensitive field effect transistor and the source electrode is connected to the ground plane for the antenna. The receiver is connected in the manner illustrated in FIG. 3.
Alternatively, a conventional field effect transistor may be used. As in FIG. 4, the connections to the gate electrode thereof are a resistor connected to a negative bias voltage and a light-variable resistor 38 connected to a positive voltage. A pulse generator 26 is then connected to a light source 40 which is housed in the same compartment with resistor 38. Pulse generator 26 cyclically causes light source 40 to light. The normally high resistance of resistor 38 is greatly reduced such that the positive voltage and the negative voltage acting through resistors 30 and 38 in voltage divider action cause the voltage on gate electrode of transistor 34 to become positive. As before, this causes transistor 34 to conduct to switch discharge the antenna as previously described.
FIG. 6 illustrates yetanother switching means for discharging the antenna 12 in accordance with present invention. As in the embodiments described above, the receiver is connected to antenna 12 and the antenna is connected to ground through the switching means, this time comprising two pairs of diodes. One pair includes diodes 52 and 54, which are connected to form a common connection with their anode junction, their cathodes being connected respectively to antenna 12 and to the ground terminal. The other pair including diodes 56 and 58 are connected with their cathodes back to back to form a common junction point therebetween. Their cathodes are respectively connected to antenna 12 and to the ground terminal. Connected to the common junction between diodes 52 and 54 is a lead 60 connected to the positive output from a pulse generating means (not shown), this signal being applied through capacitor 62. In similar fashion, a lead 64 -is connected to the common junction between diodes 56 and 58. A negative pulse from the pulse generating means is applied through capacitor 66. A positive bias voltage is applied through resistor 68 to the common junction between diodes 56 and 58 and a negative bias voltage is applied through resistor 70 to the common junction between diodes 52 and 54.
In operation of diode pair 52 and 54, the normal negative bias voltage which is applied through resistor 70 to the common anode junction maintains the diodes nonconducting. Periodically when the positive pulse signal is applied on line 60, the voltage overcomes the normal negative bias voltage to cause the diodes to conduct, thereby causing the antenna to discharge any accumulated negative precipitation static voltage which has built up on antenna 12. Likewise, the positive voltage applied to the common cathode junction of diodes 56 and 58 through resistor 68 maintains these diodes nonconducting until a negative pulse signal is applied to the junction on line 64. When this event happens, the negative voltage overcomes the positive bias voltage, causing diodes 56 and 58 to conduct and discharging any positive precipitation static voltage that has accumulated on antenna 12.
Now turning to FIG. 7, a schematic diagram is shown of a preferred embodiment of an antenna discharging switch circuit. Antenna 12 is connected to an r-f energy choke circuit comprising resistor 72, inductor 74 and resistor 76 connected in series and capacitor 82 connected from resistor 76 to ground. This circuit is connected to the drain electrode of field effect transistor 78. A neon bulb is connected between inductor 74 and resistor 76 to ground as an additional overload safety device for preventing extremely high r-f energy from nearby transmitters and the like from burning out the receiver to which the circuit is connected. The drain electrode is also coupled to an antenna coupler unit 84 which, in turn, is connected to the receiver (not shown).
The source electrode of transistor 78 is connected to ground and to the ground terminal of antenna coupler 84. The gate electrode of field effect transistor 78 is connected to resistor 86,'which, in turn, is connected to an isolating amplifier comprising pnp transistor 88, and normal current limiting bias resistors and 92. A pulse generator 94 producing an output illustrated as between either 0 and 5 volts applies its output through a resistor 96 to the base of transistor 88. It may be seen that the 0 output from the pulse generator has a much longer period than the period for the production of the -5 volts.
The application of the periodic -5 volts on transistor 88 produces the desired waveform for the control voltage as shown in FIG. 3, when the gate electrode of transistor 78 becomes positive, transistor 78 operates in the manner which has been previously described.
Although the antenna discharging or dumping technique has been discussed as a means of solving aircraft precipitation static problem, it should be obvious that the technique has broader applicability and may prove useful wherever electrostatic noise fields cause reception difficulties.
One such application is in the reduction of atmospheric noise (sferics) which results from thunderstorm activity. The electric field generated by each lightning stroke includes both an electrostatic and radiation term. At close-in ranges the electrostatic predominates (the electrostatic field decreases in a lla' relationship). Various data indicate that the electrostatic and radiation fields are equal in the neighborhood of d 26 km (roughly a distance of 1 wave length) in the Omega frequency region. The antenna discharging concept can, therefore, be expected to be most useful in the immediate vicinity of thunderstorm activity, the region where such noise is severe.
Even in fair weather there is a large vertical electric field at the surface of the earth (typically 100 volts per meter). This electric field can fluctuate tremendously in the vicinity of a thunderstorm or in a disturbed weather condition. Short term fluctuations in the elec tric field can result in apparent radio noise. Charge separation and potential build up can occur in clouds, a process that can take place without lightning actually occurring. The antenna discharging technique will effectively remove any antenna potential accumulated as a result of any of these electrostatic processes.
Although the present invention is specifically directed towards the solution of the precipitation static interference problem for an airborne Omega navigation system, it should be understood that the same technique will be effective as a means of materially reducing any interference caused by electrostatic fields from other sources and therefore is invaluable on land vehicles space vehicles, on shipboard, as well as for aircraft receivers. Also, the useful frequency range extends beyond Omega/VLF so that the technique may prove beneficial in the case of Loran-C reception at 100 kilol-lertz. The technique is also applicable to the reception of communication signals as well as navigation signals.
While particular embodiments of this invention have been shown and primarily discussed, it will be understood that the invention is not limited thereto, since many modifications may be made and will become apparent to those skilled in the art. For example, the term that switch 10 in FIG. 2 (and the equivalent switching means illustrated in the other drawings) is in the closed condition has been stated as being desirably shorter than the term that switch 10 is in its normally open condition. At the higher rates of switching, such as near 2 megaHertz, maintaining a short close term with respect to the open term becomes less important. That is, it is still important that the switch be periodically closed momentarily to efiect discharge. However, at the higher switching rates, the closed term and the open term may each approximate 50 percent of the overall switching cycle.
What is claimed is:
1. An antenna discharging switching system for a radio receiver for reducing precipitation static, comprising:
an antenna connected to the receiver for receiving the radio carrier with which the receiver operates by being sensitive to the electric field intensity of the radiation field established by the carrier;
switching means, including at least on solid state switching device, for opening and closing a connection between said antenna and the ground potential at a frequency substantially higher than the frequency of said radio carrier; and
pulsing means for operating said switching device.
2. An antenna discharging switching system in accordance with claim 1 wherein said solid state switching device includes a field effect transistor having its drain electrode connected to said antenna and its source electrode connected to said ground potential, and
said pulsing means includes a pulse generator for intermittently biasing the gate electrode of said transistor at said substantially higher frequency rate to form a low resistance path from antenna to ground potential.
3. An antenna discharging switching system in accordance with claim 1, wherein said solid state switching device includes a photosensitive transistor, and
said pulsing means includes a light source, the light intensity of which varies at said substantially higher frequency rate.
4. An antenna discharging switching system in accordance with claim 1, wherein said solid state switching device includes a positive set of diode switches and a negative set of diode switches, and said pulsing means alternately produces a positive bias signal for gating on said positive set of diode switches and a negative bias signal for gating on said negative set of diode switches.
5. In a radio receiver system including a radio receiver for operating with a radio frequency carrier and an electric field antenna for receiving the radio frequency carrier by being sensitive to the electric field intensity of the radiation field established thereby, the improvement comprising:
switching means, including at least on solid state switching device, for opening and closing a connection between said antenna and the ground potential at a frequency substantially higher than the frequency of said radio carrier, and
pulsing means for operating said switching device.
II I Patent No. 3,694,754 Dated September 26, ,1972
Invontofls) Otto altzer It is certified that error appears in the above-idcntified patent and that said Letters Patent are hereby corrected as shown below:
Col. -1, I line 49, "e.g.," should read -(e.g.,; Col. 9, line 37,
"on" should read -one; and Col. 10, line 35, "on" should read -one-.
sighed and sealed this 3rd 'day' of'April 19-73.
(SEAL) Attest: I v
EDWARD M.FLETCHEIR,JR. I ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents Patent No. 3,694,754 Dated September 26, ,1972
Inventofls) Otto altzer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
Col. '1, line 49, "e.g.," should read -(e.g.,--; Col. 9, line 37, "on" should read -one-; and Col. 10, line 35, "on" should read one.
Signed and sealed this 3rd day of April 1973.
(SEAL) Attest: v
EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents (5/1131) u v I F V a r 4 I iii ,EQ/i'i f1, 0F CGHEUJJLEEQN p 3,694,754 Dated September 26, 1972 Invcntofls') Otto altzer It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:
COl. 'l, 3 line 49, "e.g. should read (e.g. Col. 9, line 37,
"on" should read -one-; and Col. 10, line 35, "on" should read one.
Signed and sealed this 3rd day of A ri 1973.
EDWARD M.PLETCHER,JR. I ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents gww