1. AN ELECTROMAGNETIC WAVE MODULATOR COMPRISING: A LAYER OF IONIZED GASES WHICH REFLECTS A PART OF ANY ELECTROMAGNETIC ENERGY THAT IS DIRECTED INTO IT; MEANS FOR APPLYING AN UNMODULATED MAGNETIC FIELD TO SAID LAYER OF IONIZED GASES; MEANS FOR MODULATING SAID MAGNETIC FIELD; MEANS FOR GENERATING AN ELECTROMAGNETIC WAVE HAVING A CONSTANT FREQUENCY EQUAL TO THE GYROFREQUENCY OF SAID UNMODULATED MAGNETIC FIELD; AND MEANS FOR TRANSMITTING SAID ELECTRO-
Jan. 24, 1967 Filed April 29, 1964 POWER SOURCE S. L. SEATON ANTENNA OF IONIZED GASES 2 Sheets-Sheet 1 ANTENNA MODULATOR (3 PET I 4 D: LL! 2 8 INVENTOR STUART L. SEATON ATTORNEYS Jan. 24, 1967 s. L. SEATON 3,300,721
MEANS FOR COMMUNICATING THROUGH A LAYER OF IONIZED GASES Filed April 29, 1964 2 Sheets-Sheet 2 0: In 1118 5 :5 w v 8 ,2 r- J w \D V E\) 5 c O at!) O NLU (D 2 --U7 :0 z z go ATTOR EYS United States Patent Ofiice Patented Jan. 24, 1967 3,300,721 MEANS FOR COMMUNICATING THROUGH A LAYER OF IONIZED GASES Stuart L. Seaton, Hampton, Va., assignor to the United States of America as represented by the Administrator of the National Aeronautics and Space Administration Filed Apr. 29, 1964, Ser. No. 363,654 7 Claims. (Cl. 32565) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
This invention relates generally to the modulation of electromagnetic waves and more particularly concerns a method and apparatus for communicating through a layer of ionized gases.
Whenever a space vehicle traveling at a hypersonic speed (Mach 10 or above) enters or leaves a planetary atmosphere, it is surrounded by a shock-induced layer of ionized gases. This energized layer of gases or plasma prevents electromagnetic waves from passing through it. The electromagnetic Waves are either reflected or absorbed by the plasma. Consequently, electromagnetic wave communication to and from the space vehicle is either partially or totally obstructed during the times that the space vehicle is surrounded by a layer of plasma. Since a layer of plasma is formed around a vehicle during critical phases of its flights (while leaving or entering a planetary atmosphere), it is desirable to be able to communicate through the plasma.
In the past, there have been several schemes suggested for accomplishing radio wave communication through the layer of plasma surrounding a space vehicle while it is reentering the earths atmosphere. Among these schemes, is the scheme of selecting for use a signal frequency that will penetrate or pass through the hypersonically generated plasma. Extremely high radio frequencies will pass through the plasma. However, since most communication equipment presently in use is designed for the VHF- UHF bands (30 me. to 300 me. and 300 me. to 3000 me.) there is an equipment acquisition problem if the extremely high frequencies are used. Also, at the higher frequencies there is an atmospheric absorption problem.
Other schemes suggested for accomplishing radio wave communication through the layer of plasma include aerodynamic shaping, material injection and use of the layer of plasma as an electromagnetic radiator. However, none of these schemes satisfies all foreseeable requirements.
The present invention provides another scheme for communicating through the layer of ionized gases formed around a space vehicle as it enters or leaves a planetary atmosphere. This invention accomplishes its purpose by applying a modulated magnetic field to the layer of plasma whereby when electromagnetic waves are directed into the layer of plasma, the reflected electromagnetic Waves contain the same modulation as the modulated magnetic field. The application of the unique principle of this invention also provides a tool for measuring the conditions inside a layer of ionized gases.
It is therefore an object of this invention to provide a new scheme for communicating through the layer of plasma formed around a space vehicle as it either enters or leaves a planetary atmosphere.
Another object of this invention is to provide a new scheme for modulating electromagnetic waves.
A further object of this invention is to provide a tool for measuring the conditions inside a layer of ionized gases.
Other objects and advantages of this invention will further become apparent hereinafter and in the drawings in which:
FIG. 1 is a schematic drawing of the apparatus used in practicing this invention to communicate through a layer of ionized gases; and
FIG. 2 is a schematic drawing of the apparatus used in practicing this invention to measure the conditions inside a layer of ionized gases.
In disclosing this invention the physics upon which the invention depends will first be discussed, then the apparatus used to carry out the invention will be described.
It is well known that free electrons in an ionized medium, in the presence of a permeating magnetic field, when acted upon by an electromagnetic wave, gyrate about the magnetic lines of force with a frequency i given in the equation:
Where f is the gyrofrequency in cycles per second; H is the magnetic field strength in gauss; e/m is the ratio of electronic charge to mass in e.s.u.; and c is the velocity of light in a vacuum.
It is also well known that the mean frequency of collision of free electrons with other particles in a gas is given by the equations:
where E is the mean collision frequency; V is the mean thermic velocity; P is the density of the i species; Q, is the collision cross section of the i species; k is Boltzmans constant; T is the temperature in degrees Kelvin; and m is the mean molecular mass.
In terms of the mean free path:
where v is the collision frequency, and L is the electron mean free path.
If n is the number of molecules, or atoms per cubic centimeter, then,
L=1/nA (5) Where A is the molecular or atomic cross section.
If the mean thermic velocity of electrons in an ionized medium is augmented by energy from an electric field, the mean work done per collision is:
where r is equal to e /m in e.s.u.; Z is the root-meansquare value of the electric vector in e.s.u.; v is the collision frequency of the electrons; and w is the angular Wave frequency of the imposed electric field.
It has been shown by previous authors that an index of interaction,
where N is the number of electrons per cubic centimeter, exists for electromagnetic wave energies present in an ionized medium permeated by a magnetic field, and that the maximum value of this index of interaction occurs at or near the gyrofrequency. It has also been shown that when the collision frequency is less than the gyrofrequency and the wave frequency varies through the gyrofrequency, then the mean work done per electron collision increases, that is, a notable resonance occurs.
It is further well known that there exists in an ionized medium a critical frequency fc such that:
N=Bfc where B is a constant equal to 1.24 times 10 when the wave frequency) is in m'egacycles per second.
For wave frequencies f, such that f is less than fc, wave energy of frequency 1 will be reflected by or refracted in an ionized medium. For f greater than fc wave energy of frequency 1 will pass through the ionized medium. When 1 is equal to fc, the dwell time of the wave energy in the ionized medium becomes long and absorption due to collisions dissipates the wave energy. When 1 is less than or equal to v, an electron oscillation in response to the Wave energy cannot be completed before a collision disrupts the systematic electron motion and hence wave energy is dissipated. For f greater than or equal to v at least one oscillation can be completed before a collision occurs and so dissipation is reduced.
One of the conditions of the invention for purposes of communication is that the electromagnetic wave frequency, 1, lie between the collisional frequency, v, and the critical frequency, fc, of the ionized medium, that is Since in many natural and artificially generated ionized media the natural earth magnetic field is insufliciently strong to satisfy mathematical expression 9, where f is equal to 13;, it is necessary to produce in the ionized medium a local magnetic field to satisfy mathematical expression 9. If this local magnetic field is modulated, for example at voice frequencies, and if simultaneously the ionized medium is illuminated by an electromagnetic wave of frequency equal to the gyrofrequency associated with the unmodulated magnetic field, then the modulation will vary the local gyrofrequency to and fro across the illuminating wave frequency. By mathematical expression 7, when this condition occurs, wave energy returned from the ionized medium will carry a modulation in intensity consonant with the frequency of modulation of the magnetic field.
Now that the theory of this invention has been discussed, the preferred embodiments of the invention will be described. In describing the preferred embodiments of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate .in a similar manner to accomplish a similar purpose.
Turning now to the specific embodiment of the invention selected for illustration in FIG. 1, the number 11 designates generally a layer of ionized gases. This layer of ionized gases 11 can be the hypersonically generated gases formed around a space vehicle as it enters or leaves a planetary atmosphere or it can be any other natural or manmade layer of ionized gases. The purpose of the embodiment of the invention in FIG. 1 is to communicate through layer 11 with electromagnetic waves or to use layer 11 to modulate electromagnetic waves that strike it. It will be assumed for illustrative purposes that the layer of ionized gases 11 has a free electron density n equal to electrons per cubic centimeter; a critical frequency fc equal to one hundred megacycles per second; and a mean collisional frequency v equal to one megacycle per second. These values are in no way restrictive, but simply illustrate one set of conditions among many.
Located on one side of and contiguous to layer 11 is an electromagnet 12 consisting of an iron core 13, a coil 14 wound around core 13, and a power source 15 connected to coil 14. Electromagnet 12 generates a magnetic field of H gauss. Also wound on core 13 is a coil 16 which is connected to a conventional modulator 17 having a frequency output of f Core 13, modulator 17 and coil 16 generate a magnetic field of AH gauss. The combined magnetic fields generated by coils 14 and 16 is a magnetic field of HiAH gauss. This combined magnetic field consists of magnetic lines of force 18 which pass through layer 11. The magnetic field Hi-AH gauss resulting from coils 14 and 16 has an associated gyromagnetic frequency f iAf such that ,f is equal to fifty megacycles per second, H is equal to nineteen gauss, and Af is equal to five percent of 3;, all of which is common knowledge in communication engineering.
Located on the other side of layer 11 is a conventional radio transmitter 19 connected to an antenna 20. Transmitter 19 feeds antenna 20 causing it to emit in the direction of layer 11 a continuous electromagnetic wave 21 having a constant frequency 1 equal to fifty megacycles per second. Electromagnetic wave 21 is reflected by layer 11 as a continuous electromagnetic wave 22. By virtue of the magnetic lines of force 18, the energy of electromagnetic Wave 21 is altered by the unique process of having f iAf move to and fro through the frequency of wave 21. The energy lost by wave 21 in layer 11 is superimposed on the reflected wave 22. Thereby wave 22 contains a modulation consonant with AH caused by the fm produced by modulator 17. Wave 22 is picked up by antenna 23 and then applied to a conventional receiver 24. Receiver 24 removes the modulation AH from wave 22.
It can be readily seen that if modulator 17 is used to modulate the magnetic lines of force 18 in accordance with some information, this information is detected by receiver 24. Consequently, this invention affords a means for communicating through a layer of ionized gases. If the apparatus including electromagnet 12, coil 16 and modulator 17 is located on a space vehicle and the apparatus including transmitter 19, receiver 24, and antennas 20 and 23 is located on earth, then this invention provides a means for communicating through the layer of plasma formed around the space vehicle a it enters or leaves the earths atmosphere. Also this invention provides a unique means for modulating electromagnetic waves which might be useful for other purposes.
Referring now to FIG. 2 the second preferred embodi ment of this invention will be described. The embodiment of this invention shown in FIG. 2 is used for the purpose of measuring the conditions inside a layer of ionized gases 31. Located on one side of layer 31 is apparatus identical to that shown in FIG. 1. This apparatus consists of an electromagnet 32 including an iron core 33 and a coil 34 supplied by a power source 35. Also wound on iron core 33 is a coil 36 which is connected to a modulator 37. The combined magnetic fields generated by coils 34 and 36 is a magnetic field HiAH gauss. This combined magnetic field consists of magnetic lines of force 38 which pass through layer 31.
On the other side of layer 31 there are transmitters 39, 40, and 41 associated with antennas 42, 43, and 44, respectively. Antennas 42, 43, and 44 emit continuous electromagnetic waves 45, 46, and 47, respectively. Wave 45 has a frequency f wave 46 has a frequency f and wave 47 has a frequency f;,. Any number of these waves can be used. It is only necessary that the frequency of each wave be between v and fc, that is, the frequencies f (i= are such that The waves 45, 46, and 47 are directed into the layer 31. And, as in FIG. 1, these waves are reflected by layer 31 as waves 48, 49, and 50.
A magnetic field falls off in strength inversely as a power of the distance from the origin of the magnetic field. Hence, in layer 31 at each distance dk (k=0, 1, 2, 3, 4, from the electro-magnet 32, there corresponds a particular gyrofrequency. If, as in FIG. 1, modulator 37 modulates the magnetic lines of force 38 by an amount AH, then there will appear on each of the waves 48, 49, and 50 a certain percentage of modulation depending upon the distance, dk, to which waves 45, 46, and 47 penetrated into layer 31. Waves 48, 49, and 50 are picked up by antennas 51, 52, and 53 and then applied to receivers 54, 55, and 56 where they are converted into useful form in a conventional manner.
Under the same unique principle as that of the embodiment in FIG. 1, the wave energy (waves 45, 46, and 47) at each of the frequencies fj (f f f upon entering the ionized medium 38 and being acted upon as before described, will emerge as frequencies f ifm. These waves will each be modulated to an extent that those frequencies which penetrate farthest into the region of action will carry large depths of modulation, while those frequencies not so penetrating will carry lesser depths of modulation. In this way, the internal condition of the ionized medium 38 can be examined. A practical way of measuringthe depths of modulation of waves 48, 49, and 50 would be to connect the outputs of receivers 54, 55, and 56 to an oscilloscope.
It is to be understood that the forms of the invention herewith shown and described arf to be taken as preferred embodiments. Various changes may be made in the shape, size, and arrangement of parts. For example, equivalent elements may be substituted for those illustrated and described herein, parts may be reversed; and certain features of the invention may be utilized independently of the use of other features, all without departing from the spirit or scope of the invention as defined in the subjoined claims. This invention could be used to modulate lasers and other such devices which at the presentresist being modulated.
What is claimed is: i
1. An electromagnetic wave modulator comprising: a layer of ionized gases which reflects a part of any electromagnetic energy that is directed into it; means for applying an unmodulated magnetic field to said layer of ionized gases; means for modulating said magnetic field; means for generating an electromagnetic wave having a constant frequency equal to the gyrofrequency'of said unmodulated magnetic field; and means for transmitting said electromagnetic wave into said layer of ionized gases whereby the reflected electromagnetic energy contains the same modulation as said modulated magnetic field.
2. A system for communicating through a layer of ionized gases which reflects a part. of any electromagnetic energy that is directed into it comprising: means on one side of said layer for producing an unmodulated magnetic field extending into said layer; means for modulating said magnetic field in accordance with the information to be communicated through said layer; means located on the other side of said layer for generating and transmitting an electromagnetic wave, having a constant frequency equal to the gyrofrequency of said unmodulated magnetic field, into said layer in the region of said magnetic field; and means for receiving and demodulating the reflected electromagnetic energy from said layer whereby the modulation extracted from said reflected electromagnetic energy contains said information.
3. A system in accordance with claim 2 wherein said means for producing an unmodulated magnetic field comprises an iron core, a coil wrapped around the iron core, and a power source connected across said coil.
4. A systern'in accordance with claim 3 wherein said means for modulating said magnetic field comprises a second coil wrapped around said iron core and a modulator connected across said second coil.
5. A system in accordance with claim 2 wherein said means for generating and transmitting comprises a conventional radio transmitter and an antenna.
6. A system in accordance with claim 2 wherein said means for receiving and demodulating comprises an antenna and a conventional radio receiver.
7. A system for communicating through the layer of ionized gases formed around a spacecraft in an atmosphere wherein said ionized gases reflect a part of any electromagnetic energy that is directed into it comprising: means on said spacecraft for producing a magnetic field extending into said layer which is modulated in accordance with the information to be communicated through said layer of ionized gases; means remote to said spacecraft for generating and transmitting an electromagnetic wave, having a constant frequency equal to the gyrofrequency of said magnetic field, into said layer in the region of said magnetic field; and means for receiving and demodulating the reflected electromagnetic energy from said layer whereby the modulation extracted from said reflected electromagnetic energy contains said information.
References Cited by the Examiner UNITED STATES PATENTS 2,047,930 7/1936 Linder 332-57 X 2,051,537 8/1936 W011i et al. 33257 X 2,557,979 6/1951 Labin 332-54 X 3,067,420 12/1962 Jones et al. 332-57 X 3,176,227 3/1965 Bender 325- OTHER REFERENCES Terman, Electronic & Radio Engineering, 4th ed., 1955, pp. 829-834.
Rao et al., Proc I.R.E., vol. 49, No. 12, December 1961, pp. 1877-1899.
DAVID G. REDINBAUGH, Primary Examiner.
JQHN W. CALDWELL, Examiner.