US3417399A

US3417399A - Millimeter-wave radiometer for radio-astronomy

Info

Publication number: US3417399A
Application number: US632163A
Authority: US
Inventors: James E Webb
Original assignee: National Aeronautics and Space Administration NASA
Current assignee: National Aeronautics and Space Administration NASA
Priority date: 1967-04-18
Filing date: 1967-04-18
Publication date: 1968-12-17
Anticipated expiration: 1985-12-17

Description

Dec. 17, 1968 JAMES E WEBB 3,417,399

, ADMINISTRATOR OF THE NATIONAL AERONAUTICS AND SPACE ADMINISTRATION MILLIMETER-WAVE HADIOMETER FOR RADIO-ASTRONOMY Filed April 184 1967 4 Sheets-Sheet. l

44 54 f 56; 7U 76 74 f6 60 a? O I /i o I .5? 7 242 45 e M Q24 j, 25 z2 Arrow/rs De. 17, 1968 JAMES E. WEBB 3,417,399

ADMINISTRATOR oF THE NATIONAI. AERoNAuTIcs AND SPACE ADMINISTRATION ETER-WAVE RADIOMETER FOR RADIO-ASTRONOMY MILLIM Filed April 18. 1967 o# r 7 wf M fm Q2, 4r r W fw 4 @L o, M

Dec. 17, 1968 JAMES Ey WEBB 3,417,399

RoNAuTlcs sTRA-rloN ADMINISTRATOR OF THE NATIONAL AE AND SPACE ADMINI MILLIMETER-WAVE RADIOMETER FOR RADIO-ASTRONOMY Filed April 18. 1967 4 Sheets-Sheet 3 mm M mm my A Va wy m m M Dec. 17, 1968 JAMES E WEBB 3,417,399

ADMINISTRATOR oF TI-IE NATIONAL AERoNAuTIcs AND SPACE ADMINISTRATION MILLIMETER-WAVE RADIOMETER FOR RADIO-ASTRONOMY Filed April 18, 1967 4 Sheets-Sheet 4 United States Patent O 3,417,399 MILLIMETER-WAVE RADIDMETER FOR RADIO-ASTRONOMY James E. Webb, Administrator of the National Aeronautics and Space Administration with respect to an invention of Anthony Giandomenico, San Gabriel, and

Willard Van Tuyl Rusch, Pasadena, Calif.

Filed Apr. 18, 1967, Ser. No. 632,163 10 Claims. (Cl. 343-100) ABSTRACT OF THE DISCLSURE A synchronous detection system capable of detecting exceptionally weak radio-astronomical signals, the system including a Cassegrainian type microwave antenna having a mechanical switching assembly `whereby the antenna beam may be periodically switched from a distant source of radio-astronomical signals to a position in space adjacent to but displaced from the source so as to permit the detection system to distinguish an actual signal from background noise. Mechanical switching within the system is achieved by a periodically tilting or nodding of the auxiliary hyperboloidal reflecting subdish which forms a part of the Cassegrainian antenna. The synchronous detection system also includes means for thermal noise calibration of the system.

Origin of the invention The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics a-nd Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457).

Background of the invention The synchronous detection system of this invention, which affords exceptionally good capabilities for the detection of particularly weak radio astronomical signals provides radiometry capabilities which facilitate continuing studies and discoveries relating to the universe through radio astronomy.

Radio astronomy involves the study of celestial objects through observations of radio waves emitted by the celestial objects, The detection of radio waves from outer space was rst accomplished in 1932 by K. G. lanksy of the Bell Telephone Laboratories in New Jersey.

In order to practice the art of radio astronomy particularly well developed and exceptionally sophisticated scientifically conceived antennas and receivers are required. More particularly, in order to practice the art of radio astronomy exceptionally good antennas, which are sutliciently large and sensitive for accurately resolving details of signals emitted from celestial objects, and additionally, suiciently large and sensitive for capturing adequate amounts of energy so as to provide a detectable signal for observation, are required. During the approximate years of World War II, and subsequent thereto, antenna and receiver development and production facilitated the serious beginning, growth and evolution of radio astronomy.

Since high-frequency radio astronomical signals generally are particularly weak radio signals there has been a particularly basic requirement for a highly sensitive receiving system, alforded by antenna beam mechanical switching in conjunction with the synchronous detection system provided by this invention, having capabilities for detection and resolution of particularly weak radio astronomical signals which have exceptionally small magnitude.

Since 1945, radio astronomical studies have been made of the sun, moon and numerous other celestial objects.

Radio waves have been detected, for example, from the suns atmosphere. A uniform level of emission which has been detected is believed to be caused by thermal collisions between e-lectrons and ions in the hot solar atmosphere. Also, relatively sharp bursts of radio waves have been detected at certain wavelengths. Measurements of the received solar radio signals give information relating to the temperature and electron density of the solar atmosphere.

Likewise, radio signals have been observed from the moon. These signals are thermally generated in the outer layers of the moons surface. Measurement of variations in intensity of the signals with frequency, and with varying phases of the moon produces information relating to the thermal and electrical properties of the moons surface.

' It is usual in the art to detect weak radio-astronomical signals by means of a highly directional microwave receiving antenna which precisely directs an antenna beam at the distant source. It is also known in the art to provide an electrical switching arrangement which periodically causes the antenna beam to be displaced from the source to a position in space adjacent the source. Such a switching arrangement signiiicantly reduces the elects of slow changes in the sky background radiation as well as fluctuating atmospheric absorption. Also, electrical switching techniques have been used in the prior art to detect the weak radio-astronomical signals in the presence of relatively high noise contributions from the atmosphere and from the receiving equipment itself.

The detection system of the present invention is provided with capabilities for reducing losses heretofore incurred by prior art electrical beam switching systems. The improved results obtained by the detection system of this invention are achieved by antenna beam mechanical switching in conjunction with a synchronous detection systern. The mechanical switching of the antenna beam is achieved in the system of the invention by periodically tilting or nodding the hyperboloidal subdish reilector of the Cassegrainian antenna.

As described, for example, in U.S. Patent 3,243,805, l. D. Smith, Jr.; and in U.S. Patent 3,255,455, G. Von Trentini, the Cassegrainian antenna is essentially a microwave antenna assembly consisting of a paraboloid-shaped primary reector and an auxiliary hyperboloidal subdish reilector which is positioned ahead of the primary reflector. For transmitting purposes, the subdish cooperates with a primary radiator disposed near the primary reflector, and the subdish serves to ilood the primary reflector with radiations. When the antenna is used for receiving radiations from a distant source, as is the case in the system of the present invention, the radiation path is oppositely directed. That is, the radiations are first directed to the primary reflecor, and from the primary reflector to the subdish reilector, there to be directed by the subdish into the synchronous detection system.

Summary ofthe invention The synchronous detection system of the invention provides for the periodical angular displacement of the hyperboloidal subdish in a Cassegrainian antenna so as to deflect the antenna beam periodically from the distant source to a position in space displaced from but adjacent to the source. In this manner the desired switching of the antenna beam is achieved mechanically so that losses as incurred by the prior art electrical beam switching arrangements are thereby reduced.

By means of the radiometer which includes the synchronous detection system of this invention radio-astronomical signals may be detected, displayed and measured. The synchronous detection system of the invention is so conceived and constructed that radio sky and atmosphere may be used for providing a reference noise input level which thereby considerably enhances facility for radio signal detection. Also, as indicated supra, by reducing losses heretofore incurred by the prior art antenna beam electrical switching arrangements the antenna beam mechanical switching in conjunction with the synchronous detection system of this invention provides a radiometer which has capabilities for detecting radio astronomical signals of significantly smaller magnitude than heretofore has been possible with known prior art antenna beam electrical switching apparatus.

The synchronous detection system of this invention uniquely incorporates a Cassegrainian microwave antenna having a drive means for the nodding subdish of the antenna for periodcallydisplacing the antenna Ybelm from the source of radio-astronomical signals. Additionally, the system is synchronously controlled so that the two groups of received noise signals, only one group of which contains the desired signal, may be separated and the desired signal thereby detected. Also, the system of the invention incorporates an effective calibration means whereby known thermal sources are observed in order that the system may be used for the quantitative measurement of the received radio-astronomical signals.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself both as to its organization and method of operation as well as additional objects and advantages thereof, will best be understood when read in connection with the accompanying drawings.

Description of Ille drawings FIGURE 1 is a schematic representation of the geometry involved in the Cassegrainian antenna;

FIGURE 2 is a side elevational view, partly in section, of the mechanical construction of a mount for the nodding subdish assembly of the Cassegrainian antenna used in the system, in accordance with one embodiment of the invention;

FIGURE 3 is a fragmentary View of a portion of the mechanism of FIGURE 2, which is hidden in the view of FIGURE 2;

FIGURE 4 is a further fragmentary view of the nodding subdish assembly, on an enlarged scale, showing in detail certain components of the assembly;

FIGURE 4A is a perspective view of a cam wheel which is included in the assembly;

FIGURE 5 is a block diagram of one embodiment of the synchronous detection system of the present invention, including a thermal calibration means for the system;and

FIGURES 6A and 6B are graphical representations of the signals displayed by the recorder portion of the system of FIGURE 5.

Description of the invention FIGURE l shows the main, or primary, paraboloid reflector 10, and the nodding subdish auxiliary refiector 12. The mount assembly and control mechanism for the subdish 12 are shown in section in FIGURE 2, with further details shown in the fragmentary views of FIGURES 3 and 4. The mount assembly of FIGURE 2 supports the subdish 12 in a position facing the primary paraboloid reflector 10, as shown schematically in FIGURE l.

As described above, the mount assembly of FIGURE 2 must tilt the subdish 12 periodically in order to obtain the desired mechanical switching of the antenna beam. The tilt is extremely small, and may be of the order of 2, by way of example, The subdish is tilted as shown in FIGURE 1 about an axis which is perpendicular to the axis of symmetry of the mount assembly. The rate of tilting of the subdish may be from 5-15 cycles per second, for example.

As shown in FIGURE 2, the mount assembly is contained, for example, in a housing 14. The housing is provided With a tubular end portion 16, and an index plate 18 is mounted on the end of the tubular portion 16 by screws, such as the screw 20. An adjustment plate 22 is supported in coaxial relationship with the index plate, and against the index plate by means of screws, such as the screws 24. A support 26 is provided for the subdish 12, and is mounted on the adjustment plate 22 by means, for example of bolts 28. The subdish 12 is mounted on the support 26 by means, for example, of a pair of

bearings

30 and 32 so as to permit the subdish to be tilted about an axis extending perpendicular to the axis of symmetry of the mount assembly.

A counterweight 36, which has a ring-shaped annular configuration, is also supported on the support 26 by a pair of bearings, such as Vbearing 38. 'l'he annular counteriweight 36 is supported on the mount adjacent the support of the subdish 12, as shown.

A drive shaft 40 is provided for driving a bevel gear 42 which, in turn, is affixed to a shaft 44. The shaft 44 is journaled in a yoke-shaped bracket 46. A tubular shaft 48 is affixed to the bracket 46 by means, for example, of bolts 50. A cam shaft 52 extends coaxially within the tubular shaft 48. A further bevel gear 54 is mounted at the end of the shaft 52 in meshing relationship with the gear 42. A cam wheel `60 is mounted at the right hand end of the cam shaft 52 as seen in FIGURE 2. Therefore, when the drive shaft 40 is rotated, the gears 42 and 54 transmit the rotary motion to the shaft 52 which, in turn, drives the cam wheel 60.

As best shown in FIGURE 4, the subdish 12 has a cam follower 62 which extends down into a first groove in the periphery of the cam wheel 60 shown in FIGURE 4A. The annular counterweight 36 also has a cam follower 64 which extends into a second peripheral groove in the cam wheel 60 adjacent the first groove. The cam wheel 60 is best shown in the perpective view of FIGURE 4A, and the two adjacent grooves in the periphery of the cam wheel are best shown in that view.

The cam shaft 52 is supported in the tubular shaft 48 in bearings 70 and 72 at the opposite ends thereof. The shaft 48 is keyed to a sleeve 74 by means, for example, of a key 76, the sleeve 74 being mounted on one end of the housing 14. The tubular shaft 48 is slidable in the sleeve 74 so that the subdish 12 can be moved to the left or right as shown in FIGURE 2 for focussing purposes. A spring 75 biases the shaft 48 to the left as shown in FIGURE 2.

A focus gear is mounted on the side of the bracket 46 remote from the shaft 48 coaxially with the axis of symmetry of the assembly. A lead screw 82 extends through the gear 80, and the gear 80 together with the bracket 46 and tubular shaft 48 move back and forth in a guide bushing 86 as the gear 80 is rotated. Such movement causes the subdish 12 to move relative to the housing 14, `back and forth along the axis of symmetry. The spring 75 biases the bracket against the lead screw to prevent play in the assembly.

The focus gear 80 is driven by an electric motor and gear set l88 shown in FIGURE 3. The motor is mounted in the housing 14, and its drive shaft is coupled to the gear 80 through a gear 90, the latter gear being mounted on the drive shaft. The motor 88 is reversible, and can be controlled to drive the gear 80 in either direction. This enables the subdish 12 to be moved controllably towards or away from the primary reflector 10 as shown in FIG- URE 1 for focussing purposes.

It will be appreciated, therefore, that when titled, the nodding subdish 12 moves between two off-axis positions (FIGURE 1). The axis about which the subdish 12 tilts may be indexed merely by releasing the bolts 24 and moving the adjustment plate 22 relative to the index plate 18. A suitable scale is provided on the index plate so that the pivotal axis of the subdish can be adjusted with respect to any desired reference.

The cam Wheel 60 is located on the back of the subdish 12, and the subdish and annular counterweight 36 have

respective cam followers

62 and 64 which engage the respective perpiheral grooves in the cam wheel. Rotation of the cam wheel 60 causes approximate square wave motion of both the subdish 12 and the annular counterweight 36. The subdish 12 remains in one olf-axis position for about 45% of the rotation of the cam wheel; it switches to the second position for about 5% of the rotation of the cam wheel; it remains in the second position for about 45 of the rotation of the cam wheel; and it then switches back to the rst position during the remaining 5% of the cam wheel rotation. The counterweight moves in a direction opposite to that of the subdish 12 in order to complement the movement of the subdish so that vibration to the overall assembly thereby is reduced to a minimum.

As shown in FIGURE 5, the nodding subdish in the Cassegrainian antenna system of the invention directs its energy by means of a wave guide section 100 into a synchronous detection system. A switch such as, for example, a photo-electric switch is also provided in the nodding subdish assembly, so that a control pulse may be generated each time the subdish is tilted from one axis to the other. This pulse is applied to a pulse generator 102, and causes the generator to produce control pulses for the different components of the system. It will be appreciated that all the different control pulses generated by the pulse generator 102 occur at the same frequency, which is the synchronous frequency of the system.

The waveguide 100 from the nodding subdish 12 is coupled to a manually operated wave-guide switch 104, the output of which is connected to a directional coupler 106. The manual waveguide switch can be turned manually to receive energy from the waveguide section 100. Additionally, the waveguide switch can be turned to a reference source of radiation designated the hot load termination (FIGURE 5) and to a second reference source of radiation designated the ambient load termination. The hot load termination may represent, for example, a reference temperature of 120 C., whereas the ambient load termination may represent a reference temperature of C.

A rst series of synchronous pulses from the pulse generator 102 is used to trigger a gas tube 108 which, in turn, passes pulses through the directional coupler 106 with energy derived through the manual waveguide switch 104.

FIGURE 5 shows a lblock diagram of one embodiment of the synchronous detection system of the invention which includes a thermal calibration means for the system. For calibration purposes, the directional coupler 106, shown in FIGURE 5 for example, is coupled to a rst terminal of a ferrite waveguide switch 110. The latter switch is operated at the synchronous frequency by a second series of control pulses derived from the pulse generator 102. A second terminal of the ferrite waveguide switch is connected to a source of radiation also designated ambient load termination, and which, likewise, may represent a reference temperature of 20 C. The common terminal of the ferrite waveguide switch is connected to a heterodyne mixer stage 114 in the electronic synchronous detection section of the system.

The electronic section is a usual synchronous heterodyne radiometer, and it includes a local oscillator 116. The signal from the local oscillator heterodynes the incoming signals in the mixer 114 to a selected intermediate frequency signal. The resulting intermediate frequency signal is amplified in an intermediate frequency amplifier 118 of one or more stages and the output from the intermediate frequency amplifier is applied to a detector 120. The detector output from the detector 120 is then passed to a synchronous detector 122, which in turn is coupled to a recorder 124. The synchronous detector 122 is operated at the synchronous frequency of the system by a third series of pulses derived from the pulse generator 102. The recorder 124 may be a usual digital paper recorder.

The ferrite waveguide switch is used for calibration purposes only. During normal operation of the synchronous detection system, the ferrite wave-guide switch is replaced by a usual straight waveguide section extending from the directional coupler 106 to the mixer 114. The calibration of the system may, for example, take place before and after a run which might, for example, be six or eight hours of looking at the moon, or other source of millimeter radiation.

Each of the various elements of the system shown in block form in FIGURE 5 are, in themselves, known to the art. Therefore, a detailed description of these elements per se is deemed to be unnecessary for the full understanding of the present invention.

The directional coupler 106 permits power from the gas tube 108 to be carried through the ferrite waveguide switch 110 to the mixer 114, and `at the same time the directional coupler permits power from the manual waveguide switch 104, likewise, to be carried through the ferrite waveguide switch 110 to the mixer. The directional coupler prevents the energy from the two sources from taking any other path.

For calibration purposes, the manual waveguide switch 104 is rst turned to the ambient load termination (To), and the ferrite waveguide switch 110 is switched synchronously by the control pulses from the generator 102, between the output from the manual waveguide switch 104 and the ambient load termination (To). The result is a reference trace by the recorder 124 corresponding to T O-TO (FIGURE 6A), since the synchronous detector output is proportional to the difference of the two signals examined.

The manual waveguide switch 104 is then switched to the hot load termination (TH), `and the ferrite waveguide switch continues to be switched synchronously. The recorder 124 now moves up the scale to a new level corresponding to T H-TO (FIGURE 6A).

With the ferrite waveguide switch 110 still being switched synchronously, the manual waveguide switch 104 is again turned to the ambient load termination (TO), and the gas tube 108 is now fired synchronously. The signal now fed to the mixer 114 is the combined output (TO-i-TGT) from the directional coupler 106 for one position of the ferrite waveguide switch 110, and (To) for the other position of the ferrite waveguide switch. Therefore, the detected output from the system, as displayed by the recorder 124 is (TO-i-TGT)-TO=TGT, this being displayed, as shown in FIGURE 6A.

Therefore, by means of the calibration procedure outlined above, the actual reference value of the noise pulses (TGT) produced by the gas tube 108, insofar as the electronic portion of the detection system is concerned, has been determined with respect to the zero axis of the recorder. The scale factor is known because the temperature of the reference terminations To and TH are accurately known.

A typical mode of operation, by which radio emissions from celestial objects are detected by means of the synchronous detection system of this invention, is set forth hereinbelow. Initially the ferrite waveguide switch 110 is replaced with a straight waveguide section and the manual waveguide switch 104 is switched to receive its signal from the nodding subdish 12. The system is then aimed away from the source, so that the signal applied to the mixer 114 for each position of the noddling subdish is TSKY, without the source signal `being included in either of the TSKY signals. This rst reception therefore, is displayed along the zero axis of the recorder as TEKT- TSKY (FIGURE 6B).

As the next step in the detection process, the antenna is still aimed away from the source and the gas tube 108 is tired synchronously. Now the signal applied to the mixer 114 is TSKY-l- TGT for one position ofthe nodding subdish, and TSKY for the other position, since the gas tube is on for only one position of the subdish. Therefore, the

7 signal detected by the detector 122 and displayed by the .recorder iS (TSKY+TGT)TSKY=TGT 6B).

The gas tube 108 is now turned off, and the antenna is aimed at the source. Now, the signal applied to the mixer 114 is such that the detected output is TsoURcE -i- TsKY) TsKY: Ts oURcE this being represented by the corresponding curve in FIGURE 6B, as generated by the recorder 124. By using the calibrated value of TGT, the result of TSOURCE can accurately be determined.

It is understood that those familiar with the art may make modications in the arrangements as shown and described herein without departing from the true scope and spirit of the invention. Therefore, all much modifications and equivalents are deemed to fall within the spirit and scope of the invention as claimed in the appended claims.

What is claimed is:

1. A system for detecting radio signals, said system including:

an antenna having a primary reflector member and a secondary reflector member, said secondary reflector member facing said primary reflector member for receiving reflected radio Waves from said primary reflector member;

an electronic synchronous detection system coupled to said secondary reflector for detecting the radio waves received thereby;

drive means for said secondary reflector for periodically g tilting said secondary reflector Vabout a selected axis and through a selected arc;

and means for introducing control pulses to said synchronous system in synchronism with said periodic tilting of said secondary reflector.

2. The system defined in claim 1 in which said primary reflector member has a parabolic configuration, and in which said secondary reflector member has a hyper- 'bolic configuration.

3. The system defined in claim 1, and which includes a mount assembly for said secondary reflector;

a cam wheel rotatably mounted in said mount assembly and coupled to said drive means to be rotatably driven thereby;

and cam follower means coupled to said secondary reflector member and engaging said cam wheel for causing said secondary reflector member periodically to tilt about said selected axis.

4. The system defined in claim 3, and which Iincludes an annular counterweight mounted in said mount assembly, and further cam `follower means coupled to said cou-uterweight and engaging said cam wheel to cause said counter- Weight periodically to tilt in a direction opposite to that of said secondary reflector member so as to reduce vibrations to a minimum.

5. The combination defined in claim 1 and which includes a mount assembly for said secondary reflector member;

shaft means for supporting said secondary reflector member slida'bly in said mount assembly;

and focussing drive means coupled to said shaft means for causing said secondary reflector member to move reciprocally in said mount assembly so as to position said secondary reflector member at a selected position relative to said primary reflector member for focussing purposes.

6. The combination defined in claim 1 and which includes a mount assembly for said secondary reflector member;

and means including an indexing plate for mounting Said secondary reflector member on said mount assembly, so as to permit the axis of tilt of said secondary reflector mem'ber selectively to be adjusted about the axis of symmetry of the assembly.

7. The combination `defined in claim 1 in which said synchronous detection system includes a source of noise pulses to serve as a reference for the received signals.

8. The combination defined in claim 7 in which said synchronous detection system includes waveguide switching means for Calibrating said source of noise pulses against sources of predetermined reference radiations.

9. The combination defined in claim 7 in which said synchronous detection system includes a ferrite waveguide switch for selectively connecting said synchronous detection system to said source of noise pulses and to a predetermined thermal load termination for Calibrating said source of noise pulses.

10. The combination defined in claim 1 in which said .synchronous detection means includes a synchronous detector responsive to said control pulses, and which further includes a source of calibrated noise signals also responsive to said control pulses for producing reference noise pulses at the synchronous frequency of the system.

References Cited UNITED STATES PATENTS 3,167,714 1/1965 Seling 325-363 RODNEY D. BENNETT, Primary Examiner.

T. H. TUBBESING, Assistant Examiner.

U.S. Cl. X.R.