Operating Manual Microwave Radiometer Ral10

Transcript

Total-Power Microwave Radiometer RAL10 Radiometric receiver 11.2 GHz for amateur radioastronomy observations. OPERATING MANUAL Doc. Vers. 1.0 del 15.03.2013 @ 2013 RadioAstroLab RadioAstroLab s.r.l., Via Corvi, 96 – 60019 Senigallia (AN) Tel. +39 071 6608166 Fax: +39 071 6612768 Web: www.radioastrolab.it Email: [email protected] Copyright: All rights reserved. The content of this document is property of the manufacturer. No part of this publication may be reproduced in any form or by any means without the written permission of RadioAstroLab s.r.l.. 1 RAL10 was developed to bring people to amateur radioastronomy observing the most intense radiosources in the sky t a frequency of 11.2 GHz. Following our advices, is easy and cheap to make a microwave radiotelescope. WARNING Always install the product in a dry place away from sources of heat, do not expose the instrument to rain or moisture. Avoid direct exposure to sunlight. To avoid electric shock, do not place on it objects filled with liquids (such as glasses, vases, bottles, etc..). Do not install the appliance in a confined space such as a bookcase or cabinet. WARNING: the RAL10 receiver is very sensitive when equipped with an antenna large enough. To get the best performance from the instrument, it is recommended that you install the receiver in a temperature-controlled environment (ideally within  1° C), minimizing the gain fluctuations of the receiving system. A major improvement in performance is obtained by thermally stabilizing the outdoor unit RAL10_LNB, subjected to daily temperature that vary the gain with temperature, causing instability in the measurement. The standard delivery (cod. RAL10A) includes, in addition to the RAL10 indoor unit, even the external module RAL10_LNB (with feed) optimized for satellite dishes circular (F/D=0.32÷0.43) and the coaxial cable. The receiver will work with any kit antenna and LNB for receiving satellite TV in the band 10-12 GHz: for those who want to use a commercial kit antenna and LNB, it is possible to supply cod. RAL10B which includes only the RAL10 indoor unit equipped with RF-IF connector type F. The packaging of the complete supply (cod. RAL10A) contains: 1. 2. 3. 4. 5. 6. 7. 8. N. 1 Control Unit RAL10 Total-Power Microwave Radiometer (Fig. 1, 2 and 3). N. 1 Outdoor unit RAL10_LNB with adjustable feed suitable for symmetrical parabolic reflector antennas with F/D of between 0.32 and 0.43 (Fig. 2). N. 1 Coaxial cable for connection between the receiver RAL10 and the external module RAL10_LNB installed at the focal point of the antenna (length of 4 meters with F and TNC connectors). N. 1 Cable for connecting the mains supply 230 VAC - 50/60 Hz (on request 115 VAC - 50/60 Hz). N. 1 USB cable (with connectors type A and B) to connect the receiver to the station computer via a standard USB port. Spare fuses for the power supply and the low voltage power supply (12 V battery). Software DataRAL10 for the acquisition, visualization, and the automatic registration of the acquired data (operating systems Windows 32 and 64 bit, Mac OS X). N. 1 CD containing instrument and software technical documentation (user manuals), data sheets RadioAstroLab product, DataRAL10 software installation package and drivers. More and updated information can be found at www.radioastrolab.it. The delivery cod. RAL10B includes only the details described in paragraphs (1), (4), (5), (6), (7), (8). In this case, the connector for input RF-IF signal from the outdoor unit LNB is of type F (Fig. 3). 2 RadioAstroLab warrants its products for a period of one year when used according to the instructions and recommendations in this document. The manufacturer is not responsible for any malfunction or damage to the machine caused by poor installation or due to the use of external components that are not suitable for the specific application (antenna system, mechanical parts used for fixing and tracking). The manufacturer reserves the right to change, without notice, the machine and documentation in order to improve performance. To get the best results from the receiver RAL10, it is essential that you carefully read this manual, that it may not be completed because of the uniqueness and complexity of radio astronomy applications. Updates of this document and application notes that describe and investigate specific topics will be released. This manual describes the installation and the operation of the receiver RAL10 Total-Power Microwave Radiometer, explaining how to connect external modules to build a microwave radiotelescope. Using RAL10 it will be possible to see interesting and exciting radioastronomy observations of the sky. Fig. 1: Block diagram of a radiotelescope based on the receiver RAL10. The instrument is composed of circular parabolic reflector antenna where it is installed RAL10_LNB (Low Noise Block), connected to the receiver via a coaxial cable. The receiver RAL10 communicates with the computer via the USB port: management software tool captures and records the received data. To complete the telescope you have to install devices antenna orientation, managed by the PC station, not included in the delivery. Figure 1 shows the block diagram of the system: input RF-IF coaxial connector located on the rear panel of the control unit RAL10 connects, via the coaxial cable supplied, the outdoor unit RAL10_LNB (or similar commercial product) installed on the focal point antenna. The power supply to the outdoor unit is supplied directly from RAL10 through the cable and it is possible to control the receiving polarization by voltage switching. A fuse accessible on the rear panel protects the instrument against accidental shortcircuits on the cable. 3 Composition of the receiver RAL10 Fig. 2: Outdoor unit RAL10_LNB (left): the device, installed on the focal point of the antenna, is a low-noise amplifier and a frequency converter that receives the signal at 11.2 GHz, performs amplification and IF conversion to 1415 MHz (with a bandwidth of 50 MHz) and transmits the information to the indoor unit via coaxial cable (the front-panel controls and display are shown on the right). Fig. 3: Rear panel of the receiver RAL10. The indoor unit generates power, acquires and processes the signal received from the RAL10_LNB, displays the functions of the instrument on the display and accepts commands from the keyboard. The radiometric measurement, formatted as a packet of serial data is continuously transmitted to the acquisition PC via USB type B. The rules of communication are established by a proprietary protocol. Figure 2 shows the instrument's front panel with controls for operating settings, and the backlit LCD display that displays the functions of the system, Fig. 3 shows the rear panel. There are the following connections: • USB port (type B) for the connection to the PC acquisition: a red LED displays the continuous 4 • • • • • transmission of serial data. RF-IF coaxial connector for connecting the outdoor unit RAL10_LNB. Power supply socket for an external battery (nom. 12 VDC) EXT. BATTERY, alternative to the mains, useful for measuring sessions "field". Specially designed for this purpose is the portable source with rechargeable battery RAL10BT. Protection fuse for the line of low voltage power supply (12 VDC). ON-OFF POWER supply voltage (does not act on the supply voltage from the optional external battery). Power supply 230 VAC MAINS - 50/60 Hz with fuses. WARNING: make the connections between the outdoor unit RAL10_LNB and the receiver RAL10 with the power off and when the power cord is not plugged into the mains. Before turning on the receiver, check, with a multimeter set for resistance measurements, the absence of short-circuits between the outer shield of the coaxial cable and the inner core, otherwise replace the faulty connections or cable to prevent damage to the power supply interior of the instrument. The sensitivity, so the quality of the measures provided by the instrument, are dependent on the effective antenna used and the temperature experienced by the external RAL10_LNB. Interesting radio astronomy observations are possible only when using antennas of large effective area: there is no limit on the size of the antenna that can be used, if not economic factors, practical limitations of space and of installation due to the support structure and to motorization of tacking system. The market for satellite TV (in addition to surplus electronics) offers a wide choice on the size of the antenna, on tracking and motorization systems. The user will adopt the best solution for his ambitions and budget, with the possibility of future improvements. After installing the antenna, the module RAL10_LNB placed at the focal point of the parabolic reflector and connected to the drop cable to the control unit RAL10, you can connect the USB cable to the computer on which you have installed the software acquisition. The receiving station is now ready to record observations unattended by the operator with automatic data acquisition. WARNING: RAL10_LNB must be installed on the media support (in the focal point of the antenna) with the F connector of the coaxial cable just turned down: only under these conditions will be guaranteed the consistent orientation of the polarization during reception. Always use the rubber cap to protect the connector F after connecting the cable to the outdoor unit. The correct focusing of RAL10_LNB occurs by moving the device forward-backward along the axis of the antenna, maximizing the intensity of the signal from a radio source characterized by an emission relatively stationary as, for example, the Sun. Technical description The radiometer Total-Power is a very sensitive microwave receiver used for the measurement of temperature associated with the scenario intercepted by the antenna, since any natural object emits a noise power related to its temperature and its physical characteristics. Figure 4 shows the block diagram of the radiotelescope which uses the receiver RAL10. The signal picked up by the antenna, amplified and frequency converted from the outdoor unit RAL10_LNB is sent, with a coaxial cable, to the radiometer microRAL11 (basic component of the receiver RAL10) which measures its power. An amplifier of postdetection adapts the level of the detected signal to the dynamic acquisition of the analog-digital converter 5 (ADC with 14 bit resolution) that “digitizes” the radiometric information. This final block, managed by a programmable microcontroller, processes the detected signal, sets the reference for the baseline, the gain of post-detection and the constant of integration of the measure, it provides the formation of the packet of serial data that will be transmitted to computer for data logging. Fig. 4: Block diagram and internal structure of the receiver RAL10. The radiometric measurement in a receiver Total-Power involves the acquisition of small variations of the detected signal, variations superimposed on an almost continuous component, of much greater amplitude, due to system noise. To measure only the signal variations due to the radiation coming from a radio source, it must be subtracted from the detected signal the contribution due to the background noise: for this reason it is generated a voltage REF_BASELINE offset that serves to position the level of the base line radiometric to an appropriate point of the acquisition’s scale of the analog-digital converter. The last block is constituted by the electronic control board RAL102 (managed by a second programmable microcontroller) that communicates via serial with the module microRAL11 compacting the measured data and operating parameters of the instrument according to a specified communication protocol. The functionality of the system are displayed and programmed manually by the user interface that is a backlit LCD display and a keyboard controls (Fig. 2). The main board provides the power for the receiver and continuously transmits the acquired data to the PC via a USB port (Fig. 3). If measures are required where there is no supply voltage of the network, you can connect the system to an external power source of 12 V (rechargeable battery or rechargeable battery unit RAL10BT): the power level is continuously monitored by the receiver and indicated on the display. The receiver RAL10 is built on a sleek and compact aluminum housing. The following pages shown some examples of recordings made with the radiotelescope RAL10 equipped with a common antenna TV-SAT parabolic reflector. Charts provide an idea of the possibilities of the instrument: it is possible to improve performance by using an antenna of large effective area, equipped with a drive system controlled by the PC (automatic pointing and tracking) and thermally stabilizing the RAL10_LNB outdoor unit to reduce fluctuations in gain due to temperature changes. It is clear that the greatest effort in building an amateur radiotelescope results from the installation of the 6 antenna system that, due to its dimensions and the complex mechanical structures, requires the availability of space for installation, operational skills and adequate financial resources. Specifications • • • • • • • • • • • • • • • • • • • • Dimensions RAL10: [200L X 100H X 155P] mm. Weight RAL10: 1.65 Kg approximately. Operating frequency receiver: 11.2 GHz. Intermediate frequency (IF) conversion receiver: 1415 MHz. Typical gain of the IF section: 20 dB. Bandwidth of the receiver: 50 MHz. Professional TNC connector for IF input (rear panel of RAL10). Adjustable corrugate feed horn: Circular waveguide Ø 18.5 mm, flange C120. Can be used with symmetrical parabolic reflector antennas (F/D 0.32-0.43). Unit RAL10_LNB low noise: GAIN = 50-60 dB NF = 0.3 dB typ. IMAGE REJECTION > 40 dB Dual polarization (H – V). Feed-horn flange C120. Detector temperature compensated for measuring the power of the received signal. Setting of the offset for the radiometric baseline. Automatic calibration of radiometric baseline. Post-detection voltage gain programmable. Integration of the detected signal programmable: Programmable moving average calculated over 2, 8, 16 and 32 adjacent samples acquired. Acquisition of the radiometric signal: ADC resolution 14 bit. Change of receiving polarization (horizontal or vertical). N. 2 microprocessors used for the control of the receiving system. USB port (type B) used to connect to a PC using proprietary communication protocol. Keyboard for manual controls and control panel with backlit LCD display (2 lines x 24 characters). Power supply: 230 VAC – 50 Hz (opz. 115 VAC – 60 Hz) 12 VDC external set RAL10BT (rechargeable battery). Instrument functions The front panel display shows the following operating parameters (Fig. 2), selectable by keyboard: • Amplification factors of post-detection (voltage gains of the amplifier stages that follow the detector) and the integration constant of the radiometric measurement: GAIN = Axxb. The first capital letter A is the main gain of the receiver associated with the internal module radiometric microRAL11 (Fig. 4). This value must be chosen carefully according to the intensity of the observed radio source and to the antenna gain available. You can select the values A = 42, B = 48, C = 56, D = 67, E = 84, F = 112, G = 168, H = 336, I = 504, J = 1008. Assuming you have a satellite antenna with a diameter of 1.5 meters, you can set the value of G for the observation of the Sun, higher values for the observation of the Moon (usually H or I). Any variation of this parameter automatically starts the calibration microRAL11 in order to optimize the measurement and resets the default values for the settings of secondary gain xx and offset BASE. The symbol xx (values between 1 and 20) represents the secondary gain of the receiver (multiplication factor software - Fig. 5). The final value of the gain of post-detection is obtained 7 • • • • • by multiplying this value to the previous one. Generally it is not necessary to change the value of secondary gain xx (and offset BASE) in comparison with the default values (xx = 1, BASE = 0): in all cases you should set values for xx greater than 1 only after it has been optimized the choice for the parameter A (see notes below). The last lowercase letter b is the constant of integration of the radiometric measurement. Setting the values a, b, c, d in ascending order. The operation is performed by software doing a moving average of the signal samples acquired and selecting the number of samples used for the calculation of the average. Increasing this value, it is reduced the importance of the statistical fluctuations of the noise on the measurement because it’s introduced a “leveling” in the received signal that improves the sensitivity of the system. Offset parameter is necessary to place the reference value of the baseline on a suitable point of the measuring scale: BASE = yyyyy, where yyyyy is a value between 0 and 50000. This is an indication that represents the value of the signal used to “cancel” the noise contribution of the instrument on the detected signal by setting an appropriate scale value for the baseline radiometric. The base line is defined as the level of the output signal considered as a “zero” of the scale, relative to the situation in which the antenna receives only the radiation of the “cold sky” in the absence of radiosources. You need to change BASE only after the choice for the gain parameter A has been optimized (see notes below). Setting the reception polarization: the center of the display will show the symbol PH if you have selected the horizontal polarization, symbol PV if you have selected vertical polarization. This command, that acts on RAL10_LNB, allows you to select the preferred direction of the electric field component (horizontal or vertical) in the process of measurement of radiation received by the antenna. Automatic positioning of the baseline radiometric: the center of the display will show the symbol CL. Automatic calibration of the radiometer microRAL11: the center of the display will show the symbol CR. This command, that can be set manually or automatically activated each time you change the gain parameter A, positions the base line of the module radiometric microRAL11 near the center-scale. Resetting the compensation signal baseline for the module microRAL11: the center of the display will show the symbol ZZ. This reset is useful to measure the temperature of the receiver noise canceling any offset voltage at the input of the ADC (more details on the usefulness of such a command will be provided in a specific application note). Figure 5 summarizes the structure and the meaning of the operating parameters of RAL10: the main parameter which influences the response of the system is the gain of post-detection A of the radiometer microRAL11, that establishes the amplification factor for the detected signal. As the receiver also amplifies the noise of the receiver (Total-Power), the correct positioning of the “zero” reference on the scale of measurement is ensured by the internal calibration of microRAL11 that adjusts the value Vrif to position the response of the instrument near the center-scale, when the antenna "sees" a region of sky free from radio sources. The output signal from RAL10 (rad) is calculated by multiplying the factor xx with the difference between the response (radio) of microRAL11 and the offset BASE. Putting xx = 1 and BASE = 0 you obtain the response of the module radiometric microRAL11 without further elaboration: this setting is typically used for most of the observations. In some cases it may be useful to increase the gain xx simultaneously adjusting the value BASE in order to optimize, on the measuring scale, the contrast between the useful signal and background noise. In these cases, you must ensure that the response of the internal module microRAL11 remains close to the center-scale despite the drifts due to temperature: repeated corrections setting the CR command will solve the problem up to the stability of the system. 8 The second line on the display shows a horizontal bar that indicates the received signal level: signal’s increases correspond to movement of the bar to the right. This is a convenient indication which allows, in parallel to the view provided from the PC screen with the acquisition software, an immediate control of the level of the received signal. The right end of the horizontal bar displays the level of any external power source (such as, for example, the battery module RAL10BT): 0 = external accumulator discharge, 8/9 = accumulator external load, 10 = AC power. CAUTION: the number bb will indicate 10 only when the instrument RAL10 is powered from the mains, while generally indicate a number between 0 and 8/9 when the instrument is powered from an external accumulator with a value of voltage of between 11 V and 14 V. Fig. 5: Structure and meaning of the operational parameters for RAL10. Setting of operating parameters When you turn on the receiver, it will beep and the display will illuminate and show the start screen with the device name and version of the software. Next you will see a waiting screen that indicates the activation of the internal initialization parameter. RadioAstroLab RAL10_xx .Total-Power Radiometer. Setting Parameters PLEASE WAIT… After a few seconds the instrument displays the following parameters: 9 GAIN = Axxa PH BASE = yyyyy ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ b10 with the default values. For each specific installation, parameter values to be set depend on the characteristics of the antenna and the emissive characteristics of the observed radiosource. The performance of RAL10 are influenced by the temperature of the environment where the equipment is installed: you should set the final values of operating parameters at least one hour after powering the receiving system, when the internal electronic circuits are thermally stabilized. The main factor that limits the stability of the radiometric response is the daily temperature range experienced by the outdoor unit RAL10_LNB: these variations in temperature cause very small variations in the gain of the front-end, however they are sufficient to cause significant fluctuations in the reference level, due to the substantial amplification of the receiver. The amount depends on the amplitude of the fluctuations of the temperature and on the gain values setted. You can have the best performance from the telescope when the outdoor unit RAL10_LNB is thermally stabilized. This action is crucial to the quality of measurements. By repeatedly pressing the SEL key, you select the parameter to be changed: on the left of the parameter symbol appears > to underline the selection (just on the left of the GAIN parameter, will appear in sequence, the symbols >, *, - that indicate, respectively, the change values xx, A, a). You can then change the value, in ascending or descending order, using the + and - buttons. The selection process is sequential. The display shows the following information: >GAIN = Axxb PH BASE = yyyyy ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ b10 Setting Values xx (gain of post-detection software) GAIN (+ and - buttons to change the value). *GAIN = Axxb PH BASE = yyyyy ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ b10 Setting the values A (gain of post-detection radiometer microRAL11) parameter GAIN (+ and - buttons to change the value). -GAIN = Axxb PH BASE = yyyyy ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ b10 Setting the values b (constant of integration of radiometric measurement) of the GAIN (+ and - buttons to change the value). GAIN = Axxb PH >BASE = yyyyy ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ b10 Setting the parameter BASE (+ and - buttons to change the value). GAIN = Axxb >PH BASE = yyyyy ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ b10 Setting the parameter POLARIZATION (+ and - buttons to change the value). GAIN = Axxb CL BASE = yyyyy 10 ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ b10 Selecting the CALIBRATION procedure (press the + button to activate). GAIN = Axxb PH BASE = yyyyy ▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒ b10 End of the selection procedure of the parameters (return to the operating screen). When the display shows the symbol CL it is possible to activate the calibration by pressing and holding the + button until you hear a confirmation beep: the receiver will start the procedure by adjusting the parameter BASE until the output signal is not positioned at the center of the scale measure. The system alerts you with a beep and the display shows the following screen: PARAMETERS CALIBRATION ! SUCCESS ! that indicates the success of the operation. The value of the radiometric baseline, after the instrument is thermally stabilized and the antenna is oriented in a region “cold” sky, represents the minimum level (reference) of the response of the receiver, respect to it , will be recorded signal’s increases due to the flow received from radiosources. Pressing again the SEL button, the display shows CR that is the command for automatic calibration of the internal module microRAL11: the command is confirmed by pressing the + button, then start the procedure. This command (together with the reset of the default values for the parameters xx and BASE) is automatically started every time you change the gain for the module microRAL11. Additional push of the SEL button predisposes the zeroing procedure of the compensation signal of the baseline for microRAL11 (which will be confirmed by pressing, as usual, the key +), canceling each offset input of the ADC. Push the SEL button to exit from the operating parameters menu. The receiver monitors the supply voltage of the external battery when using the socket EXT. BATTERY: The battery charge status is indicated on the right on the second line of the display. The display shows: !! WARNING !! LOW BATTERY when the level of charge of the battery is lower than that of the security one. In this case the operator must quickly terminate the measurement session and turn off the instrument, by disconnecting the battery for recharging. Performance optimization The signal at 11.2 GHz received by RAL10 is directly proportional to the power associated with the incident radiation mediated within the passband of the instrument, then to the brightness temperature of the region of sky "seen" by the antenna beam. The radiometer behaves like a thermometer that measures the temperature equivalent of noise of scenario celestial observed. If the antenna is pointed at a region of clear skies and dry, where radio sources are absent (the so-called clear atmosphere, with negligible absorption - Fig 6), you measure a very low noise equivalent 11 temperature (due to fossil radiation to 3 K of the cosmic background), usually of the order of 6-10 K (cold sky), that corresponds to the minimum temperature measured by the instrument and takes into account the losses instrumental. Even taking into account the minimum contributions due to small radio sources, and to the noise of the atmosphere, if your antenna is kept at least 15°-20° above the horizon, away from the Sun and the Moon, we can assume a temperature of noise antenna between a few degrees and a few tens of degrees (due mainly to the secondary lobes - Fig. 6). Pointing the antenna on the ground, the temperature rises to values of the order of 300 K if it covers all the receive beam. When an antenna of medium size (beam width of the order of 2°-3°) is oriented toward the Sun (apparent size of 0.5° and characterized, at a frequency of 11.2 GHz, a noise temperature approximately equal to that surface of 6000 K), the system “sees” a source with a temperature of about 396 K. The radiation of the cosmic background “dilutes” the powerful radiation of the Sun if the antenna beam is wider at the point of collecting a significant contribution of external radiation (with much lower brightness). The background radiation, captured in good percentage from the outer crown of the antenna receive beam, decreases the amplitude of the received signal like if it comes from a source with a temperature below that the real one. A similar situation occurs if, between the “cold sky” and the observer, are localized cloud formations (hydrometeors in general) variables in composition, thickness and height from the ground: in the operating band of RAL10 the disturbing contribution of the troposphere (with its fluctuations and irregularities) can be appreciably. Before starting any radio astronomy observation, you should observe the following rules: Switching on the receiving station and wait until the receiver has reached thermal stability. The instability of the system are mainly caused by changes in temperature: before you begin any radio astronomy observation is necessary to wait at least one hour after switching on the instrument in order to achieve the operating temperature in the system of internal electronics. One can verify this condition by observing a long-term stability of the radiometric signal when the antenna tip a region of sky “cold” (absence of radio sources): drifts in the signal indicated by the bar on the display or by the graphic trace on the program acquisition appear minimum. 2. Initial setting of the gains of post-detection GAIN to minimum values (typically from F01a to H01a). As they are not predictable a priori the optimal values for the parameters of the system, each installation will be characterized by different performance. To properly set RAL10 you should adjust experimentally the gains of post-detection starting with minimum values of proof (to avoid saturation), optimizing the setting with successive and repeated scans of the same region of the sky. To observe the Sun it is advisable to initially set GAIN = F01a, to observe the Moon you should start with GAIN = H01A. It is recalled that these settings are strongly influenced by the size of the antenna used. The most important gain adjustments are those relating to the values A, B, C, D, E, F, G, H, I, J of the radiometer microRAL11. These settings must be made first, setting BASE = 0 (see note below) and values xx = 01. After every change of gain A, ..., J, the system automatically activates the calibration procedure for microRAL11: before performing any adjustments, it is necessary to wait until the end of the procedure. When the appropriate values for the gains of post-detection are found, it is possible to change the value of the integration constant to stabilize the measurement. The system is initially set to the measurement with a short integration constant (a). This value, corresponding to the moving average calculation on the radiometric signal using a few samples, is generally desirable in most cases. You can improve the sensitivity of the measurement, with a system response slower than signal changes, adopting a longer time constant: it is recommended to set the value a during the initial calibration and tuning of the system, subsequently adopt the time constant d during the session measurement of radio sources characterized by emissions stationary. When you measure events that rapidly change (such as, for example, solar flares), it is appropriate select the shorter time constant. Usually there is an additional radiometric signal integration processing the samples 1. 12 acquired from software installed on your PC. Setting of the parameter BASE fixing the reference level (zero) of the radiometric baseline. Also for this parameter the foregoing considerations are valid, because its correct setting depends on the value of the overall gain of the chain of pre-detection, including the antenna system. As a general rule, the BASE parameter should be set so that the minimum level of signal at the output of RAL10 corresponds to the “cold sky” (ideal reference), in conditions of clear atmosphere, when the antenna “sees” a region of sky-free radio sources: a signal increase respect to the reference level would be representative of a scenario characterized by higher temperature (radiosource). Note how the final position of the baseline on the scale of measurement is a function of two parameters: the set value for the gain of post-detection of the radiometer microRAL11 and the set value for the parameter BASE (Fig. 5). If, due to internal drifts, the signal is localized outside the scale of measurement (start-scale or full scale), it is necessary, as first operation, repeat the calibration of microRAL11 (CR command) and, subsequently, change the value for BASE or activate the automatic calibration with the CL command. 4. Initially, the system is set to receive with horizontal polarization (PH). It's always possible to change the polarization for the study of radio sources emission where a specific component of polarization dominates. In most of the observations accessible at amateur level, the radio sources emit with random polarization: in these cases the change of polarization in reception may be useful to minimize the possibility of interference with signals of artificial origin. 5. Optimization of the response of the feed (outdoor unit RAL10_LNB). It's important to optimize the response of the feed according to the specific ratio F/D of the antenna. The simplest way is to target exactly the antenna in the direction of a radio source sample (such as the Sun or the Moon) and adjust the position back and forth respect to the focal point of the outdoor unit and the position of the corrugated ring feed so that to record a maximum intensity signal. After the operation (performed by minimizing sighting errors with repeated measures) it should fix the ring with silicone sealant. 3. The correct setting of parameters RAL10 requires registration of many test observations before starting the work session. This procedure, which is normally also used by radio-professional observers, allows you to “calibrate” the receiving system so that its dynamic response and the scale factor are adequate to record the observed phenomenon without errors. If properly executed, this initial setting (especially necessary when are forecasted long periods of observation with automatic recording of acquired data) will allow you to adjust the gain of post-detection and the scale offset for an adequate record of the evolution of the phenomenon, avoiding risks of saturations or signal resets with consequent loss of information. The typical and most simple radioastronomical observation involves the orientation of the antenna system to the south and its positioning at an elevation such as to intercept a specific radio source during its transit to the meridian, that is the passage of the apparent radio source for the local meridian (the one that contains the poles and the installation point of the radiotelescope). Our instrument, generally characterized by a large antenna beam few degrees, “forgives” us a lack of knowledge of the position of the radio sources: it is therefore acceptable a precision pointing less than the one used in optical observations. Setting a sampling rate that, in the acquisition program, you obtain approximately a screen every 24 hours, you can verify if, during the day, the antenna beam intercepts radiosources desired and if the values chosen for the system parameters (gains of post-detection and level of the baseline) are suitable for the observation. You might increase GAIN = Axx to amplify the track, or change the level of the baseline BASE so that, at some point on the graph, the signal goes out of the scale of measurement. When you are sure of the correct parameter setting, you can run long sessions of automatic registration. A primary verification on the functionality of the radio telescope provides the antenna pointing towards a region relatively “cold” (eg, the sky) and, subsequently, the pointing towards the ground: it 13 should be noted a very large deflection on the recording track of the signal since the instrument measures the temperature difference between the ground (about 300 K) and the temperature of the cosmic microwave (a few degrees K). This simple procedure illustrates, although in a simplified and approximate way, the technique that can be used to calibrate the scale of the receiver (Fig. 8). Calibrate a radio telescope means create a correspondence between the levels measured at the output of the instrument (expressed in units of ADC acquisition [count]) and the temperature range of noise expressed in degrees K. You can think to many interesting experiments to check the sensitivity of the receiving system RAL10. One of these provides the outdoor unit RAL10_LNB pointing towards incandescent or neon lamps: these components emit a significant amount of microwave radiation (according to different mechanisms emissive, some of which are not simply related to the physical temperature of the source) easily detectable by our tool. Powering and turning off the lamp, an appreciable variation of the received signal is recorded, proportional to the intensity and to the angular size of the source. Ultimately, using any source placed at a known temperature and characterized by an angular extension sufficiently broad to cover the whole of the receive beam of the antenna, it is possible to calibrate the instrument. Detailed information on calibration procedures used to define the scale of the radiometer RAL10 will be published on the site http://www.radioastrolab.it. 14 Fig. 6: Attenuation due to the absorption of the gases present in the atmosphere (to the left). You notice the disturbing contribution of the Earth's troposphere (absorption lines due to H 2O molecules) for frequencies just above 10 Ghz. To the right is shown the brightness temperature of the sky as a function of frequency and of the elevation angle of the antenna. Fig. 7: Operational capabilities of the instrument RAL10. 15 The flow units Jy (in honor of Karl Jansky) is 10-26 W/(m2 Hz), a measure that quantifies the emissivity of the radio sources. Are shown the main radio sources accessible to RAL10 when it is equipped with an antenna of appropriate effective area. References • • • • • Kraus J.D., Radio Astronomy (2° ed.), Cygnus-Quasar Book (Powell, Ohio). Sinigaglia G., Elementi di Tecnica Radioastronomia, CeC (1990, Faenza). Falcinelli F., Radioastronomia Amatoriale, Comprendere le basi della radioastronomia, costruire gli strumenti, pianificare le osservazioni, Il Rostro (2003, Milano). Falcinelli F., Tecniche Radioastronomiche, Sandit Edizioni (2005, Albino - BG). Falcinelli F., Radioastronomia amatoriale: utilizziamo il kit microRAL10+RAL126 per costruire un radiotelescopio a microonde, Edizioni interne RadioAstroLab (2013, Senigallia). Fig. 8: Transit of the Sun. 16 Fig. 9: Diagram of a lunar transit. The thermal radiation of the moon is visible: its emission is a result of the fact that the object emits approximately as a black body characterized by a temperature of the order of 300 K. If the visible emission of the Moon is almost exclusively due to the reflected light of the Sun, in the microwave there is an issue due to the temperature of the object that contrasts with that of the sky “cold”. Fig. 10: Transit of the radiosource Taurus A. Doc. Vers. 1.0 del 15.03.2013 @ 2013 RadioAstroLab RadioAstroLab s.r.l., Via Corvi, 96 – 60019 Senigallia (AN) Tel. +39 071 6608166 Fax: +39 071 6612768 Web: www.radioastrolab.it Email: [email protected] Copyright: all rights reserved. The content of this document is the property of the manufacturer. No part of this publication may be reproduced in any form or by any means without the written permission of RadioAstroLab s.r.l.. 17