Bellarmine College Preparatory
San Jose, California 95126-1899
Astronomy and Physics Dept.
Saint Mary's University
Halifax, N.S., Canada, B3H 3C3
|Figure 1: Block diagram of the system as actually used for this report. The dish and LNB (with associated supports) were commercial units. The homemade I.F. amplifier and diode detector had a bandwidth of over 500 MHz (the circuit is given at the end of this article).|
|Figure 2: Block diagram of a system that is somewhat less sensitive than the above, but quite capable of producing all the results described in this article. Here, rather than a home-made wide-band I.F. amplifier, a commercial receiver (matching the LNB) is used, in which the effective bandwidth associated with the tuning meter is around 25 MHz.|
|Figure 3: Figure 3: Schematic diagram of an 'voltage-offset module'. In this case, the a battery polarity will offset positive voltage appearing between the coax input connector and ground. To offset a negative voltage, simply reverse the battery.|
|Figure 4: Angular area is taken to be proportional to (D/2)^2, where D is the angular diameter. For the Sun, D = 0.5 deg. arc; for the telescope, D = 3 deg arc. The ratio of the angular areas is approximately 36. Hence, if the telescope voltage output corresponds to a temperature of 340 K (on the basis of the cali-bration curve), then the equivalent blackbody temperature of the Sun at 12 GHz is 340¥36 = 12,200 K.|
|Figure 5: Idealized chart-recording showing the initial maneuvers to calibrate the telescope at the cold-sky temperature, indicated by A. The telescope is pointed in various directions until the minimum output is obtained; this is the first calibration point. The temperature is nominally 6 K. The telescope is then pointed at the ground to obtain another calibration point, B. Finally, at C, the telescope is pointed at the Sun.|
|Figure 6: Idealized chart recording for Experiment #2. At first, the telescope is adjusted so that it is looking about 20 degrees above the horizon at an empty sky (no trees, satellites, etc. allowed), and then back and forth along azimuth (about 10 degrees or so) until it is looking at the coldest part of the sky; this produces a minimum deflection of the pen, marked A on the chart. Then a person climbs up on a short ladder and stands in front of the dish in such a way as to fill as much of the telescope beamwidth as possible. Stand about a meter or so away from the LNB. The deflection is marked B for this particular person. The deflection for another person is marked C, etc.|
|Figure 7: A block diagram of the experimental arrange-ment to measure the microwave emission from some object (e.g., water) at various temperatures. A suitable beaker holder needs to be fitted more or less as shown. In our experiment, we simply filled the beaker with water at several different temperatures and measured the telescope (we could also call it a microwave radiometer) output each time. It is rather important to ensure that no contaminating microwave radiation (e.g., from fluorescent lights) is entering the feed-horn.|
|Figure 8: An idealized graph obtained using the procedure described above. The voltage-offset module was used to set the output voltage of the system (input voltage at the chart recorder or equivalent) to zero volts when the substance was at 0o C. Note that the data does not follow a straight line, perhaps contrary to expectation. The basic reason is that the blackbody radiation depends on the fourth power of the blackbody temperature. Also, the LNB itself produces microwaves consistent with an object at a temperature specified by the 'Noise Temperature' of the LNB (given by the manufacturer; usually about 50 K for typical off-the-shelf units). For the purposes of the experiment, however, it suffices to find that the temperature of the substance and its associated blackbody microwave emission are related in a fairly simple manner.|
|Figure 9: Parts Values|
C1: 10 uF
C2: 0.1 uF
C3: 0.001 uF
C4: 4.7 pF
L: 6 uH
R1: 470, 1/8 watt
R2: 100, 1/4 watt
Note that C3 is a feed-through capacitor.
|Figure 10: Schematic diagram of the home-built detector module.|
C1: 4.7 pF
C2: 0.01 uF
D1: 1N34 or equivalent.
R: 100 ohm, non-inductive.