Astrometric Observatory (The Online Observer and Instrument Builder)

Simple solar thermal radioastronomy with a C-band satellite dish.


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The C-band satellite dish at the right was purchased at the local Raton junk yard for $10 - including the low-noise amplifier (LNA). The normal price is $20, but a tractor had RUN OVER IT and almost folded it over !!! Fortunately, this made it very easy to put in my pick-up; and it gave me a once in a lifetime opportunity to "make" an 8-foot diameter parabola. After a few hours with some assorted blocks of wood and a rubber mallet it was a pretty good F/0.5 parabola.

The mount is about 8 feet high at the axis, and it is adjustable in altitude. I rotate the whole mount to change azimuth. The dish is counterbalanced with a 6 foot long pipe bolted at the center, and a sliding weight on the pipe. The altitude axis uses a 2-inch pipe resting in bearings which are pressed into a 2 by 6. It can be easily moved in altitude with 1 finger - so I have to lock it.

The amplifier is powered by 15 VDC at 1 amp. The signal is then boosted by a small in-line amplifier made for driving long lengths of coax cable. It is then rectified with a microwave diode and small-value capacitor. This rectified signal is fed to a Radio Shack multimeter that has PC output capability. The meter does an analog to digital conversion at 3 Hz. I wrote a GW-BASIC program to sense the meter output, build a data file, and draw a graph on the PC screen in real time.

C-band satellite dish

My C-band satellite dish for crude radioastronomy (photo taken 10/2001). There is usually 1 to 3 days each year when the cottonwoods are this yellow AND the sky is cloudless. Raton's skies really are this blue !!! (No color or contrast adjustments were made)

A drift scan of the sun

A drift scan of the sun on 10/31/2000. The dish was pointed due south and elevated to the calculated solar declination. I started recording the data 26 minutes before the expected solar transit, and recorded data for just over 56 minutes (96 min. is the maximum possible record time). I then ran a post-processing program using the data file as input and created the graph at the left.

This program finds the times and values (in millivolts) of maximum, minimum, and the 2 points on the curve that are -3dB below the maximum value. The program then computes the "derived data" and lists it at the right. The derived data are the beamwidth (in degrees) for the dish, and the system temperature in degrees Kelvin. Formulas for computing the derived data can be found in The ARRL Antenna Book ($30), and The ARRL UHF/Microwave Experimenter's Manual ($20) - both published nearly annually; and these prices are for 1997.

The sun displays a nice symmetrical bell-curve about the time of transit. The 3-4 small sharp spikes or blips are caused by aircraft microwave collision avoidance radar, and/or aircraft weather radar. This system responds to microwaves in the 3.7 to 4.2 gigahertz (GHz) region. This is the "C-band" used by satellites in geosync orbit to downlink TV signals. This frequency was very popular 10 and 20 years ago, but is now almost entirely superseded by the higher frequency "Ku" band downlinks in the 14.0 to 14.5 GHz region. For this reason, these dishes can be obtained almost for nothing - and sometimes free for the asking.