Chris R. Brown's physics and astronomy stuff

280 MHz Radio Interferometer

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Radio Interferometer detects sun!

Chris R. Brown

July 2006

Well, I finally did it, after about 10 years of procrastinating. I built a pair of Yagis cut for 280 MHz, and T'd them together at my receiver's RF input jack. The organization of all the stuff is shown in figure 1. The reciever, an old Watkins-Johnson 906A, has a bandwidth of 300 KHz.

Figure 1. 280 MHz solar interferometer and associated equipment

The antennas, each with a half power beamwidth of about 50 degrees, were separated by 20 meters on a line running NW to SE (dictated by convenience) and data was collected for two hours in mid-afternoon.

Figure 2. Yagis and receiver setup outside our condo. Temporary, of course.

The VHF receiver was outside, sheltered from the sun halfway between the antennas. The 21 MHz IF signal was sent inside to a noisy computer about 30 meters from the reciever. First results are shown below.

Figure 3. First light! Solar interferometer fringes at 280 MHz

For this first effort, there was no attempt to reduce noise. The demodulator and low pass filter had been used for radio meteor work, with a time constant of about 0.1 seconds, and I left it unchanged just to see what the noise spectrum looks like. The A/D took a sample of the demodulator voltage every five seconds during the two hour observation.

The presence of the sine wave pattern in Figure 3. is the icing on the cake. It is an unambiguous indicator of a radio source in the sky, presumably the sun. At about 100,000 Janskys, there's not another likely candidate. (1 Jansky = 10E-26 W/m^2 Hz)

Subsequently, I increased the time constant to 14 seconds, still sampling once every five seconds, and added an op amp, with gain of about 9. A portion of a solar observation is shown in figure 4. below. There are interesting periodic signals riding on the sun's signal, aircraft perhaps, or satellites.

Figure 4. Improved solar fringes, with other signals

To further improve the signal to noise ratio in anticipation of a search for other radio sources, I added some software averaging to the 14 s time constant in the low pass filter. Specifically, I sampled once per second, and saved the average of these samples every thirty seconds. The op amp (or its supply) caused some drift, so I removed it in favor of passive components in the demodulator and low pass filter. The signal for the sun with this arrangement is shown below.

Figure 5. Solar fringes with software averageing.

I'm not certain that the increased sensitivity is sufficient to detect much less powerful radio sources than the sun. In summer, CAS A and CYG A are not well placed for observation from my site. When the seasons change, or when I find a better site, I'll press on. Also needed is better electronics between the antennas and the computer. Any suggestions would be welcome.

The software that operates the A/D card and saves the data is a homebrew in BASIC. The Yagi design program was inspired by Guenter Hoch's design in Ch. 9 of the ARRL UHF/ microwave experimenter's handbook. Both of these programs are available (for free of course) upon request.

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