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Chapter 6: Technology

Reducing RFI

Dear Dr. SETI:
There is simple circuit that I had once used in audio recording. I am wondering if it might prove useful for SETI. I used it to get rid of noise from an otherwise wonderful recording. I created a 3rd channel by using an isolation (1:1 transformer,) and running a wire to the positive side of the left and right channels. The new channel only showed what was different between the two channels.) If there were an isolated voice in the left and a bell on the right; the third channel had a voice and a bell. And the noise, (or other common-mode signal that was shared by the left and right channels) had diminished or disappeared altogether.

I am imagining the use of a small dish (with perhaps an LNA) placed somewhat perpendicular to the main dish. Any radio frequency interference that enters both dishes, any noise that they both share would similarly disappear or diminish, leaving the difference only to be downshifted and sent through the system. Is this a workable solution to the RFI problem?

Rick (a Project ARGUS participant)

The Doctor Responds:
You're on to something there, Rick. It happens that it's not original, but that should not discourage you; your thoughts parallel those of some of the great minds of SETI. What you have described is not only useful, but an established RFI reduction technique. A little history:

In April of 1960, Frank Drake received a strong pulsed signal when first he pointed at Epsilon Eridani during Project OZMA. Suspecting RFI, he rigged a second receiver, tied to a fairly broad beamwidth (i.e., low gain) waveguide horn antenna, pointed straight up. When the signal reappeared a couple of days later, it was present in both the 85 foot dish and the horn. Hence, it was verified to be RFI, and disregarded.

During the two decades that the Big Ear radio telescope was online at Ohio State doing a continuous SETI all sky survey, there was a three-channel RFI elimination technique in use. The big antenna had two feedhorns, offset slightly along the Right Ascension axis, feeding matched LNAs. The outputs of the two were added 180 degrees out of phase, cancelling any local interference that made it into both horns simultaneously. In addition, an omnidirectional discone antenna was rigged, driving a third LNA. Any signal present both in Big Ear and the discone was similarly rejected as local interference. (It's significant to note that this is how RFI was ruled out as an explanation for the famous 'Wow!' signal. It passed both those tests.)

It's not clear how an amateur radio telescope could apply this technique, but it certainly bears consideration. Thanks for raising the issue.

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