The "Plumber's Delight" Radio Astronomy Interferometer
For the past year and a half, I have been experimenting with radio astronomy using converted Ku band (12 GHz) satellite TV equipment. This has allowed me to detect the Sun and Moon, and to experiment with different antenna arrangements. This article describes an interferometer using copper pipe and galvanized sheet metal cones called the "Plumber's Delight." The idea was suggested to me by William Lonc, professor emeritus of physics at St. Mary's University in Halifax, Nova Scotia.
An interferometer uses interference patterns to determine information about the source being observed. A basic design involves separating two antennae pointed skyward on an east - west line joined by a cable or waveguide. Radio waves received at the antennaes from cosmic sources are fed into the satellite TV receiver. The output that results consists of a dc voltage that rises and falls as the source passes in front of the antennaes. The changing lengths of the signal paths from the passing source cause the signals to arrive in and out of phase with time. This causes the overall signal strength entering the amplifiers to rise and fall so that, when graphed, it produces an interferogram.
Inteferometers have several advantages over single antenna designs. They allow observers to distinguish between ‘real’ cosmic sources and noise caused by earth based radio signals. The fringe pattern in the output created by the antennaes is predictable, it follows formulae that indicate the distance between the fringes in either degrees or time. A pattern that does not conform to the predicted response is probably not a cosmic radio source.
Another advantage is that the size of the source can be determined with some accuracy. Interferometers have a beamwidth, or observational area, of varying sizes depending on the individual antennaes being used and the spacing between them. The further apart the antennaes, the narrower the beamwidth. If the source is larger than the beamwidth, no fringes will appear although a signal increase will be seen overall. As the source size approaches the size of the beam some fringes will appear. A point source produces quite clear fringes. The source size can therefore be deduced by the characteristics of the interferogram.
These advantages can be demonstrated with the Plumber’s Delight, although with my system, only the Sun can be observed. The basic components of my radio telescope antenna consist of two galvanized sheet metal cones joined by copper tubing. When pointed at the Sun microwave energy is collected by the cones and fed along the copper waveguide. The signal is mixed at a T-junction halfway between the cones and carried into a Low Noise Block downconverter (LNB). The LNB amplifies the signal and converts it to a lower frequency where there will not be as much signal loss when it travels via coaxial cable to the satellite TV receiver. Signal strength is the primary observational data of radio astronomy and in most cases, it is measured at the receiver stage either by a signal strength meter, or a detector. Finally, a recording device is used to note the data.
The two cones are made of a light gauge galvanized sheet metal. I cut each cone so that it measured 12 inches across at the open end and 24 inches from the open end to the vertex.
These measurements were somewhat arbitrary. It appeared to me in pictures of microwave horns that the length tended to be about double the opening, so I let that observation guide me. I used a dry eraseable white board marker to scribe the sheet metal, cut the metal using tin snips, and then fastened the two ends of the metal together with pop rivets to make the cone. See figure x for the dimensions.
Copper pipe was used for waveguide. There are formulae to determine waveguide dimensions but I didn't (and don't now) know what these are. So I measured the dimensions the designers used for the waveguide cavity at the mouth of the LNB - 11/16 by 3/8 of an inch. In order to find copper tubing that would give me those dimensions, I had to purchase refridgeration grade pipe, elbows and T with an inside dimension of 5/8 inch. This pipe is expensive. I have not tried using 3/4 inch copper water pipe but I think it might work. I cut the pipe to give a separation of 150 cm between the cones, fastened the pipe, elbows, and T together (figure x) and hammered the tubing so that it formed an elipse with an inside dimension of 11/16 of an inch (1.8 cm) in the horizontal plane and 3/8 of an inch (1 cm) in the vertical.
Note in the picture above that the attachments on the ends of the waveguide are part of a different experiment and were not used in the Plumber's Delight. The LNB is attached to the T in the middle of the waveguide.
The cones were soldered at the vertices to the ninety degree copper elbows with a plumbers propane torch. You need to cut the vertices back enough so the cone fits snuggly over the mouth of the elbow allowing a good solder joint and smooth signal transition. Once all the pieces were fastened together I attached the LNB to the mouth of the T and mounted the assembly on the LNB support arms of the dish system from which I gleaned parts. Any other mounting arrangement would work; this was the most convenient way for me to do it.
I ran coaxial cable from the LNB to the analog satellite receiver in the house. To measure the signal strength, the output must be converted to a dc voltage. This can be done one of two ways. A detector circuit can be attached to the video output, or the signal strength meter or Automatic Gain Control (AGC) circuit can be used to extract the dc voltage. I picked the signal out of the receiver from the AGC pin on the receiver's tuner box as measured to the receiver ground. The signal voltage was read by using a Radio Shack RS 22-168A digital multimeter with computer interface. The meter comes with software to record data on a PC collected by the meter.
In July, 1998 I made several observations of the sun that showed the interference fringe patterns that one can expect to see with an interferometer. Because of the small collecting area presented by the cones the signal was barely discernable above the baseline voltage created by the combination of system noise and sky temperature. But after viewing the data using an Excel spreadsheet and chart, it was clear I had the results I hoped to see. In two of the sightings taken in late afternoon about 7 fringes were visible. When I moved the antenna so the sun moved horizontally through the beam of the antenna at noon, 12 or 14 fringes could be seen.
Given an antennae spacing of 150 cm, the predicted fringe spacing should have been 1.72 degrees or 6.87 minutes elapsed time between fringes. The average observed fringe spacing from maxima to maxima measured over the eight strongest fringes was 6.8 minutes, pretty close to predicted.
The parabolic antenna I used previously had a beamwidth (the cone of received energy in front of the antenna) of between 1.5 and 2 degrees. The beamwidth of the Plumber's Delight was substantially wider, approximately 51 minutes wide or 12 degrees. Taken on their own, the beamwidth of an individual cone, without the added resolution created by using two cones, would be substantially broader.
This project shows it is relatively easy to construct an antenna system that will demonstrate the principles of radio interferometry using parts from a used Ku band satellite TV system.
For a good introduction to amateur radio astronomy including interferometry and basic formulae, see Bill Lonc's Radio Astronomy Projects ISBN 1-889076-00-7 published by Radio-Sky Publishing, P.O. Box 3552, Louisville, KY 40201-3552. Also see my article describing my first radio telescope published in the February, 1999 Journal of the Royal Astronomical Society of Canada.