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In 1894, Sir Oliver Lodge first demonstrated waveguide microwave transmission lines at London's Royal Institution. Three years later at the University of Calcutta, Indian physicist J.C. Bose (seen here) flared out the end of a waveguide, demonstrating the horn antenna. Photo from Acharya Jagadis Chandra Bose, Birth Centenary, 1858-1958. Calcutta: published by the Birth Centenary Committee, printed by P.C. Ray, November 1958

Dr. Jagidas Chandra Bose's first waveguide horn antenna. Being a low-Q structure, the Bose horn offered respectable gain over perhaps an octave of bandwidth. photo courtesy of Dr. D. T. Emerson, AA7FV

Although the Bose horn was cylindrical, the pyramidal rectangular configuration is today favored for ease of construction. As a rule of thumb, a horn aperture of three wavelengths in the e-field, by four wavelengths in the h-field, by five wavelengths long, will yield about +20 dBi of gain at its design center frequency. AA2TX sketch

In one of radio astronomy's landmark experiments, Harvard University graduate student Harold I. "Doc" Ewen built this horn antenna to first measure the 1420 MHz hyperfine transition line of interstellar hydrogen. He based his 1951 antenna design on the earlier work of University of California professor Samuel Silver at the MIT Radiation Laboratory, with its physical dimensions constrained by the size of the parapet in Harvard's Lyman laboratory where his receiver apparatus was installed. photo courtesy of Dr. Harold I. Ewen

This is the crude receiving equipment used by Ewen in 1951, to first detect the interstellar hydrogen line. For his efforts, Ewen received his Ph.D. His thesis advisor, Professor Edward Purcell, was later awarded the Nobel prize. photo and diagram courtesy of Dr. Harold I. Ewen

The radiation characteristics (gain and beamwidth over frequency) of a pyramidal waveguide horn are purely a function of its physical dimensions. Ewen thoroughly documented the physical dimensions of his horn. Reverse-engineering the Ewen horn design by analyzing its dimensions in software, we find that it exhibited nearly +24 dBi of gain at the hydrogen line, while producing symmetrical e-field and h-field beamwidths of 10 degrees. horn dimensions courtesy of Dr. Harold I. Ewen

Harold Ewen's waveguide horn antenna is now on display at the National Radio Astronomy Observatory, Green Bank WV. Doc Ewen visited it there in 2001, fifty years after achieving the first measurement of the 21 cm hydrogen line. NRAO photo

At the 2002 Amsat Space Symposium, Anthony Monteiro, AA2TX, demonstrated a technique for manufacturing disposable horn antennas of negligible cost, out of corrugated cardboard, aluminum kitchen foil, and packing tape. The antenna seen here, used for 2.4 GHz satellite reception, was an inspiration for the SETI Horn of Plenty. Unfortunately, cardboard and foil proved insufficiently robust for our purposes, and we had to resort to 26 gauge galvanized sheet steel. Tony writes, "I agree that while cardboard is fun and a neat demo, for a serious radio telescope, the metal antenna is the way to go. After all, what if ET calls and it is raining that day?" AA2TX photo

A replica of the Ewen horn would seem to provide ideal performance for an amateur radio telescope, but for its size. The horn length of ten feet is definitely unwieldy, and provides little advantage over the standard satellite TV dish we are attempting to replace. It was decided to scale the dimensions of the Ewen horn, in search of a reasonable compromise between performance and size. A somewhat arbitrary horn length of four feet, and width of three feet, were selected, constrained by the standard size of available materials (26 gauge galvanized sheet steel is readily available in 3 foot by 4 foot sections, at under $10 per sheet from a local fabricator of heating, ventilation and air-conditioning ductwork.) SETI League drawings

Gain and beamwidth for the selected dimensions are analyzed in software. The resulting performance is nominally +20 dBi of gain across the radio astronomy Water Hole, with beamwidths on the order of 16 degrees. SETI League image

A primary design objective is that the SETI Horn of Plenty be constructed of readily available materials, with common hand tools. Before taking tinsnips to sheetmetal, a 1/12th scale mockup was constructed out of 3" by 5" index cards, to verify that all the parts would fit together properly. SETI League photos

The four horn sections are joined to 1" wide by 1/8" thick aluminum angle stock (sold locally at 75 cents per foot) with pop rivets. I like to place the pop rivets about a quarter wave apart at the operating frequency, which equates to about two inch spacing. SETI League photos

A quarter-wavelength monopole probe is used to excite the horn. It is fabricated out of a Type N flange mount coaxial connector and a 1.8 inch length of brass hobby tubing. The assembly is inserted on the centerline of the e-field horn face, 3 inches from the apex of the truncated pyramid. SETI League photos

Don't forget to rivet a 3 1/4" by 6 1/2" sheetmetal short at the back of the horn. SETI League photos

This horn design exhibits gain on a par with that of a quad helix array, a single long yagi, or a one-meter dish. However, the horn's excellent broadband impedance match, inherent lack of overspill, and low sidelobes mean its ground noise pickup will be significantly less than that of alternative antennas, providing a greater signal to noise ratio than one would expect from gain figures alone. SETI League photos

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