[Doc Ewen looks into the horn antenna, 1950]
Image courtesy of Doc Ewen


Harvard Cyclotron: 1948-1951

Detection of HI Line: 1951

Harvard 24ft and 60ft and NRAO founding: 1952-1956

1950s and 1960s: Two Roads that Crossed

Microwave & Millimeter Wave Applications in the 1970s and 1980s

Mm Wave Radiometry in the 1990s

May 2001 visit to NRAO Green Bank



[Doc Ewen and horn antenna, 2001]
Image courtesy of Doc Ewen

Doc Ewen: The Horn, HI, and Other Events in US Radio Astronomy

by Doc Ewen, © 2003

Slide 9: Detecting the Interstellar Hydrogen Line, 1951

[Line detection technique]

The "switch frequency" technique is a method for detecting weak lines like the hydrogen line. A switch is introduced in the receiver to cause the receiver to tune rapidly (30 Hz) back and forth between two frequencies displaced by 75 KHz (in the case of hydrogen). The two frequencies are then tuned together to move slowly from lower to higher frequencies, beginning at a frequency well below the rest frequency of hydrogen. A synchronous detector in the video circuit assigns a positive output to one frequency and negative to the other. As the two displaced frequencies pass over a line, the output will be positive when the line is at the first receiving frequency and negative when it passes through the second. This (+) followed by (-) feature is referred to as the "S-Curve".

Normally, the switch frequency is applied to the first local oscillator in a superheterodyne mode. For several technical reasons, I elected to switch the second local oscillator in a double conversion superheterodyne. That had its own set of problems, but they were manageable.

Slide 10
Modified on Tuesday, 28-Sep-2004 15:34:10 EDT by Ellen Bouton