Watching the movie "Contact", someone at the VLA calls out signal strength of the vegan A.M. signal as 112 "Janskies"(?).What is a jansky? Is it a real unit?, how does it compare to dbm? is it a relative or absolute measurement?

It's a real unit, basically a power spectral density. At the output of an antenna, you measure power (dbm for instance or watts). This power is measured within a certain bandwidth measured in Hz. The antenna will have an effective aperture which is measures it's collection area. If we take the power output, divide by the bandwidth, and divide by the area, we get the power flux falling on the antenna. This number should be the same for any two observers, regardless of their equipment.

http://www.unc.edu/~rowlett/units/dictJ.html

<font size=-1>[ This Message was edited by: Karl on 2002-11-29 23:50 ]</font>

Grote Reber implied to me that he had something to do with getting the unit name Jansky after the Bell labs guy who made the discovery that radio waves were being received from outer space.

Thankyou very much, clear, concise and makes perfect sense...

I just thought I'd mention that most astronomical radio sources (at least the ones I've looked at) are measured in milliJanskys, or even microJanskys. A 100 Jansky source is *incredibly* bright, and the characters in the film are appropriately astonished.

Don

Jansky was not looking for radio waves from outer space. He was interested in learning what the natural background noise level was for radio transmissions, so he could set a noise level sensitivity limit. He discovered that thunderstorms were a big source, but he also discovered a source that he eventually correlated to the position of the Milky Way. Grote Reber was so impressed by Jansky's discovery that he built the first real radio parabolic reflector and did the first real radio astronomy. Jansky used a movable antenna using parts from an old Ford that Grote Reber insisted be reproduced at Greenbank when they moved Reber's antenna there for display. (Reber had to keep the workers from making "improvements" to Jansky's design in bulding the memorial to keep it faithful to Jansky's original drawings.) I have had a good relationship with Reber over the years and have first hand stories about him told to me by Frank Drake. Fred Whipple told me that when Reber's maps of the night sky were published in the Astronomical Journal he and another guy tried to put a radio receiver at the focus of a Smithsonian (visible) telescope to see if they could receive anything, but that they weren't good enough electrical engineers to know that what they were doing wouldn't work.