Re: [amsat-bb] focal point of the dish?

[Franklin Antonio <antonio@xxxxxxxxxxxx>]

At 06:23 AM 3/30/2003, Tyler Harpster wrote:
>My question is this:   The helical feed is a few inches long.  The focal 
>point of a dish is theoretically a point.  Where should that point be in 
>relation to the feed?  Along the centerline of the feed, yes, but at the 
>reflector, the end of the helical, or somewhere in the middle?

The point you are searching for is called the "phase center" of the 
helix.  Ye Olde rule-of-thumb is one fourth of the helix length from the 
helix's feed point.  (So claims Antenna Engineering Handbook, by Richard C. 
Johnson.)

For the archeologists:
S. Sander & D.K. Chang, "Phase Center of Helical Beam Antennas", IRE Conv 
Rec, Part I, 1959, pp 138-146.


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