[NLRS] Parabolic Question

Mon, 10 Nov 2003 16:18:11 -0500

Scott,

Many of the offset dishes I've seen (which admittedly isn't all that 
many) have the center of the parabola at the bottom of the dish. These 
means that your feed will be on a horizontal line that goes through the 
bottom of the dish.  You might look at W1GHZ's handbook if you haven't 
already--he gives dimensions for many popular dishes including where to 
put the feed and all the other useful info you might want.

http://w1ghz.cx/

Andy K0SM/2

[email protected] wrote:

>I use a DISH500 offset dish that is 20.5".   Check the internet but you can
>buy a new one (less LNA) for $40 to $50 plus shipping.   Here is one site
>that has them for $49.99
>http://dishdepot.com/Cart/description.php?II=492276&UID=20031110081350192.28.2.17
>     I bought mine from some place in Eagan, MN, that Doug, N0NAS, pointed
>me to (that I have since forgot).    I think I paid $35 on sale at the
>time.
>
>One (possible) advantage with the DISH500 offset dish is that the back
>mounting plate appears to be at 90 degrees vertical when the dish is on the
>horizon .... I am purely guessing by my thought is that when the dish
>designers CADed the design they may have used this back mounting plate
>surface as a reference line.  I mounted a simple bubble level flush with
>this surface.    This helps alot with figuring out elevation.   Azimuth is
>largely right down the feed arm.    After aiming the feed arm and leveling
>my bubble, its then a matter of (hopefully) small adjustments ..... once
>I've found the signal I peak with azimuth, lock it in, unlock elevation and
>peak it then lock it back in, the go back and unlock, peak, and relock
>azimuth .... all this can be done in 15 seconds or so.
>
>If you buy this assembly, the little angle mount at the bottom front of the
>feed arm that mounts the LNA is not provided so you will have to figure out
>where the focal point is.    The website that Donn pointed you towards
>contains lots of great information and data including a method to determine
>where the feedpoint should be based on physical measurements.   The trick
>is to make your feedhorn mount adjustable, do the calculations, mount the
>feedhorn where the math says it should go, then show up at one of the NLRS
>antenna parties and tweak it.
>
>73, Jon
>W0ZQ.
>
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