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Re: No Stupid Questions, right? (focal point)



Kevin and...

Good questions and not sure I have a definitive answer.  I have heard the
"first-turn" position.  For a dipole with reflector the dipole is most
often quoted.  I think you will find that the dipole in a yagi is also
given, though not often used for a dish feed.  For horns the position is
just inside the opening or "throat", so does a helix resemble a dipole or a
horn..??  

So best advise is to mount it so one can adjust its position in-or-out
while observing a reference signal [whatever that is]...well a signal
source is nice, or a neighbor's carrier, or even sun noise if the dish is
large enough.  Adjusting on the satellite signal is probably a recipe for
frustration due to varying signal.

In the final analysis, my experience is that the feed will work even when
not at the optimum focus of the dish.  Get it close and mount it securely
aimed at the correct point on the dish [this varies a little whether its a
center-fed or offset fed dish].  Then..."kill the engineer"...and have fun
using it!  At some point its time to stop fooling with it and just operate
;-)  {giving myself advice, here}

Ed


>From: "Kevin J. Smith" <kjsmith@home.com>
>I asked what part of a 3.25 turn helix should be at the focal point should
>be at the focus of a parabolic dish a few months ago.  It was suggested that
>the focus should be at the end of turn 1 of the helix.  But in the next
>sentence, it was suggested to experiment by moving the helix through the
>focus while listening to a signal generator.
>
>Kevin Smith
>N3HKQ
 

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