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Re: helix location vs focus



IIRC -- you want the main lobe of the helix to illuminate just the 
dish, so the helix should be just long enough to narrow the main lobe 
to where it just fully illuminates the dish.  The sense of the helix 
should also be *opposite* that of the bird, because the reflection 
will reverse it (i.e. LHCP feed for an RHCP bird, and vice versa) and 
the helix should be positioned so the center of its roughly spherical 
wavefront in free space (the "phase center") is at the focal point of 
the dish.  Where that is exactly on a helix I can't tell you for 
sure, but I think it's near the feedpoint if not right at it -- 
putting the feedpoint/groundplane of the helix at the focal point 
should at least get you in the ballpark.  You can slide the helix 
toward or away from the dish when you get a bird in the boresight of 
the dish and peak it up on the S-meter for your final adjustment. 
(The sun is a fairly good test source, because you can see when the 
dish is lined up on it by the feed's shadow on the dish, and it will 
give you enough signal to check your focus, as it were.)

hope this helps .. (goes back to kicking his C-band dish)

>Once again I am back at trying for a better AO40 RX antenna.
>I just finished building a 5 turn helix that I am placing
>at the front of a 1 meter Channel master dish.  I know
>I read somewhere (can't remember) that you should place the
>bottom (i.e. feedpoint) of the helix at the focus of the dish.
>Now that I am back to playing with helices, I cannot find
>a single printed word indicating this.  I am wondering if that
>really is the case or if there are more opinions/suggestions
>on it's placement in regards to the dishes focus.  I have
>been using a DBS 18 inch dish with a 5 turn helix for about 8 months
>with great success, but would like to squeeze every bit of performance
>gain I can out of the new dish.  TIA

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