USING THE CHAPARRAL 11 GHz SUPERFEED AT 10.368 GHz

                        N1BWT 1995  wade@tiac.net
                                
        The best feed I have tested for a prime-focus dish is a Ku-band
        TVRO scalar feed from Chaparral, which they call the 11 GHz
        Superfeed (catalog #11-0148).  Although it is specified for 11
        GHz, it works as well as a Chaparral feed that K1DPP made from
        the ARRL UHF/Microwave book, dimensioned for 10.3 GHz.  Since it
        took Don 6 hours work to machine a feed, the TVRO one seems like
        a bargain.

        The Chaparral feed works well on dishes with f/D ~ 0.39 to 0.45;
        we measured efficency as high as 60% on a 24-inch dish at 10.368
        GHz.  For slightly shallower dishes, f/D ~ 0.45 to 0.47,
        efficiency was improved slightly by shortening the projecting
        central ring so it was level with the outer rings.  With this
        modification we measured efficiency of 64% on a 30-inch dish 
        of  f/D = 0.45.  In a direct comparison on a 25-inch dish with 
        f/D = 0.45, efficiency increased to 61% with the modified
        Superfeed compared to 56% unmodified.  On the other hand, a 
        22" dish of f/D = 0.39 showed better efficiency with an
        unmodified feed.         
        
        As shown in the sketch, the input waveguide to the feed is
        narrower and taller than standard WR-90 waveguide, but we have
        obtained good results by centering the alignment in both
        dimensions by eye, then drilling and tapping holes in the feed
        flange to mate with the waveguide flange.  Another approach is
        to bore out the flange to mate with 3/4 inch copper pipe used
        as circular waveguide;  I have not tried this, and the example
        I've seen did not work as well when measuring sun noise.
        
        With no further tuning, a return loss of 10 to 12 dB is obtained,
        which is usable, but could cause a reflection loss of about 0.3
        dB.  The return loss is easily improved with a tuning screw, as
        shown in the sketch.  To reduce corrosion, I used an aluminum
        screw to match an aluminum waveguide; brass screws are better for
        brass or copper waveguides, and stainless steel is lossy.  The
        location of the screw is quite critical;  I measured the
        impedance with a WR-90 slotted line and calculated the location.
        For the unmodified Superfeed, the screw distance is 11 mm. from
        the flange interface, centered in the broad face of the
        waveguide.  If the Superfeed is modified by cutting the central
        waveguide ring level with the outer rings, then the screw
        distance is 14.5 mm.  For any other modification or frequencies
        other than 10.368 GHz, you are on your own.  The screw is
        adjusted for best return loss -- better than 20 dB is normal.
        All efficiencies cited above were measured with the tuning screw
        adjusted for best return loss, which did increase gain by about
        the expected 0.3 dB.  However, if you are unable to measure
        return loss, no tuning screw is better than a misadjusted one.
        
        We experimented with the focal distance for this feed, using one
        that N1IOL modified to make the outer choke rings adjustable.  It
        appears that the focal point is controlled by the outer choke
        rings, labelled "focal point reference" in the sketch,
        independent of the projection of the center ring, which makes
        small adjustments to the illumination angle for different f/D.
        My best estimate for the focal point of this feed is 3 mm. 
        behind the "focal point reference," inside the feed horn.