[Moon-net] septum feed question

[Rastislav Galuscak]

  Hello Peter,
   
    Function of the distance from the waveguide aperture to the front edge of the septum. it is better to explain with conjuction of whole structure function. While at the feed  area , where the probes are placed, fundamental waveguide mode TE is excited, on the septum due to rapid waveguide discontinuity, higher modes TE and TM are arising. Some of them are attenuated behind the septum and some are coupling together just at mentioned area. As the coupling result  are two perpendicular TE11 modes (considering feed with round cross-section) at feed aperture.  On length of this area depends purity of  circular polarization. This length has a minimal or critical value, when deterioration of circular polarization will occur. For my and Mr. Hazdra design this length  is about 0.71 λg (wave length in WG). But for others design this length may differ. For feed with square cross-section , according to Mr. Kysela and OK1CA design, published by OK1DFC, this length is only 0.595
 λg., The analyses of RA3AQ designed feed, based on WG with square cross-section with improved parameters
 ( this lenght is extended by round waveguide), indicates that this length was chosen too short.  
   
     Sincerely,
                        Rasto, OM6AA
  

peter blair <g3ltf at btinternet.com> wrote:        Referring to Zdenek's ( OK1DFC) septum  calculation file  which  you will find at http://www.ok1dfc.com/eme/technic/septum/calc.xls   can anyone tell me what is the significance or importance of the dimension K,  the distance from the end of the last tooth of the septum to the open end of the  guide.
   Im not questioning Zdeneks excellent work but I  cant understand  why this should be a specified or critical   parameter.
 If it is a critical parameter in the original ( square )  design is it also critical in a circular guide?
   Im asking the question widely because I guess by  now quite a few people will have built these  and there should be some  experience out there Thanks, 73 Peter G3LTF 


       
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