[Moon-net] septum feed question

[Dmitry Dmitriev, RA3AQ]

On Sunday 06 May 2007 13:43:03 Rastislav Galuscak wrote:
>   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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Hello, Peter.

 Square waveguide  in my feed is matched with stepped dual mode section 
operating to open air (S11 around -40dB ). That is the main.  So axial ratio 
and isolation does not depend (or depend very weak) on  distance  from end of  
the last tooth of the septum to  end of the square waveguide if this distance 
is greater than some value. Simulation shows that futher increasing of this 
distance does not improve axial ratio.

-- 
73 de RA3AQ
Dmitry