[Moon-net] septum feed question

[peter blair]

----- Original Message ----- 
From: "Dmitry Dmitriev, RA3AQ" <ra3aq at vhfdx.ru>
To: <moon-net at list-serv.davidv.net>
Cc: "Rastislav Galuscak" <om6aa at yahoo.com>; "peter blair" 
<g3ltf at btinternet.com>
Sent: Sunday, May 06, 2007 6:07 PM
Subject: Re: [Moon-net] septum feed question


> 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 &#955;g
>> (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 &#955;g., 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
>