[Moon-net] 13cm Septum measurements

[Walter Crauwels (ON4BCB)]

Hi,

Peter,

Eddy and I build a square septum feed directly after the publication with 
good results but not obtimum.
After some discussions with Paul WA6PY and Paul W1GHZ,
we noticed that the sidelobe illumination of the square septum is far from 
obtimum.
Paul Ghz did some simulations and later his new publication about
septum polarizers in a round WG which completly match with our findings.
Later we have build 2 prototype's for 13cm in red copper.
We build the following models for CF 2320mhz:
1 with WG diameter of 80mm (standard tube) and 1 with 0.71 lamda diameter as 
suggested by W1GHZ.
The septum dimensions where scaled down to fit this diameter.
Results following measurements with networkanalyzer ( no anechoic chamber) 
where:

Circularity is good better than 0.6 dB at CF and -16 mhz, RL loss > 26dB, 
only the Isolation was poor maximum 18dB with normal probes.
Bandwidth: Circularity was still 1.5dB at 2422 with poor RL

Now we are building 2 models following rasta's modeling,dimensions and
experiances which we will ship soon to Prague for additional measurements.

73 Walter ON4BCB, Eddy ON7UN
See you at the conference.





----- Original Message ----- 
From: "Rastislav Galuscak" <om6aa at yahoo.com>
To: "peter blair" <g3ltf at btinternet.com>
Cc: <moon-net at list-serv.davidv.net>
Sent: Friday, August 11, 2006 10:45 AM
Subject: Re: [Moon-net] 13cm Septum measurements


>    Hello gentlemen,
>
>      I would like to share with some  experiencies, I have acquired with 
> the 13cm septum feed with  square cross-section /modeled according to 
> Zdenek, OK1DFC, dimension/ . The septum feed was modeled by CST Microwave 
> Studio software at the Czech Technical University in Prague and some 
> computed analyzes  was verify with measurement on  real  feed on 23 cm in 
> university`s anechoic chamber.
>
>      It possible to achieve good isolation /S21/  and good impedance 
> match/S11/ since S paremeters  are good within the frq. range 2300 MHz 
> (S21=26.5  dB, S11=23.7dB), 2350 MHz( S21=36dB ,S11=25dB), up to 2424 MHz 
> where S21=32 dB,  S11=26 dB.
>      For wide band impedance matching, probe with  high outer diameter 
> must be used /up to 10 mm for 13cm/
>      For septum feed with round cross section what  is  our present 
> project, conical shape of  probe is used with very good wideband impedance 
> match /below 20 db within freq.  range 210 MHz on 23 cm band/
>
>      Septum feed with square cross section has very  good axial ratio for 
> main axes. The axial ratio does not exceed - 1 dB within  range Theta 
> = -68 deg. Up to 68 deg. for Phi =0, 90 deg.in freq. range 2300/2424  Mhz.
>      Axial ratio is becaming worser for cuts with Phi=  45, 135 deg. 
> where -1 dB range Theta=
>        - 26.4  up to 26.4 is. This is the worst case for freq. 2424MHz.
>      This is the limitation of  square septum feed , since circular 
> polarized  feed should have low axial ratio within feed substended angle 
> for appropriate  f/D dish ratio.
>      Better axial ratio and better copolarization  efficiency in parabolic 
> antenna assembly is possible to achieve  with septum placed in wavegiude 
> with round  cross- section.  This project is  finishing now with the tests 
> in anechoic chamber. The results will be published  very soon in DUBUS.
>
>      Noting the polarization properties of feed  described above is not 
> possible to investigate with sun noise measurement  techniques.
>
>          Detailed computed results / in 2D or 3D or ASCII format/ of 
> square  septum feed I can send on request.
>
>                73, Rasto OM6AA
>
>
> peter blair <g3ltf at btinternet.com> wrote:   I would like to hear from 
> anyone who has measured the match of a 13cm septum feed at 2304 or 2320 
> and also at 2424 MHz.
> thankyou.73 Peter G3LTF
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