[Moon-net] septum feed question

[vladimir.masek at volny.cz]

Hello Peter,
just a short note.
I am interested to hear about that as well.
It is obviously recommended to maintain the length of the square cross-section
or cylindrical WG more than or at least equal to the respective WG wavelength.

The reason is to form homogenous E-H field in the WG before entering
WG aperture.
I do not know why the length of WG under the question (septum feed) relates
just to "air" lambda.
By my understanding, as smaller the diameter of cylindrical or square
feed section is, as longer becomes the WG lambda and the section length
should be prolonged.
I think it is valid not only for this section but to the septum itself
as well.
In other words the relative dimensions recalculation is valid in the
radial(perpendicular) direction but the axial dimensions have to be
related to the respective WG lambda.
Of course, probably the function for our ham-radio is not as much critical
to be noticed before a precise measurement is provided.
For example, the critical wavelength of standard septum feed with square
cross-section is simply double of the square's side (say, 2a).
So, if I change the square size to get different radiation beamwidth,
I should not recalculate the dimensions along the feed axis in relation
to lambda in air but to WG lambda.
The similar attempt has to be used for going to cylindrical shape from
the square one.
I hope, somebody of antenna experts will drop down more about it to delight
that issue.
73,
Vladimir
OK1DAK/OK1KIR  

----- PŮVODNÍ ZPRÁVA -----
Od: "peter blair" <g3ltf at btinternet.com>
Komu: "moonbounceboard" <moon-net at list-serv.davidv.net>, "freetalk moonnet"
<moon at moonbounce.info>, w1ghz at arrl.net, "Rastislav Galuscak" <om6aa at yahoo.com>
Předmět: [Moon-net] septum feed question
Datum: 2.5.2007 - 11:57:34

> 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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