C Band LNB rectangular horn design

[radiogareth]

I picked up a BNIB Norsat Gold (DRO) LNB originally intended for cruise ship duties. I want to use it for my local repeater GB3JV but it could do with a little bit of gain to assist. I've seen 3-D printed horns and read the drawbacks of the surface imperfections (less relevant and easier to resolve at 3.4GHz than at 22) so thought I'd have a go at designing/printing something.
Its a rectangular cavity 29.4 x 58.5 mm with a gasket in the surrounding flange. I'll need to seal it against weather and was thinking some thin (1.6mm) PTFE sheet (I have an abundance of the stuff) or acrylic.
I'll line the printed horn with self adhesive aluminium tape, but I need some dimensional guidance please. Could I scale from a similar X band horn and expect results? Or I could fab it from DS GRP PCB, solder joints on the outside.
Not found any particular design software (read=free) to assist, unlike almost any other type of antenna....
Guidance welcome...
Thanks
Gareth

[G7JTT]

This may help http://www.microwavetools.com/pyramidal ... alculator/

73 John

[radiogareth]

Thanks John, that will do once I get my 3-D head around it.....
Gareth