Antenna and Feed simulation

The basics of prime focus and offset dishes can be found in "The W1GHZ Online Microwave Antenna Book". Paul Wade has written an excellent antenna book with a lot of detailed explanations about different feeds. Paul also made a lot of simulations for each feed.

Formula to calculate a dual mode horn for an offset antenna: e.g. 122 GHz

Simulation by W1GHZ:

Nowadays it is easy to make such simulations without any extra costs at home. Only your computer speed/memory limits your own simulations.
I use open software OpenEMs and Octave to do my own feed simulations. On the OpenEMS website there are a tutorials available with a lot of simulation examples.

Another commercial software is CST studio suite. There is a free student version but limited to max. mesh cells of 100 000.

Here are some of my own simulations with openEMS:

Dual Mode Horn @76 GHz, optimized for an offset dish f/D=0.66:

Coax to rectangular waveguide transition @10.368 GHz:

WR90 waveguide.

Backshort (distance back wall to coax pin) : 6.1 mm
Coax pin length (inside of wave guide) : 5.73 mm

Coax to circular waveguide transition @10.368 GHz:

Copper pipe with inside diameter of 20.1 mm.

Backshort (distance back wall to coax pin) : 12.0 mm
Coax pin length (inside of wave guide) : 6.3 mm
Length between coax pin and open end : 53.3 mm (= 1x wave guide wavelength)

The end cap is moveable to adjust S11.

As I wasn't so happy with OpenEMS I tried some other simulation tools, e.g. GRASP, CST student edition, Ansys HFSS student, QuickWave student and CENOS.