Cassegrain Dish Calculator with Blockage Analysis
Optimized for amateur radio microwave station homebrewing.
System Inputs
Derived Engineering Specs
- Main Focal Length (F):-
- Auto Feed Distance (Lr):-
- Main Dish Feed Angle (Total):-
- Sub-Reflector Feed Angle (Total):-
- Linear Aperture Blockage:-
- Area Loss Blockage:-
Dynamic CAD Ray Overlay Window
Ham Microwave Dish Builder's Guide
1. Blockage Targets
The sub-reflector naturally casts a shadow over the main dish, blocking incoming/outgoing energy.
- Ideal Target: Keep Linear Blockage below 10% to 15% if possible.
- Area Loss: Keep total reflective area loss under 2% to 3% to avoid degrading your side-lobe levels and antenna gain performance.
2. Min Sub-Reflector Size
If a sub-reflector is too small relative to the target wavelength ($\lambda$), the radio waves will wrap right around it via diffraction instead of clean bouncing.
- Rule of Thumb: Sub-reflector diameter ($D_s$) should be at least $5\lambda$ to $10\lambda$.
- Example (10 GHz / 3cm): Minimum $D_s$ is roughly $150\text{ mm}$. Cassegrain systems are generally unviable for small dishes on 1.2 GHz or 2.3 GHz bands.
3. Ideal Ham F/D Ratios
Choosing the correct dish shape fundamentally dictates your feed engineering challenges.
- Deep Dishes (F/D 0.25 – 0.35): Excellent for standard prime-focus feeds, but make terrible Cassegrain conversions because the sub-reflector gets forced too close to the main dish.
- Shallow Dishes (F/D 0.40 – 0.60): The gold standard for Cassegrain configurations. Gives clean, narrow focus angles making feed placement highly practical.