A dish's published gain is a ceiling, not a promise. Gain is 20 log10(D/λ) + 10 log10(η) + 9.94 dBi, and the only term a catalogue cannot know is η, the aperture efficiency. Measured reclaimed dishes run 10 to 50 percent against the 55 percent the books assume. Set a dish and a band, and read what the link budget should actually carry.
| Wavelength | |
| Diameter in wavelengths | |
| Half-power beamwidth |
Gain formula and the 0.13 to 0.8 efficiency spread: Antenna Introduction / Basics, Electronic Warfare and Radar Systems Engineering Handbook (2004), equation [13]. Measured reclaimed-dish efficiencies and the 55 percent book value: Paul Wade W1GHZ, Microwave Antenna Book, chapter 4. Beamwidth factor 70 λ/D: see Module 06 Unit 07.