Blockage Efficiency
Understanding Blockage Efficiency
Aperture blockage has two effects: gain reduction (blocked area contributes no signal) and sidelobe degradation (diffraction from the obstruction edges raises near-in sidelobes). The gain effect scales as the area ratio squared; the sidelobe effect depends on the blockage geometry and edge treatment.
Offset-fed reflectors (commonly used in satellite TV dishes) eliminate blockage entirely by placing the feed below the aperture edge. This gives the highest blockage efficiency (100%) but introduces beam squint and cross-polarization that must be managed.
ηb = (1 − (db/D)2)2
Example: db=0.3 m, D=3 m:
ηb = (1 − 0.01)2 = 0.98 (98%)
Gain loss = −10·log10(0.98) = 0.09 dB
Blockage by Reflector Type
| Type | Blockage Source | ηb | Sidelobe Impact |
|---|---|---|---|
| Prime-focus | Feed + 4 struts | 85-92% | High |
| Cassegrain | Subreflector + struts | 90-95% | Medium |
| Gregorian | Subreflector + struts | 90-95% | Medium |
| Offset-fed | None | ~100% | Minimal |
Frequently Asked Questions
What causes blockage?
Feed/subreflector in aperture center, support struts, and cables. Blocked area loses gain; edges cause diffraction raising sidelobes.
How calculated?
ηb = (1−(db/D)2)2. 10% diameter ratio: 98% efficiency, 0.09 dB loss. Struts add 1-5% more loss.
How to minimize?
Offset feed (100% efficiency), small subreflector, thin/serrated struts, or dielectric strut material.