Why can't aperture efficiency reach 100 percent?

Multiple independent loss mechanisms each take a percentage. Illumination taper (the feed pattern is stronger at the center than the edges) typically costs 1 to 2 dB. Spillover (energy that misses the reflector) costs 0.3 to 1 dB. Blockage by the feed and struts costs 0.2 to 0.5 dB. Surface roughness costs 0.1 to 0.5 dB depending on frequency. Feed mismatch and cross-polarization each add 0.1 to 0.3 dB. These losses multiply, making 55 to 70 percent typical for front-fed dishes and 70 to 80 percent for offset-fed designs where blockage is eliminated.

What aperture efficiency should I expect from a phased array?

A well-designed planar phased array with half-wavelength element spacing achieves 70 to 85 percent aperture efficiency at broadside. As the beam scans off-axis, the projected aperture area shrinks by cos(theta), and element patterns reduce the gain further. At 60 degrees scan, the combined effect typically reduces the effective aperture efficiency to 35 to 45 percent of the broadside value. Active phased arrays can partially compensate by increasing element drive levels, but thermal limits cap the recovery.

How does the Ruze equation relate surface errors to efficiency?

The Ruze equation gives the gain loss due to random surface errors: G_actual = G_ideal times exp(-(4 pi sigma / lambda)^2), where sigma is the RMS surface error. For a surface accuracy of lambda/20 (typical for a well-made dish), the efficiency loss is about 0.4 dB. At lambda/10, it rises to 1.5 dB. This becomes critical at millimeter-wave frequencies: a dish with 0.3 mm RMS error works well at 10 GHz (lambda/100) but loses 3.5 dB at 100 GHz (lambda/10).

Antenna Technology

Aperture Efficiency

A 3-meter parabolic dish at 12 GHz has a physical area of 7.07 m², which should produce 48.5 dBi of gain if every square centimeter contributed equally. It does not. The feed illuminates the center more strongly than the edges. Some energy spills past the rim. The feed and its support struts block part of the aperture. Surface imperfections scatter energy out of the main beam. After all these losses, the dish delivers about 45.5 dBi, roughly 65% of its theoretical maximum. That 65% is the aperture efficiency, and understanding what eats the other 35% is the key to better antenna design.

Category: Antenna Technology

Symbol: η_ap

Typical Range: 50 to 80%

Six Losses That Shrink Your Antenna Gain

Gain from physical aperture:
G = η_ap × (4πA / λ²)

Aperture efficiency breakdown:
η_ap = η_illum × η_spill × η_block × η_surface × η_xpol × η_feed

Loss Mechanism	Cause	Typical Loss	How to Minimize
Illumination taper	Feed pattern stronger at center	1.0 to 2.0 dB	Optimize f/D ratio and feed beamwidth
Spillover	Energy past reflector rim	0.3 to 1.0 dB	Narrower feed beamwidth (trade-off with taper)
Blockage	Feed, struts shadow aperture	0.2 to 0.5 dB	Offset-fed geometry eliminates blockage
Surface errors	RMS deviation from parabola	0.1 to 1.5 dB	Better manufacturing; Ruze limit: σ < λ/20
Cross-polarization	Feed radiates unwanted polarization	0.1 to 0.3 dB	Corrugated horn feed, septum polarizer
Feed mismatch	VSWR at feed-to-waveguide junction	0.05 to 0.2 dB	Proper impedance matching of feed

The Illumination vs. Spillover Trade-Off

The feed horn's beamwidth controls a fundamental trade-off. A narrow feed beam concentrates energy on the dish center and reduces spillover, but the edges receive much less power, increasing taper loss. A wide feed beam illuminates the edges more uniformly but spills more energy past the rim. The optimum is an edge taper of −10 to −12 dB, which balances these two effects and typically yields a combined illumination-plus-spillover efficiency of 75 to 82%.

Ruze Surface Error Limit

Gain degradation from surface roughness (Ruze equation):
η_surface = exp(−(4πσ/λ)²)

Example: σ = 0.3 mm RMS at different frequencies:
At 10 GHz (λ = 30 mm): η = exp(−(4π×0.3/30)²) = exp(−0.016) = 0.984 (−0.07 dB)
At 30 GHz (λ = 10 mm): η = exp(−(4π×0.3/10)²) = exp(−0.142) = 0.868 (−0.62 dB)
At 100 GHz (λ = 3 mm): η = exp(−(4π×0.3/3)²) = exp(−1.58) = 0.206 (−6.9 dB)

The same dish is nearly lossless at 10 GHz but unusable at 100 GHz. For mmWave operation, surface RMS must be <0.05 mm.

Common Questions

Frequently Asked Questions

Why can't aperture efficiency reach 100%?

Multiple independent losses multiply: illumination taper (1 to 2 dB), spillover (0.3 to 1 dB), blockage (0.2 to 0.5 dB), surface errors (0.1 to 0.5 dB), and cross-polarization (0.1 to 0.3 dB). Combined, these bring typical front-fed dishes to 55 to 70% and offset-fed designs to 70 to 80%.

What efficiency should I expect from a phased array?

At broadside, 70 to 85% for half-wavelength spacing. As the beam scans to 60 degrees, projected area shrinks by cos(θ) and element pattern effects reduce the effective aperture efficiency to 35 to 45% of broadside value.

How does the Ruze equation work?

G_actual = G_ideal × exp(−(4πσ/λ)²). A dish with 0.3 mm RMS error loses 0.07 dB at 10 GHz but 6.9 dB at 100 GHz. For mmWave dishes, surface RMS must be below 0.05 mm.

Related Terms

← Aperture-Coupled Patch Archimedean Spiral →

Antenna Design Services

Get a Reflector Efficiency Audit

Submit your dish geometry and feed pattern data. We will compute the full efficiency budget and identify which loss mechanism has the most room for improvement.

Request an Audit