Diffraction effects on prime focus radio dishes
This comes up a lot in both radio astronomy and satellite communication when using prime-focus reflectors.
On a prime-focus parabolic dish, diffraction effects mainly arise from:
1. Feed Support Legs (Struts)
- The struts that hold the feed in place obstruct the aperture.
- They act like straight diffracting edges, producing diffraction spikes in the far-field beam pattern, much like the spikes on stars in optical Newtonian telescopes.
- The spikes are oriented along the directions of the struts.
- At radio frequencies, they appear as faint sidelobe structures extending away from the main beam.
Effect:
- Typically increases sidelobe level by a few dB in certain azimuths.
- Reduces aperture efficiency by ~2–5%, depending on strut thickness and geometry.
2. Feed Blockage
- The feed horn itself blocks the center of the aperture.
- This produces a shadow in the aperture distribution, which leads to:
- A broader main lobe (slightly reduced gain).
- Increased sidelobe levels (particularly in a concentric ring pattern).
Effect:
- Gain reduction of ~0.5–2 dB depending on the feed/dish size ratio.
- Sidelobes elevated typically by 3–6 dB compared to an unblocked aperture.
3. Edge Diffraction
- At the rim of the dish, diffraction occurs since the parabolic surface is a finite aperture.
- This is inevitable in any reflector and defines the basic diffraction-limited beamwidth (~λ/D).
- If the feed illumination is too sharp (under-illumination), edge diffraction is suppressed but aperture efficiency drops.
- If too shallow (over-illumination), efficiency is high but edge spillover noise and diffraction sidelobes increase.
Effect:
- First sidelobe usually around −17 to −20 dB relative to the main beam in a well-illuminated dish.
- Poor edge taper can raise sidelobes to −10 dB or worse.
4. Surface Irregularities
- Not strictly diffraction but causes scattering of wavefronts.
- If surface RMS error is >λ/16, noticeable reduction in gain and rise in scattered sidelobes occurs.
✅ Overall Impact (typical prime focus Cassegrain-style feed with struts):
- Aperture efficiency: 55–65% (vs 70–80% for offset dishes).
- Extra sidelobes and diffraction artifacts mainly show up at the few-percent level of the main lobe — small but noticeable in precision work (e.g., radio astronomy).
- For satellite TV and comms, usually negligible. For hydrogen-line or interferometry work, you do want to minimize blockage.
👉 In practice, astronomers often prefer offset-fed reflectors (no central blockage, no struts in beam) for high dynamic-range imaging, exactly because they eliminate these diffraction artifacts.