How Do Cassegrain and Gregorian Antennas Differ?

A classical Cassegrain antenna uses a convex hyperboloidal subreflector before the main reflector’s prime focus; a classical Gregorian uses a concave ellipsoidal subreflector beyond it. In reception, the Gregorian’s reflected waves pass through a real intermediate focus before reaching the subreflector, while the Cassegrain intercepts them before they converge. For comparable main-reflector geometry, this generally makes the Cassegrain arrangement shorter. Neither design automatically provides higher gain, less blockage, or a wider usable field of view.

How the Two Reflector Layouts Work

Both designs use a main reflector and a subreflector to redirect energy to a feed horn. In conventional centered layouts, the feed can sit near the main reflector’s vertex, giving easier receiver access than a prime-focus arrangement. The descriptions below refer to classical paraboloidal main reflectors; shaped-reflector designs need their actual surface profiles and optical drawings to be assessed.

Cassegrain: intercepting the waves before focus

The main reflector sends incoming parallel rays toward its prime focus. A convex hyperboloidal subreflector intercepts the converging rays and redirects them toward the feed at the secondary focus near the vertex. The rays therefore never reach the main reflector’s prime focus on this receiving path. A hole through the main reflector is one possible feed arrangement, not a defining requirement. NRAO’s radio-astronomy textbook illustrates this geometry and explains how the secondary reflector increases the effective focal ratio and allows accessible receiver placement. [1]

Gregorian: reflecting the waves after focus

The main reflector first brings the incoming rays through a real prime focus. They then diverge toward a concave ellipsoidal subreflector, which refocuses them toward the feed. Placing the subreflector beyond the prime focus generally adds axial extent compared with intercepting the rays before focus. That intermediate focal region is a genuine optical distinction, but it does not by itself guarantee better scanning, lower sidelobes, or easier calibration. NRAO presents both classical geometries together. [1]

A documented Gregorian example is the 100-m Effelsberg radio telescope. Its technical specifications list a 6.5 m ellipsoidal Gregory subreflector with 24.5 m between its focal points. This illustrates an actual implementation, not a dimensional rule for other Gregorian antennas. [2]

Cassegrain vs. Gregorian at a Glance

Does Either Design Have Higher Gain?

Not at the same aperture, wavelength, and aperture efficiency. For either design, G = η(πD/λ)², where G is linear gain, D is aperture diameter, λ is wavelength, and η is aperture efficiency. Convert to dBi using 10 log₁₀(G). Wavelength is λ = c/f, with c = 299,792,458 m/s. The aperture-area and gain relationships are described in NRAO’s antenna fundamentals. [1]

The following is a calculated illustration for a 3.7 m circular aperture at an assumed 65% efficiency, not a product specification or a comparison of measured antennas. Wavelengths are rounded for display; gain uses the unrounded values.

A longer free-space ray path is not, by itself, an extra waveguide insertion loss. Compare illumination, spillover, blockage, surface accuracy, alignment, and any actual feed-network losses rather than assigning a gain penalty simply because one antenna is Gregorian.

Blockage, Surface Accuracy, and Field of View

Centered versus offset geometry is a separate choice. A centered secondary and its supports can obstruct the aperture. An offset layout can move them outside the projected aperture; this is not a benefit exclusive to the Gregorian name. NRAO illustrates the unblocked offset Gregorian geometry of the Green Bank Telescope. [1]

For a simple uniformly illuminated aperture model, the blockage factor is approximately [1 − (d/D)²]², where d is the obstruction diameter and D is the aperture diameter. A diameter ratio of 0.10 blocks 1% of the area and gives about 0.09 dB loss in this model; a ratio of 0.15 gives about 0.20 dB. These calculated estimates exclude support struts, diffraction details, and illumination taper. They are not predicted losses for every antenna of either type.

For random RMS reflector-surface error σ, the Ruze factor exp[−(4πσ/λ)²] estimates the efficiency reduction. It is not a model for feed misalignment or every possible structural deformation. NRAO discusses the surface-error relation and its assumptions. [1]

At 30 GHz, λ/50 is approximately 0.2 mm RMS. That tolerance matters to either reflector layout. Likewise, a wide usable field of view depends on aberrations, focal ratio, feed arrangement, and the complete optical design. Do not use “single beam means Cassegrain” or “multiple beams mean Gregorian” as an absolute selection rule: NRAO explicitly describes multiple-feed operation with a Cassegrain system. [1]

A Worked Datasheet Comparison

Consider a hypothetical procurement check for a 3.7 m antenna operating at 6 GHz. Suppose two candidate datasheets quote 45.5 dBi and 45.7 dBi at the same reference plane. Using η = 10^G_dBi/10/(πD/λ)², those rounded gains imply approximately 65.6% and 68.6% aperture efficiency. The difference is 0.2 dB, or about 4.7% in linear gain. These are calculated scenario values, not vendor measurements.

The useful next question is not which optical name appears on the cover. Check whether the quoted gains use the same frequency, polarization, reference plane, and treatment of feed-network losses. Then compare sidelobes, cross-polarization, noise temperature under equivalent conditions, pointing accuracy, and wind-dependent performance. A small gain advantage may not resolve a requirement for low noise or tighter sidelobes.

A document check also matters. The published information for an 11.3 m earth-station antenna uses both Cassegrain and ring-focus wording in its optical descriptions. For that equipment, obtain the approved reflector-and-feed drawing before treating the labels as interchangeable. The drawing should establish the actual optical configuration.

Which Antenna Should You Choose?

For satellite ground-station antennas, begin with the required gain, noise performance, sidelobe envelope, frequency coverage, and site envelope. A classical Cassegrain is a useful candidate when a compact dual-reflector layout is important. A Gregorian is worth considering when its beyond-focus geometry serves the intended optical system, but the name alone does not establish superior beam quality.

• Limited installation space: compare actual depth, moving envelope, mass, and wind ratings rather than assuming either design meets the site limits.
• Low blockage: inspect projected secondary and support obstruction, and compare centered with offset options.
• Multiple beams or scanning: request off-axis patterns, usable focal-region dimensions, and cross-polarization performance.
• High-frequency operation: verify surface accuracy, focus adjustment, pointing, and deformation limits at the operating wavelength.

The Bottom Line

The defining difference is convex-before-focus versus concave-beyond-focus. Compactness often favors the classical Cassegrain arrangement, while the Gregorian supplies a real intermediate focus. Choose between actual antenna designs using their optical drawings and warranted performance, not assumed advantages attached to either name.