A waveguide antenna system combines a source interface, a waveguide feed line, any required distribution or switching network, a radiating aperture, matching and monitoring hardware, and mechanical support. These six functional groups help organize the component list; they are not six mandatory assemblies. A fixed horn installation may need only a few parts, while a dual-band, steerable reflector needs a much more complex feed.
- Source interface: coaxial-to-waveguide adapters, diplexers, filters, orthomode transducers, and optional isolators or circulators.
- Feed line: rigid or flexible waveguide, bends, twists, transitions, flanges and seals.
- Distribution and switching: power dividers, hybrids, switches, phase trimmers and, where movement requires them, rotary joints.
- Radiating aperture: an open-ended guide, horn, slotted waveguide array, or a feed horn and reflector assembly.
- Matching and monitoring: tuners, matched loads, directional couplers and compatible power sensors.
- Mechanical and environmental hardware: brackets, gimbals, pressure windows, protective covers and thermal provisions.
A documented example is NASA’s 1978 Shuttle Millimeter Wave Communications Experiment (MWCE) antenna design. Its report identifies an orthomode transducer, a waveguide diplexer and a pair of single-pole double-throw switches as the major feed components. At system level, it groups the hardware into a waveguide network, corrugated feed horn, dish reflector and gimbal. The numerical examples below belong to that design or to the other explicitly named studies, not to every waveguide antenna. [1]
The distinction between a feed and an antenna matters. An enclosed waveguide transports energy; the aperture radiates it. NTIA describes an open-ended waveguide antenna and a flared horn as different members of the aperture-antenna family. A waveguide-fed reflector or slot array is a larger system built around that energy path. [2]
Table of Contents
Component map: what each group does
| Group | Purpose | Main selection questions |
|---|---|---|
| Source interface | Connects the RF port and separates frequency or polarization channels | Which connector, guide size, bands and polarization states must mate? |
| Feed line | Transports RF between the equipment and aperture | What are the route, loss allowance, flange details and movement requirements? |
| Distribution and switching | Divides power, selects paths and controls relative phase | How many branches are needed, with what amplitude balance, phase balance and isolation? |
| Radiating aperture | Produces the required beam | What gain, beamwidth, polarization and sidelobe performance are required? |
| Matching and monitoring | Controls reflections and provides measurement access | Where is the reference plane, and how will reflected and forward power be checked? |
| Mechanical and environmental | Supports, points and protects the assembly | What loads, pressure differences, temperatures and sealing conditions apply? |
Use this map to build a bill of materials, then assign specifications to the actual waveguide components in the signal path. A feed network can serve several groups at once. Its insertion loss should appear once in the budget, even if its functions appear in several rows.
Source interfaces: adapters, diplexers and polarization hardware
Adapters and transitions
A coaxial-to-waveguide adapter joins unlike transmission-line interfaces. Other transitions change guide dimensions or cross-section. Specify their operating band, mode conversion, insertion loss and return loss rather than treating them as lossless connectors.
NASA’s slotted-array report provides a useful measurement example: its L-band module gain was corrected for the dissipative losses of a stepped half-height-to-full-height transition and a coaxial-to-waveguide adapter, while input mismatch loss remained in the reported data. Those measurement boundaries matter when comparing antenna gains. [3]
Diplexers, orthomode transducers and hybrids
A diplexer separates frequency bands; an orthomode transducer (OMT) separates orthogonal polarization components. They solve different problems. In the MWCE feed, two identical branching-filter diplexers separated the 20 and 30 GHz signals, with specified isolation above 30 dB and insertion loss below 1 dB. [1]
The OMT handled two orthogonal TE11 polarizations at both bands. One polarization coupled through a circular-guide-to-finline transition into WR-34; the other reached a separate WR-34 branch through a circular-to-rectangular transition. TE11 describes the circular-guide modes here, not the dominant mode of the rectangular WR-34 branches. [1]
A 90° hybrid in each frequency channel and a pair of switches enabled selection of circular-polarization sense. The report specifies hybrid coupling of 3.0 dB, amplitude balance of ±0.25 dB and isolation of at least 30 dB. The 3 dB power split is distinct from dissipative insertion loss, listed as at most 0.10 dB over 19.70–20.20 GHz and 0.15 dB over 29.50–30.00 GHz. [1]
Feed lines: guide size, loss and mechanical interfaces
Match the guide to the complete operating band
Start with exact frequency limits, not just a band name. ITU-R V.431-8 defines SHF as above 3 GHz through 30 GHz, and EHF as above 30 GHz through 300 GHz. Those labels classify frequencies; they do not select a waveguide size. [4]
The MWCE design used WR-42 for its nominal 20 GHz transmit channel, WR-28 for its nominal 30 GHz receive channel, WR-34 in branches carrying both bands, and circular guide at the horn interface. The report’s frequency ranges below describe that application, not universal operating-band limits for those guide sizes. [1]
| Guide | Inside dimensions | Application frequency | Tabulated dominant-mode cutoff | Attenuation and status |
|---|---|---|---|---|
| WR-42, RG-53/U | 0.420 × 0.170 in; 10.67 × 4.32 mm | 19.95 ±0.25 GHz | 14.051 GHz | 0.73 dB/m, measured at 20 GHz |
| WR-34 | 0.340 × 0.170 in; 8.64 × 4.32 mm | 20–30 GHz | 17.328 GHz | 0.87 dB/m, estimated at 26 GHz |
| WR-28, RG-96/U | 0.280 × 0.140 in; 7.11 × 3.56 mm | 29.75 ±0.25 GHz | 21.077 GHz | 1.25 dB/m, measured at 30 GHz |
For an ideal air-filled rectangular guide, the dominant TE10 cutoff is fc = c/(2a), where a is the inside broad-wall dimension and c = 299,792,458 m/s. Using WR-28’s 0.280 in width gives approximately 21.077 GHz. For WR-34, the printed 0.340 in width gives approximately 17.357 GHz, slightly different from the report’s 17.328 GHz. The table preserves the published value; calculations should state which dimensional input they use. [5][1]
Above cutoff, the ideal guide wavelength is λg = λ0/√(1 − (fc/f)²). It exceeds the free-space wavelength because axial phase velocity exceeds c, while group velocity is below c in this ideal model. A longer guide wavelength does not mean that phase travels more slowly. [6]
Check higher-order modes as well as cutoff
A guide can support a mode without that mode necessarily being excited. NASA’s circular-guide table gives a 1.12 cm inside diameter with cutoffs of 15.7 GHz for TE11, 20.6 GHz for TM01 and 26.1 GHz for TE21. Its feed discussion acknowledges higher-order propagation capability but states that the configuration was expected not to generate those modes. This is a design expectation, not a measurement proving their absence. [1]
Do not infer a different feed diameter by demanding single-mode operation across both bands. Confirm the intended modes, discontinuities and mode-conversion performance of the actual assembly when selecting standard waveguide or a custom guide.
Keep attenuation values tied to their sources
The MWCE report gives measured WR-42 attenuation of 0.73 dB/m at 20 GHz. University of Houston course notes give an approximate value of 0.37 dB/m at the same frequency. The available records do not establish the cause of the difference or matching material, finish and measurement conditions. Neither value is a universal guarantee. [1][6]
For procurement, request attenuation over the required band for the specified construction, plus losses for bends, joints and transitions. An unrelated calibration-standard uncertainty cannot establish the measurement uncertainty of either published result.
Flanges, bends and flexible sections
The MWCE circuitry specified a UG596/U flange on its 20 GHz WR-42 output, a UG599/U flange on its 30 GHz WR-28 output, and a special circular-guide flange mating with the horn. These are historical interface designations, not a substitute for checking current mating drawings. [1]
Specify the flange face, bolt pattern, alignment features, gasket arrangement and guide orientation at every junction. Choose waveguide bends to fit the route, and use flexible sections where the mechanical design requires movement. In NASA’s X-band test panel, eight flexible waveguides connected the feed to eight subpanels to accommodate subpanel movement. [3]
Distribution: dividers, switches and phase control
A distribution network sets the power and phase delivered to each branch. NASA’s X-band panel used a three-piece, eight-way waveguide power divider. That architecture belongs to the array; a single horn does not automatically need an equivalent divider. [3]
The MWCE waveguide switches were specified at 60 dB isolation, with losses of 0.2 dB at 20 GHz and 0.3 dB at 30 GHz. Its phase trimmers used thin dielectric slabs to compensate phase deviations between paths. The report allowed changes to the feed layout provided equal line lengths were maintained in the appropriate parts of the network. [1]
A rotary joint is required only when the chosen RF route must cross a rotating interface. The MWCE two-axis Cassegrain budget included rotary-joint loss. Its alternative one-axis shaped-reflector design carried the electronics and antenna together on the gimbal and used no rotary joints. Movement alone does not dictate the component list. [1]
Radiating apertures: horns, reflectors and slots
Horns and reflector feeds
A horn flares the waveguide opening to shape the aperture field and beam. NTIA distinguishes rectangular or square pyramidal horns from circular conical horns. The flare geometry affects beam size and efficiency. [2]
The MWCE design used a corrugated conical feed horn to obtain equal E- and H-plane beamwidths at both frequencies. Its Cassegrain main reflector had a 27.56 in (approximately 0.700 m) aperture and a 9.646 in focal length. The hyperboloidal subreflector aperture was 3.106 in, with dimensionless eccentricity 1.490. These optical parameters must be considered together; eccentricity is not a length that can be added to the feed spacing. [1]
Using a 0.700 m circular aperture and λ0 = c/f, the ideal uniform-aperture gain 4πA/λ0² is approximately 43.33 dBi at 20 GHz. This is a calculated ideal value, not realized system gain. The report tabulates 43.3 dB area gain and 39.4 dB net overall antenna gain at 20 GHz after its aperture-efficiency, surface, feed-horn and thermal allowances. [5][1]
Slotted waveguide arrays
In a slotted array, openings in the guide form the radiating elements. Slot geometry and excitation replace the horn-and-dish combination as the means of shaping the beam. NASA’s L-band module measured approximately 94% aperture efficiency at 1.270 GHz, with the transition and adapter dissipative-loss corrections described earlier. That result is specific to the tested module. [3]
A separate NASA Glenn and University of Puerto Rico study illustrates the mass-versus-loss trade. Its simulated 20 mm guide sections at 37.5 GHz gave the following per-metre values. These are simulations, not measured product ratings. [7]
| Construction | Mass | Attenuation |
|---|---|---|
| Standard WR-28, Al 6061 | 70.00 g/m | 0.65 dB/m |
| Aerogel-filled guide | 4.00 g/m | 5.16 dB/m |
| RT-Duroid 5880 substrate-integrated guide | 6.98 g/m | 8.54 dB/m |
| Aerogel substrate-integrated guide | 1.26 g/m | 7.20 dB/m |
The standard hollow guide was 17.5 times heavier than the aerogel-filled guide, while the aerogel-filled guide had about 7.94 times its attenuation. The authors considered the mass savings attractive for short aerospace runs. That trade depends on the installation’s mass and RF-loss budgets; it is not a general rule that the lighter guide is preferable. [7]
Matching, monitoring and environmental protection
Measure at a defined reference plane
A waveguide tuner adjusts the impedance match, a matched load terminates a port, and a directional coupler provides a sampling path for a compatible sensor. Their required ratings depend on where they sit in the system. Record the measurement reference plane and whether adapter, cable and mismatch losses are included.
Matching hardware is not mandatory in every guide. NASA reported VSWR below 1.4:1 over most of the L-band module’s bandwidth without a matching or tuning device. Adding a tuner should solve an identified mismatch rather than fill a presumed slot in a standard parts list. [3]
Pressure windows and power handling
A pressure window maintains an environmental boundary while passing RF. JPL’s 2013 Deep Space Network design used a WR-28 copper body and a bonded foam insert. The finished device had loss below 0.08 dB and return loss above 25 dB at 32 GHz, with helium leakage below 3 × 10⁻³ cm³/s. Those values belong to that construction and test frequency. [8]
Power handling requires a separate specification. NASA’s X-band array study used a 6 kW total design level and a conservative 1 kW assumption for each of eight coupling slots. Equal division would be 750 W per slot, but actual field concentrations and pressure-dependent breakdown still had to be assessed. These figures are not generic continuous-wave or peak ratings for a WR size. [3]
Include average power, peak power, duty cycle, pressure or altitude, temperature and cooling in the component enquiry. Mechanical supports and sealing hardware must suit the installation as well as the RF interfaces.
Practical lessons from documented engineering work
The following lessons use published design and service records, plus explicitly stated calculations. They do not represent first-hand testing of Dolph Microwave products.
Build a system budget before choosing the feed-line length
| Budget item | 20 GHz | 30 GHz |
|---|---|---|
| Antenna gain before the following allowances | 39.6 dB | 44.9 dB |
| Rotary-joint loss | −0.5 dB | −0.5 dB |
| Waveguide-run loss | −0.5 dB | −0.5 dB |
| Waveguide feed-network loss | −0.9 dB | −0.9 dB |
| Thermal degradation | −0.2 dB | −0.2 dB |
| Antenna system gain | 37.5 dB | 42.8 dB |
For a straight-guide-only calculation, the 0.5 dB run allowance corresponds to 0.5/0.73 ≈ 0.68 m at NASA’s measured WR-42 attenuation, or 0.5/0.37 ≈ 1.35 m using the university’s approximate figure. These are sensitivity estimates. They exclude additional joint and bend losses and do not establish a safe maximum length for untested hardware. [1][6]
Now assume a hypothetical 3 m WR-42 run at 20 GHz, keeping every other budget item fixed. At 0.73 dB/m, its loss is 2.19 dB and the revised budget is 37.5 + 0.5 − 2.19 = 35.81 dB. At 0.37 dB/m, loss is 1.11 dB and the result is 36.89 dB. The 1.08 dB difference shows why the selected guide needs an applicable loss figure. The 3 m length is an illustrative assumption, not a published MWCE dimension. [1][6]
Translate path-length differences into phase differences
For ideal air-filled WR-28 at 30 GHz, using a = 0.280 in gives λg ≈ 14.04 mm. An assumed 1 mm difference between otherwise identical paths gives Δφ = 360° × 1/14.04 ≈ 25.6°. For WR-42 at 20 GHz, a = 0.420 in gives λg ≈ 21.06 mm and approximately 17.1° per millimetre. These calculations illustrate sensitivity; 1 mm is not a recommended manufacturing tolerance. [5][6][1]
The practical requirement is electrical phase balance across the operating band, not merely equal external lengths. The MWCE report describes dielectric compensation for differing phase-frequency characteristics in its two transition paths, as well as phase trimmers near the hybrids. [1]
Treat a pressure window as a reliability component
JPL developed its replacement window after mechanical failures of earlier windows caused lost tracking time. At publication in February 2013, 40 replacement windows had been produced, with 12 in operation for more than 30,000 combined hours without failure. That is fleet-total service experience, not 30,000 hours per window or a lifetime guarantee. It demonstrates why an enquiry should cover bonding, leakage and mechanical reliability alongside RF loss. [8]
What to include in a component enquiry
- Exact frequency limits, channel directions and polarization requirements.
- A signal-path drawing showing every transition, branch, switch and rotating interface.
- Guide dimensions, flange drawings and mating details at each port.
- Required gain, beamwidth, sidelobes, VSWR and cross-polarization performance.
- A complete insertion-loss budget with a defined measurement reference plane.
- Average and peak power, duty cycle, pressure, temperature and cooling conditions.
- Mechanical envelope, support loads, movement range and environmental sealing.
- Acceptance measurements, calibration arrangements and the supplied test documentation.
Frequently asked questions
Does every waveguide antenna system need all six groups?
No. The groups organize functions rather than prescribe a fixed component count. For example, a fixed single-polarization feed may not need polarization switching or a rotary joint. Even NASA’s two moving reflector designs differed: one included rotary joints, while the other moved the electronics with the antenna. [1]
Is a horn always the radiating component?
No. An open-ended guide or slotted array can radiate without a horn. In a reflector system, the horn commonly illuminates the dish. JPL’s dual-band feed used a corrugated horn for the outer 8–9 GHz guide and a dielectric rod insert for the inner 30–40 GHz guide, with nominal design frequencies of 8.45 and 32 GHz. [2][3][9]
Is waveguide always lower-loss than coaxial cable?
No universal comparison covers every construction and frequency. The university notes list approximately 1.5 dB/m for RG-59 coaxial cable and 0.11 dB/m for WR-90 at 10 GHz. That is a comparison of those examples, not every coaxial cable against every waveguide. Compare actual assemblies, including adapters and joints, at the required frequency. [6]
Which component usually dominates the loss budget?
It depends on the architecture and run length. In the MWCE system budget, the 0.9 dB feed-network allowance exceeded either the 0.5 dB waveguide-run allowance or the 0.5 dB rotary-joint allowance. A longer run can reverse that order, as the 3 m example shows. Aperture-efficiency losses are a separate part of the antenna gain calculation. [1]
Send Dolph Microwave your frequency band, waveguide and flange details, power requirements and mechanical drawing. Ask for a component list and an assembly-level loss budget matched to your installation.
References
- NASA Goddard Space Flight Center, Shuttle Millimeter Wave Communications Experiment (MWCE) Antenna System Development — Antenna System Requirements Report, prepared by Hughes Aircraft Company, contract NAS5-24211, report P78-281 (NTRS 19790014179, N79-22350), March 1978. ntrs.nasa.gov
- National Telecommunications and Information Administration (NTIA), Antenna Models For Electromagnetic Compatibility Analyses, NTIA TM-13-489, C. W. Wang and T. Keech, October 2012. ntia.gov
- NASA, X-band slotted array test panel and test fixture, Hughes Aircraft Company, August 1979 (NTRS 19790022252). ntrs.nasa.gov
- ITU-R Recommendation V.431-8, Nomenclature of the frequency and wavelength bands used in telecommunications (08/2015). itu.int
- NIST, CODATA value of the speed of light in vacuum, 299 792 458 m/s (2022 CODATA recommended values). physics.nist.gov
- University of Houston, ECE 5317-6351 Microwave Engineering course notes, Waveguiding Structures Part 2: Attenuation, Notes 7, Fall 2026 (Prof. D. R. Jackson, adapted from notes by Prof. J. T. Williams). courses.egr.uh.edu
- R. A. Rodríguez-Solís, H. L. Pacheco (University of Puerto Rico) and F. A. Miranda and M. A. B. Meador (NASA Glenn Research Center), Slotted Polyimide-Aerogel-Filled-Waveguide Arrays, conference paper, publication date July 7, 2013 (NTRS 20140010643). ntrs.nasa.gov
- NASA Jet Propulsion Laboratory, High-Reliability Waveguide Vacuum/Pressure Window, NASA Tech Briefs NPO-48372, February 2013 (NTRS 20130009805). ntrs.nasa.gov
- NASA Jet Propulsion Laboratory, Dual-Band Feed for a Microwave Reflector Antenna, NASA Tech Briefs NPO-41061 (NTRS 20110015092); work by D. Hoppe and H. Reilly, Caltech. ntrs.nasa.gov
