Select a Ku-band satellite antenna from the operator’s coverage and access requirements, a site-specific link budget and the minimum service rate required during bad weather. Dish diameter alone cannot guarantee 99.9% availability. At equal frequency and aperture efficiency, increasing diameter from 1.2 m to 1.8 m adds about 3.52 dB of gain; whether that closes the link depends on the remaining budget.
Collect the site coordinates and altitude, satellite or constellation, transmit and receive bands, polarization, elevation range, coverage contours, required data rates and annual availability target. Keep fixed GEO terminals and moving or NGSO terminals separate: their tracking, handover and transmit-control requirements differ.
Weather Fade
Table of Contents
Rain Fade Characteristics
Rain absorbs and scatters microwave energy. A rain rate measured at ground level is not, by itself, the loss through the complete satellite path. Frequency, polarization, elevation, rain height and the spatial distribution of rainfall affect the result.
ITU-R P.618 provides Earth-space propagation prediction methods. Use site-specific rainfall statistics and geometry within the model’s stated scope; include gaseous, cloud and scintillation effects where applicable. The result is a predicted attenuation distribution, not a promise that every storm will produce the same loss.[1]
ITU-R P.838-3 gives specific rain attenuation as γR = kRα, in dB/km, with R in mm/h. Its coefficients depend on frequency, polarization and path geometry.[2]
| Calculated reference case | Specific attenuation |
|---|---|
| 12 GHz, 50 mm/h | H: 2.44 dB/km; V: 1.98 dB/km. |
| 14 GHz, 50 mm/h | H: 3.23 dB/km; V: 2.66 dB/km. |
| 12 GHz, 100 mm/h | H: 5.53 dB/km; V: 4.30 dB/km. |
| 14 GHz, 100 mm/h | H: 7.11 dB/km; V: 5.56 dB/km. |
These examples use the horizontal-path H/V coefficients in P.838-3 Table 5, rounded to two decimals. They are not total satellite-path losses. Use the elevation and polarization transformations for a slant path, followed by the applicable path-reduction and probability calculations.
For a simple geometric illustration, a 4 km vertical height difference corresponds to 11.70 km at 20° elevation and 5.66 km at 45°, using length = height difference / sin(elevation). This does not imply uniform rain over that length or justify multiplying a local peak rain rate by the entire path.
The operator assigns polarization. Do not rotate to another polarization solely to reduce modeled rain loss. Correct skew alignment and cross-polarization verification protect the assigned channel and other users.
Translate the availability target into a time budget. For a 365-day year, 99.9% permits 8.76 hours below the defined service threshold; 99.99% permits about 52.6 minutes. Specify whether the target concerns carrier lock, a minimum data rate or the complete service, including power and equipment outages.
Snow and Ice Crystal Loss
Distinguish propagation through precipitation from accumulation on the dish, feed or radome. Snow wetness, coverage and temperature change the RF effect; a fixed snow thickness cannot be assigned one universal gain loss.
Specify the manufacturer’s snow and ice operating conditions, approved de-icing method, heater power and environmental limits. Check drainage and feed-window condition. Include the heater load in the power design, and follow the approved cleaning procedure rather than scraping or heating the reflector arbitrarily.
Weather can reduce carrier power and increase received noise. In an illustrative calculation, raising system noise temperature from 50 K to 200 K adds 10 log10(200/50) = 6.02 dB of noise degradation, separately from carrier attenuation. Avoid counting the same atmospheric effect twice when combining link-budget terms.
Aperture Gain Compensation
For a circular aperture, G = η(πD/λ)2, where η is aperture efficiency, D is diameter and λ is wavelength. NASA’s antenna reference distinguishes physical area from effective collecting area; the latter includes efficiency.[3]
At constant efficiency and frequency, the gain change is 20 log10(D2/D1). Doubling diameter adds 6.02 dB. Increasing 0.9 m to 1.2 m adds 2.50 dB; increasing 1.2 m to 1.8 m adds 3.52 dB. An equal increase in meters does not always produce the same gain increase.
A 1.2 m circular dish has a physical area of about 1.13 m². At an assumed 65% aperture efficiency, its effective area is about 0.74 m². Calling the entire physical area “effective area” overstates the calculation.
AGC adjusts receiver signal level; it cannot restore carrier-to-noise ratio already lost in the channel. Adaptive coding and modulation can retain a usable link at a lower rate. Uplink power control needs available amplifier headroom and operator permission within the applicable transmit limits.
High Throughput
Beam Coverage Technology
HTS systems increase aggregate capacity through techniques such as spot beams and frequency reuse. Aggregate satellite capacity is not the rate allocated to one terminal. Obtain the operator’s usable coverage, receive G/T, downlink EIRP, access policy and service-rate commitments for the exact site.
A fixed site’s position in a satellite spot beam and the terminal antenna’s pointing error are different losses. Moving toward a coverage edge changes the available satellite signal; mispointing the ground antenna reduces its gain toward the satellite.
Check gateway dependence as well as local weather. A clear terminal can lose service during a gateway outage. If site diversity is part of the design, evaluate weather correlation, backhaul, switching time and shared power dependencies; 20 km or 50 km separation alone does not guarantee independent outages.
Spectral Utilization Efficiency
DVB-S2X adds finer modulation-and-coding choices, sharper roll-off filtering and options such as channel bonding. Support for the standard does not mean that every modem implements every option or achieves one fixed spectral efficiency.[4]
For a preliminary comparison, coded information rate before framing and other overhead is approximately symbol rate × bits per symbol × code rate. At 10 Msymbol/s, QPSK with rate 3/4 gives 15 Mbit/s; 16APSK with rate 2/3 gives about 26.67 Mbit/s. Net user throughput is lower and also depends on service allocation and network overhead.
For raised-cosine bandwidth planning, B ≈ Rs(1 + roll-off). At 10 Msymbol/s, roll-off values of 0.35 and 0.05 give 13.5 MHz and 10.5 MHz respectively: a 22.2% bandwidth reduction at fixed symbol rate. Confirm the waveform, filtering and spacing the operator permits.
Use the modem’s required Es/N0 and implementation margin for the selected waveform, frame and error-rate target. A universal SNR-to-MODCOD table cannot cover different receiver implementations, nonlinear distortion, phase noise and interference conditions.
Hardware Installation Standards
Match the feed, LNB, BUC, modem and IF cable as a system. Specify the LNB’s frequency coverage, local-oscillator accuracy, phase-noise mask and noise performance over temperature. A 0.2 dB noise-figure claim alone does not establish HTS suitability.
Compare receive performance through G/T in dB/K = antenna gain in dBi − 10 log10(system noise temperature in K), with both quantities referred to the same plane. System temperature includes antenna, feed-path and receiver contributions, not just the LNB. NASA’s noise-temperature reference explains this system figure of merit.[5]
For illustration, 42.06 dBi gain and 100 K system temperature give 22.06 dB/K. Substituting an LNB-only temperature into that calculation would omit other noise contributions and input losses.
Use the IF impedance and connectors specified by the equipment. A 50 Ω cable is not a drop-in replacement for a specified 75 Ω chain. Check loss at the actual IF frequencies, DC voltage drop to the outdoor unit, current capacity, control signaling and weather sealing.
Confirm usable BUC output with the required waveform and backoff, rather than relying on saturated watts. Document supply voltage, peak current, startup behavior and cooling requirements. Use manufacturer-specific connector torque and cable bend radius.
Obtain a structural design for the site wind, ice, roof or ground support and local installation requirements. Dish diameter alone cannot specify concrete depth, ballast mass, anchor capacity or grounding resistance. Distinguish operating wind, which includes tracking or pointing performance, from survival wind.
Dish Size
Aperture’s Impact on Gain
The following values are calculations at 12.5 GHz and 65% aperture efficiency, not measured specifications for a Dolph model. They let you compare diameter on one consistent basis.
| Dish diameter | Calculated gain |
|---|---|
| 0.60 m | 36.04 dBi |
| 0.75 m | 37.98 dBi |
| 0.90 m | 39.56 dBi |
| 1.20 m | 42.06 dBi |
| 1.80 m | 45.58 dBi |
| 2.40 m | 48.08 dBi |
In the same ideal comparison, doubling diameter lets a transmitter use one quarter of the RF power for equal on-axis EIRP: 8 W becomes 2 W. This does not imply a 75% reduction in DC consumption or automatic compliance; feed loss, efficiency, backoff and the off-axis pattern still matter.
Use the measured radiation pattern to set pointing tolerance. Half-power beamwidth is the full angular width between the two −3 dB points, so the corresponding one-sided offset is half that width for a symmetric main beam. Do not apply a fixed “0.2° causes 3 dB” rule to every aperture and frequency.
Specification Comparison
When comparing SATCOM antenna configurations, request gain across both bands, G/T conditions, sidelobe and cross-polar patterns, pointing performance under operating wind, and the supported environmental range. List the feed and radio configuration used for each quoted result.
For a feed network, include the OMT, diplexer, waveguide and connection losses in the same budget. A high dish-gain figure cannot recover loss or mismatch that was omitted from the comparison.
Price the complete installation: antenna, mount, foundation, radio, modem, power, heaters, access and maintenance. Calculate bandwidth and outage costs from an operator quote and the service design; a larger dish does not guarantee a fixed percentage reduction in monthly charges.
Compliance and Interference Limits
For U.S. deployments, antenna-pattern requirements and off-axis EIRP-density requirements are separate checks. The 2025 annual edition of 47 CFR Part 25 contains these in §§25.209 and 25.218, with scope and exceptions that depend on the system. Confirm current rules and the station’s authorization before transmitting.[6]
Do not treat 2° orbital spacing, one gain-mask equation, a 1.2 m minimum diameter or a single power-density number as a worldwide rule. Obtain the operator’s approved antenna pattern, polarization, carrier bandwidth, power limits and commissioning procedure for the intended service.
Moving terminals also need the applicable tracking and transmit-inhibit behavior. Verify the approved terminal configuration and loss-of-pointing response instead of imposing an unsupported universal update rate or shutdown time.
Before requesting a custom antenna-feed configuration, supply the coverage data, site and elevation range, service-rate target, availability definition, RF bands and interfaces, power budget and installation loads. Require the link-budget assumptions, predicted weather performance and measured antenna data to be identified separately in the acceptance package.
