Antenna Control Unit

The Antenna Control Unit (ACU) manages a parabolic dish antenna - the fundamental interface between your ground station and satellites in space. The antenna performs two critical functions: it focuses and amplifies weak signals received from satellites (like a giant light collector for radio waves), and it concentrates and directs your transmitted signals toward the satellite (like a searchlight beam).

A satellite dish works on the same principle as a reflecting telescope or satellite TV dish, but at much larger scale and with precision pointing requirements. Signals from geostationary satellites travel ~38,000 km and arrive incredibly weak (often -100 dBm or less - a millionth of a millionth of a watt). The large parabolic reflector collects this energy over its entire surface area and focuses it to a single point (the feed horn), providing 40–60 dB of gain - amplifying the signal by a factor of 10,000 to 1,000,000 times.

Key controls

• POWER - enables or disables the antenna control unit. When powered off, the antenna cannot be moved and all control is disabled. Power-on takes ~3 seconds to initialize the servo systems.
• Azimuth (Az) - horizontal pointing angle in degrees, measured clockwise from north. Range: -270° to +270° (or 0° to 360° when normalized). Adjustable in 0.1° increments. For example, 0° = North, 90° = East, 180° = South, 270° = West. Moving the azimuth knob breaks satellite lock and disables auto-tracking.
• Elevation (El) - vertical pointing angle in degrees above the horizon. Range: -5° to 90°. 0° = horizontal (looking at the horizon), 90° = zenith (straight up). Adjustable in 0.1° increments. Typical geostationary satellites are at 30–50° elevation depending on your latitude. Moving the elevation knob breaks satellite lock and disables auto-tracking.
• Skew - rotates the feed horn to align polarization with the satellite signal. Range: -90° to +90° in 1° increments. When a satellite’s signal polarization is rotated relative to your antenna (due to the satellite’s orbital position and your ground location), skew adjustment recovers the signal. 0° = no rotation, ±45° = significant rotation typically needed at extreme orbital positions.
• Loopback to OMT - a test mode switch that routes transmitted signals directly to the receive path instead of radiating them. When enabled, your BUC/HPA output goes to the LNB input, creating a local RF loop for testing modems, signal processing, and troubleshooting without transmitting to the satellite. The blue indicator light turns on when loopback is active.
• Auto Track - enables automatic satellite tracking. When switched on, the ACU scans for the strongest satellite signal at the current pointing position and locks onto it, then continuously adjusts azimuth and elevation to maintain lock (compensating for satellite orbital drift, thermal expansion of the structure, wind loading, etc.). The blue indicator light shows auto-track status. If auto-track fails to acquire lock, the indicator shows a fault condition (red).

Status indicators

• Power LED (top right) - shows overall system status:
  • Green - powered on and operational
  • Amber - powered off or fault condition
  • Red - critical error (antenna not operational)
• Loopback indicator (blue light) - illuminates when loopback mode is active. Signals are being routed internally for testing instead of transmitted/received via satellite.
• Auto Track indicator (blue light) - illuminates when auto-tracking switch is enabled. If the light is on but auto-track failed to acquire lock, a fault indication appears (red border). This means the signal is too weak or no satellite is present at the pointing position.
• Bottom status bar - displays operational status and alarms:
  • LOCKED ON SATELLITE (green) - successfully tracking a satellite
  • Manual Tracking Enabled (blue) - operating in manual pointing mode
  • ACQUIRING LOCK… (yellow) - auto-track is attempting to lock
  • X SIGNAL(S) DEGRADED (yellow) - received signals have interference or poor C/I
  • HIGH SKEW (X°) (yellow) - skew exceeds 45°, check alignment
  • ANTENNA NOT OPERATIONAL (red) - hardware fault, antenna disabled
  • AUTO TRACK FAILED (red) - could not acquire satellite lock

Polar plot display

The circular plot on the right side shows the antenna’s current pointing position in a polar coordinate system. Azimuth is shown as angle around the circle (0° at top = North, rotating clockwise), and elevation is shown as radius (center = 90° zenith, edge = 0° horizon). A marker indicates where the antenna is currently pointed. This provides a quick visual reference for antenna orientation.

What is antenna gain and why does it matter?

Antenna gain (measured in dBi - decibels relative to an isotropic radiator) quantifies how well the antenna focuses energy in a specific direction compared to radiating it equally in all directions. A larger dish with better surface accuracy provides higher gain:

• 3-meter dish at 4 GHz: ~40 dBi (10,000× power concentration)
• 9-meter dish at 4 GHz: ~50 dBi (100,000× power concentration)
• 13-meter dish at 4 GHz: ~54 dBi (250,000× power concentration)

Gain is frequency-dependent - it increases with frequency (shorter wavelengths allow tighter focusing). The formula is: G = η(πD/λ)² where D is diameter, λ is wavelength, and η is aperture efficiency (typically 0.55–0.70). Every time you double the diameter, gain increases by 6 dB (4×). Every time you double the frequency, gain increases by 6 dB (4×).

On receive, higher gain means you can detect weaker signals - crucial for satellite links where signals arrive at -90 to -110 dBm. On transmit, higher gain means your signal arrives stronger at the satellite, allowing lower transmit power or higher data rates. Gain is the single most important parameter in link budget calculations.

Understanding beamwidth and pointing accuracy

Beamwidth (measured in degrees) describes how narrow the antenna’s beam is - the angular range over which the antenna effectively radiates or receives. The half-power beamwidth (HPBW) is the angle between points where gain drops by 3 dB (half power). For a parabolic dish:

HPBW ≈ 70λ/D degrees

• 9-meter dish at 4 GHz (λ = 7.5 cm): HPBW ≈ 0.58°
• 9-meter dish at 12 GHz (λ = 2.5 cm): HPBW ≈ 0.19°

Narrower beamwidth means tighter focus and higher gain, but also more stringent pointing requirements. A 9-meter C-band dish with 0.5° beamwidth must be pointed to within ~0.1–0.2° for optimal performance. Pointing error of even 0.5° (one beamwidth) causes 3 dB gain loss - half your signal strength.

This is why large antennas need motorized pointing and tracking systems. Geostationary satellites appear stationary but actually drift slightly due to orbital perturbations, and ground structures expand/contract with temperature, requiring continuous tracking corrections. Auto-track systems use signal strength or phase measurements to maintain peak pointing accuracy.

What is G/T and why is it important?

G/T (pronounced “gee over tee”) is the receive system figure of merit, measured in dB/K (decibels per Kelvin). It combines antenna gain (G) and system noise temperature (T):

G/T = G_antenna (dBi) - 10×log₁₀(T_system in Kelvin)

Higher G/T means better receive performance. System noise temperature includes contributions from:

• Sky temperature (~8–12 K at zenith in C-band, higher at low elevation)
• Atmospheric absorption noise (~5–15 K depending on elevation and frequency)
• Feed horn and waveguide losses (contributes 10–30 K)
• LNA noise figure (contributes 15–80 K depending on LNA quality)

Typical values: 9-meter C-band antenna with good LNA achieves G/T ≈ 30–35 dB/K. Every 3 dB improvement in G/T allows you to receive 3 dB weaker signals or use 3 dB less transmit power from the satellite - directly impacting link budget and cost.

Polarization and skew alignment

Satellite signals use polarization to allow frequency reuse - two signals at the same frequency can coexist if they use orthogonal polarizations (Horizontal/Vertical or Left/Right circular). Your antenna must align its polarization with the satellite signal to receive it.

For linear polarization (H/V), the satellite’s polarization plane appears rotated when viewed from different ground locations due to the geometry of the satellite arc. A satellite at your meridian (directly south in northern hemisphere) requires 0° skew, but satellites far east or west require significant skew adjustment (±30–60°).

Polarization mismatch causes loss:

• 0° skew error: 0 dB loss (perfect alignment)
• 10° skew error: 0.15 dB loss (negligible)
• 30° skew error: 1.25 dB loss (noticeable)
• 45° skew error: 3 dB loss (half power - significant)
• 90° skew error: ∞ dB loss (cross-polarized - no reception)

The skew knob compensates for this geometric rotation. When tracking a satellite, optimize skew by adjusting for maximum signal strength or minimum cross-polarization interference.

Auto-tracking vs manual pointing

Manual pointing mode: You directly control azimuth and elevation with the knobs. This is used for initial satellite acquisition, testing, or when tracking is not needed (very short sessions, star tracking for calibration, etc.). Manual mode requires you to maintain pointing as satellites drift and the structure moves.

Auto-track mode: The ACU takes over pointing control. When you enable auto-track:

1. The system scans the current pointing position for satellite signals
2. It identifies the strongest signal (assuming it’s the desired satellite)
3. It performs a lock sequence: pointing is adjusted to maximize signal strength
4. Status changes to “LOCKED ON SATELLITE” (green status bar)
5. The ACU continuously monitors signal level and makes small corrections to maintain peak

Auto-track requires a strong signal (typically >-100 dBm) to acquire lock. If the signal is too weak or absent, auto-track will fail and show “AUTO TRACK FAILED” alarm.

Breaking lock: If you manually adjust azimuth or elevation while auto-tracking, the system breaks lock and disables auto-track. This prevents fighting between manual control and automatic tracking. You must re-enable auto-track to resume tracking.

Loopback test mode

Loopback mode creates a local RF path from transmit to receive, bypassing the antenna’s radiating elements. When enabled:

• Your transmitted signal (from BUC/HPA) is routed to the LNB input
• No RF is actually transmitted to space (prevents interference and conserves power)
• You can test modems, signal processing, encoding, modulation, etc. end-to-end
• Link budget is drastically different - signal doesn’t travel to satellite and back
• Useful for validating configurations before live transmission

Loopback is a development and troubleshooting tool. Never operate in loopback during live satellite operations - you won’t be transmitting or receiving real satellite signals.

RF performance metrics (when displayed)

The antenna can display detailed RF performance metrics computed from the current configuration and operating frequency:

• Gain (dBi) - antenna gain accounting for aperture efficiency, surface accuracy (Ruze formula), and blockage from the feed/subreflector
• HPBW (degrees) - half-power beamwidth, indicating pointing precision requirements
• G/T (dB/K) - receive figure of merit combining gain and system noise temperature
• Pol Loss (dB) - polarization mismatch loss due to current skew setting
• Atmos Loss (dB) - atmospheric absorption loss at current elevation and frequency
• Sky Temp (K) - sky noise temperature contribution (elevation-dependent)

These metrics use realistic RF physics models and update based on antenna configuration, frequency, elevation, and skew. They’re valuable for link budget analysis and performance optimization.

Alarms and warning conditions

The ACU monitors for several alarm conditions:

• ANTENNA NOT OPERATIONAL (critical) - Hardware fault or power sequencing error. The antenna is disabled. Check power, servo systems, and control cables.
• ACQUIRING LOCK… (warning) - Auto-track is enabled and attempting to lock onto a satellite. This is normal during initial acquisition and typically resolves within 3–10 seconds. If it persists for >30 seconds, the signal may be too weak or no satellite is present.
• AUTO TRACK FAILED (warning) - Auto-track switch is enabled but lock could not be acquired. Signal is too weak (< -100 dBm) or antenna is not pointed at a satellite. Manually adjust pointing or verify satellite is transmitting.
• DISCONNECTED (warning) - No signals are being received and the system is not in loopback or auto-track mode. Check antenna pointing, verify satellite is transmitting, and ensure LNB is powered and connected.
• X SIGNAL(S) DEGRADED (warning) - Received signals have poor carrier-to- interference ratio (C/I < 10–15 dB) or significant overlap with stronger signals. This causes increased bit errors and reduced throughput. Investigate frequency coordination, pointing accuracy, and adjacent satellite interference.
• HIGH SKEW (X°) (warning) - Skew exceeds ±45°, which is unusual for most satellite locations. Verify you’re tracking the correct satellite and that skew is properly set. Extreme skew (>45°) causes >3 dB polarization loss.
• NO SIGNALS RECEIVED (warning) - Antenna is locked but no signals detected. Satellite may not be transmitting, frequency may be wrong, or there’s an equipment failure in the receive chain (LNB, OMT, cabling).
• LOOPBACK ENABLED (info) - Loopback mode is active. Signals are routed locally for testing, not transmitted/received via satellite. This is normal for testing but should not be active during live operations.

Simple troubleshooting steps

1. No signals received - Verify power is on (green LED). Check azimuth and elevation are pointed at a known satellite (use satellite position calculator or known good coordinates). Enable auto-track to automatically acquire the satellite. If auto-track fails, the satellite may not be transmitting or antenna may have a hardware fault.
2. Auto-track won’t lock - Signal is too weak. Manually point the antenna closer to the satellite’s expected position using azimuth and elevation knobs. Verify the satellite is above the horizon (elevation > 0°). Check that RF front-end is powered and LNB is operational. Ensure no obstructions block the antenna’s view (buildings, trees).
3. Signal is weak (low power) - Check skew alignment - incorrect skew causes polarization loss. Verify antenna is at peak pointing (use auto-track or maximize signal manually). Check for atmospheric attenuation (rain, snow, fog degrade signals especially at Ku/Ka-band). Inspect antenna surface for debris, ice, or damage affecting gain.
4. Lost lock during operation - Check for obstructions that moved into the beam (crane, vehicle, etc.). Verify antenna structure hasn’t shifted due to wind, thermal expansion, or foundation settling. Check power to servo motors - loss of power causes tracking to fail. Re-enable auto-track to re-acquire.
5. Cross-polarization interference - Adjust skew to maximize wanted signal and minimize cross-polarized signal. Use a spectrum analyzer or signal quality meter to monitor cross-pol rejection (should be >25 dB for good performance). If skew adjustment doesn’t help, feed alignment may be incorrect - this requires professional service.
6. Loopback doesn’t work - Verify loopback switch is enabled (blue indicator on). Check that BUC/HPA is transmitting (output power > 0 dBm). Ensure LNB is powered and locked to reference. Verify OMT is properly routing loopback signals. Check modem frequencies match (BUC upconverted frequency should be in LNB downconversion range).

Short examples

• Example A - Normal tracking operation: Power ON, Az = 195.3°, El = 42.7°, Skew = -12°, Auto-Track ON (blue light), Status = “LOCKED ON SATELLITE” (green). Antenna is locked onto a satellite at 195° azimuth (south-southwest), 42.7° elevation, with -12° skew compensation for polarization alignment. Auto-track is maintaining lock. Signal quality is good.
• Example B - Manual pointing: Power ON, Az = 180.0°, El = 45.0°, Skew = 0°, Auto-Track OFF, Status = “Manual Tracking Enabled” (blue), receiving 3 signals. Operating in manual mode. Antenna pointed due south at 45° elevation. No auto-track, operator must maintain pointing manually. Three signals are being received from satellites in this direction.
• Example C - Acquiring lock: Power ON, Az = 203.5°, El = 38.2°, Auto-Track ON, Status = “ACQUIRING LOCK…” (yellow). Auto-track enabled and searching for satellite. Antenna is scanning and adjusting pointing to maximize signal. Expect lock within a few seconds. If this persists >30 seconds, signal may be too weak or no satellite present.
• Example D - Auto-track failed: Power ON, Az = 270.0°, El = 10.0°, Auto-Track switch ON, Auto-Track indicator ON with fault (red border), Status = “AUTO TRACK FAILED” (red). Auto-track switch enabled but lock acquisition failed. Pointing may be wrong (270° west at 10° elevation may not have a satellite), or signal is too weak. Adjust pointing manually or verify satellite is transmitting.
• Example E - High skew warning: Power ON, Az = 240.0°, El = 35.0°, Skew = 58°, Auto-Track ON, Status = “LOCKED ON SATELLITE” (green) + “HIGH SKEW (58°)” (yellow). Antenna is locked but skew is extreme (58°). This causes ~4 dB polarization loss. Verify you’re tracking the correct satellite and that skew is properly calibrated. Such high skew is unusual unless tracking a satellite at extreme orbital position.
• Example F - Loopback test: Power ON, Az = 0.0°, El = 0.0°, Loopback ON (blue), Status = “LOOPBACK ENABLED” (blue). Testing configuration. Loopback routes transmitted signals directly to receiver without going through space. Azimuth/elevation don’t matter in loopback mode. Use this to validate modems and signal processing before live operation.
• Example G - Degraded signals: Power ON, Az = 198.2°, El = 41.5°, Auto-Track ON, Status = “LOCKED ON SATELLITE” (green) + “2 SIGNAL(S) DEGRADED” (yellow). Tracking normally but two received signals have interference or poor C/I ratio. Adjacent satellite may be transmitting on overlapping frequencies. Check frequency coordination, or adjust pointing to minimize adjacent satellite pickup (may need to sacrifice some signal strength to reduce interference).

Advanced: How parabolic reflectors work

A parabolic reflector has a special mathematical property: all rays parallel to the axis (coming from a distant source like a satellite) reflect to a single point called the focus. Conversely, a source at the focus produces a parallel beam. This is the same principle used in flashlights, car headlights, and telescope mirrors.

For satellite dishes, the feed horn is placed at the focus. On receive, incoming plane waves from the satellite reflect off the parabolic surface and converge at the feed horn, where they’re collected and sent to the LNB. On transmit, the feed horn radiates outward from the focus, the reflector converts this into a parallel beam aimed at the satellite.

The larger the reflector, the more energy collected (higher gain). But there are practical limits: larger dishes are heavier, more expensive, more affected by wind, and require higher surface accuracy (the Ruze formula shows that surface errors of λ/16 RMS cause ~1 dB gain loss).

Advanced: Link budget and the antenna’s role

A satellite link budget is an accounting of all gains and losses from transmitter to receiver. The antenna contributes to both transmit and receive budgets:

Transmit (uplink):

• Transmit power (HPA output): +50 dBm (100W)
• Feed loss: -1 dB
• Antenna gain: +50 dBi
• EIRP (Effective Isotropic Radiated Power): +99 dBW

Receive (downlink):

• Satellite EIRP: +50 dBW
• Free-space path loss (38,000 km at 4 GHz): -196 dB
• Atmospheric loss: -0.5 dB
• Polarization loss: -0.5 dB
• Antenna gain: +50 dBi
• System noise temperature: 50 K (G/T = +33 dB/K)

The antenna’s gain appears on both sides - higher gain allows lower transmit power or better receive sensitivity. This is why large ground antennas are cost-effective: one expensive antenna can save on satellite power, which is extremely expensive (limited solar panel area, battery capacity, and thermal management in space).

Final notes

The antenna is often the most visible and expensive component of a satellite ground station, but it’s a passive device - it’s fundamentally just a precisely-shaped metal reflector. The sophistication lies in the mechanical design (pointing accuracy, wind resistance, thermal stability) and the RF design (feed illumination, polarization purity, sidelobe suppression).

Best practices for antenna operation:

• Always use auto-track for continuous operations to compensate for drift and structural movement
• Optimize skew for each satellite - don’t assume 0° is correct
• Monitor signal quality metrics (C/I, signal strength) to detect degradation early
• Keep the reflector surface clean - dirt, ice, water, or debris degrades gain
• Protect the feed horn and waveguide from moisture, insects, and corrosion
• Verify pointing periodically using beacon signals or known satellites
• In adverse weather (high wind), reduce power or cease operation to prevent damage
• Use loopback mode to test system configurations before live transmission

Remember: the antenna is the “front door” to space. Everything downstream (LNB, receiver, demodulator) depends on the antenna capturing the signal in the first place. Poor antenna performance cannot be recovered by better electronics - invest in quality antennas, maintain them properly, and operate them within specifications.