Parabolic Satellite Dish Antenna 3dB Beamwidth Measurement Method

Background Information

When tracking satellites using a beacon receiver, the -3dB Beamwidth in degrees is a very important parameter for the system. Almost all of the calculations performed when optimizing signal strength are based on this parameter. The more accurate this parameter, the better the system will perform. By default the parameter is populated using the theoretical calculation for the -3dB Beamwidth which is 70 D where λ is the wavelength of the signal in meters and D is the diameter of the parabolic dish in meters.

• λ = velocity ÷ frequency
• λ = speed of light m/s ÷ (Frequency in MHz) x 1000000
• λ = 299792458 ÷ (Frequency in MHz) x 1000000

The 70 in the equation is a constant representing a generally accepted value. Some people incorrectly call this efficiency of an antenna but it is not related to efficiency. What one finds is that the less perfect the antenna the lower this number will be in value.

Potential issues could be associated with:

Panel misalignment
Clamshelling
Sub-reflector misalignment
Waveguide or feed issues

Setup Procedure

Schedule Downtime: Doing this measurement will cause the signal to drop significantly so it is best to not be carrying traffic during this test.
Target selection: Find a target that is well station kept, meaning the target has an Inclination of less than 0.1°. This is important because as you are running the test in both AZ and EL you don’t want the target to move and distort the data.
Attach the Spectrum Analyzer: Even though one could use the beacon receiver it would be better to use a spectrum analyzer.
Access the sheet we have created for you here.

Measurement Procudure

Initial Scan (Peak Power Detection):

Set the Spectrum Analyzer to the desired operating frequency of the target.
Rotate the AUT using the ACU over a degree range covering the expected main lobe and 1st set of side lobes to confirm the AUT is on the peak, in both AZ and EL.
Identify the angle in both AZ and EL at which the received power is at its maximum. This will take some time for the signal to settle, turning on averaging on the Spec-An will help with unstable signals. This is the boresight or peak of the main beam. Record this maximum power level Pmax and the corresponding Beam Center angles AZ_BC and EL_BC.
Calculate P3dB by subtracting 3 from PMax

Scan for -3dB Points:

- - Starting from the pointing angles AZ_BC and EL_BC, rotate the AUT in the EL up direction until the received signal is P3dB. This will take some time for the signal to settle, turning on averaging on the spec-an will help with unstable signals. Record this angle as EL_UP.
  - Return to the pointing angles AZ_BC and EL_BC, and confirm that signal strength returns to Pmax. Then rotate the AUT in the opposite direction, EL down, until the received signal is P3dB. This will take some time for the signal to settle, turning on averaging on the spec-an will help with unstable signals. Record this angle (EL_DW).
  - Return to the pointing angles AZ_BC and EL_BC, and confirm that signal strength returns to Pmax. Then rotate the AUT clockwise in AZ until the received signal is P3dB. This will take some time for the signal to settle, turning on averaging on the spec-an will help with unstable signals. Record this angle (AZ_CW).
  - Return to the pointing angles AZ_BC and EL_BC, and confirm that signal strength returns to Pmax. Then rotate the AUT counter-clockwise in AZ until the received signal is P3dB. This will take some time for the signal to settle, turning on averaging on the spec-an will help with unstable signals. Record this angle (AZ_CCW).

Data Analysis and Reporting

If you have been entering the data into the sheet we provided, it will automatically calculate the 3db beamwidth which can be entered in the target menu under Beamwidth (deg).
If you are not entering the data in the sheet, you must manually calculate:
- 3dB Beamwidth
  - EL = EL_UP - EL_DW
  - AZ =
- Next you average the AZ and EL 3dB Beamwidths together for the targets -3dB Beamwidth

NOTE: If the AZ and EL -3dB Beamwidths are different by more than 25%, Radeus Labs’ recommendation is to review the path of the target in the sky from your perspective and give 75% weight to the primary axis of motion for the anilemma.

Use the table below as a reference for expected -3dB beamwidths based on dish size and frequency.

Frequently Asked Questions

What is 3dB beamwidth and why does it matter for antenna tracking?

The 3dB beamwidth (also called half-power beamwidth) is the angular width of your antenna's radiation pattern where signal power drops to exactly half of its peak value, corresponding to a 3 decibel reduction. This parameter is the foundation of how modern antenna control systems optimize tracking performance. Modern tracking algorithms use beamwidth to calculate step sizes during peak-seeking operations, determine when to stop stepping, predict where the true peak lies, and set orbital tracking limits. When your configured beamwidth doesn't match your antenna's actual physical beamwidth, the algorithm's predictions become inaccurate, leading to constant re-peaking, failure to converge on target, or slow drifting off peak.

Why can't I just use the theoretical beamwidth formula instead of measuring?

The theoretical formula (Beamwidth = 70λ ÷ D) assumes a perfectly formed parabolic reflector with ideal feed illumination, which no real antenna achieves, especially as it ages. The formula can't account for mechanical degradation like panel misalignment from thermal cycling, clamshelling, sub-reflector droop, or feed horn position changes. It also can't account for manufacturing variations (the "70" constant actually varies from 65 to 75 depending on design) or environmental factors like corrosion, wind loading, or UV exposure. Two supposedly identical antennas can have measurably different beamwidths, which is why measurement provides the accurate data your tracking system needs.

When should I measure or re-measure my antenna's 3dB beamwidth?

Measure proactively before mission start (new installations, new satellite targets, controller retrofits, or feed horn replacements), after significant events (high wind, any maintenance touching the reflector or feed, structural repairs, or re-painting), when you see tracking symptoms (constant need to manually re-peak, step track never converging, or signal levels varying unexpectedly), and for aging infrastructure (antennas over 20 years old should be re-measured every 3-5 years). Don't wait for tracking problems to appear. The measurement takes only a few hours of scheduled downtime but provides foundational data for everything your tracking system does.

What equipment do I need to measure 3dB beamwidth?

You need three components: (1) The antenna under test capable of controlled movement in azimuth and elevation, (2) A spectrum analyzer (preferred over beacon receiver) for better measurement accuracy, real-time signal visibility, noise averaging, precise -3dB point identification, and main lobe verification, and (3) An antenna control unit (ACU) capable of precisely moving the antenna to specific angles. The measurement also requires a well station-kept target satellite (inclination less than 0.1°) and scheduled downtime of 2-4 hours when the antenna is not carrying traffic.

How do I interpret my azimuth and elevation beamwidth results?

If azimuth and elevation beamwidths match closely (within 10-15%), your antenna is in good condition with symmetrical pattern—use the average value. If they differ by 15-25%, some asymmetry exists from slight panel misalignment, feed positioning offset, or normal manufacturing variations—use the average but monitor tracking closely. If they differ by more than 25%, significant mechanical issues are present (major clamshelling, severe panel misalignment, or structural deformation). In this case, use weighted averaging: give 75% weight to the primary axis of target motion (azimuth or elevation) and 25% to the other axis, then schedule mechanical inspection and repair.

What is the measurement procedure for finding the half-power points?

The procedure has three steps: (1) Initial scan—confirm you're at the peak by scanning several beamwidths in both directions, record maximum power (Pmax) and calculate P3dB = Pmax - 3.0 dB, (2) Elevation scan—from beam center, move slowly up until reaching exactly P3dB and record angle (EL_UP), return to center, then move down to P3dB and record angle (EL_DW), (3) Azimuth scan—from beam center, rotate clockwise to P3dB and record angle (AZ_CW), return to center, then rotate counter-clockwise to P3dB and record angle (AZ_CCW). Always return to beam center between measurements and verify you're back at Pmax before proceeding.

How does frequency affect antenna beamwidth?

The same physical antenna has different beamwidths at different frequencies—as frequency increases, beamwidth decreases dramatically. A 13-meter dish at 4 GHz (C-band) has a beamwidth around 0.40°, providing plenty of margin for error. That same dish at 20 GHz (Ka-band) has a beamwidth of 0.08°, less than one-tenth of a degree, where tiny errors become critical. This frequency dependency is why you must measure beamwidth for each operating frequency, and why Ka-band tracking requires much more precision than C-band tracking. You cannot use a C-band beamwidth measurement for Ka-band operations.

Parabolic Satellite Dish Antenna: 3dB Beamwidth Measurement Method

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