Antenna Pattern Measurement

A geostationary satellite is effectively a point source at infinity. Its angular size is negligible compared to the dish beam width, which means scanning the dish across the satellite and recording RSSI at each position maps the antenna’s radiation pattern directly. This tells you the 3 dB beam width (half-power point), first sidelobe level, and overall beam shape — numbers that matter for every other experiment in this section.

What you’ll measure

The dish’s far-field radiation pattern in one or two planes (AZ cut, EL cut, or both). The key parameters:

Parameter	Expected value (33” x 23” dish at 12 GHz)	Why it matters
AZ beam width (3 dB)	~2.0—2.5 degrees	Determines angular resolution for sky mapping
EL beam width (3 dB)	~3.0—4.0 degrees	Wider because the dish is shorter in the EL plane
First sidelobe	-15 to -20 dB below peak	Sets dynamic range for nearby source separation
Beam symmetry	Slightly elliptical	Reflects the physical dish geometry

The theoretical beam width for a uniformly illuminated circular aperture is approximately lambda / D radians. At 12 GHz (lambda = 0.025 m) with an 84 cm dish, that gives about 1.7 degrees. The practical beam is wider because the feed illumination tapers toward the dish edges, and the reflector is elliptical rather than circular.

Prerequisites

• Motors homed and calibrated (see Calibration & Homing)
• TV search disabled (see Disabling TV Search)
• LNA enabled (dvb then lnbdc odu)
• A strong geostationary satellite locked with RSSI > 1000 and Lock = 1
• Serial logging to a file with timestamps
• A spreadsheet or plotting tool (Python/matplotlib, gnuplot, LibreOffice Calc)

Why geostationary, not the Sun?

The Sun works as an alternate target (see Solar Radio Monitoring), but it has two disadvantages for pattern measurement: its 0.5 degree angular diameter partially fills the beam, which broadens the measured pattern; and its signal is weaker than a strong geostationary transponder, giving worse SNR on the sidelobes. A geostationary satellite is a true point source with a strong, stable signal — ideal for this measurement.

Finding and locking a target satellite

1. Calculate look angles for a geostationary satellite. DISH Network 110W, 119W, or 129W are strong targets in North America. Use dishpointer.com or Stellarium with your latitude/longitude.

2. Position the dish at the computed AZ/EL.

   TRK> mot
   MOT> a 0 218.7
   MOT> a 1 38.5

3. Enable the LNA and verify signal.

   MOT> q
   TRK> dvb
   DVB> lnbdc odu
   DVB> rssi 50
   Reads:50  RSSI[avg: 1305  cur: 1311]

   If RSSI is near the noise floor (~500), nudge AZ and EL in 0.5 degree increments until you find the peak. A locked signal should be well above 1000.

4. Fine-tune for peak RSSI. Adjust AZ in 0.2 degree steps, recording RSSI at each position. Park at the maximum. Repeat for EL. This is the beam center — record this position as your reference (AZ_0, EL_0).

   DVB> q
   TRK> mot
   MOT> a 0 218.5
   MOT> q
   TRK> dvb
   DVB> rssi 50
   Reads:50  RSSI[avg: 1320  cur: 1318]

   Record: AZ_0 = 218.5, EL_0 = 38.5, RSSI_peak = 1320.

Azimuth cut (E-plane pattern)

Scan in azimuth while holding elevation constant at EL_0. This traces the antenna pattern in the azimuth plane.

1. Decide your scan range and step size. A range of +/- 5 degrees from beam center covers the main lobe and first sidelobes. A step size of 0.1—0.2 degrees gives good resolution on the main lobe; 0.5 degrees is sufficient for the sidelobes.

   For a high-resolution scan: 100 positions across 10 degrees = 0.1 degree steps.

2. Move to the starting position.

   TRK> mot
   MOT> a 0 213.5

   This is AZ_0 - 5.0.

3. Step through azimuth, recording RSSI at each position. At each step:

   MOT> a 0 213.5
   MOT> q
   TRK> dvb
   DVB> rssi 50
   Reads:50  RSSI[avg: 502  cur: 498]
   DVB> q
   TRK> mot
   MOT> a 0 213.7
   MOT> q
   TRK> dvb
   DVB> rssi 50
   Reads:50  RSSI[avg: 504  cur: 501]

   This is tedious manually. For a complete scan, use azscan in the MOT submenu:

   TRK> mot
   MOT> azscan 10 0 500

   This sweeps 10 degrees in AZ at the current EL, with a 500 ms delay between positions. The output includes RSSI at each position. Log the serial output.

4. Record all (offset, RSSI) pairs. Offset = AZ - AZ_0 for each position.

Elevation cut (H-plane pattern)

Same procedure, but scan in elevation while holding azimuth constant at AZ_0.

1. Return to beam center.

   TRK> mot
   MOT> a 0 218.5

2. Step through elevation. The EL range is constrained by the firmware limits (18—65 degrees on the G2). Scan as far as you can on each side of EL_0.

   MOT> a 1 33.5
   MOT> q
   TRK> dvb
   DVB> rssi 50

   Repeat in 0.2 degree EL steps across +/- 5 degrees (or to the firmware limit). There is no built-in EL scan command, so this must be done manually or scripted over serial.

3. Record all (offset, RSSI) pairs. Offset = EL - EL_0 for each position.

Scripting the measurement

Manual stepping through 50—100 positions is slow and error-prone. Birdcage’s rotctld interface accepts position commands over TCP, and the RSSI extension (R 10 on port 4533) returns signal strength. A simple Python loop that sets position, waits for settle, reads RSSI, and logs the result automates the entire scan in minutes. See the Radio Telescope Guide for the rotctld RSSI protocol.

Processing the data

Normalize to dB

Convert raw RSSI to a relative power scale in dB, referenced to the peak:

pattern_dB(offset) = 10 * log10(RSSI(offset) / RSSI_peak)

The peak is 0 dB by definition. The 3 dB beam width is the angular distance between the two points where pattern_dB = -3.

Example AZ pattern data

AZ offset (deg)	RSSI	Pattern (dB)
-3.0	510	-4.13
-2.0	620	-3.28
-1.5	850	-1.91
-1.0	1150	-0.60
-0.5	1290	-0.10
0.0	1320	0.00
+0.5	1295	-0.08
+1.0	1140	-0.64
+1.5	860	-1.86
+2.0	615	-3.31
+3.0	505	-4.17

In this example, the -3 dB points are near +/- 2.0 degrees, giving a 3 dB beam width of approximately 4.0 degrees in azimuth. (These are illustrative values — your actual pattern will depend on the dish surface accuracy, feed position, and frequency.)

Plotting

Plot pattern_dB vs. offset angle for both AZ and EL cuts on the same graph. The AZ cut will be narrower (larger dish dimension in AZ) and the EL cut wider (smaller dish dimension in EL). Overlay the theoretical pattern for a uniformly illuminated elliptical aperture if you want a comparison reference.

Interpreting the pattern

Feature	What it tells you
3 dB beam width	Angular resolution for sky mapping and satellite discrimination
Main lobe symmetry	Feed alignment quality — asymmetric main lobe means the feed is off-axis
First sidelobe level	How well the dish rejects signals from adjacent satellites (typically 2 degrees apart in geostationary arc)
Sidelobe asymmetry	Feed offset direction or reflector surface deformations
Noise floor offset	Pattern measurement dynamic range — if sidelobes disappear into the noise floor at -10 dB, your effective dynamic range is 10 dB

If the beam is significantly wider than expected, the feed may not be at the reflector’s focal point. Adjust the feed position along the focal axis and re-measure. The optimal position minimizes beam width and maximizes peak RSSI simultaneously.

Going further

Full 2D pattern. Combine AZ and EL scans at multiple cross-cuts (e.g., AZ cuts at EL_0, EL_0 + 1, EL_0 + 2) to build a 2D contour map of the beam. This reveals any coma or astigmatism from feed misalignment.

Dual-polarization pattern. Measure the pattern at both H-pol and V-pol using peak rssits at each position. The cross-pol pattern (sidelobes in the orthogonal polarization) indicates the dish’s polarization purity, which matters for experiments comparing polarization channels.

Frequency dependence. If you can select different transponder frequencies with dvb t <n>, measure the pattern at two or three frequencies. The beam should narrow at higher frequencies (shorter wavelength). The ratio of beam widths should be approximately proportional to the ratio of wavelengths.

Calibrating sky maps. The beam pattern is the point spread function (PSF) of your radio telescope. Deconvolving the PSF from a sky map (from the Radio Telescope Guide workflow) improves angular resolution and separates closely spaced sources. This is the same technique used by professional radio observatories.

Solar Radio Monitoring The Sun as an alternate beam pattern target -- useful when no strong satellite is available at your elevation.

Radio Telescope Guide The azscanwxp workflow that produces sky maps. Beam pattern data directly improves the quality of those maps.

Console Command Reference Full details on mot a, azscan, dvb rssi, and peak rssits commands.