Noise & Y-Factor Measurements
Quick Start Guide — EMETest
EMETest provides two complementary noise measurement tools: a real-time Noise Monitor for watching noise power as the antenna moves, and a Noise Test function that takes longer averaged measurements from reference sources and computes Y-factors. Both use the 500–2500 Hz audio band, so no additional hardware is needed beyond the sound card already used for echo measurements.
Background: What is a Y-Factor?
A Y-factor is simply the ratio in dB between noise power measured from two different sources. By comparing noise from a known cold (quiet) reference against hotter sources, you can characterise how much noise each source contributes and, indirectly, what the noise figure of the receive system is.
EMETest computes three Y-factors:
| Y-Factor | Formula | What it tells you |
|---|---|---|
| Moon | Pmoon / Pcoldsky | How much the Moon raises the noise floor above cold sky. Useful for correcting echo SNR to a standard Moon distance (perigee) and for estimating antenna beamwidth effects. |
| Sun | Psun / Pcoldsky | A sensitivity diagnostic. The value depends on antenna gain and solar flux; it can be compared with predictions or other stations' results. |
| Ground noise | Pcoldsky / P50 Ω | A direct indicator of receive system noise figure. The higher this value, the better the system noise performance. |
Typical values at 1296 MHz with a 3-metre dish:
- Moon Y-factor: +0.1 to +1 dB
- Sun Y-factor: +10 to +16 dB (depends on solar activity)
- Ground noise: +6 to +8 dB
Using the Noise Monitor
In the Settings panel, set the Noise avg time using the drop-down (0.5 s to 8 s). Shorter periods update faster; longer periods reduce scatter.
Click Noise Mon. The button turns red and readings begin appearing in the Noise level display and on the plot below the SNR history. The plot shows the last 50 readings so short-term trends are visible.
Move the antenna between different sky positions, or switch the LNA input between the antenna and a 50-ohm load, and watch the noise level change. The values are in dBFS (dB relative to full-scale sine wave at 0 dBFS). Typical readings are in the −30 to −59 dBFS range.
Click Stop when done.
Using the Noise Test (Y-Factor Measurements)
The Noise Test window takes longer, more accurate measurements from up to four reference sources. Complete the measurements in the order shown; cold sky must be done first as it is the baseline.
Open the Noise Test window
Click the Noise Test button in the main window. The Noise Test window opens showing four measurement slots and a Y-factor results panel.
Cold sky measurement
Point the antenna at a quiet region of sky, well above 45 degrees elevation and away from the Galactic plane. The integration time is set by the Int. time field (default 60 s; longer gives lower uncertainty). Click Measure Cold Sky. When complete, the measured dBFS value appears in the Cold Sky slot.
Up to four cold-sky positions can be measured (slots 1–4). The coldest value is automatically selected as the reference. You can use this to bracket the Galactic plane or to verify that a position is genuinely quiet.
On-Moon measurement
Point the antenna at the Moon and click Measure Moon. The difference from cold sky is the Moon Y-factor. At most frequencies and with most dish sizes the Moon raises the noise only slightly (<1 dB at 1296 MHz), so good repeatability requires adequate integration time.
Sun measurement (optional)
Point the antenna at the Sun and click Measure Sun. Note the current 10.7 cm solar flux value — EMETest can retrieve this automatically from the DRAO Penticton data feed — since solar emission varies and the measured Y-factor cannot be properly interpreted without knowing the flux at the time of measurement.
50-ohm load measurement
Connect a 50-ohm termination to the LNA input (or switch an antenna relay to a load) and click Measure 50 Ω. Note the ambient temperature of the load environment, since the load's thermal noise depends on temperature.
Read the Y-factors and save
Y-factors are computed and displayed automatically as measurements are
added. When all desired measurements are complete, optionally add a note in
the Cold sky notes text box (e.g., the azimuth and elevation
of the cold sky position), then click Save Y-factors.
A timestamped text file is saved in the noise/ subdirectory
of the data folder.
Interpreting Results
Power values are shown in dBFS. A consistent reading in the green zone (−30 to −59 dBFS) is normal. A sudden rise usually means interference; a gradual drift in the 50-ohm load reading suggests gain or temperature changes in the system. Comparing today's ground noise Y-factor against a historical baseline quickly reveals whether the receive system has degraded.