Why Moving the EUT Changes the Emissions Plot

1. Introduction – why a small change can make great differences in the plot

Did this happen to you. You go to the testing lab. you do a full RE scan and the scan looks great. Then you

• Rotate the EUT 90°

• Lift a cable

• Move the EUT by 20 cm

• Change cable routing

And the emissions plot changes.

This is not randomness. It is physics.

Radiated EMC measurements are strongly influenced by how the product couples to its environment. I want to discuss this today, so, next time, when you are in a chambe and youmove a cable, or move the EUT, change a cable, etc. you can quickly identify the issue.

2. The Cable Is the Real Antenna

Most electronics products:

• The PCB is small

• The cables are long

Long conductors carrying common-mode current behave as antennas.

Why Common-Mode Current Radiates

Common-mode current flows:

• In the same direction on all conductors

• Returning through parasitic capacitance to ground

• Using the chamber ground plane as a return path

Even a few milliamps of CM current at 30–200 MHz can radiate significantly.

In a RE chamber the common mode path is artificially controlled by the test setup.

What Happens When You Move the Cable

Changing cable position changes:

• Its effective antenna length

• Its height above the ground plane

• Its orientation relative to the measurement antenna

• Its coupling to the EUT enclosure

The radiated field changes accordingly.

This is why lifting a cable off the table can increase emissions.

3. Height Above Ground Plane: Image Theory in Practice

In a semi-anechoic chamber:

• The floor is conductive

• The EUT sits on a table (typically 80 cm high)

The conductive floor creates an “image” of the cable (image theory do you remember we study this at the uni).

When you change:

• EUT height (by adding a cardboard box under the EUT)

• Cable droop

You change:

• The effective antenna structure

• The radiation pattern

• The field strength at the measurement antenna

Small height changes can produce several dB variation.

So... by moving the EUT of ... say 10 cm "up" you have a emission reduction. then CM emission is happening.

4. Rotating the EUT: Polarization and Radiation Pattern

Rotating the EUT changes:

• The orientation of the dominant radiating structure

• The polarization of the field

• The coupling between cable and antenna

  
  

Radiation is directional.

If the dominant current path is aligned with the antenna polarization, the measured level increases.

If it is orthogonal, it decreases.

This explains why:

• 0°, 90°, and 180° positions can show different peaks

• Some frequencies are more sensitive than others

  
  

5. How to Interpret These Changes During Debugging

When emissions change after moving the EUT:

If rotating the product changes amplitude significantly:

The product has directional radiation (likely cable-driven).

If lifting the cable increases emissions:

Strong common-mode coupling to the ground plane.

If twisting the cable reduces emissions:

Radiation dominated by loop area.

If touching the enclosure changes emissions:

Poor enclosure bonding or floating metal parts.

Movement is a diagnostic tool — not a nuisance.

6. Practical Debug Strategy

During pre-scan:

• Intentionally move cables

• Rotate the EUT

• Lift and drop cables

• Lift the EUT

• Change cable routing

If emissions are unstable:

• You likely have common-mode current problems

• Filtering or return path control needs improvement

A robust design shows limited sensitivity to cable position.

7. Conclusion

If your emissions plot changes when you move the EUT, it does not mean the test is unreliable.

It means:

• The product is interacting with its environment.

• Cables are radiating.

• Common-mode current exists.

Understanding this allows you to use movement as a diagnostic method rather than treating it as noise in the measurement.