Active Impedance
Understanding Active Impedance
If you measure the impedance of a single Wi-Fi antenna sitting on a desk, it will read perfectly at 50 Ohms. However, if you bolt 100 of those antennas together onto a metal board and turn them all on at the same time, the math completely breaks down. You must now calculate their Active Impedance.
| Version | Data Rate | Range | Key Feature |
|---|---|---|---|
| BT 4.0 (BLE) | 1 Mbps | 50 m | Low Energy intro |
| BT 5.0 | 2 Mbps | 200 m | 4x range, 2x speed |
| BT 5.2 | 2 Mbps | 200 m | LE Audio, LC3 codec |
| BT 5.4 | 2 Mbps | 200 m | PAwR, ESL support |
The Mutual Coupling Chaos
Antennas packed tightly together violently interfere with each other (Mutual Coupling).
- Antenna 1 is trying to push a radio wave forward into the sky.
- At the exact same microsecond, Antenna 2 (right next door) blasts a massive wave. Part of that wave bleeds sideways and physically hits Antenna 1.
- This invading wave violently collides with the electricity inside Antenna 1, mathematically altering the ratio of Voltage to Current.
- Because Impedance is exactly Voltage divided by Current (Z = V / I), the impedance of Antenna 1 instantly shifts from a perfect 50 Ohms to a chaotic, unpredictable number (e.g., 73 + j12 Ohms). This new, chaotic number is the Active Impedance.
The Nightmare of Beam Steering
Active Impedance is not a fixed number. It changes dynamically based on where the radar is pointing.
If a military phased-array radar points straight ahead (Boresight), the mutual coupling is relatively mild, and the Active Impedance stays close to 50 Ohms. If the radar attempts to steer its beam hard to the right (e.g., a 60-degree scan angle), the phases of the antennas shift drastically, causing the sideways mutual coupling to become violently destructive. The Active Impedance can instantly spike to 500 Ohms, creating a massive brick wall that causes 100% of the radio energy to violently reflect backward into the radar instead of shooting into the sky (Scan Blindness).
Frequently Asked Questions
Can you fix Active Impedance with a matching network?
Not easily. A standard matching network (like an LC circuit) is static; it can only fix a fixed impedance error. Because Active Impedance changes dynamically every time the radar beam sweeps to a new angle, a static matching network is useless. Engineers must either carefully redesign the physical spacing of the antennas, or use highly complex, dynamically tunable matching circuits that shift gears in real-time.
How is Active Impedance simulated?
Using massive S-Parameter matrices. In 3D simulation software (like CST Microwave Studio), the engineer calculates the exact S-parameters between every single antenna and every single neighbor. By applying the planned beam steering phase shifts to this massive matrix, the software can mathematically predict exactly what the Active Impedance of every element will be at any given scan angle.
Does Active Impedance affect receiving antennas?
Yes, but the physics are slightly different. When an array is receiving a weak radar echo from a stealth jet, the mutual coupling causes the elements to 'steal' energy from each other. This warps the receiving footprint (the Active Element Pattern), requiring the central radar computer to use massive calibration matrices to un-warp the math and accurately determine where the jet is located.