Advanced RF Coordination & Fiber Backbone (RFoF)
I. Theoretical Context
1. Inter-Zone Interference and Frequency Re-farming In a massive festival environment (e.g., Coachella or Glastonbury), we aren't just coordinating one stage; we are managing an entire "RF Ecosystem." The biggest challenge is Inter-Zone Interference. A transmitter on Stage A might not be loud enough to "hit" Stage B's receiver, but it can still raise the Noise Floor.
We use Frequency Re-farming. Mics that require the most range (Lead Vocals) are placed in the "Cleanest" parts of the UHF spectrum. Tech Comms and less critical channels are moved to the "Edges" of the TV channels or into the 900 MHz and 1.2 GHz bands. We utilize Zone Coordination in WWB7. This allows the software to calculate the "Distance Loss" between stages. If Stage A and Stage B are 200 meters apart, we can "reuse" frequencies, provided the intermodulation products (IM3) are calculated across both zones.
2. RF over Fiber (RFoF) and Distributed Antenna Systems (DAS) Coaxial cable (RG-8/LMR-400) has significant attenuation at 600 MHz (approx. 10-15 dB per 100m). For stadium shows where the "Antenna Farm" is 150 meters from the FOH rack, copper is unusable. We use RF over Fiber (RFoF). An RFoF link converts the electromagnetic waveform into light pulses via a laser diode. Fiber has almost zero loss (approx. 0.3 dB per kilometer).
The Transducer: The RF signal modulates the intensity of a laser. At the other end, a photodiode reconstructs the RF signal.
Gain Staging: This is critical. If the laser is "over-driven," it creates "Optical Intermodulation," which is far more destructive than standard RF intermod. We use "Optical Attenuators" to ensure the light levels are within the photodiode’s linear range.
3. Passive Intermodulation (PIM) and the "Environment as a Transmitter" In high-power environments (stadiums with hundreds of transmitters), we encounter Passive Intermodulation (PIM). This occurs when two high-power RF signals hit a "non-linear junction" in the environment—such as a rusty fence, a loose shackle, or even a specific type of metal roof. These junctions act as a "Diode," mixing the signals and radiating new "Ghost Frequencies" back into the air. PIM is the hardest interference to troubleshoot because it isn't coming from a device; it's coming from the building itself. We mitigate this by lowering transmitter power and using "High-Q" filters at the antenna.
II. Practical Lab: The Multi-Stage Scan
Tool: WWB7 / tinySA.
Tasks: * Coordinate 60 channels of Shure Axient and 24 channels of PSM1000 across three "Zones": Main Stage, Stage B, and Press Room.
RFoF Simulation: Calculate the "Link Budget" for a fiber run. If the laser has a +3 dB output and the fiber loses 1 dB, but the photodiode requires -5 dB for linear response, calculate the required attenuation.
Intermod Sweep: Use the tinySA to identify a "Ghost" frequency created by two handhelds held 10cm apart.
III. Daily Assessment (Wednesday)
Theory: Explain the physical difference between "Carrier-to-Noise Ratio" (CNR) and "Signal-to-Noise Ratio" (SNR).
RFoF: Why is "Optical Gain Staging" more critical than "Coaxial Gain Staging"?
Synthesis: Describe the impact of "Zone Bleed" on a True Diversity receiver’s ability to switch antennas.