Active RX and TX Antenna Proximity: Safe Distances and RF Protection

When deploying active receive (RX) antennas and transmit (TX) antennas in close proximity, managing RF exposure and preventing overload in sensitive receivers is crucial. At RF.Guru, all our active RX antenna designs incorporate an RF limiter using anti-parallel diodes at the input to protect against excessive signal levels. In this article, we discuss practical safe distances for active active RX antennas near high-power TX antennas and the role of RF limiters in ensuring reliable operation.

The Challenge of Close-Proximity RX and TX Antennas

Placing an active RX antenna too close to a TX antenna can result in:

  • Receiver front-end overload
  • Nonlinear distortion or intermodulation
  • Potential damage to sensitive components

The main concern is the electric field strength (E-field) at the active RX antenna location. When the E-field exceeds the breakdown voltage of components in the RX circuit, it can cause degradation or even failure.

Safe Distances for TX Power Levels

A practical guideline for safe distances is based on the clamping effect of the RF limiter diodes. Regardless of the transmit power, the diode limiter ensures that only a fixed maximum power reaches the active RX antenna. The following guidelines are based on practical experience with close-circuit coupling effects, where the TX signal directly impacts the active RX antenna components. However, since all our designs are RF shielded, the limiter diodes ensure that the received signal remains within safe levels.

  • 100W PEP TX Power → 6 meters (practical distance)
  • 2kW PEP TX Power → 8 meters (practical distance)

The Role of RF Limiters

To mitigate the risk of receiver overload, all our active active RX antennas integrate an RF limiter with anti-parallel diodes at the input. These diodes clamp excessive RF voltages before they can reach sensitive front-end amplifiers.

We use 1N4148WF diodes in our RX designs, which effectively limit the power reaching the amplifier:

  • Echotracer | SkyTracer | VerticalVortex (2 diodes, 1 per polarity): Clamps at ~6.9 dBm (~5 mW)
  • Octaloop | TerraBooster | GroundWeaver (4 diodes, 2 per polarity): Clamps at ~12.9 dBm (~19.6 mW)

These diodes provide consistent protection, meaning that even with very high TX power levels, the maximum power reaching the RX amplifier remains within safe limits.

Practical Considerations

Free-space path loss is only relevant for radiated energy and does not determine safe distances for active RX antennas. The only real issue with active antennas is direct coupling, in real-world conditions, the dominant effect at close range is direct coupling. Testing has shown that 6 meters for 100W and 8 meters for 2kW provide sufficient protection against direct coupling when using our RF limiter designs.

Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.