Is Radiating Resistance as Important for RX Antennas as It Is for TX Antennas?
What is Radiating Resistance?
Radiating resistance (Rr) is the portion of an antenna’s impedance that accounts for power radiated as electromagnetic waves rather than being lost as heat. It is a key factor in determining the efficiency of a transmitting (TX) antenna.
Why is Radiating Resistance Important for TX Antennas?
In transmitting antennas, higher Rr improves efficiency because it means more power is radiated rather than wasted. Efficient TX antennas, such as half-wave dipoles or quarter-wave monopoles, have relatively high Rr, typically in the range of tens of ohms (e.g., 73Ω for a free-space dipole).
Is Radiating Resistance Equally Important for RX Antennas?
No, radiating resistance is not as critical for receiving antennas, especially for small active antennas that are much shorter than a tenth of a wavelength.
In RX applications:
• The received signal strength depends more on the effective height and impedance matching rather than Rr.
• External noise (atmospheric, man-made, or cosmic) dominates over the antenna’s inherent thermal noise.
• Active RX antennas use high-impedance, low-noise amplifiers to compensate for low radiation efficiency.
Why Do Small RX Antennas Work Well Despite Low Rr ?
Even though a small receiving antenna has very low Rr, it can still work effectively because:
• It doesn’t need to radiate power efficiently—only capture incoming electromagnetic waves.
• High-impedance amplifiers increase voltage gain, improving signal-to-noise ratio (SNR).
• The received signal power is usually limited by environmental noise rather than the antenna’s efficiency.
Can a Small RX Antenna Work as a TX Antenna?
Not efficiently. If used for transmission, a small antenna’s low means that most of the input power is lost in resistive and reactive components rather than being radiated. This makes it impractical for transmitting significant power.
Conclusion
Radiating resistance is a crucial factor for TX antennas but much less important for RX antennas. Small active RX antennas rely on high-impedance amplification rather than efficient radiation to function effectively. For reception, the focus is on signal capture and amplification rather than minimizing power loss.
Article written by Joeri Van Dooren, ON6URE – RF engineer, antenna designer, and founder of RF.Guru. With extensive experience in active and passive antenna systems, high-power RF transformers, and custom RF solutions, Joeri shares insights into cutting-edge radio communication technologies.