Is Radiating Resistance as Important for RX Antennas as It Is for TX ?
What Is Radiating Resistance?
Radiating resistance (Rr) is the component of an antenna’s input resistance that represents power radiated as electromagnetic waves (rather than dissipated as heat). TX efficiency increases with higher Rr for a given loss resistance Rloss.
Why Rr Matters Greatly for TX
Efficient TX antennas (e.g., ½-wave dipole, ¼-wave monopole) exhibit Rr in the tens of ohms (≈73 Ω free-space dipole). A higher Rr relative to Rloss means more of the transmitter power leaves as radiation rather than heat.
Is Rr Equally Important for RX?
Not generally. On receive, especially with electrically short active antennas, the limiting factor is rarely the thermal noise set by Rr. HF bands are dominated by atmospheric and man-made noise. What matters is coupling (effective height/aperture), low loss resistance, front-end impedance, and common-mode control.
Key point for RX: Keep Rloss low and avoid loading the element. Use a high-Z, low-noise, high-linearity front end so the captured voltage isn’t squandered. Good CMRR and grounding/choking often improve SNR more than chasing Rr.
Why Small RX Antennas Work Despite Low Rr
- They don’t need to radiate power — only to sample the field.
- High-impedance active buffers convert small element currents to useful voltages without heavy loading.
- System noise is dominated by the external environment; the LNA’s NF and linearity, and the installation’s CMR hygiene, set the outcome.
Can a Small RX Antenna Serve as a TX Antenna?
Not efficiently. Very small Rr plus reactive impedance means most input power is lost in Rloss or matching networks. Practical TX requires much higher Rr (or resonant/loaded geometries) and careful loss control.
Design Focus for RX
- Effective height/aperture: geometry and height set coupling to the field.
- Minimize Rloss: good conductors, joints, and corrosion control (e.g., AL-1100/CU-800 where applicable).
- Front-end: high-Z, low-noise, high-IP3 buffer; sensible filtering.
- CMRR & grounding: proper RF reference + shack-end common-mode choke.
Conclusion
Rr is a cornerstone of transmit efficiency but is not the main lever for receive performance on HF. Design receive antennas around field coupling, low loss, linear, quiet front-ends, and excellent common-mode control.
Mini-FAQ
- Does higher Rr always improve RX? — No. On HF, external noise dominates; prioritize loss reduction, coupling, and CMRR.
- When can Rr matter on RX? — If Rloss is comparable to or exceeds Rr, you waste captured signal as heat. Keep joints/connectors low-loss.
- Why use high-Z active buffers? — To avoid loading low-Rr elements, preserving the induced voltage for a good SNR and dynamic range.
- Do tiny RX antennas need perfect SWR? — No. SWR is a TX metric; for RX, stability, linearity, and noise control matter far more.
- Can improving CMRR beat improving NF? — Often yes; choking the feedline and setting a solid RF reference typically yields larger SNR gains than sub-dB NF tweaks.
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