Is Radiating Resistance as Important for RX Antennas as It Is for TX ?
What Is Radiating Resistance?
Radiating (radiation) resistance (Rr) is the portion of an antenna’s input resistance that accounts for power converted into radiated electromagnetic waves. It isn’t a physical resistor — it’s a convenient way to represent radiated power in the terminal power balance. For a given loss resistance Rloss, radiation efficiency is approximately η ≈ Rr / (Rr + 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 (≈72–73 Ω for a free-space dipole; ≈36 Ω for an ideal ¼-wave monopole over a very good ground). A higher Rr relative to Rloss means more of the transmitter power is radiated rather than dissipated as heat in conductors, joints, matching networks, or the ground system.
Is Rr Equally Important for RX?
Not in the same way. On receive, Rr mainly matters through overall antenna efficiency and how strongly the element is loaded by the front end. On HF, the external noise temperature (atmospheric/galactic/man-made) is often far above 290 K, so moderate efficiency losses often reduce both the desired signal and the dominant external noise by similar amounts — leaving SNR nearly unchanged. Rr (and Rloss) becomes important when you are in a quiet location, on higher bands/VHF+, or when losses/loading are large enough that the receiver/LNA noise starts to dominate.
Key point for RX: Keep Rloss low and avoid unnecessary loading of the element so the induced signal isn’t thrown away as heat.
Then pick the right front-end interface: often a high‑Z voltage buffer for short “electric” elements, or a current‑sensing/low‑Z approach for small loops. Prioritize linearity, sensible filtering, and overload behavior.
Finally, strong common‑mode rejection (good CMRR, grounding, and feedline choking) often produces bigger real-world improvements than chasing a higher Rr.
Why Small RX Antennas Work Despite Low Rr
- They don’t need to radiate power — they only need to sample the field and deliver a usable signal to the receiver.
- With the right interface (high‑Z voltage sensing for short electric elements, or current sensing for loops), you avoid heavy loading that would otherwise waste the captured signal.
- In many HF installations, the outcome is dominated by external noise and local interference; front-end linearity, filtering, and common‑mode hygiene often matter more than maximizing Rr.
Can a Small RX Antenna Serve as a TX Antenna?
Not efficiently. Very small Rr plus a strongly reactive feed impedance means large currents/voltages are required for a given radiated power, and most input power is lost in Rloss and practical matching networks (and sometimes the ground system). Practical TX typically requires much higher effective Rr (often via resonant or properly-loaded geometries) and very tight control of losses.
Design Focus for RX
- Effective height/aperture: geometry and installation height largely set coupling to the field.
- Minimize Rloss: good conductors, joints, and corrosion control (e.g., AL-1100/CU-800 where applicable).
- Front-end: use the right impedance interface (voltage buffer vs current sensing), low-noise where it matters, high linearity (IP3), and sensible filtering/protection.
- CMRR & grounding: define a solid RF reference and control feedline common-mode currents (e.g., a shack-end common-mode choke; good cable routing).
Conclusion
Rr is a cornerstone of transmit efficiency because it directly competes with loss resistance. For receive, the more practical question is whether your system is external-noise-limited or receiver-noise-limited. When external noise dominates (common on HF), optimizing coupling, minimizing loss/loading, improving linearity, and controlling common‑mode pickup usually delivers more benefit than chasing a higher Rr. When external noise is low or losses are high, antenna efficiency (and thus Rr vs Rloss) can matter a lot.
Mini-FAQ
- Does higher Rr always improve RX? — No. Higher Rr generally improves efficiency, but on HF you’re often external-noise-limited, so SNR may change little until losses/loading make receiver noise significant.
- When can Rr matter on RX? — When Rloss is large, the antenna is heavily loaded, you’re in a quiet site, or you’re operating on higher bands/VHF+ where external noise is lower and receiver noise becomes a factor.
- Why use high-Z active buffers? — For short electric elements, a high‑Z buffer senses the induced voltage without loading. For loops, a current-sensing/low‑Z approach is often the better match.
- Do tiny RX antennas need perfect SWR? — Usually no. SWR is critical for TX and for calibrated 50 Ω power transfer. Many RX front-ends aren’t operated as a strict 50 Ω power match, and interference/noise control tends to dominate. (For passive 50 Ω systems or measurements, matching can still matter.)
- Can improving CMRR beat improving NF? — Often yes. Reducing common-mode pickup and local EMI can lower the dominant noise/interference. Once you’re already external-noise-limited, chasing sub-dB NF improvements rarely helps.
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