RX vs TX Antennas: A Fundamental Difference
In the world of antenna technology, we distinguish between three main categories of antennas: passive RX antennas, active RX antennas, and transmission (TX) antennas. While they all receive or transmit electromagnetic waves, their design principles differ fundamentally due to aspects such as skin effect, resonance, impedance, and their behavior with respect to the electric field (E) and magnetic field (H).
1. Skin Effect and Skin Depth
The skin effect describes how alternating currents at higher frequencies tend to flow near the outer surface of a conductor. This reduces the effective cross-section where current flows, increasing resistance.
- TX antennas: These carry significant current, so the skin effect has a major impact on losses. Low skin depth requires thick conductors or litz wire to minimize losses, especially at HF and higher.
- Passive RX antennas: These don’t handle power, but the signal-to-noise ratio is important. Skin effect may have slight influence on weak signal conduction, but it is generally not critical if impedance distortions are controlled.
- Active RX antennas (such as E-field probes or active magnetic loops): The skin effect is negligible here. With extremely low currents and immediate buffering (e.g., with a FET input), the current path losses are virtually irrelevant.
2. Resonance Behavior
- TX antennas: Typically designed to operate at or near resonance for maximum efficiency and minimal SWR. The antenna acts as a tuned resonator.
- Passive RX antennas: May be resonant or broadband. Resonant types require tuning but have looser matching tolerance. Broadband designs prioritize wide frequency coverage over peak sensitivity.
- Active RX antennas: Operate essentially non-resonantly. The small element detects the field, and an active buffer provides broadband amplification. Any frequency selection occurs later in the signal chain. Element resonance is generally undesirable.
3. Impedance and Field Sensitivity
Active RX antennas fall into two subcategories:
- E-field probes (high-Z): These act as electric field sensors, capacitively coupling with the field. They present a very high input impedance and respond to potential differences. They must be connected directly to a high-impedance buffer stage.
- Active magnetic loops (low-Z): These respond mainly to the magnetic field (H). The loop has low impedance, and the current in the loop induces a voltage that is actively amplified. These are more sensitive to nearby metal objects and need shielding to suppress E-field pickup.
- TX antennas: Designed for a standard 50-ohm impedance. Both E and H fields contribute to radiation. Feedpoint impedance must match the transmitter for maximum power transfer.
4. Summary Table
| Property | Passive RX | Active RX (E/H) | TX Antennas |
|---|---|---|---|
| Power | None | None | High (W to kW) |
| Skin Effect Relevance | Moderate | Negligible | High |
| Impedance | Varied | Very high (E) / Low (H) | 50 ohm (standard) |
| Resonance | Possible | No | Yes |
| Element Size | Frequency-dependent | Can be very small | Size is critical |
| Mechanical Requirements | Light | Extremely light | Very robust |
5. Conclusion
Although all antennas deal with electromagnetic energy, the design principles of RX and TX antennas differ significantly. Skin effect, resonance, and impedance play varying roles depending on the type. Understanding these differences is crucial when designing or selecting the right antenna for a given application.
Active RX antennas, such as E-field probes and H-loops, show that alternative approaches combining small elements with smart electronics can provide excellent performance over wide frequency ranges, without the limitations of resonance or mechanical size that traditional TX antennas face.
For serious listeners and experimenters, this opens up a creative and technically elegant range of solutions.
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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.