Ground Lug on EndFeds in RX: The Overlooked Factor in Noise Reduction

In most discussions around End-Fed Half-Wave (EFHW) antennas and similar high-impedance TX antennas, much focus is placed on matching, power handling, and radiation efficiency. However, the receive performance of such antennas is often assumed to be inherently good due to the principle of reciprocity. This assumption ignores one key non-reciprocal factor: common-mode (CM) noise.

Reciprocity Is Not the Whole Story

While antenna theory tells us that antennas are reciprocal devices – what works well for TX should work equally well for RX – this ignores the impact of common-mode noise on the coaxial feedline. On transmit, differential mode currents dominate, and return currents are managed through proper matching and impedance control. (counterpoise and 1:1 unun)  But on receive, especially in noise-dense environments, CM pickup becomes the limiting factor.

What the Ground Lug Actually Does During RX

The ground lug, tied to the cold side of the transformer (coax shield), provides a critical role during reception. Without a defined drain path for common-mode noise, the coax shield can act as a secondary, uncontrolled receive antenna. This results in high levels of unwanted broadband noise and RF pickup from nearby equipment, power lines, or domestic sources.

Connecting the ground lug to a local RF drain – even a short radial, wire to a tree trunk, or garden fence – offers a low-impedance path for these stray CM currents. This doesn’t affect the differential-mode operation of the antenna but significantly improves the signal-to-noise ratio by suppressing noise that would otherwise ride along the shield and leak into the receiver.

Why TX Seems Fine but RX Is Noisy

On TX, you may see a perfect match, low SWR, and good power transfer. But receive performance is degraded by factors invisible to your SWR meter: household EMI coupling to the shield, PC monitor hash, switch-mode PSU noise, etc. These do not show up as mismatches, but they do get picked up by the unchoked or ungrounded shield. In this context, the ground lug serves as a preventative measure against this insidious type of coupling.

Practical Example: EFHW on 40m

An EFHW for 40m may work excellently for DX on transmit. However, if the feedpoint isn’t properly choked and the ground lug floats, a significant amount of CM noise from the shack and house wiring can be conducted into the receiver. Simply bonding the lug to a local garden rod or metallic object can drop the noise floor by several S-units.

Best Practices for Clean Receive Paths

  • Use a 1:1 choke at 0.05 to 0.1 wavelength down the feedline to stop CM currents from reaching the receiver.
  • Always provide a CM drain path via the ground lug. Even a capacitive connection to nearby mass (e.g., metal frame, pipe, wire along soil) is beneficial.
  • Do not confuse differential-mode current return paths (which affect TX efficiency) with common-mode noise rejection (which affects RX clarity).

Conclusion

The ground lug is not just about safety or TX efficiency. In receive scenarios, it plays a vital role in keeping your noise floor low. Don’t assume that because your EFHW transmits well, it also receives well – without addressing CM noise through proper lug grounding and coax choking, you may be losing up to 20 dB of SNR in the noise. Smart use of this lug helps your receiver hear what your transmitter can reach.

 

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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.