Antenna Performance: Height, Ground Loss, and Resonance

Many radio amateurs struggle to understand why certain antennas perform poorly when mounted too low or without sufficient ground clearance. One key reason lies in how ground losses interact with current distribution and antenna resonance. In this article, we explore how various antenna types behave at different heights and how this impacts their efficiency and radiation pattern. We'll compare resonant vs non-resonant antennas and examine the crucial differences in how they interact with the ground.

Resonant Antennas

EFHW & Dipoles (End Fed or Center Fed)

Resonant dipoles and EFHW (End-Fed Half-Wave) antennas have well-defined current distributions, with maximum current typically occurring at or near the center of the antenna for a dipole, and at about 1/3 of the way for an EFHW.

  • Current Distribution: The current peaks are high in the air if the antenna is mounted high enough, reducing interaction with lossy ground.
  • Height Relation: To radiate efficiently, these antennas need to be placed at least 0.2 to 0.25 wavelengths above ground. Below this, the current maximum approaches the lossy earth, increasing ground losses.
  • Ground Loss Sensitivity: High. At low heights, both EFHWs and dipoles suffer efficiency loss and distorted patterns.

Vertical (e.g., 1/4 Wave)

A vertical antenna uses the ground (or radial field) as its return path.

  • Current Distribution: Maximum current is at the base, making the interaction with ground extremely sensitive.
  • Ground Loss Sensitivity: Very High. Ground-mounted verticals without an adequate radial field (ideally >16 radials or a solid mesh) will radiate poorly.
  • Height Relation: Not about raising the element, but improving the ground system. Elevated verticals with tuned radials reduce ground loss dramatically.

Resonant Loops (Delta and Quad Loops)

Closed-loop antennas like Delta loops and Quad loops can be resonant on their design band.

  • Current Distribution: Loops have circulating current with maxima distributed along the loop. Vertex-fed loops (point down) often have vertical polarization with some omnidirectionality.
  • Height Relation: Loops are less sensitive to absolute height than dipoles but still benefit from >= 0.2 wavelengths for optimal takeoff angles.
  • Ground Loss Sensitivity: Moderate to Low. Their closed structure and distributed current make them more forgiving.

Non-Resonant Antennas

Doublets (non-resonant dipoles)

Doublets fed with ladderline and tuned via a balanced tuner are classic non-resonant antennas.

  • Current Distribution: Variable, depending on frequency. Standing waves occur on both arms.
  • Height Relation: Same rules apply as with resonant dipoles. If the current maxima fall close to the ground, ground losses increase.
  • Ground Loss Sensitivity: High. Especially when used below 0.2λ.

EFOC (End Fed Off-Center, e.g., 29m with 4:1)

These antennas are non-resonant by design, but exhibit semi-resonant behavior on multiple bands.

  • Current Distribution: Complex, with shifting current peaks depending on the frequency.
  • Height Relation: More tolerant than EFHW due to less severe impedance swings, but still impacted by ground proximity.
  • Ground Loss Sensitivity: Moderate. More robust than EFHW, less efficient than center-fed dipole.

Rybakoff / 6M Non-Resonant Vertical

A single 6m vertical pole connected via a 4:1 unun (often with no radials).

  • Current Distribution: Poorly defined, usually high common-mode currents.
  • Ground Loss Sensitivity: Very High. Without a radial system or elevated feedpoint, performance suffers heavily.
  • Height Relation: Raising it reduces ground losses but common-mode radiation often dominates.

Low-Mounted Non-Resonant Loops (e.g., Delta Loop close to ground)

Used for NVIS or as a compromise low-profile multiband antenna.

  • Current Distribution: Circulating and diffuse, not sharply peaking near ground.
  • Ground Loss Sensitivity: Low to Moderate. Performs relatively well even mounted low.
  • Height Relation: Can be as low as 0.05λ, particularly for horizontal polarization and NVIS use.

Summary Table

Antenna Type Resonant Current Peak Location Height Requirement Ground Loss Sensitivity Efficiency Potential
EFHW / Dipole Yes Mid-wire / ~1/3 wire > 0.2 λ High High if elevated
Vertical (1/4 wave) Yes Base Ground level OK w/ radials Very High Low to High (radial dependent)
Resonant Loops Yes Distributed > 0.2 λ Moderate High
Doublet (non-resonant) No Varies per frequency > 0.2 λ High Medium to High
EFOC Semi Variable / off-center > 0.15 λ Moderate Medium to High
Rybakoff Vertical No Near base, high CM > 0.15 λ + choke Very High

Low to Medium
(radial dependent)
Low Delta Loop No Distributed 0.05–0.15 λ Low Medium to High

Key Takeaways

  • Ground loss is proportional to how much current flows near the ground. Antennas with current maxima near or on the ground (low EFHW, verticals without radials) will lose significant power to heating the soil.
  • Resonant antennas with properly placed current peaks are more efficient, especially when elevated above 0.2 wavelengths.
  • Loops are more forgiving due to their closed geometry and distributed currents.
  • Verticals demand radial systems or elevation to be effective.
  • Non-resonant antennas can work well, but height and matching networks (like tuners) play a larger role.

Knowing these interactions helps set realistic expectations. A half-wave antenna only performs like a half-wave when installed with the right height and grounding context. Otherwise, it behaves more like a dummy load than a radiator.

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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.