Understanding Radiating Wire Length vs Wavelength in Sloper, Flat-Top, and Inverted-L Antennas
The efficiency, radiation pattern, and impedance behaviour of wire antennas are strongly influenced by the relationship between the physical wire length and the wavelength (λ) at the operating frequency. This article explores how this relationship manifests across three popular HF antenna types:
- Sloper
- Flat-Top
- Inverted L (and its reversed variant)
We examine how the vertical (V) and horizontal (H) portions of each layout contribute to radiation, how current distributes along the wire, and how these factors affect impedance transformation using either a 4:1 or 49:1 UNUN.
1. The Basics: λ and Radiating Wire
A half-wave dipole is resonant at ≈ 0.5λ and has a current maximum at the center and voltage maxima at the ends. A full-wave wire (1λ) may develop complex multi-lobe patterns and higher impedance at the feedpoint. End-fed antennas, often matched with a 49:1 or 64:1 UNUN, exhibit high voltages at the feedpoint and operate well when the total wire length is near half-wave, 1.5λ, or similar odd multiples.
Geometry matters. A 20-meter wire at 14 MHz behaves very differently in sloper vs flat-top vs inverted-L form.
2. Sloper Antenna
Geometry
A sloper is a single wire descending diagonally, often from a tower or mast (high end) down to a support or stake (low end). It is typically fed at the high end.
V/H Components
The diagonal orientation means the wire has both vertical and horizontal radiation components. The vertical projection favors low-angle DX radiation, while the horizontal adds mid-to-high angle radiation.
Current Distribution
The current is highest near the feedpoint. When cut to ~0.5λ, this resembles an off-center-fed sloping dipole with a manageable impedance (~50–100Ω depending on height and angle). Longer lengths (0.7λ–1λ) introduce voltage-fed conditions and higher feed impedance, necessitating a 49:1 UNUN.
Matching
~0.5λ: 4:1 UNUN or even direct coax (if near 50–70Ω)
~λ or more: 49:1 UNUN required due to high-Z feedpoint
3. Flat-Top Antenna
Geometry
The flat-top wire is strung horizontally between two supports. This is a classic layout for resonant half-wave end-fed and center-fed antennas.
V/H Components
The entire structure is horizontal, so horizontal polarization dominates. It is well-suited for NVIS and medium-distance contacts, less ideal for DX unless installed very high above ground.
Current Distribution
Center-fed flat-tops (e.g., dipoles) have a current maximum at the center and voltage max at ends. End-fed flat-tops (EFHW) have current minimum at the feedpoint and voltage maximum, with high impedance.
Matching
49:1 or 64:1 UNUN needed for EFHW at 0.5λ, 1.5λ, etc.
OCF (Off-Center-Fed): Use a 4:1 balun with a ~33% offset feedpoint
4. Inverted-L and Reversed Inverted-L
Geometry
The inverted-L has a vertical segment rising from the feedpoint, then bending horizontally. The reversed inverted-L is fed at the far horizontal end, with the wire going horizontal first, then dropping vertically. Both are asymmetrical, resembling an off-center-fed long wire.
V/H Components
Inverted-L: The vertical segment contributes low-angle DX radiation. The horizontal segment contributes mid-angle radiation and modifies feed impedance.
Reversed variant: Vertical radiation is reduced. Ground loss increases if feedpoint is far from earth. Less favorable for DX due to less vertical component near the high-current region.
Current Distribution
Current maximum is always near the feedpoint. In a typical inverted-L, the vertical portion carries more current and is the primary radiator for DX. The horizontal portion is more reactive, shaping impedance and pattern.
Matching
~0.25λ total wire: 4:1 UNUN or coax feed
0.5λ or more: 49:1 UNUN preferred, especially for EFHW usage
Summary Table
| Antenna Type | λ Length Typical | Feed Impedance | Matching Type | Best Use Case |
|---|---|---|---|---|
| Sloper | 0.5λ–1.5λ | 50–600Ω | 4:1 or 49:1 UNUN | DX at low elevation |
| Flat-Top EFHW | 0.5λ, 1.5λ | 2–5kΩ | 49:1 UNUN | Multi-band, stealth installations |
| Inverted-L | 0.25–1λ | 30–2kΩ | 4:1 or 49:1 UNUN | Low-band DX with vertical boost |
| Reversed Inv-L | 0.5λ+ | 1–5kΩ | 49:1 UNUN | Compromise, less V radiation |
Final Thoughts
The geometry of a wire antenna—not just its length—has a major impact on performance. A 20-meter wire may behave like a great DX antenna in an inverted-L form but be mediocre when laid flat at low height. The interaction between vertical (V) and horizontal (H) sections affects takeoff angle, polarization, and common-mode current risks. Understanding current distribution helps determine the best matching network—saving both signal and sanity.
For best results, always consider:
- Height above ground
- Wire length in relation to λ
- Feedpoint location and access to good grounding or counterpoise
- Impedance transformation needs and power levels
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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.