Wire Antenna Geometry – Sloper, Flat-Top & Inverted-L
The efficiency, radiation pattern, and impedance behavior 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 (or an appropriate balun/choke where applicable).
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.
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 can be fed at the high end (end-fed sloper) or center/offset-fed (sloping dipole/OCF).
V/H Components:
The diagonal orientation yields both vertical and horizontal radiation components. The vertical projection favors low-angle DX radiation; the horizontal adds mid- to high-angle radiation.
Current Distribution:
Current is highest near the feedpoint. For a center-fed or clearly off-center-fed sloper at ~0.5λ, the feedpoint impedance is often in the tens of ohms to low hundreds (≈ 50–100 Ω depending on height/angle). For an end-fed sloper near ~0.5λ (or longer), the feedpoint is a high-voltage/high-impedance point.
Matching:
• Center/offset-fed (~0.5λ): Often near 50–100 Ω — may take a 1:1 current balun (choke) and direct coax feed or a modest ATU.
• End-fed (~0.5λ or ≥ ~0.7–1λ): High-Z at the feed — typically requires a 49:1 (sometimes higher) UNUN and a quality common-mode choke.
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 structure is horizontal, so horizontal polarization dominates. It is well-suited for NVIS and medium-distance contacts; for DX, install it as high as practicable.
Current Distribution:
Center-fed flat-tops (dipoles) have a current maximum at the center and voltage maxima at the ends. End-fed flat-tops (EFHW) exhibit a current minimum and voltage maximum at the feedpoint, yielding high impedance.
Matching:
EFHW at 0.5λ, 1.5λ, etc.: 49:1 or 64:1 UNUN (or higher) recommended.
OCF dipole (~33% offset): use a 4:1 unun with a good 1:1 choke downstream.
Inverted-L and Reversed Inverted-L
Geometry:
An 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 adds mid-angle radiation and modifies feed impedance.
Reversed variant: Vertical radiation is reduced; ground loss can increase if the feedpoint is far from earth or lacks a good return path. Often less favorable for DX due to less vertical component near the high-current region.
Current Distribution:
Current maximum is 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 (classic quarter-wave L against ground/counterpoise): Often ~30–60 Ω with a good RF ground. Use a 1:1 current choke and, if needed, simple matching (series capacitor or ATU). A 4:1 UNUN is usually not required here unless impedance is higher in your install.
• ≥ 0.5λ (EFHW-style lengths): High-Z at the feed — a 49:1 UNUN (sometimes 64:1 or even higher) plus a choke is preferred.
Summary Table
| Antenna Type | Typical λ Length | Feed Impedance | Matching Type | Best Use Case |
|---|---|---|---|---|
| Sloper (center/offset-fed) | ~0.5λ | ~50–100 Ω | 1:1 current balun + ATU or direct coax | Broad coverage; simple matching |
| Sloper (end-fed) | ~0.5λ to ≥1λ | High-Z (kΩ) | 49:1 (or 64:1) UNUN + choke | Compact multiband with EFHW behavior |
| Flat-Top EFHW | 0.5λ, 1.5λ | 2–5 kΩ | 49:1 (or 64:1) UNUN | Multi-band, stealth installs |
| Inverted-L (~0.25λ against ground) | ~0.25λ | ~30–60 Ω | 1:1 choke + simple matching/ATU | Low-band DX with vertical boost |
| Inverted-L (≥0.5λ EFHW-like) | ≥0.5λ | High-Z (kΩ) | 49:1 (or 64:1) UNUN + choke | Multiband end-fed operation |
| Reversed Inverted-L | ≥ 0.5λ | 1–5 kΩ | 49:1 UNUN + choke | Space compromise; less vertical component |
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 ground or counterpoise
- Impedance transformation needs and power levels
Mini FAQ – Sloper vs. Flat-Top vs. Inverted-L
- Which layout is best for DX? Inverted-L or sloper (thanks to the vertical component). Elevate the vertical section and add a quality 1:1 common-mode choke.
- Do I need a choke or line isolator? Yes—especially for end-fed and asymmetrical layouts to reduce RF on the feedline and lower receive noise.
- What feedline should I use? For end-fed/high-SWR coax runs, keep coax short and choked. For center-fed multiband, ladder-line minimizes loss.
- Which transformer ratio do I choose? EFHW near half-wave or 1.5λ: typically 49:1 (sometimes 64:1 or higher). OCF dipole: 4:1 at ~33% offset.
- Is a 4:1 UNUN right for a ¼-wave inverted-L? Usually not. With a good ground/counterpoise the feed is often ~30–60 Ω; use a 1:1 choke and simple matching or an ATU.
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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, with experience in telecom and broadcast hardware including set-top boxes, transcoders, and E1/T1 switchboards.