Wire Length and Wavelength: Sloper, Flat-Top or Inverted-L?

I often see a wire described by two facts—its length and its shape—and then given a complete personality: the sloper is for DX, the flat-top is for NVIS, the Inverted-L has a vertical radiator and a horizontal loading wire. Those pictures can start a discussion. They cannot finish the engineering.

Wavelength Is the Scale, Not a Cutting Chart

Free-space wavelength is:

Here c is the speed of light in vacuum—exactly 299,792,458 m/s in SI—and f is frequency in hertz. At 14 MHz, for example, the free-space wavelength is about 21.4 m, so a 20 m conductor is physically close to one wavelength. That statement does not yet identify a resonance, feed impedance or pattern.

An installed wire's electrical behaviour also depends on conductor diameter and insulation, end hardware, bends, height, ground, nearby structures and coupling to other conductors. A familiar half-wave dipole cutting formula is therefore a starting estimate for one mode and geometry, not a universal conversion from metres to resonance. Do not apply a coaxial-cable velocity factor blindly to a radiator.

As electrical length increases, the wire can support additional current regions. The points of maximum and minimum current depend on the excitation and boundaries. “Half wave,” “full wave” and “three half-waves” describe useful modal scales, but the real installation decides where the current actually flows.

The Feedpoint Selects More Than an Impedance

A centre-fed dipole presents two explicit radiator terminals. Near its familiar lowest mode, current is large near the centre and approaches zero near the open ends. Move the feedpoint off centre and the source samples a different place in the current distribution; the terminal resistance and reactance change.

An end-fed wire commonly places the source near a low-current, high-voltage region for a half-wave-like mode. That is not true for every electrical length or every end-fed geometry. A base-fed quarter-wave-like wire over a defined return system instead has substantial current near the feedpoint. “End-fed” describes where the source connects, not one guaranteed impedance.

The other source terminal cannot be omitted. Current returns through a counterpoise, radial or ground network, another conductor, capacitance to the environment, the exterior of coax, station wiring—or some combination. Until that branch is drawn, the antenna length and feed impedance are incomplete descriptions.

A Sloper Is an Orientation, Not a Feed System

A sloper places all or part of the wire on a diagonal. It may be centre-fed, offset-fed or end-fed, and each choice excites a different terminal condition and return-current structure. A nearby tower or mast can also carry induced current and reshape both impedance and pattern.

The diagonal current has horizontal and vertical vector components, so mixed polarisation is possible. It does not follow that the vertical projection automatically creates a low-angle DX lobe while the horizontal projection creates a higher-angle lobe. The far field is a phase-sensitive vector sum over the whole wire, return path, feedline and coupled structures.

Height at both ends, slope angle, azimuth, ground conductivity and terrain determine the realised elevation and azimuth response. Feedpoint position can place a high-current region near the higher or lower end, changing which part of the wire contributes most strongly. Model the installed sloper rather than assigning it one pattern from its name.

A Flat-Top Is Not Automatically an NVIS Antenna

A horizontal flat-top can be a centre-fed dipole, an off-centre-fed wire, an end-fed wire or a multiband doublet. Its orientation favours horizontal polarisation in a symmetric installation, but height in wavelengths and the ground reflection strongly affect the elevation pattern.

At a low electrical height, a horizontal wire often has strong high-angle response. As height increases, lower-angle lobes and elevation nulls can form. On higher bands, the same physical wire becomes several half-waves long and develops more azimuth and elevation lobes. A flat-top can therefore support local, regional or long-distance paths depending on band and installation; “horizontal” is not a propagation mode.

End-fed and centre-fed versions also need different feed and return-path analysis. A high terminal impedance at one end does not identify a fixed transformer ratio, and an offset percentage does not guarantee a four-to-one load across bands.

An Inverted-L Is One Continuous Current Structure

An Inverted-L bends a wire from a vertical section into a horizontal section. In a common base-fed quarter-wave-like installation, current can be large near the base, making the vertical part influential in the far field while the horizontal part changes electrical length, terminal impedance and the total pattern. That description is conditional on the mode and return network.

At other lengths or feed positions, the current maximum need not be at the base, and the horizontal section need not be merely reactive. Both sections can radiate. Their fields combine with phase that varies along the conductor, so moving the bend can change polarisation, azimuth pattern, elevation pattern and impedance together.

Feeding the other end—sometimes called a reversed Inverted-L—changes the source and return-path boundary. It does not simply turn down a universal “vertical component.” The installed current distribution, feedline participation, matching network and ground coupling decide the result.

Projection Does Not Divide the Radiated Power

Breaking a bent wire into vertical and horizontal projections is useful for visualising local current direction. It is not an independent power budget. Fields from every current element add as vectors, including their phase and propagation delay. Nearby ground and conductors add reflected and induced fields.

That is why a long horizontal section cannot be labelled “mid-angle radiation” while the vertical section is assigned “DX radiation” without a solved or measured current distribution. A weak-current section may contribute little despite its physical length; a shorter high-current section may dominate some directions; and cancellation can create deep nulls.

LLNL's Numerical Electromagnetics Code can represent wires, conducting surfaces, networks, transmission lines and homogeneous ground and can report segment currents and patterns. The useful model includes the feed, return structure, real wire height and nearby conductive objects, then demonstrates convergence rather than relying on a sketch alone.

Match the Measured Load, Then Control Common Mode

Do not select a transformer solely because the wire is called a sloper, OCF, EFHW or Inverted-L. Measure or model the complex load at the intended transformer terminals across every operating band. Select the ratio and topology for that load range, then verify insertion loss, voltage, current, temperature and duty-cycle margin.

Impedance transformation and common-mode suppression are separate functions. A transformer that produces a convenient differential impedance does not automatically stop current on the exterior of coax. A current choke may be appropriate where the intended return or counterpoise ends, but its location and impedance must follow the installed common-mode circuit.

A balanced two-terminal radiator can still develop common mode through unequal surroundings or feed routing. An intentionally unbalanced antenna needs a declared return conductor before any choke establishes the boundary. Adding a choke without understanding that return path can change the antenna's effective length, feed impedance and pattern.

Keep the Measurement Planes Separate

The bare radiator terminals, transformer input, feedline input and shack connector are different reference planes. A feedline transforms impedance and adds loss. A matching network can present 50 + j0 Ω to the transmitter while the radiator remains reactive at its own terminals. A low SWR at the radio does not reveal the current distribution or efficiency.

Use open-short-load calibration at the connector where the impedance result is required, or a validated de-embedding method for the intervening fixture. Save resistance and reactance as well as SWR. Record feedline type and length, matching state, choke position, cable route, ground/return network, wire geometry and weather.

Measure current on the intended conductors and on possible unintended paths. A clamp around coax can reveal exterior-shield current; corresponding probes on balanced conductors can check magnitude symmetry and phase. Repeat after rerouting the cable or changing the return network. A large impedance or pattern change is evidence that the changed conductor was part of the antenna.

Compare Geometry Band by Band

For each candidate layout, I would compare the same complete set:

• physical length and electrical length at the actual frequency;
• feedpoint location and every intentional and incidental return path;
• complex impedance at named radiator, transformer and station planes;
• current magnitude and phase along the wire and feed conductors;
• transformer, tuner, feedline, ground and conductor loss;
• realised azimuth and elevation patterns over defensible ground;
• common-mode current and sensitivity to cable routing;
• voltage, current, heating and RF-exposure limits at operating power; and
• repeatable A/B/A field or received-signal comparisons for the intended paths.

A 20 m wire at 14 MHz can be arranged as a flat-top, sloper or Inverted-L, but those outlines do not promise which one will be better. The useful answer belongs to the installation that delivers the required realised pattern and loss while keeping currents, voltages and exposure within verified limits.

Questions, antenna-factor records or height trials to share? Contact RF.Guru.