Inverted-L Current Distribution: Where the Antenna Really Radiates

A base-fed quarter-wave inverted-L, an end-fed half-wave folded into an L, and an off-centre-fed wire may share the same garden silhouette while supporting very different standing-wave patterns.

Radiation Is a Vector Sum

Current magnitude is a useful map of where conductor loss and magnetic field can be important. It is not a map of “radiation per metre” by itself. The far field is the vector sum of contributions from the entire conductor, including each segment's:

• current magnitude;
• current phase;
• orientation and polarisation;
• position relative to the observer;
• ground-reflected field; and
• coupling to the return conductor and nearby objects.

A high-current horizontal segment may contribute strongly at high elevation angles while a shorter vertical segment contributes a different low-angle, vertically polarised field. Fields can reinforce in one direction and cancel in another.

Three First-Order Current Models

Let s be distance measured along a thin wire. For an isolated resonant wire with open ends, the simplest sinusoidal approximation is:

I(s) ≈ Imax × sin(mπs/L)

where m is the number of half-wave sections along total electrical length L. This is an intuition tool, not a finished model.

Real ends have capacitance; real feed systems have transformers and return conductors; ground is lossy; and bends couple the two legs. Maxima therefore shift and minima need not be zero.

Classic Base-Fed Quarter-Wave Inverted-L

       horizontal top section
             ┌──────────────── open end
             │
             │ vertical section
             │
feed/return  ┴

First-order current: high near base, falling toward the open end

This structure is a monopole with part of its conductor bent horizontally. The base current is normally high. The horizontal section provides top loading and also radiates; it is not merely a capacitor.

Its return current must flow through elevated radials, buried conductors, a ground screen or some less desirable combination of soil, feedline exterior and nearby metal. A ground rod may be valuable for electrical safety or lightning bonding, but one short rod is generally a poor replacement for an RF radial system on 160 or 80 m.

Lowering the horizontal wire increases capacitance and coupling to earth and nearby objects, usually changes resonance and feed impedance, and can increase loss. The base-region current maximum often remains, but neither its exact position nor efficiency is fixed.

End-Fed Half-Wave Inverted-L

feed end                                         far end
transformer                                         open
  │                                                   │
  └──── vertical ─── bend ─── horizontal/sloping ─────┘

Half-wave intuition: low → maximum near electrical centre → low

For an ideal half-wave wire, the main current maximum lies near the electrical midpoint measured along the entire conductor—not “halfway across the horizontal span.” If the physical wire is 81 m and behaves as a half wave, 40.5 m from an end is a useful first estimate.

With a 12 m vertical leg, that path coordinate would fall about 28.5 m beyond the bend; with a 16 m vertical leg, about 24.5 m beyond it. Those numbers are geometry bookkeeping, not predictions accurate to half a metre.

Insulation, end loading, transformer capacitance, ground, nearby trees and metal, wire slope and the selected frequency all change electrical length and current position. A bent half-wave also couples its vertical and horizontal sections, so a full electromagnetic solution is preferable.

The Same Long Wire on Its Full-Wave Mode

If that wire operates near a full-wave resonance, the idealised pattern has two maxima at one-quarter and three-quarters of the electrical path and a central minimum. For an 81 m physical reference, the first estimates are about 20.25 m and 60.75 m from the end.

A 12 m vertical leg would place the first path coordinate roughly 8.25 m beyond the bend. Again, the useful conclusion is not the decimal: there are multiple current-rich regions, and a low early horizontal section may carry substantial current.

On multiband operation, every band needs its own current and phase plot. Scaling one 160 m diagram is not enough when traps, compensation capacitors, transformer parasitics or non-harmonic operation are present.

Off-Centre-Fed Inverted-L

short arm / return       feed          long arm
───────────────┬────────────────────────────────
               │
            transformer

Moving the feed away from the centre does not by itself drag the natural half-wave current maximum to the feedpoint. In the simplest 80 m open-open half-wave example, the maximum remains near the electrical centre of the complete conductor while the feed samples a different current and voltage ratio.

For an illustrative 80 m total path with a feed 24 m from one end, the electrical centre is 40 m from that end: about 16 m into the long arm from the feed. If the long arm rises vertically for 12–16 m before bending, the first-order maximum may fall near the upper vertical section or bend.

This explains why an off-centre geometry can place a current-rich region differently from an end-fed one. It does not prove that a particular 4:1 transformer is correct, that the system is efficient, or that 12 m is a universal support requirement.

The Return Path Changes the Antenna

No two-terminal source drives current into only one conductor. The return may be an explicit short arm, radials, a counterpoise, capacitance to the environment, the outside of the coax shield, or a combination.

An EFHW does not need the same radial field as a base-fed quarter-wave monopole, but “no radials” does not mean “no return current.” The transformer, stray capacitance, counterpoise and feedline exterior form the other side of the circuit.

A common-mode choke inserts impedance into the feedline-exterior path. It can move current nodes and change feed impedance as well as reduce shack current. If no adequate alternative return exists, adding a very high impedance at the feedpoint can materially change operation.

Scale Height to Wavelength and Current Geometry

A physical height becomes meaningful only when it is expressed in wavelengths and related to the complete current geometry. The same support height can place a current-rich section in a very different electrical and environmental position on 160, 80 or 40 metres.

At 1.85 MHz, 12 m is only about 0.074 free-space wavelength; at 3.6 MHz it is about 0.144 wavelength. The same physical support therefore represents very different electrical heights on 160 and 80 m.

Results also depend on:

• vertical and horizontal current magnitude and phase;
• wire height along the complete span;
• soil conductivity and permittivity;
• radial geometry or alternative return path;
• trees, buildings, utility wiring and metal structures;
• conductor and transformer loss; and
• desired elevation and azimuth pattern.

Use physical metres for construction; use electrical wavelength and a model for performance.

Takeoff Angle Is Not at One Point on the Wire

A classic inverted-L often has a strong vertically polarised component from its vertical leg, while its horizontal leg contributes horizontally polarised and higher-angle fields. An end-fed or off-centre-fed version can place more current in the horizontal section and therefore change the mixture.

Ground reflection combines with those direct fields. Lossy earth changes magnitude and phase; it does not merely “absorb whatever is close.” The elevation pattern must be calculated from the complete geometry over the specified ground or measured in the far field.

“Current maximum higher equals lower takeoff angle” is therefore not a general law. Raising current-rich wire often reduces near-object and ground coupling, but the far-field result depends on orientation and phase.

Radials: Important, but 120 Is Not Magic

A larger and longer buried radial system generally lowers ground-return resistance for a monopole, with diminishing returns. Broadcast practice often uses 120 radials, but that count is not a universal optimum for amateur installations.

Elevated radials behave differently and can work efficiently with far fewer conductors when tuned, symmetric and installed clear of people and objects. Buried short radials, long radials, ground screens and elevated counterpoises cannot be compared by count alone.

Measure or model radial current and loss. Do not infer antenna efficiency from a pleasant SWR; ground loss can broaden the match while reducing radiation.

How to Model the Three Systems

Lawrence Livermore National Laboratory describes NEC as a code that can model wire structures, lossy ground, loads, networks and transmission lines and output currents, near fields and radiation patterns. The original NEC-2 user guide likewise includes induced currents, radiated fields and a Sommerfeld/Norton treatment for antennas near lossy ground.

A useful model should include:

1. every radiator and intentional return conductor;
2. the actual feedpoint and transformer/network representation;
3. the coax exterior when it carries common-mode current;
4. wire diameter, conductivity, insulation approximation and loads;
5. vertical, horizontal, sloping and bent coordinates;
6. radials or ground screen and an appropriate ground model;
7. nearby conductors that materially couple; and
8. a segmentation-convergence check.

Use Sommerfeld/Norton ground for close-to-earth accuracy where the program supports it. A simplified reflection-coefficient or MININEC-style ground can give misleading loss and near-ground current results.

What to Plot on Every Band

How to Validate the Model

• Measure feed impedance with the final transformer, coax route and choke state.
• Use a non-contact RF current probe at accessible points on the feedline exterior and intentional return.
• Compare resonance and current changes when choke position or counterpoise length changes.
• Measure relative field strength at stable distant locations and several azimuths; one local reading can sit in a multipath null.
• Check transformer temperature and loss separately from radiator performance.
• Repeat after rain or seasonal foliage changes if the antenna is close to soil and trees.

Agreement in feed SWR alone is weak validation. Several different loss and common-mode configurations can present the same input impedance.

A Defensible Comparison

There is no universal winner. A serious radial-fed monopole can be excellent; an off-centre-fed system may suit a medium support and constrained ground area; an EFHW may suit a long high span. The ranking follows the completed installation, not the product family.

The Practical Verdict

Use the sinusoidal sketches to ask better questions, then model the real conductor and return path. Locate current magnitude and phase on every band, map those values back onto physical orientation and height, and calculate the polarised far field over real ground.

The best inverted-L is the one whose complete current distribution, loss budget and pattern suit the site—not the one with the most persuasive feedpoint label.

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