Inverted-L Feedpoint Height: Follow the Whole Current Path
Inverted-L Feedpoint Height: Follow the Whole Current Path
The height of an end-fed transformer is not the same as the height of the current-carrying wire. Feedpoint position can matter, but the complete wire, return path, feedline exterior, ground and terrain determine the installed result.
A frequent question is whether moving an inverted-L feedpoint from near ground to a few metres higher will “lift” the radiation and spoil low-angle DX. The intuition mixes up three different things: feedpoint height, the height of each current-carrying section and the elevation pattern of the complete antenna. They are connected, but they are not interchangeable.
The engineering position: do not predict an inverted-L from transformer height alone. Solve or measure the current on every conductor that participates in the RF circuit, then calculate the vector sum of its fields over the actual ground and terrain.
There Is More Than One Relevant Height
An inverted-L has a vertical or sloping section, a bend and a horizontal or further-sloping section. An end-fed version also has a transformer, a return path and usually some exterior feedline current unless that branch is deliberately controlled.
Record at least these geometrical facts separately:
- transformer and feed-terminal height above local ground;
- height and length of the rising wire section;
- bend position and angle;
- height, route and slope of the upper wire;
- counterpoise, mast, bond and nearby-conductor geometry;
- coax route and the location of any common-mode choke; and
- ground profile and terrain in each important azimuth.
Two installations with the same feedpoint height can have different current distributions and patterns. Two installations with different feedpoint heights can be similar when the electrically important geometry and return-current division remain similar. Neither result should be assumed before the complete structures are compared.
Radiation Follows Current on the Complete Structure
Every wire section that carries time-varying current contributes an electromagnetic field. The far field is the vector sum of those contributions, including their magnitude, phase, orientation and position. Calling one section “vertical” and another “horizontal” is useful for visualising the layout, but it does not make the upper wire electrically passive.
The vertical section may contribute strongly to vertically polarised, low-elevation radiation in some geometries. The horizontal or sloping section can also contribute substantially, and its field can reinforce or cancel the vertical section at particular azimuth and elevation angles. Ground reflection and terrain then reshape that combined field.
That is why “the vertical leg creates the DX pattern” is too categorical. The correct question is how current is distributed along the complete installed conductor and what field that distribution produces in each wanted direction.
The Current Maximum Is Not Fixed at One Midpoint
A thin, isolated, straight half-wave wire in a simplified model suggests a familiar standing-current distribution with a current maximum near its centre and current minima near its ends. An installed inverted-L is not that ideal object.
Bending the conductor, moving it near ground or structures, changing conductor diameter, adding a transformer, changing the return path and allowing feedline current all change the boundary conditions. Loading, loss and off-resonance operation can further move or flatten current maxima. On a multiband wire, each band has its own current distribution.
Use “near the centre” only as an initial picture for a simple half-wave case. For the installed antenna, inspect the modeled or measured magnitude and phase of current along each segment.
Moving the Feedpoint Can Change More Than Impedance
Raising or lowering the transformer may change several things at once:
- the length and average height of the rising radiator;
- the route or height of the upper wire when its supports stay fixed;
- capacitance to ground and nearby objects;
- the geometry and impedance of the intended return conductor;
- the amount and distribution of coax-exterior current;
- transformer and choke surroundings; and
- feedline coupling into the radiated field.
If a small physical move is electrically small and leaves those variables nearly unchanged, the pattern change may also be small. That is a measured or modeled result for that installation—not a general statement that feedpoint height never matters.
Conversely, a modest height change can matter when it changes wire slope, coupling to a metal roof, a ground-loss region, choke placement or the length of coax participating in the return path. Impedance, current distribution and pattern can then move together.
An EFHW Still Needs a Return Path
An end-fed half-wave does not pass current into nowhere. The transformer secondary drives current into the long wire. Its other terminal connects—intentionally or parasitically—to some combination of a deliberate counterpoise, coax exterior, enclosure capacitance, mast, bonding, soil coupling and station wiring that completes the RF-current path.
A common-mode choke changes the impedance of the coax-exterior branch at its location. It does not erase every field or force a universal current split. Choke impedance is complex and frequency-dependent, and the other return branches also have complex impedances. Moving the feed assembly or choke can therefore change both matching and radiation.
For a repeatable system, decide which return branch is intentional, place the choke to define its boundary, and measure exterior current on every required band. A low SWR at the transmitter cannot prove that the intended branch carries the intended current.
Ground Is Important, but It Is Not the Only Loss
Loss can occur in soil, conductors, the transformer, a matching or compensation network, the feedline, the choke and lossy nearby objects. Poor return-current geometry can increase current in unwanted structures without making soil the sole cause.
Ground also affects more than efficiency. Conductivity, permittivity, surface profile and terrain change the reflected field and therefore the elevation pattern. ITU-R BS.705-2 treats ground, topography and surrounding structures as practical antenna-pattern variables rather than one universal “good” or “bad” ground number.
A useful power budget places each loss at a declared reference plane. Measure transformer or network loss with representative source and load impedances, measure feedline loss over the actual route, inspect choke current and temperature, and use an appropriate field model for soil and terrain. Do not infer efficiency from SWR.
The Transformer and Choke Are Part of the Geometry
The end of a half-wave wire is a high-voltage, low-current region in the simplified resonant case. The actual feed impedance varies with layout, height, surroundings, return path and frequency. The transformer must therefore be selected and tested for the measured complex load, operating bandwidth, power, waveform, duty cycle, mismatch, insulation, cooling and enclosure.
The choke has a different job: it presents common-mode impedance where the designer wants to bound exterior feedline current. Transformer ratio and choke impedance cannot be chosen from the antenna label alone. A choke placed immediately at the transformer, farther down the coax or omitted creates a different return-current structure.
Measure both devices in their installed context. A transformer that gives a convenient SWR can still dissipate power, and a high bench-measured choke impedance can still be inadequate at the installed current, voltage or temperature.
There Is No Universal Best Height
A fixed recommendation such as “put the transformer 1–3 m high” confuses access, mechanics, electrical geometry and safety. The right position must satisfy all of them:
- the modeled and measured pattern for the required paths;
- the intended return-current and choke arrangement;
- clearance from people, conductors, buildings and vegetation;
- protection against accidental contact with high RF voltage;
- feedline bend radius, weather loop and independent strain relief;
- transformer mass, support loads, wind and ice;
- safe installation and maintenance access; and
- the applicable national exposure, electrical and structural rules.
RF safety: an end-fed terminal can support substantial RF voltage, and the antenna’s near field depends on frequency, power, duty cycle and geometry. Prevent access while transmitting and evaluate human exposure under the rules that apply at the installation. ICNIRP’s current RF guidelines provide a health-protection framework; the competent national authority determines the legal compliance method.
Model and Measure the Change You Actually Make
A useful comparison begins with two complete geometry files. Include the entire wire, transformer location, deliberate counterpoise, the coax-exterior section before the choke, conductive supports where relevant, nearby structures and a defensible ground model. LLNL’s Numerical Electromagnetics Code family can calculate wire currents, radiation patterns and fields above modeled ground, but the result is only as complete as the geometry and material assumptions.
For each configuration, compare:
- complex feed impedance at the antenna reference plane;
- current magnitude and phase along vertical, sloping, horizontal and return conductors;
- azimuth and elevation patterns at each operating frequency;
- realised gain in the wanted path directions and angle ranges;
- feedline-exterior current before and after the choke;
- transformer, choke, feedline and conductor loss; and
- voltage, temperature and exposure limits at operating power.
After installation, measure impedance at a declared reference plane and map common-mode current. Field-strength or rapid A/B receive comparisons can check important directions, but propagation, receiver settings and local noise must be controlled. A single contact report cannot isolate a one-metre feedpoint change.
Primary technical and safety references
- IEEE 145-2025 — standard definitions for antennas
- ITU-R BS.705-2 — HF transmitting and receiving antenna characteristics and diagrams
- ITU-R P.533-14 — prediction of HF-circuit performance and path elevation angles
- Lawrence Livermore National Laboratory — Numerical Electromagnetics Code capabilities
- ICNIRP — Guidelines for Limiting Exposure to Electromagnetic Fields, 100 kHz to 300 GHz
Follow the current, not the mounting label. Feedpoint height is one coordinate in a complete RF structure. Change it when pattern, return-current control, mechanics and safety justify the change—and verify the resulting antenna rather than assuming it stayed the same.
Mini-FAQ
- Is feedpoint height the same as radiation height? No. The complete current distribution and the position of every radiating and return-current section determine the field.
- Is the current maximum always at the midpoint of an EFHW? No. That is an approximation for a simple straight half-wave; bends, ground, loading, loss, return paths and nearby structures can move or reshape it.
- Does only the vertical leg produce low-angle radiation? No. Vertical, sloping, horizontal and return-current sections all contribute fields whose magnitude and phase combine by direction.
- Can raising the feedpoint change the pattern? Yes. It can change wire geometry, ground coupling, the return path, choke position and coax-exterior current, although an electrically small change may have a small effect.
- Is ground loss the only important efficiency loss? No. Conductors, transformer, matching network, feedline, choke and nearby lossy objects can all dissipate power.
- Is a feedpoint height of 1–3 m universally best? No. Choose height from the complete modeled and measured pattern, return path, clearances, RF safety, mechanics and applicable rules.