How Height Changes End-Fed Feedpoint Impedance

I do not use one minimum feedpoint height for every end-fed antenna. The same physical height is a different fraction of a wavelength on every band, and moving the transformer usually changes more than its distance from the soil. The way forward is to define the whole antenna system, measure resistance and reactance at a known plane and map the return current.

Feedpoint Impedance Is R+jX at a Declared Pair of Terminals

At one frequency, the antenna terminal impedance is written as Z = R + jX. The resistance R contains both radiation resistance and every loss referred to that terminal pair. The reactance X represents net stored electric or magnetic energy at that plane. Neither term belongs to the wire alone; both describe the installed structure connected to those terminals.

An end-fed system still has two terminals. One terminal connects toward the long wire. The other must connect through an intentional return conductor, a defined section of coax exterior, local capacitance to the surroundings, a mast or bonding structure, or some combination. If the second terminal is treated as “ground” without identifying its current, the measured impedance includes an unknown part of the station.

SWR compresses R and X into a mismatch magnitude referenced to a line impedance. It is useful for the transmitter, but it does not reveal which term moved or why. A resistance/reactance sweep is far more useful when studying height.

There Are Several Heights, and They Do Different Jobs

The phrase “feedpoint height” often hides several independent geometric variables:

• Terminal height: the height of the matching-network or wire terminal above local ground and nearby objects.
• Average wire height: the distribution of the radiator above the ground profile, not simply the support height.
• Height of current-rich sections: the position of the conductor regions carrying substantial current on the frequency being studied.
• Return-path height and route: the position of the counterpoise, coax exterior, mast and other conductors carrying return current.
• Height above effective ground: an electromagnetic quantity influenced by soil layers, terrain and structures, not just tape-measure distance to the surface.

Moving only the feed-end support may also change wire slope, bend location, distance to a wall, counterpoise angle, coax route and the spacing between conductors. A changed impedance cannot then be attributed to vertical height alone.

Electrical Height Changes from Band to Band

Electrical height is the physical height divided by wavelength. Using the free-space approximation λ = c/f, the same 10 m terminal is about 0.06λ at 1.8 MHz, 0.12λ at 3.6 MHz, 0.24λ at 7.1 MHz, 0.47λ at 14.2 MHz and 0.95λ at 28.5 MHz. Those values are orientation guides, not impedance predictions.

Near the lower bands, a practical feed end may be electrically close to ground while much of the wire follows a different height profile. On higher bands, the wire can support several current maxima and minima. A small geometric change can then move lobes and nulls as well as terminal impedance.

The conductor's electrical length also includes diameter, insulation, end effects, bends and coupling to its environment. Treat the simple wavelength ratio as the start of a model, not a promise that a given resistance or pattern will appear.

Why Moving a Voltage-Rich End Often Moves R and X

A wire operated near an end-voltage maximum has strong electric-field coupling around the terminal and matching network. Bringing that region close to soil, a wall, gutters, branches or support hardware changes the distributed capacitance and loss. The zero-reactance crossing can shift, and the resistance at that crossing can change.

The effect is not universally larger or smaller than it is at a more current-rich feedpoint. A different feed position changes the transformation from the full antenna current distribution to the terminal. Geometry, conductor loss, soil, the return branch and feedline participation determine the result. Labels such as EFHW, off-centre end-fed and random wire are not substitutes for the installed R+jX sweep.

A low terminal can therefore work well, but height alone does not certify that outcome. A low SWR may coexist with ground loss, matching-network loss or unintended feedline radiation. A higher SWR may coexist with strong useful field if the matching and feedline system handles the load efficiently and safely.

The Return Branch Can Move More Than the Radiator

Many “height changed my SWR” reports are really current-division experiments. When the feedpoint moves, the intentional return wire may change angle and distance from soil. The coax may acquire a different length or route before the first effective common-mode boundary. Capacitance to a mast or wall may change. Each change alters the complete two-terminal load.

A counterpoise is not defined only by its cut length. Its electrical role depends on current magnitude and phase, height, route, coupling and the point at which current is forced to stop or divide. A ground rod is not an infinite RF sink, and protective earth or lightning bonding must not be rearranged merely to improve an analyser trace.

To make a height comparison meaningful, keep the return conductor, coax route, matching network and choke position fixed relative to the antenna. Then measure current on the intended return and at several points on the coax exterior. If those currents change, the experiment changed more than feedpoint height.

A Choke Creates a Boundary, Not a Missing Terminal

A common-mode choke impedes current on the feedline exterior at its installed location. It does not eliminate the need for return current between the antenna terminals. If a length of coax exterior is intentionally part of the antenna-side return, the choke marks the far end of that branch only when its complex common-mode impedance is adequate at the frequency and drive level involved.

Placing the choke directly at the transformer and placing it several metres away create different antenna systems. Neither location is universally correct. Choose the boundary deliberately, characterise the choke over the required bands and measure exterior current on both sides.

Impedance transformation and common-mode suppression remain separate functions even when they share one enclosure. The transformer handles the differential load between its ports. The choke controls current on the outside of the feedline. A suitable transmitter match proves neither common-mode suppression nor transformer efficiency.

The Transformer Changes What the Analyzer Sees

The impedance on the coax side of a matching network is not the bare wire-end impedance divided by a perfect fixed number. A real transformer has magnetising impedance, leakage inductance, winding capacitance, conductor loss and core loss. Its response changes with frequency, complex load, power and temperature.

Nominal impedance ratio is a design starting point. It should follow the measured load envelope rather than define it. At a voltage-rich feed, terminal and winding voltage can be severe. At a lower resistance, current and core flux can become limiting. Reactive loads can increase circulating energy even when the station tuner presents 50 Ω to the transmitter.

For height studies, measure the antenna-side R+jX when practical, then measure through the matching network at a second declared plane. That separates an antenna change from a change introduced by the transformer and its parasitics.

The Measurement Plane Must Travel with the Question

A VNA calibrated at the shack sees the matching network through the feedline. The line transforms impedance with electrical length, and loss reduces the magnitude of the reflected wave on its return journey. Changing cable length or routing while changing feedpoint height can move the measured R+jX even if the antenna terminals stayed the same.

Use at least these declared planes:

• Wire and return terminals: the installed antenna load before transformation.
• Matching-network coax port: the load presented to the feedline.
• Station end of the feedline: the transformed load arriving at the tuner or transmitter.

Calibrate at the plane that answers the question or de-embed a characterised test cable within its validated frequency and uncertainty limits. Save resistance, reactance, reflection magnitude and phase. Record the cable, calibration, tuner state, choke position, weather and geometry with every sweep.

Height, Efficiency and Pattern Are Separate Results

Feedpoint impedance is one boundary condition on the current distribution; it is not a radiation-efficiency meter. The resistive term combines radiated power and multiple losses. Subtracting a textbook resistance from measured R does not isolate soil loss because the ideal resistance itself changes with geometry, ground and participating conductors.

The pattern follows current magnitude and phase over the entire wire, return structure, feedline exterior and nearby coupled conductors. Average height and terrain matter. On a multiband wire, the same physical layout can be electrically short on one band and many half-waves long on another, producing entirely different lobes and nulls.

That is why a convenient impedance after lowering the feedpoint does not prove better efficiency, lower take-off angle or quieter reception. Model the complete installed geometry over realistic ground, then verify important directions with equal-accepted-power field measurements.

A Controlled Height Experiment

• Choose one frequency or narrow band first: a multiband conclusion needs separate evidence on every band.
• Document the complete geometry: wire, supports, feed terminal, return conductor, coax route, choke, mast, building and nearby metal.
• Change one height variable: preserve wire length, matching network, return route and cable configuration as closely as possible.
• Measure at defined planes: record R+jX at the antenna-side and coax-side ports, then account for the feedline to the station.
• Map common-mode current: measure the coax exterior and intended return before and after each height change.
• Check loss and stress: measure delivered power and equilibrium temperature with representative complex loads, power and duty cycle.
• Verify the field: compare equal accepted power using rapid A/B/B/A switching, fixed receiver settings and a restored baseline.

If only the SWR trace was recorded, report only the SWR trace. If the current map and field measurement were also controlled, a stronger conclusion becomes possible.

Low Feedpoints Need Extra Safety Attention

End-fed terminals and matching networks can carry high RF voltage even when feed current is modest. A low feedpoint increases the chance of contact by people or animals and can place strong electric fields close to combustible material, wet vegetation or a building surface.

Use a closed, strain-relieved assembly; prevent access while energised; maintain clearances from power conductors and utilities; de-energise and verify before adjustment; and assess RF exposure for the actual power, duty cycle, pattern and accessible geometry. Weather protection does not make an energised terminal touch-safe.

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