An Asymmetric Inverted-V Is a Three-Conductor Problem

My practical rule is blunt: if the installed antenna does not remain balanced, do not pretend that a component name will make it balanced. Give the RF current a defined route. For many real amateur installations, that means an impedance-transforming UNUN, a deliberate counterpoise or return conductor, and a separate current choke whose position is chosen from measurement.

Geometry Does Not Guarantee Balance

A textbook centre-fed dipole has equal arms in a symmetric environment and supports approximately equal-and-opposite terminal currents. An Inverted-V changes the geometry but can still be well balanced when its two sides, surroundings and feed arrangement are sufficiently symmetric.

An asymmetric Inverted-V is different. One wire may be longer, lower, closer to a roof or tree, or coupled more strongly to the mast and ground. The current in the shorter leg is not automatically smaller; amplitude and phase follow the complete electromagnetic boundary conditions. The feedline, mast, equipment bonding and nearby conductors can supply an additional route. When current flows on the outside of the coax shield, that exterior conductor participates in reception and radiation.

This is why a neat 50 Ω reading cannot certify balance. The analyzer sees the impedance at its calibrated plane. It does not identify which conductors carried the current that produced that impedance.

The Coax Has Two Electromagnetic Surfaces

Inside a coaxial cable, the centre conductor and the inner surface of the shield support the intended differential transmission-line mode. Current on the outer surface is a different mode. That exterior current can continue onto the station wiring, mast, bonding conductors and operator environment.

The exterior current is not created merely because one dipole arm is shorter. It appears when the installed system offers an unequal terminal condition and an external return path with finite impedance. Its magnitude and phase therefore change with feedline length and route, choke placement, counterpoise geometry, frequency, ground and nearby conductors.

Possible symptoms include pattern movement, tuning that changes when the coax is touched or rerouted, RF voltage in the station, conducted or radiated interference, and receive-noise changes. None of these symptoms proves the mechanism alone; measure common-mode current and repeat the test after one controlled configuration change.

Why I Separate the Two Jobs

In many amateur installations, antennas that look balanced on paper do not stay balanced after they are suspended near real ground, buildings, supports and feedlines. My default is therefore to keep impedance transformation and common-mode suppression separable.

An UNUN can transform the complex impedance presented between the radiator terminal and the selected return terminal. A separate 1:1 current choke can then establish a high common-mode impedance at the chosen point on the coax exterior. This architecture lets each part be measured for its own job and changed without assuming that one winding does everything.

That is a practical preference, not a universal theorem that an UNUN always beats a current balun. A properly designed current balun can be the right interface for a genuinely balanced installed load when its differential transformation, common-mode impedance, loss, voltage, current and temperature are verified across the intended operating range.

Choose the Transformer From the Measured Load

A label such as 4:1 or 9:1 states a nominal impedance transformation. For an ideal transformer, the impedance ratio is the square of the turns ratio:

The installed antenna is not an ideal fixed resistor. Its resistance and reactance vary with frequency, geometry, loss and the return path. Measure complex impedance—R + jX—at a declared plane with the intended counterpoise and cable arrangement connected. Select the topology and nominal ratio to keep the resulting loads inside the tuner and transformer operating domains.

Do not automatically progress from 4:1 to 9:1 to 49:1 until the SWR looks attractive. A higher ratio can produce a convenient input match while increasing winding voltage, circulating current, core loss or sensitivity to stray capacitance. A transformer that measures well with a resistive bench load may behave differently with the antenna's reactive, frequency-dependent load.

The Short Leg Is a Return Conductor, Not a Disposal Bin

One useful way to commission an intentionally asymmetric V is to treat the longer wire as the principal radiator terminal and the shorter wire as an intentional return or counterpoise terminal. That gives the system a stated topology, but it does not make the return conductor non-radiating. Any conductor carrying time-varying current contributes to the electromagnetic field.

The counterpoise length should not be selected from one universal wavelength fraction. It interacts with the radiator, transformer enclosure, coax exterior, ground and nearby objects. A very short return can force more current onto the feedline exterior; an electrically significant return can develop its own standing-wave distribution and reshape the pattern.

Start with mechanically credible geometry, then measure. Record current on the intended return, current on the coax exterior at several positions, complex impedance and the stability of those results when cable routing changes.

Place the Choke Where It Defines the Boundary

A choke is useful where it separates the intended antenna conductors from an unwanted continuation of the return path. That may be close to the transformer, or it may be after a deliberate length of feedline exterior that forms part of the antenna. “Immediately after the UNUN” is not a universal placement rule.

Evaluate the choke as a common-mode impedance across frequency, not only as a single dB number. Its resistive and reactive parts, winding parasitics, connected load, voltage, current, duty cycle and temperature matter. The differential insertion path and the common-mode path are not interchangeable measurements.

With the antenna at low power, map exterior-shield current before and after adding the choke. Repeat the map after moving the choke or changing the intended counterpoise. The best position is the one that produces the required current boundary without creating excessive voltage, heating or a new unintended resonant section.

Matching Does Not Prove Efficiency or Pattern

A tuner can make the transmitter see an acceptable load. It cannot reveal transformer dissipation, feedline loss, ground loss, unintended current or where the far field went. Likewise, lower common-mode current at one probe position does not by itself prove higher radiation efficiency.

An asymmetric V can retain a useful pattern, or the unequal conductor and feedline currents can move lobes and nulls. The answer depends on the complete current distribution. Model the installed wires, feedline, mast and representative ground where practical. Validate important directional claims with calibrated field measurements or time-controlled A/B/A comparisons at equal accepted power.

Commission the Complete Current System

• Document the geometry: both wire lengths, heights, V angle, orientation, conductor, insulation, transformer location, counterpoise and cable route.
• Measure the load: calibrate at the stated reference plane and record resistance and reactance across every intended band.
• Characterize the transformer: use representative complex loads and verify loss and temperature at the intended duty cycle.
• Map exterior current: measure at several points on the coax and attached conductors before and after placing the choke.
• Test sensitivity: reroute a controlled section of feedline or alter one return conductor, then restore the baseline and repeat.
• Verify the radiated result: compare at equal accepted power and keep propagation changes from becoming the result.
• Respect safety limits: assess RF exposure, inaccessible high-voltage wire ends, transformer heating, mechanical loads, lightning protection and overhead-line clearance.

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