A 44 m Doublet on a 16 m Pole With Open-Wire Feedline

The attraction is straightforward: one central support can carry a full-size 44 m wire as an inverted-V, while low-loss two-wire line brings a wide range of impedances to a tuner. I would build from those dimensions, but I would not call them an all-band performance recipe. They define the first drawing, not the final current distribution.

What the Nominal Dimensions Actually Mean

The radiator starts as two 22 m legs, for approximately 44 m tip to tip. Its installed electrical length changes with conductor diameter and insulation, the centre and end hardware, sag, apex angle, height, wet foliage, nearby structures and ground. The 16 m centre height and 16 m feedline length are equally real geometric constraints, but neither is an electrically neutral number across HF.

On some bands the wire will be near a useful resonance; on others it will be electrically short or several half-waves long. The tuner must therefore encounter a frequency-dependent complex load. Whether it can match that load—and what voltage, current and loss occur while doing so—must be checked at the end of the actual feedline.

A nominal 600 Ω line is not automatically 600 Ω in place. Characteristic impedance depends mainly on conductor diameter, spacing and the dielectric environment. Spacers, rain, ice, vegetation and nearby conductive or dielectric objects change the line. Measure or calculate the actual construction rather than using the name as a calibration standard.

The Inverted-V Is a Practical Shape, Not a Universal Optimum

With one tall centre support, the inverted-V usually needs less horizontal space than a flat top and gives the wire ends two accessible anchor directions. The apex angle, end height and leg symmetry then affect feedpoint impedance, current distribution, polarisation mix and azimuth/elevation pattern.

There is no universal best apex angle. Bringing the legs close together increases their electromagnetic interaction and can change the terminal impedance and pattern; it can also increase lateral and downward loads on the support. A wider V may better preserve the broadside behaviour of a horizontal dipole, but it demands more site width. Choose the angle from the available footprint, modelled pattern, end clearance and the support manufacturer’s load limits.

Keep both ends outside accessible areas and high enough for the maximum RF voltage, sag, wind motion and local safety rules. Do not place anchors where a failed rope or falling wire can reach people, vehicles, buildings or overhead power lines. The UK Health and Safety Executive warns that flashover can occur without contact and directs users to the network operator for the required clearance; the applicable local authority and line owner control the actual requirement.

A Carbon-Fibre Pole Belongs in the RF Model

Carbon-fibre composite should not be assumed to be RF-transparent. Hexcel’s carbon-fibre data, for example, specifies finite electrical resistivity. The complete mast behaviour depends on fibre type, lay-up, resin, joints, wall thickness, moisture and attached hardware, but a long carbon support close to a feedpoint and balanced line can carry induced current or introduce loss and imbalance.

Keep the centre insulator and both line conductors clear of the pole according to a measured or modelled design. Do not fasten one conductor against the mast while the other hangs in free space. Include metallic guys, halyards, clamps, cable routes and ground anchors in the installation record.

RF suitability does not establish mechanical suitability. Verify the exact pole’s permitted vertical and side loads, guying method, wind area, extension limits and inspection schedule with its manufacturer. The wire, centre hardware, feedline and weather load can create bending moments well beyond their static weight.

Route Open Wire as a Balanced Transmission Line

The wanted transmission-line mode has approximately equal and opposite currents on the two conductors. The external fields then cancel substantially at distances large compared with the spacing. That cancellation depends on symmetry. If one conductor couples more strongly to a carbon pole, wall, gutter, metal roof, wet tree or ground, part of the differential mode can convert to common mode.

A vertical drop from the centre can be a good starting route because it keeps the first section geometrically similar for both conductors. It is not necessary to prescribe one fixed drop length. Continue the line with gentle bends, constant conductor spacing and comparable surroundings for both wires. Avoid coiling spare line, running it flat against a surface or twisting it around metalwork.

Likewise, one fixed clearance such as 10 or 15 cm cannot cover every line spacing, frequency and nearby material. Increase separation until moving the line or nearby object no longer changes measured impedance or current balance materially, then preserve that route. Where a crossing is unavoidable, a near-right-angle crossing usually limits the length of close parallel coupling, but measurement still decides whether it is acceptable.

The Feedline Length Transforms the Load

The 16 m line is not only a low-loss connection. It transforms the antenna-terminal impedance according to its characteristic impedance, propagation constant and electrical length. In the lossless approximation:

Here Z_L is the doublet terminal impedance, Z₀ is the line characteristic impedance, β is phase constant and ℓ is physical length. Real line also has attenuation. A load that is manageable at the antenna may become an extreme voltage or current at the tuner, while a difficult antenna-terminal value may transform to something easier.

That is why adding or removing line is a legitimate system-design adjustment, not folklore. Measure the tuner-plane impedance across every intended band and, where a point lies outside the tuner’s load region or stress limit, change the line length deliberately. Re-measure current balance and pattern because the physical route and common-mode boundary may also have changed.

Choose the Tuner Plane Deliberately

A genuinely balanced tuner can connect directly to balanced line when its voltage, current and impedance ranges cover the transformed load and its enclosure and control connections do not create a dominant common-mode path. An unbalanced tuner requires a suitable transition whose ratio and common-mode behaviour are selected from the actual load, not from a universal balun recipe.

A remote tuner at the end of the open-wire line can keep the following coax near its intended characteristic impedance. That can reduce coax loss under mismatch and keep high standing-wave voltage and current out of a long coax run. It is not automatically the best arrangement: the remote tuner must survive the measured load, power, duty cycle, weather and transient environment and needs a controlled RF, DC and protective-earth installation.

An indoor tuner can be a valid choice when the balanced line reaches it safely and symmetrically. If coax is inserted between the high-SWR balanced section and the tuner, its loss must be calculated under the actual mismatch and the balanced-to-unbalanced transition must be measured. A low SWR at the transmitter does not remove loss or high voltage on the antenna side of the tuner.

Keysight’s VNA calibration guidance treats calibration as establishing a reference plane. Apply the same discipline here: identify the doublet terminals, line input, tuner output and tuner input as different planes. Save complex impedance or S₁₁, not only the final SWR shown by the radio.

Balanced Geometry Still Needs a Common-Mode Check

A centred doublet and symmetric open-wire line are a strong starting point for balanced current. The installation can still convert mode through unequal leg surroundings, feedline skew, mast coupling, tuner capacitance, control cables, station bonding or the operator. Roy Lewallen’s W7EL experiments demonstrate that feedline movement and asymmetry can change measured balance and that the feed interface materially affects the result.

Measure both conductors with the same calibrated current probe at corresponding positions. For the wanted differential mode, magnitudes should be similar and phases opposite. A clamp around both conductors responds primarily to their net current, but fixture symmetry, probe position, conductor spacing and nearby material still matter. Repeat after rain, rerouting, changing band and connecting station cables.

A current-balancing device may be appropriate at the tuner or another transition, but its common-mode impedance, wanted-mode insertion loss, voltage and temperature must cover the measured operating conditions. Adding a device because the word “balanced” appears in the antenna name is not a completed diagnosis.

Pattern Changes by Band and Geometry

On a band where the doublet is near a half-wave current distribution, the broadside pattern is a useful starting picture. Tilting both legs downward changes the current geometry and can fill some azimuth directions while changing elevation response. On higher bands, several current regions form along the wire, producing more lobes and nulls whose directions depend on the V angle, height, orientation and environment.

A flat top is not automatically a better NVIS antenna, and an inverted-V is not automatically a low-angle DX antenna. Electrical height, ground conductivity, terrain and surrounding structures control the realised elevation pattern. ITU-R BS.705 treats ground and site environment as practical pattern variables, while LLNL’s Numerical Electromagnetics Code can include wires, conducting surfaces, loads, networks, transmission lines and homogeneous ground.

Model the carbon mast, complete wire, feedline and nearby conductors when they carry meaningful current. Compare realised gain at the path elevation angles of interest rather than ranking the two shapes from their names.

Commission the Complete System

A disciplined installation record should include:

• leg length, sag, apex angle, end height, orientation and centre-hardware dimensions;
• the exact mast, guy, halyard, line and spacer materials;
• open-wire conductor diameter, spacing, route, crossings and wet/dry condition;
• complex impedance at antenna, line and tuner planes where accessible;
• tuner state, load range, component voltage/current and temperature;
• paired-conductor and net common-mode current measurements;
• feedline and transition loss under representative loads;
• converged current and realised-pattern models over defensible ground; and
• repeatable A/B/A field or received-signal comparisons for important bands and paths.

Only after those checks can the station claim useful band coverage, efficiency or pattern. A tuner finding a match is necessary for many configurations, but it is not the final measurement.

Keep High Voltage and Access in the Plan

Open-wire systems can develop high differential voltage at standing-wave maxima, tuner components and wire ends. Keep the line and radiator inaccessible during transmission, use insulation and spacing suited to the measured voltage and environment, provide strain relief, and de-energise before touching or moving any part.

Evaluate RF exposure for the actual frequency, power, duty cycle, current distribution, pattern and accessible area under the national rules that apply. ICNIRP’s RF guidelines cover 100 kHz to 300 GHz, but they do not replace local requirements or an installation-specific assessment.

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