An 80 m-Long Doublet on a 16 m Pole With Open-Wire Feedline

The attractive part of this installation is practical: one 16 m support can raise the centre of a very long wire, and an open-wire feeder can tolerate standing waves with less loss than many small coaxial cables. That does not make two 40 m legs an automatic 160-to-10 m solution. I treat every stated dimension as the first build, then measure what the complete site has made.

Start With the Physical Geometry

The radiator begins as two nominal 40 m legs, about 80 m tip to tip. That span is close to a half wavelength in the 160 m amateur band before end effects, insulation, centre and end hardware, sag, apex angle, height, soil and nearby objects are included. Around 80 m it is closer to a full-wave wire, and on higher bands it supports progressively more current regions.

Those observations are useful for planning, but they do not fix an exact resonance. Cut the legs with adjustment available, record their installed geometry and sweep the complex feedpoint or tuner-plane impedance. A frequency at which the reactance crosses zero is not by itself proof of low loss, balanced current or a useful pattern.

A 16 m centre support makes an inverted-V practical where an 80 m flat-top span is not. The wire ends still need generous horizontal distance if the legs are not to hang steeply. Select the apex angle and end positions from the available footprint, modelled pattern, accessible-area clearances and the support's real side-load limit; there is no universal 120–140° optimum or 90° threshold.

Treat the Carbon Pole as Part of the Antenna

Carbon-fibre composite is not automatically transparent to RF. Manufacturer data for carbon fibre specifies finite electrical resistivity, while the behaviour of a finished pole also depends on fibre type, lay-up, resin, joints, wall construction, moisture and attached fittings. A long mast close to the feedpoint or feeder can therefore carry induced current, dissipate energy or disturb balance.

Keep the centre hardware and the two feeder conductors geometrically comparable with respect to the mast. Include the pole, metallic guys, halyards, clamps and other nearby conductors in the model when practical, then verify the installed result with current and impedance measurements. Moving the feedline while watching those measurements is often more informative than assigning the mast a generic label.

RF behaviour and structural capacity are separate questions. Check the exact support manufacturer's extension, guying, wind-area and lateral-load limits. The weight and wind load of 80 m of wire, the feedline, insulators, wet ropes, ice and hardware can create substantial bending moment even when the static assembly feels light.

Route Open Wire as a Transmission Line

The wanted mode on a balanced two-wire line has approximately equal currents in opposite directions. Its external fields largely cancel when both conductors see comparable surroundings. If one conductor lies closer to the carbon mast, a wall, gutter, tree, ground or metalwork, some differential current can be converted to common mode.

A symmetric vertical drop from the feedpoint is a sensible first route, but there is no mandatory three- or four-metre section. Keep conductor spacing constant, use gentle bends, keep both wires in similar surroundings and avoid coiling spare line. Where a crossing is unavoidable, crossing another conductor near a right angle usually reduces the length of close parallel coupling, but the installed measurement still decides whether it is acceptable.

Likewise, 10 or 15 cm is not a universal clearance. The required separation depends on line spacing, frequency, voltage, nearby material and how much impedance or balance change the installation can tolerate. Increase separation until moving the line or the nearby object no longer changes the measured result materially, then secure and document that route.

“600 Ω” is a nominal construction description, not a guarantee at the tuner. Characteristic impedance depends on conductor diameter, spacing and dielectric environment. Spacers, rain, ice, foliage and nearby surfaces can alter impedance, velocity factor, attenuation and balance.

The 16 m Feeder Transforms Every Band Differently

The open-wire feeder is not merely a low-loss extension cord. In a lossless transmission-line approximation, its input impedance is:

Z_L is the impedance at the doublet terminals, Z₀ the line characteristic impedance, β the phase constant and ℓ the line length. Real line adds attenuation and may be non-uniform. Sixteen physical metres therefore represent a different electrical length on every band and can transform a moderate antenna-terminal load into an extreme tuner voltage or current—or transform a difficult load into something easier.

If a band falls outside the tuner's documented load region or produces unacceptable loss, voltage, current or temperature, changing the line length is a legitimate system adjustment. It is not a promise that one “magic” length exists. Recheck the other bands and current balance after every change because the electrical transformation and the physical common-mode boundary have both moved.

Choose the Tuner Reference Plane Deliberately

A genuinely balanced tuner can feed the open wire directly when its impedance, voltage, current and thermal limits cover the transformed load. An unbalanced tuner needs an appropriate transition and a controlled common-mode path. The transformer's wanted-mode ratio and the choke's common-mode impedance are separate functions and should be selected from the measured installation rather than from a universal one-to-one recipe.

A remote tuner at the open-wire transition can keep a following coax run close to its intended impedance and may reduce coax loss under mismatch. It is not automatically best: it must survive the measured complex load, operating power and duty cycle, weather, control wiring and transient environment. An indoor tuner can also be valid when the balanced line reaches it safely and symmetrically and the transition is characterised.

Declare the reference plane for every result. The doublet terminals, open-wire input, tuner output, tuner input and transmitter are different planes. Calibrated complex impedance or S₁₁ at the relevant plane says far more than the final SWR at the radio; a successful match does not reveal feed-system loss or radiation efficiency.

Measure Balance Instead of Assuming It

A centred doublet and symmetric feeder are a strong starting point, not a guarantee. Unequal leg surroundings, feedline skew, mast coupling, tuner capacitance, control cables, station bonding and even the operator can create a common-mode current path.

Measure each conductor with the same calibrated current probe at corresponding positions. In the wanted differential mode, the magnitudes should be similar and the phases opposite. A probe around both feeder conductors responds mainly to their net current, although fixture symmetry, conductor spacing, position and nearby materials still affect the reading. Repeat the check after rerouting, rain, a band change and connection of the station cables.

If common-mode current is significant, trace the complete return path before adding hardware. A separately specified choke may be useful at a mode-conversion point, but its common-mode impedance, wanted-mode insertion loss, voltage and heating must be adequate at the actual frequencies and loads.

Expect the Pattern to Change Across HF

Near a half-wave current distribution, a broadside pattern is a reasonable free-space starting picture. Around a full wavelength and above, multiple current regions create additional lobes and nulls. The inverted-V angle, electrical height, orientation, ground conductivity, terrain, mast and nearby structures alter both azimuth and elevation response.

That is why this geometry cannot be labelled universally “DX”, “NVIS” or omnidirectional. Model realised gain at the elevation and azimuth angles that matter for each intended path, include defensible ground parameters and nearby conductors, and check the model against repeatable field or received-signal comparisons.

Build the Mechanics for the Real Loads

Use centre and end hardware with strain relief so conductor tension is not carried by electrical joints. Choose rope, insulators, pulleys, guys and anchors for UV, moisture, wind, ice and the expected service interval. Allow controlled sag and motion without letting the wire, feeder or rope abrade the pole or nearby structures.

Keep the radiator and its possible fall zone away from people, vehicles, buildings and overhead power lines. Do not infer a safe clearance from antenna dimensions. The UK Health and Safety Executive notes that flashover can occur without physical contact and directs work planners to the network operator; the local authority and line owner determine the applicable clearance.

Commission the Complete Installation

Before calling the system multiband, record:

• both leg lengths, conductor and insulation, sag, apex angle, end height and orientation;
• the mast, guys, halyards, anchors, centre hardware and their load limits;
• open-wire conductor diameter, spacing, spacer material, route, crossings and wet/dry condition;
• complex impedance at declared antenna, line and tuner reference planes;
• tuner state, match range, insertion loss, component voltage/current and temperature;
• paired-conductor current and net common-mode current on every intended band;
• transition and feedline loss under representative transformed loads;
• converged current and realised-pattern models with defensible ground; and
• repeatable A/B/A received-signal or field comparisons for the paths that matter.

The operating band set comes from those results. Open wire can keep mismatch loss usefully low, but it is not lossless, and a tuner match cannot make an arbitrary installation perfectly efficient.

Control Voltage, Access and RF Exposure

Standing waves can produce high differential voltage at the feeder, tuner components and wire ends even when transmitter SWR is low. Keep the antenna and line 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.

Assess RF exposure for the actual frequency, power, duty cycle, current distribution, pattern and accessible area under the national rules that apply. ICNIRP's RF guidance covers 100 kHz to 300 GHz, but it does not replace local requirements or a site-specific assessment.

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