A 31 m Doublet on a 10 m Pole With Open-Wire Feedline
A 31 m Doublet on a 10 m Pole With Open-Wire Feedline
Two 15.5 m legs, a 10 m centre support and roughly 16 m of nominal 600-ohm open-wire line form a compact doublet system. The dimensions define the starting layout; the installed impedance, tuner stress, balance, loss and pattern decide what the station can actually use.
This layout is attractive because one centre support carries the radiator and the low-loss balanced line tolerates substantial mismatch better than many coaxial runs. That does not make the system self-balancing, omnidirectional or automatically efficient from 160 through 10 metres. The line and tuner are part of the antenna system.
The installation rule: preserve electrical symmetry where the site permits, route the two-wire line in a clear and consistent environment, and measure the complex load at the tuner plane. A tidy drawing does not guarantee equal conductor currents once the mast, ground, house and station cables are included.
Start With the Dimensions, Then Measure the Antenna
The nominal radiator is 31 m tip to tip: two approximately 15.5 m conductors joined at the centre insulator. A 10 m support naturally produces an Inverted-V when the ends descend toward separate anchors. Those numbers are construction inputs, not resonance or coverage specifications.
Insulation, conductor diameter, centre and end height, apex angle, soil, nearby metal and the electrical connection to the open-wire line all change the input impedance. On different bands, the radiator may be electrically short, near a resonance or several half-waves long. The tuner therefore sees a frequency-dependent complex load transformed by the feedline.
Let the Inverted-V Fit the Site
An open apex and reasonably similar leg environments are useful starting conditions. There is no universal 120-degree optimum and no hard 90-degree efficiency cliff. Pulling the ends closer changes coupling between the legs, current distribution, feedpoint impedance and pattern; lowering either end also changes ground coupling and accessible RF voltage.
Choose end positions from available span, safe touch clearance, pattern goals and mechanical load. Keep each end beyond casual reach, use UV-resistant non-conductive rope and provide strain relief that does not transfer conductor tension into the feedline connection. Do not place anchors or conductors where a failure can reach an overhead line or occupied path.
The 10 m Carbon Support Is Part of the RF Environment
A carbon-fibre mast is not electrically invisible. Fibre orientation, resin, joints, moisture and construction produce a frequency-dependent lossy conductor that can carry induced current. Separation between the open-wire line and mast, and the route used below the feedpoint, can alter balance, impedance, loss and pattern.
Use the mast only within its manufacturer's guying, wind, compression and section-overlap limits. Measure the assembled system with its actual mast and support hardware. If a feedline-current or pattern change appears when the line route moves relative to the mast, treat that as coupling evidence rather than a tuning inconvenience.
Open-Wire Line Transforms the Load
“600 ohms” is a nominal characteristic impedance. Conductor diameter and spacing, spacer dielectric, moisture, bends and nearby material determine the installed value. The 16 m line also has an electrical length set by frequency and propagation velocity.
Lossless starting equation:
Zin = Z0 · (ZL + jZ0 tan βl) / (Z0 + jZL tan βl)
ZL is the doublet-terminal load, Z0 the line impedance, β the phase constant and l the line length. Real line loss and imbalance require the lossy-line model and measured installation.
A line length that gives a comfortable tuner load on one band can transform another band toward high voltage, high current or the edge of the tuner's range. Do not trim the feedline merely to obtain a pretty SWR at the transmitter. Record complex impedance and tuner state across every intended operating window.
Choose the Tuner Plane Deliberately
A remote balanced tuner at the end of the open-wire line can keep any following coax close to its design impedance. That can reduce coax mismatch loss, but the tuner must survive the measured complex load, power, duty cycle, weather and transient environment. It also needs a safe and controlled RF, DC and protective-earth installation.
An indoor tuner is valid when the balanced line can reach it with safe clearance and reasonably symmetric surroundings. If coax is inserted between the mismatched balanced line and tuner, calculate its loss under the actual load. A low SWR at the transmitter does not erase loss or high standing-wave voltage on the antenna side.
Keep the doublet terminals, open-wire-line input, tuner output and transmitter input as separate reference planes. Calibrate or de-embed to the plane whose impedance you are discussing and save resistance and reactance, not only SWR.
Balanced Geometry Still Needs Current Verification
A centred doublet encourages equal and opposite differential currents, but unequal leg surroundings, a carbon mast, asymmetric line routing, tuner capacitance and attached cables can convert some energy into common mode. A matching network can transform impedance without fixing that current division.
Where the system becomes unbalanced, use a current-balancing device whose common-mode impedance, wanted-mode insertion loss, voltage, current and temperature cover the measured conditions. Map current on both line conductors and on the exterior of any coax or control cable before and after the change.
Pattern and Band Coverage Come From the Commissioned Geometry
The 31 m doublet is a multiband candidate, not a promise of efficient 160–10 m operation. On a band where it is electrically short, tuner loss and high current or voltage may dominate. On higher bands, several current maxima create additional lobes and nulls. Ground, height and surroundings modify every pattern.
A flat top is not automatically the better NVIS antenna, and an Inverted-V is not automatically omnidirectional. Model the complete installed geometry with defensible ground parameters, then verify important paths at equal accepted power using simultaneous receivers or rapid A/B/B/A switching.
Commission the Whole System
- Record geometry: measure both legs, centre and end heights, apex angle, line spacing, line route, mast separation, anchors and nearby conductors.
- Sweep the antenna plane: capture complex impedance across each intended band with the installed support and return environment.
- Measure the tuner plane: include the complete 16 m line and identify loads near tuner voltage, current and range limits.
- Account for loss: distinguish open-wire, tuner, transition, balun, connector and any coax loss at declared planes.
- Map common mode: compare current on both line conductors and exterior current on coax, control and power cables.
- Check heat and voltage: test representative power and duty cycle while watching tuner, transition, line and connection temperature.
- Verify the field: compare wanted-signal field or SNR with equal accepted power, fixed receiver settings and a restored baseline.
Safety: open-wire line and doublet ends can carry high RF voltage. Maintain clearances from people, combustible material, wiring, structures and overhead lines; prevent climbing or accidental contact; de-energise before adjustment; assess RF exposure; and design guying and anchors for the actual wind and failure envelope.
Bottom line: a 31 m doublet on a 10 m support with about 16 m of well-routed open wire is a useful compact multiband starting system. Its real usable bands, tuner stress, loss, balance and patterns belong to the installed measurements—not to the dimensions alone.
Primary technical references
- IEEE Std 145-2025 — Standard for Definitions of Terms for Antennas
- IEEE Std 149-2021 — Recommended Practice for Antenna Measurements
- Recommendation ITU-R BS.705-2 — HF Antenna Characteristics and Diagrams
- Lawrence Livermore National Laboratory — Numerical Electromagnetics Code
- Keysight — Network Analyzer Basics
- Roy Lewallen, W7EL — Baluns: What They Do and How They Do It
- UK Health and Safety Executive — Working Safely Near Overhead Electric Power Lines
- ICNIRP — Guidelines for Limiting Exposure to Electromagnetic Fields
Mini-FAQ
- Is 31 m a resonant all-band length? No. It is a practical doublet length. The installed antenna and line present a different complex load on every band.
- Must the Inverted-V apex be exactly 120 degrees? No. Apex angle changes impedance, coupling and pattern, but there is no universal optimum or hard efficiency boundary independent of the site.
- Is nominal 600-ohm open wire always 600 ohms? No. Conductor diameter, spacing, spacers, moisture and nearby materials determine the installed characteristic impedance and balance.
- Is a remote tuner always the best choice? No. It can keep following coax near its design impedance, but total loss, range, voltage, current, weather, power and maintenance decide the result.
- Does a centred doublet guarantee no common mode? No. Unequal surroundings, mast coupling, line routing, tuner capacitance and connected cables can convert part of the differential current.
- Can this installation cover 160 through 10 metres efficiently? Not by dimension alone. Confirm tuner range and loss, current balance, component stress and installed pattern separately on every intended band.