Half-Square and Bobtail Curtain: Choosing the Feedpoint

A half-square or bobtail curtain can be fed at a current-rich, moderate-impedance point or through a tuned network at a high-impedance end. Neither method is inherently correct for every installation. The practical choice is the one that provides a defined two-terminal RF path, a suitable match, controlled feedline current and safe voltages without compromising the intended pattern.

Geometry and Current Distribution

A conventional half-square has two approximately quarter-wave vertical sections joined at their tops by an approximately half-wave horizontal phasing section. Its wire is open at both lower ends. On the fundamental mode, current is approximately zero at those open ends, reaches maxima near the two top corners and falls to a minimum near the middle of the top wire.

Half-square: approximate fundamental current magnitude

open end       corner        top midpoint       corner       open end
   0  ──────── maximum ───── minimum ──────── maximum ──────── 0
       vertical          horizontal wire          vertical

The two vertical sections carry currents that reinforce in the broadside directions. Their radiation is predominantly vertically polarised. Current on the horizontal section is part of the phasing system; its radiation tends to cancel in useful directions but does not vanish perfectly in a finite antenna above real ground.

A conventional bobtail curtain extends the arrangement to three approximately quarter-wave vertical sections. Adjacent verticals are about a half wavelength apart, so the total top span is about one wavelength. In the idealised fundamental mode, the vertical currents are in phase for broadside radiation and their approximate outer-to-centre-to-outer magnitudes are 1:2:1. The exact distribution changes with dimensions, conductor diameter, feed network, height, ground and nearby objects.

Practical Feedpoint Choices

Published models illustrate why no single impedance should be treated as a design constant. L. B. Cebik’s half-square studies show corner-feed resistance changing with geometry, height and ground. His bobtail examples likewise show a top feed below 50 Ω, a higher value partway down the centre vertical and a high, strongly reactive impedance at its bottom. Those values demonstrate trends; they are not substitute dimensions for another site.

Moderate-Impedance Feeds

A corner feed on a half-square is made by breaking the conductor at a top corner and connecting the two feed terminals across that gap. A top-centre bobtail feed similarly separates the centre vertical from the two top phasing sections. In either case, the feedpoint is a two-terminal network—not one wire connected to an undefined “ground.”

These current-rich positions often have lower voltage and lower impedance than an open lower end. That reduces the leverage of a few picofarads of accidental shunt capacitance, but it does not guarantee 50 Ω, zero reactance or independence from the surroundings. Measure the installed complex impedance and design any transformation for the operating bandwidth and power.

The coax route has no universal 45-degree prescription. Route the cable away from antenna conductors, supports and metalwork in a direction established by modelling and controlled trials. Keep that route fixed while comparing measurements. A changed SWR or pattern after moving the cable is evidence that the complete installation changed, not proof of one specific cause.

High-Impedance End Feeds

An open lower end is a current minimum and voltage maximum. Feeding there is a valid technique when a tuned network transforms the high complex impedance, provides an intentional RF return and uses components and insulation rated for the resulting voltage and current. Ground-level access can be a real maintenance advantage.

High impedance makes small admittances significant. At 14 MHz, the magnitude of the reactance of 1 pF is about 11.4 kΩ; for 5 pF it is about 2.27 kΩ:

Capacitance to wet supports, foliage, an enclosure, a mast or earth can therefore move the tuning of a high-impedance feed. Leakage and dielectric loss can add dissipation. A robust installation controls clearances, drainage, insulation material, mechanical strain and the position of every nearby conductor, then verifies performance in dry and wet conditions.

For a purely resistive input, a first estimate is V_rms = √(PR). A reactive feed and a high-Q tuned network can have higher circulating voltage and current than that simple port estimate suggests. Determine stress from the measured or modelled complex network, include component tolerances and mismatch, and apply a documented safety margin.

Feedline Current and Choking

Coax carries the intended differential current on the centre conductor and inner surface of the shield. Current on the shield exterior is a separate common-mode path involving the antenna, cable route, mast, station wiring and surroundings. That path can change the input impedance and radiation pattern.

A feedpoint choke is often useful, but “install any 1:1 choke” is not a complete specification. The choke presents a frequency-dependent complex common-mode impedance, dissipates some power and can develop significant RF voltage. Select its materials and winding topology from measured data across the intended band, then check installed exterior current and temperature at realistic power and duty cycle. A low SWR alone does not show that the feedline is decoupled.

Use a calibrated clamp-on RF current probe or another characterised method at repeatable cable positions. Compare the final feed arrangement with a controlled cable-route or choke change while holding frequency, power and geometry constant.

Ground, Height and Pattern

The ideal half-square and bobtail conductors form self-contained, open-ended antenna networks; they do not require a buried radial field to act as the missing half of a ground-mounted monopole. A particular bottom-feed circuit may nevertheless require an RF reference such as a counterpoise, radial system, earth connection or isolated coupling winding. That return path must be drawn and measured rather than left implicit.

Earth remains an electromagnetic boundary even when it is not the feed-current return. Conductivity and permittivity affect reflection and loss. Height changes the combination of direct and ground-reflected fields, so the elevation pattern, gain and secondary lobes can move substantially. Nearby conductors can disturb current distribution and symmetry.

These arrays are normally bidirectional broadside to the plane of their vertical elements, with a predominantly vertical component. “Low angle” is not automatic: the useful elevation angle depends on electrical height, ground and the installed environment. Raising the antenna can strengthen some low-angle directions while also creating additional higher-angle lobes.

Maximum-gain geometry, resonance and a convenient 50 Ω input are separate goals. Adjusting element lengths or spacing solely to improve SWR can trade away pattern performance. Model the installed antenna over representative ground, then verify complex impedance and field behaviour rather than selecting the design by minimum SWR.

Dimensions Are a Starting Point

Free-space wavelength provides the first scale:

These are wavelength scales, not cut lengths. End effects, wire diameter, insulation, sag, feed gaps, matching hardware, height and mutual coupling alter the required dimensions. Lower frequencies demand more space, but no fixed band range is inherently superior: the useful choice depends on available supports, desired azimuth and elevation pattern, bandwidth, losses and comparison with antennas that can be installed at a suitable height.

A Repeatable Design and Test Workflow

1. Define the objective. State frequency range, power and duty cycle, desired directions, acceptable elevation pattern, bandwidth and mechanical limits.
2. Draw the complete RF circuit. Include both feed terminals, matching network, intended return path, feedline, choke, mast, bonds and protective conductors.
3. Model the installed geometry. Use realistic wire dimensions, height, ground and nearby conductors. Inspect element currents and three-dimensional patterns, not only SWR.
4. Choose a feedpoint deliberately. Compare accessible voltage, current, complex impedance, environmental sensitivity and matching-network stress.
5. Measure at low power. Calibrate at, or accurately de-embed to, a declared reference plane and save complex impedance over the full band.
6. Control common mode. Fix the cable route, measure exterior current and qualify any choke over frequency and power.
7. Increase power in steps. Monitor matching components, insulation and choke temperature with suitable instruments while maintaining exclusion distances.
8. Verify the pattern. Use repeated, reciprocal field observations or calibrated measurements in several azimuth and elevation directions.
9. Repeat after weather changes. Compare dry and wet tuning, leakage, current and mechanical condition.

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