Half-Square Feedpoints: Corner Current Feed or End Voltage Feed
Half-Square Feedpoints: Corner Current Feed or End Voltage Feed
A half-square can be driven near a current maximum at a top corner or near a voltage maximum at an open lower end. Both can excite the intended structure, but they demand very different matching, insulation and return-path control.
I like the half-square because it puts two vertical current regions about half a wavelength apart and connects them with one wire. On its design band it can provide useful bidirectional, vertically polarised radiation without a conventional quarter-wave ground-plane layout. The important practical question is where we introduce the source.
For a straightforward monoband installation, I begin at a top corner. That is the comparatively low-impedance current-feed region. An open lower end is a valid high-voltage feedpoint, but it needs a matching network designed from the measured complex impedance—not a transformer ratio chosen by antenna name.
The Half-Square Is One Continuous Current System
The familiar starting geometry uses two approximately quarter-wave vertical sections joined by an approximately half-wave horizontal section. The wire is continuous except at the chosen feedpoint. Current magnitude is low at the open lower ends and high near the upper corners. The vertical sections carry the current that produces the principal vertical polarisation; the horizontal section establishes the spacing and phase relationship but is not electromagnetically invisible.
“Quarter wave” and “half wave” are starting descriptions. Wire diameter, insulation, bends, height, ground, supports and nearby conductors change the electrical length, current distribution, impedance and pattern. A finished antenna therefore needs a model or measurement of its actual geometry.
The classic broadside pattern is also conditional. On the design band, suitable geometry can produce two broad azimuth lobes normal to the plane of the wire. Height and ground alter the elevation pattern; harmonic operation can introduce more lobes. A low SWR does not prove that the desired pattern survived.
Corner Feed: Work Near the Current Maximum
At a top corner, open one connection between a vertical and the horizontal wire and place the source across that break. Cebik's model families put resonant corner-feed resistance roughly in the 60–80 Ω region for several declared 40 m and 80 m geometries, with reactance changing with dimensions and height. That is evidence for those models—not a universal 50 Ω promise.
This point is attractive because ordinary coax can often be matched with modest adjustment on one design band. It also avoids the extreme voltage and impedance of the open end. The two source terminals still have to be explicit: one drives the horizontal section and the other drives the adjacent vertical. If current escapes onto the coax exterior, mast or other conductors, those paths join the antenna and can change the pattern.
A characterised 1:1 current choke at the intended coax boundary is therefore a sensible starting tool, provided its common-mode impedance, differential loss and powered limits suit the installation. Route the feedline so it does not run closely beside a vertical current region, then measure exterior-shield current rather than assuming the routing or choke label solved it.
End Feed: Work at a High-Voltage Point
The bottom of either vertical is an open end and therefore a current minimum and voltage maximum in the intended fundamental mode. It can be fed, but the source impedance is high and complex. In Cebik's specific 3.6 MHz model, the design-frequency end feed was about 4100 − j4000 Ω while the corner feed was about 71 + j3 Ω. Those values illustrate the scale of the change; they are not portable specifications for another build.
A parallel-tuned or other suitable network can transform this high-impedance point. A broadband transformer may also be part of a design, but its nominal impedance ratio handles only a declared resistive relationship. It does not cancel arbitrary reactance, define the RF return, suppress common mode or prove low loss.
Ideal starting ratio: Zhigh / Zlow = n2
The expression is useful only after both port impedances and the reference plane are defined. A real transformer adds magnetising impedance, leakage, winding resistance, capacitance, core loss and voltage/current limits.
For example, a nominal 49:1 network maps 50 Ω to 2450 Ω only in the ideal ratio model. It may produce a usable match when the measured end impedance and reactance fall inside the transformer's demonstrated load domain. If the feedpoint is several kilohms with large reactance, moving automatically to 64:1, 81:1 or 100:1 is not a complete solution. Measure R + jX, design the network for that load and verify transfer loss and temperature under power.
The end-feed network and its return conductor also operate at elevated RF voltage. Provide spacing, insulation, weather sealing, strain relief and touch protection for the measured peak and RMS conditions. Protective earthing and lightning protection remain separate safety systems; neither should be improvised as an RF matching element.
The Top Centre Is Not a Second Corner
The midpoint of the horizontal wire lies between the two upper current regions. It should not be described as another simple near-50 Ω current feedpoint. Opening and feeding the wire there creates a different port, often in a high-impedance region for the intended mode.
A balanced line and tuner can drive that port if the complete system is deliberately designed and measured. That makes a valid tuned-wire installation; it does not make the centre electrically equivalent to the corner. State which point is used so that impedance and pattern claims remain reproducible.
Choose Geometry Before Choosing a Ratio
For a first monoband model, start with a horizontal section near 0.5λ and each vertical near 0.25λ. Treat these as electrical-length targets, not cut guarantees. Cebik also showed that changing the horizontal-to-vertical ratio can move a corner-fed model closer to 50 Ω while changing gain and elevation behaviour. Matching the feedpoint and optimising the pattern are not automatically the same optimisation.
| Decision | Corner current feed | Open-end voltage feed |
|---|---|---|
| Typical design-band region | Tens of ohms in many declared models | Several kilohms and often strongly reactive |
| Matching starting point | Direct coax or modest network after measurement | High-impedance tuned or transforming network from measured R + jX |
| Main stress | Feed current and common-mode boundary | High RF voltage, network loss and insulation |
| Return-path task | Keep coax exterior out of the intended antenna current | Define the network return and keep uncontrolled conductors out |
| Evidence needed | Complex impedance, exterior current and pattern | The same, plus network transfer loss, voltage and temperature |
Scaling a monoband antenna across 30–10 m is straightforward in wavelength terms, but each installed version still needs trimming and verification. Reusing one physical half-square on several harmonically related bands is a different problem: feed impedance and current distribution change, and the azimuth/elevation pattern can become multi-lobed. A tuner can present an acceptable load to the transmitter without restoring the fundamental-band pattern.
A Measurement Sequence That Preserves the Pattern
- Freeze the geometry. Record conductor dimensions, height, ground clearance, supports, feedline route and nearby metal.
- Measure at the chosen port. Save calibrated R + jX across the intended band before selecting a transformer or tuning network.
- Characterise the matching device separately. Use representative complex loads and record insertion loss, port voltage/current and temperature.
- Map unintended current. Measure coax-exterior, mast and bonding-conductor current at several positions with the same power and frequency.
- Check the radiating result. Use a controlled field-strength, beacon or receive A/B/A comparison in the intended directions; do not infer pattern from SWR.
- Repeat after weather and movement. High-impedance end feeds are especially sensitive to moisture, clearances and nearby objects.
A 49:1 can be plausible at an end feed, but it is not the definition of a half-square. The corner remains my clean monoband starting point because its impedance and voltage are usually easier to manage. Whichever feedpoint you choose, prove the complete current path, loss, stress and pattern in the installed antenna.
Primary and authoritative technical references
Mini-FAQ
- Where is the simplest monoband half-square feedpoint? A top corner is the usual starting point because it is a comparatively low-impedance current-feed region. Measure the installed complex impedance rather than assuming exactly 50 Ω.
- Can a half-square be fed at an open lower end? Yes. The lower end is a high-voltage, high-impedance port and needs a matching network designed for its measured complex impedance, plus suitable insulation and return-path control.
- Is a 49:1 transformer always correct for end feeding? No. Its ideal 2450 Ω-to-50 Ω relationship is only a starting ratio. Reactance, transformer loss, common mode, voltage and temperature must be measured for the actual load.
- Is the midpoint of the top wire another 50 Ω feedpoint? No. It is a different, often high-impedance port for the intended mode. A balanced line and tuner can drive it, but that system is not equivalent to corner feed.
- Does low SWR prove the half-square keeps its low-angle pattern? No. SWR describes the selected reference plane. Pattern depends on installed current, geometry, height, ground, feedline route and unintended conductors.
- Can one half-square be used on several bands? It can be matched on additional bands, but impedance and current distribution change and the pattern may become multi-lobed. Model and measure each intended band.