Halo and Small-Loop Transmit Antennas on HF
Halo and Small-Loop Transmit Antennas on HF
A halo and a small transmitting loop can both fit where a straight dipole cannot. Their similar outline hides different electrical sizes, current distributions, patterns and loss mechanisms. Choose between them from the installed measurements—not a generic efficiency percentage.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
When someone asks me whether a halo or a small transmitting loop “works” on HF, I do not start with a promise. Both can radiate and make contacts. The useful question is whether the installed antenna produces the required pattern and link performance with acceptable loss, bandwidth, tuning effort, voltage, current, temperature, coupling and exposure risk.
My short version: classify the antenna by electrical circumference and measured current distribution. A halo is normally a bent, approximately half-wave dipole with a gap. A small transmitting loop has a circumference much smaller than a wavelength and is tuned as a high-current resonator. Their outlines may be circular, but they are not interchangeable circuits or radiators.
The Outline Does Not Define the Antenna
A ring of conductor can support many electromagnetic modes. The operating frequency, circumference, gap, conductor diameter, tuning and matching parts, feed, mounting plane and surroundings determine which mode is excited. Calling every circular conductor a “loop” erases the distinction that matters most.
| Engineering boundary | Halo | Electrically small transmitting loop |
|---|---|---|
| Electrical circumference | Approximately one half wavelength as a first geometric description; the feed gap, conductor size and coupling shift the actual resonant length | Much less than one wavelength while the small-loop approximation is intended to apply |
| Current distribution | Non-uniform, broadly related to a bent dipole; it must go to zero at the open ends | Approximately uniform in magnitude and phase only while the loop is electrically small and symmetrically excited |
| Primary tuning issue | Conductor circumference, gap/end capacitance, loading if used, and feed/matching geometry | Loop inductance resonated by a low-loss capacitor, plus coupling or impedance transformation |
| Loss sensitivity | Conductor, joints, loading and matching loss matter; bending and shortening can reduce radiation resistance and raise circulating stress | Small radiation resistance competes directly with conductor, joint, capacitor and matching resistance |
| Pattern model | Needs the full current distribution, mounting plane, feed, height, ground and nearby conductors | Approaches the magnetic-dipole pattern only when current is nearly uniform; larger electrical size changes the pattern |
Do not confuse either antenna with a resonant full-wave loop. Once a loop circumference is no longer electrically small, current phase varies around it and the magnetic-dipole approximation no longer predicts its impedance or pattern. A full-wave loop, a half-wave halo and a small tuned loop may all look like rings while behaving very differently.
A Halo Is a Bent Dipole, Not a Small Magnetic Loop
The practical halo begins with roughly half a wavelength of conductor bent into a circle or rounded shape, leaving a small gap between its high-voltage ends. The bend brings sections of conductor and the end fields close together, so the actual resonant circumference and feed impedance depend on conductor diameter, gap, feed arrangement and surroundings. “Half wavelength” is a starting model, not a cut-length guarantee.
The size remains substantial on HF. Using free-space wavelength and C ≈ λ/2 only as a geometric first estimate gives:
| Example frequency | Approximate half-wave circumference | Approximate circular diameter |
|---|---|---|
| 3.65 MHz | 41 m | 13.1 m |
| 14.2 MHz | 10.6 m | 3.4 m |
| 28.5 MHz | 5.3 m | 1.7 m |
Those figures explain why a physically complete halo becomes awkward on the lower bands. They do not predict resonance, efficiency or match. Loading or tighter folding can reduce physical size, but the complete current distribution, radiation resistance, loading loss, stored energy and component stress then have to be recalculated or measured.
A halo mounted in a horizontal plane is normally intended to provide horizontal polarization near the horizon and broad azimuth coverage. “Broad” is not “perfectly omnidirectional.” The gap, feed network, support, feed-line exterior current, height, ground and nearby structure can produce azimuth ripple, pattern tilt, mixed polarization and loss. Turn the same conductor into a vertical plane and its polarization and directional cuts change. Always state the mounting plane and observation direction.
A Small Transmitting Loop Is a High-Current Resonator
An electrically small loop is described by its area and by a current that is approximately uniform around the conductor. NBS calibration work shows how quickly symmetry, stray capacitance and current distribution become part of the measurement; it treated a circumference no greater than about one sixteenth wavelength as essentially uniform for its calibrated-loop context. Amateur transmitting designs can be larger, but the further C/λ grows, the less safely the ideal small-loop equations describe them.
For the ideal small-loop model, radiation resistance decreases very rapidly as loop area becomes small compared with λ². The accepted power then sees radiation resistance in series with real losses from the conductor’s RF resistance, joints, tuning capacitor, matching/coupling structure and any nearby lossy material. The useful first-order efficiency relation is:
ηrad = Prad/Paccepted ≈ Rrad/(Rrad + Rloss)
That series-resistance expression is only as good as its declared current reference and loss model. A DC ohmmeter will miss skin effect, proximity effect, contact resistance under RF current, capacitor equivalent series resistance and environment loss. Calculate to choose a plausible geometry; measure the assembled antenna to claim a result.
At resonance, the tuning capacitor cancels the loop’s inductive reactance at the feed reference plane. Large circulating current can exist in the loop and large RF voltage can exist across the capacitor even when transmitter power is modest. Conductor diameter, joints, capacitor construction, plate spacing, weather, support dielectric, matching loop and feed balance are therefore not minor construction details. They set loss, detuning, arcing and thermal limits.
Pattern and Polarization Follow the Plane
The ideal electrically small loop has a magnetic-dipole far-field pattern. Its field is maximum in the plane of the loop and has nulls along the loop axis, normal to that plane. The familiar “doughnut” is centred on the loop axis. That three-dimensional statement is safer than calling the antenna omnidirectional.
For a loop mounted in a vertical plane, the loop axis is horizontal. The ideal azimuth pattern near the horizon therefore has two deep nulls broadside to the loop—normal to its plane—and maxima in directions lying in the plane. In those low-elevation in-plane directions, the electric field is normally vertical. A vertical-plane small loop is not an omnidirectional horizontally polarized shortcut.
For a loop mounted horizontally, the axis is vertical. The ideal horizon cut is then broad in azimuth and horizontally polarized, while the axial null points toward zenith and nadir. At other elevation and azimuth angles, describe polarization as a vector rather than assigning one word to the entire three-dimensional field.
Real null depth is installation-dependent. Non-uniform current, asymmetrical coupling, feed-line common mode, a conductive support, ground, building metal and nearby wiring can fill, rotate or tilt the nulls. A larger electrical loop develops a different current phase distribution and can acquire additional lobes. Pattern and polarization require at least the relevant azimuth and elevation cuts; one field-strength point is not enough.
Radiation Resistance, Loss and Match Are Separate
Radiation resistance is an input-equivalent resistance tied to a chosen current reference. It represents radiated power; it is not the same as conductor loss, and it is not the same as the resistance shown by a VNA when matching, feed-line transformation and environmental loss are present.
For both antennas, measure complex input impedance R + jX at a declared plane. De-embed or characterize the feed line. A low SWR tells us that the input is close to the analyzer’s reference impedance at that plane. It cannot separate radiation resistance from conductor, capacitor, matching, ground or feed-line loss.
The halo normally avoids the extreme small-area radiation-resistance penalty of an ideal tiny loop because its conductor is electrically longer. That does not guarantee higher installed efficiency. A lossy loaded halo can underperform a carefully built loop, while a high-current loop with poor joints or capacitor loss can turn accepted power into heat. Compare complete specimens at the same accepted power, frequency, installation and measurement uncertainty.
Loaded Q Explains Bandwidth Only with a Declared Metric
Near a single well-behaved resonance, loaded quality factor QL relates stored energy to total power removed by radiation, dissipation and external coupling. A rough inverse relation between fractional bandwidth and QL is useful only after stating the response and limit: SWR, return loss, impedance, realized gain, efficiency, pattern or something else.
An efficient electrically small loop can be narrow because the stored reactive energy is large relative to radiated power. Added resistance lowers Q and can broaden the SWR curve, but that apparent bandwidth may have been bought with dissipation. Stronger external coupling and a different matching network also change the loaded response. A broad match is therefore not independent evidence of high efficiency.
Do not transfer one loop’s bandwidth to every band or installation. Record the capacitor setting, frequency, complex impedance, accepted power, realized-gain or efficiency criterion, temperature and surroundings. For a mechanically tuned loop, retuning frequency and repeatability are operational requirements, not footnotes.
Voltage, Current and Heat Define the Power Limit
The small transmitting loop concentrates current in its conductor and joints and voltage across its tuning capacitor. A halo concentrates electric field near the open gap and can create high current or voltage in loading and matching parts. In both cases, a good shack SWR says little about peak internal stress.
A defensible power envelope declares waveform, PEP and average power, duty cycle, transmission duration, ambient temperature, wind or enclosure, wetness and tuning state. Monitor loop or halo conductor, joints, capacitor, matching parts, connector and feed-line choke for temperature rise and impedance drift. Stop on arcing, corona, unstable SWR, unexpected heating or permanent detuning.
Safety boundary: never touch or manually retune an energized transmitting loop. Guard exposed conductors and capacitor terminals, use remote tuning or interlocks where appropriate, maintain electrical and mechanical clearance, and evaluate RF exposure for the actual frequency, power, duty cycle, pattern, distance and accessible area under the rules that apply at the site. Compact does not mean low field.
Installed Coupling Can Dominate the Comparison
Walls, reinforced concrete, roofs, vehicles, gutters, masts, solar wiring, station bonds and soil can detune either antenna, absorb power or become coupled conductors. A balcony halo and an indoor loop are not being compared only as topologies; they are being compared as two different installed electromagnetic systems.
Feed-line exterior current deserves its own test. Use a characterized RF current probe around the complete cable at several marked positions. Change routing or add a characterized common-mode choke in an A/B/A sequence without moving the antenna. If input impedance, current map or pattern changes beyond uncertainty, include the cable and mounting structure in the model and in the published installation description.
Ground also changes pattern and loss. The ideal free-space magnetic-dipole pattern is a reference, not a backyard guarantee. Declare height, plane, azimuth, ground model or measured properties and nearby geometry when comparing results.
Decide with a Measurement Workflow
- Define the job: band segment, mode, accepted power, duty cycle, required directions, polarization, bandwidth, tuning speed, available volume and accessible-person boundary.
- Record geometry: circumference, conductor cross-section and material, gap, capacitor and match, supports, mounting plane, height, feed line, common-mode control and surrounding conductors.
-
Measure complex impedance: calibrate at the antenna feed or de-embed the line, then record
R + jX, not only SWR. Repeat after the antenna reaches thermal equilibrium. - Check current and balance: measure feed-line exterior current at several positions and, where practical, compare conductor-current distribution with a calibrated probe or validated full-geometry model.
- Establish loss and efficiency: use a complete gain/directivity, Wheeler-cap, reverberation-chamber, calorimetric or other validated method appropriate to the specimen. Include mismatch only when reporting total efficiency and state the reference plane.
- Measure pattern and polarization: use a suitable range or validated near-field transformation, take the required azimuth and elevation cuts, and report test-site uncertainty. Rotate the antenna and restore the baseline to separate pattern from propagation variation.
- Run thermal trials: increase average power in controlled steps while recording current, capacitor and joint temperature, impedance drift and evidence of discharge. Repeat at the intended waveform and duty cycle.
- Compare fairly: place both candidates at the same site and height where possible, use the same accepted power and receiver/transmitter conditions, and evaluate the link directions and operating time that matter.
Efficiency is not the only decision variable. A narrower, manually tuned loop may still be the practical answer where only a small rotating frame fits and directional nulls help. A halo may be attractive where its larger circumference fits and broad horizontal-plane coverage is the goal. Neither earns the word “better” until the objective, installation and measurements are declared.
The Practical Conclusion
Halos and small transmitting loops do work on HF. That sentence is true but incomplete. The halo asks whether an approximately half-wave bent radiator, its gap, feed and mounting can fit and remain acceptably low-loss. The small loop asks whether enough radiation resistance can be obtained relative to conductor and capacitor loss while managing high circulating current, capacitor voltage, tuning and narrow response.
So I will not publish one efficiency number for either family. Give me circumference in wavelengths, current distribution, complex impedance, calibrated efficiency or gain evidence, pattern cuts, temperature and the installed geometry. Then the compact-antenna comparison becomes engineering instead of folklore.
Primary and authoritative technical sources
- NBS Technical Note 370: Calibration Principles and Procedures for Field Strength Meters—small-loop far-field pattern, mounting-plane polarization, symmetry, balance, electrical size and current-uniformity boundaries.
- NIST Technical Note 1506: Electromagnetic Theory of Reverberation Chambers—small-loop receiving function, angular dependence and radiation resistance under the ideal electrically small model.
- NBS/NIST: Effect of Antenna Size on Gain, Bandwidth, and Efficiency—fundamental size, stored-energy, gain, bandwidth and loss boundaries.
- Harold A. Wheeler: Fundamental Limitations of Small Antennas and L. J. Chu: Physical Limitations of Omni-Directional Antennas—primary small-antenna size and Q foundations.
- IEEE 145-2025—current definitions for impedance, radiation resistance, efficiency, gain, pattern, polarization and antenna Q.
- IEEE 149-2021 and NIST reverberation-chamber efficiency methods—gain, pattern, efficiency, test-site and uncertainty measurement practice.
- ICNIRP 2020 radiofrequency exposure guidelines—frequency-dependent exposure quantities and assessment boundaries; the legally applicable local rules still control each installation.
Mini-FAQ
- Is a halo just another small transmitting loop? No. A halo is normally an open, approximately half-wave bent dipole with non-uniform current; a small transmitting loop is much smaller electrically and uses an approximately uniform high circulating current.
- Is a vertical-plane small loop horizontally polarized and omnidirectional? No. Near the horizon, the ideal loop has deep azimuth nulls normal to its plane, maxima in its plane and normally vertical electric-field polarization in those maximum directions.
- Can SWR bandwidth tell me loop efficiency? Not by itself. Loss can broaden the impedance response. Efficiency requires radiated power relative to accepted power at a declared reference plane.
- Why can a small loop have high voltage and current? Resonance stores and exchanges reactive energy. The conductor can carry large circulating current while the tuning capacitor supports large RF voltage, even at modest transmitter power.
- Which is better from 80 to 10 metres? It depends on electrical circumference, available space, band coverage, pattern, polarization, loss, tuning, power, safety and installation. Compare measured specimens against the same objective.
- What measurements should I require? Require complex feed impedance, feed-line current, calibrated efficiency or gain evidence, azimuth/elevation pattern and polarization, temperature and stress tests, plus the complete installed geometry and uncertainty.