80 m Lazy Loop: Why I Use a 4:1 UNUN and a Separate Choke

My practical case is the familiar full-wave horizontal loop, hung low, fed at a corner and used on several HF bands. Four sides near 21 m and a height near 10 m describe one 80 m installation, not a universal recipe. Shape, total electrical length, feedpoint, height, soil, conductor, supports and nearby objects all move the impedance and current distribution.

A Corner Feed Does Not Make the Loop Unbalanced

Cutting a feed gap in a closed conductor creates a two-terminal port. In an ideal symmetric environment, the terminal currents are equal in magnitude and opposite in direction. Feeding at a corner changes the current distribution and feedpoint impedance, but it does not by itself convert that port into an unbalanced antenna.

The installation can break the symmetry. One side may run nearer soil, a roof, tree, gutter, mast or house wiring. The feed line may leave parallel to one loop side. Unequal support insulation and wet foliage can add different capacitive and loss paths. At roughly 10 m above ground on 80 m, ground coupling can be important, but height alone does not predict the resulting balance, impedance or pattern.

The useful distinction is between geometry and modes:

• Differential antenna current flows into one loop terminal and returns through the other.
• Wanted coaxial current is equal and opposite on the centre conductor and the inner surface of the shield.
• Exterior common-mode current uses the outside of the shield plus some return through the antenna, mast, station wiring or environment.

A clamp around the complete coax measures the net enclosed current and is therefore useful for mapping the exterior mode. It does not, by itself, tell us the two loop-terminal currents or prove where the conversion occurred.

Choose the Ratio From R + jX

An ideal transformer with a 2:1 voltage ratio has a 4:1 impedance ratio. If the antenna terminal presents 200 + j80 Ω, the ideal low-impedance side sees 50 + j20 Ω—not a resistive 50 Ω. If the loop presents 110 − j35 Ω or 350 + j120 Ω, the same network produces a different and possibly less useful result.

A full-wave loop’s feedpoint impedance depends on shape, feed location, electrical length, height, ground and environment. On harmonic bands, the current distribution and impedance can change dramatically. Therefore, measure or model the installed complex load across every intended band before selecting 1:1, 2:1, 4:1 or another transformation.

Calibrate at the antenna-side plane or characterize and de-embed the fixture. A shack-end SWR includes feed-line transformation and attenuation; it cannot by itself identify the impedance needed at the loop.

What the Three Feed Options Actually Do

A Guanella 4:1 current balun commonly uses two nominal 1:1 transmission-line sections, parallel-connected at the low-impedance side and series-connected at the high-impedance side. The line sections can perform impedance conversion while their choking impedance resists the unwanted mode. That can be a valid combined function for a genuinely balanced installed load, but it does not also cover the unbalanced case merely because the ratio is 4:1.

An unun can be excellent at a defined impedance transformation. It does not force the loop’s two terminals to behave symmetrically relative to the environment, and it does not make exterior coax current disappear. Adding a 1:1 choke can isolate the coax farther along the path, but the combination must be measured as one installation. Transformer capacitance, enclosure, mounting and the short interconnect between networks can still couple common mode.

After two decades studying HF baluns and UNUNs, I no longer choose the default from the ideal loop drawing. Unequal height, soil, supports, wet foliage, nearby metal and feed-line routing usually disturb the balance. A current balun is then solving only the case we hoped we had. The UNUN-plus-choke pair covers the unbalanced installation we normally measure and still covers the rare balanced case when the choke is deliberately arranged to isolate the transformed port.

This does not make every UNUN-plus-choke assembly successful. The choke must isolate the relevant load-side coupling, present sufficient common-mode impedance and survive the differential voltage and current at its chosen location. The combined network must be tested as a hybrid when it is asked to feed a balanced load.

Choke Placement Follows the Current Map

There is no universal placement at 0.05λ, 0.10λ or a fixed number of metres. A choke at the feedpoint constrains the coax exterior immediately. A choke farther down the cable defines a segment that can still carry common-mode current. A choke near the station controls current entering that boundary but cannot remove radiation, coupling or loss from the cable section above it.

Place the choke at the boundary you intend to create, then map net coax current at several marked positions on every operating band. If the current maximum simply moves, the remaining return path is resonant or insufficiently controlled. Change one position at a time and repeat the first configuration afterward; without that A/B/A return, propagation or instrument drift can masquerade as an improvement.

Measure Differential and Common Modes Separately

A single-ended VNA measurement answers the match question at its calibration plane. It does not fully characterize balance or mode conversion. A calibrated three- or four-port setup can measure the two balanced terminals independently and convert the results to mixed-mode parameters.

For a single-ended-to-balanced device, useful quantities include differential transmission, common-mode transmission, differential return loss, common-mode return loss and conversion between modes. Keysight’s balanced-measurement guidance derives these from complex single-ended measurements; true-mode drive can additionally apply calibrated differential or common-mode excitation at the device plane.

For the installed antenna, combine that bench evidence with a current map:

• measure R + jX at the declared loop-feed plane across every intended band;
• measure the two loop-terminal currents with matched probes when practical;
• clamp around the complete coax at multiple marked positions;
• repeat after reversing the balanced terminals to expose fixture or winding asymmetry;
• repeat after one controlled feed-line-route or choke-position change; and
• restore the first configuration and confirm that the readings return within uncertainty.

Complex Loads Set the Voltage and Heat

A device that matches a 200 Ω resistor on the bench has not yet been qualified for a multiband loop. Reactive load, internal standing waves and common-mode excitation can increase core flux, winding current, insulation voltage and local heating. Guanella sections also need appropriate line impedance and close amplitude/phase tracking; ferrite permeability, loss and parasitic capacitance change with frequency, flux and temperature.

Run a load matrix based on measured loop impedances. At each frequency, record input and output power at calibrated planes, differential and common-mode response, core and winding temperature, connector temperature and drift. State waveform, duty cycle, accepted power, mismatch, ambient, enclosure, cooling and test duration. Do not infer a high-power rating from small-signal SWR or insertion loss alone.

Do not use touch as a thermometer. RF voltage can be hazardous at the loop feed and transformer terminals. Keep the assembly inaccessible while transmitting, use insulated sensors, isolate measuring equipment from transmitter power and discharge the network before service.

A Repeatable Lazy Loop Decision

• Describe the loop. Record total length, shape, corner geometry, feed gap, conductor, insulation, height, slope, supports, soil and nearby structures.
• Declare the bands. A transformation chosen for 80 m may be unsuitable on harmonic bands.
• Measure the installed complex load. Use a declared plane and uncertainty; do not select 4:1 from antenna type alone.
• Choose a topology. Decide whether one Guanella network or separate transformation and choking makes the required functions easier to meet and verify.
• Map the unwanted current. Measure along the coax before and after the chosen boundary.
• Stress the real loads. Verify loss, voltage, current and temperature at the intended power and duty cycle.
• Run A/B/A changes. Compare ratio, topology, cable route or choke placement one variable at a time, then return to the baseline.
• Judge the whole system. Match, balance, common-mode current, pattern, station RF, loss and thermal margin are separate acceptance results.

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