1. Executive Summary

This guide presents a comprehensive technical analysis of integrating multiband horizontal loop antennas with the Collins 180S-1 Antenna Tuner using the Balun Designs Model 4114 4:1 current balun at the tuner output. The analysis covers impedance transformation theory, common-mode current suppression strategies, and practical installation guidelines for amateur operators seeking optimised multiband HF performance with minimal station RFI.^[1][2][3]

The combination of a properly configured multiband loop, the Collins 180S-1, a Model 4114 4:1 current balun at the tuner output, and a Model 1115d common-mode choke provides exceptional multiband coverage from 160 through 10 metres with a stable, repeatable noise floor and improved pattern predictability across all bands.

2. Introduction

Multiband horizontal loop antennas have been a reliable, cost-effective HF solution for amateur operators for decades. Fed with balanced transmission line through a high-quality antenna tuner, these antennas provide consistent performance across multiple amateur bands from a single wire structure. However, achieving optimal results requires careful attention to impedance transformation, common-mode current suppression, and proper integration of balun and choke components.^[4][5][6]

The Collins 180S-1, originally produced as a companion to the S-Line amateur transceiver family, remains a highly capable matching network for modern amateur installations. When properly configured with external current baluns and common-mode chokes, the 180S-1 can drive multiband loop antennas at continuous power levels up to 1 kW with exceptional efficiency and minimal station RFI.^[7]

3. Collins 180S-1 Antenna Tuner

Design and Configuration

The Collins 180S-1 is a high-power antenna coupler designed as a companion to the S-Line and KWM-2/2A station equipment.^[7] The unit employs a pi-network matching configuration — used in most cases as an L-network, with the full pi circuit engaged for more demanding antenna impedances. A variable vacuum capacitor in the output circuit can be switched between series and shunt connection with the antenna, extending the tuner’s matching range considerably.

Output Configuration for Balanced Lines

The 180S-1 provides an unbalanced output through a standard coaxial connector. When driving balanced transmission lines such as ladder line or open-wire feeders, an external current balun is required at the tuner output to transform the unbalanced output to a balanced configuration, provide high common-mode choking impedance, prevent feedline radiation, and maintain proper current balance on the balanced line.^[1][9]

The 180S-1 does not include integrated baluns suitable for high-power, high-SWR multiband operation with ladder line. External baluns are therefore mandatory for this class of operation.

4. Multiband Horizontal Loop Antenna Theory

Loop Geometry and Impedance Characteristics

A horizontal loop antenna exhibits complex impedance behaviour that varies significantly across the HF spectrum. Fundamental resonance occurs when the loop perimeter equals one wavelength at the operating frequency. At this frequency, feedpoint impedance is typically 50–150 ohms resistive — the precise value depending on loop geometry, height, and feed-point location (corner vs. centre-of-side).^[10][11]

As operating frequency increases, the loop operates as a multiple-wavelength structure and feedpoint impedance varies dramatically. Table 2 presents representative impedance values for an 80-metre full-wave loop (approximately 84 metres total wire) operated across the amateur HF bands. Individual installations will differ based on height, geometry, and soil conductivity.

Multi-Wavelength Operation and Radiation Patterns

On bands above the fundamental, the loop supports multiple current maxima and voltage nodes around its perimeter. The radiation patterns evolve progressively:^[11][12]

• 80m (1λ): Four-lobe broadside pattern through the corners; 1–2 dB gain over a dipole. Useful for NVIS.
• 40m (2λ): Eight-lobe pattern; lower take-off angle; 2–3 dB advantage over dipole.
• 20m (4λ): Multiple lobes; excellent DX characteristics when mounted at quarter-wavelength height or above.
• 15m and 10m (6λ and 8λ): Highly directional multi-lobe patterns; very low take-off angles — highly competitive for DX.

The key practical advantage of horizontal loops for multiband operation is the progressive reduction in radiation angle as frequency increases, providing NVIS coverage on the lower bands and low-angle DX performance on the upper bands from a single, fixed installation.^[11][12]

Optimal height for an 80m loop is approximately one quarter-wavelength at the fundamental frequency — roughly 20 metres (66 feet). Practical installations at 4–6 metres (12–20 feet) still provide excellent performance, particularly on 20m and above.

5. Balun Designs Model 4114 — Technical Analysis

Design Architecture and Heritage

The Balun Designs Model 4114 is a 4:1 current balun engineered for high-power multiband antenna applications with elevated SWR conditions. The design originates from the Guanella current-balun topology, subsequently refined by Dr. Jerry Sevick (W2FMI, SK).^[2]

By using Thermaleze wire — a polyamide-imide insulated conductor — inserted in Teflon tubes, Sevick’s refinements substantially increased voltage breakdown between conductors, enabling reliable high-SWR operation at sustained power levels.^[2][13]

Current Balun Topology and Why It Matters

Current baluns maintain equal and opposite currents in the balanced line conductors regardless of load impedance. This is critical for multiband antenna systems where impedance varies widely across operating frequencies. A voltage balun, by contrast, attempts to maintain equal voltages — a much less reliable approach when the load is not symmetrical or not resonant.^[1][2][14]

The 4:1 transformation ratio is well-matched to multiband loops, where feedpoint impedances typically span 100–600 Ω across the operating bands. Dividing by four presents the 180S-1 with impedances in the 25–150 Ω range — well within the tuner’s comfortable operating envelope.^[1][10]

High-SWR Performance

The Model 4114 is explicitly designed for elevated-SWR operation, making it ideal for non-resonant multiband antennas. Key design features enabling this include low-permeability ferrite cores that resist saturation under high circulating currents, heavy-gauge Thermaleze conductors with high-temperature high-voltage insulation, and the sealed weatherproof enclosure allowing outdoor mounting directly at the tuner-to-ladder-line transition.^[2][13]

Balun Designs specifies the 4114 for continuous operation at 1,000–1,500 W into multiband antennas with elevated SWR — a rating that matches the Collins 180S-1’s design power level.

6. System Integration: Loop + 180S-1 + Model 4114

Signal Path

The recommended architecture places the Model 4114 current balun immediately at the Collins 180S-1 output, with a short high-quality coaxial jumper between tuner and balun. This minimises losses and voltage stress in the interconnecting coax running under high-SWR conditions.

Impedance Transformation Example

Consider an 80-metre horizontal loop operating on 20 metres. The feedpoint impedance might measure approximately 215 + j180 Ω at the antenna. After 15 metres of 450-Ω ladder line, the impedance at the shack end could transform to something in the region of 300–350 Ω complex.

Without the 4:1 balun, the 180S-1 would need to match this impedance directly — placing significant stress on tuner components. With the Model 4114, the impedance presented to the tuner is approximately 75–90 Ω complex — comfortably within the tuner’s optimal matching range with low component stress and efficient power transfer.^[1][2][10]

Coaxial Interconnection Best Practices

• Use highest-quality low-loss coax — LMR-400, Belden 9913, or equivalent
• Keep length under 3 metres (10 feet); 1–2 metres is ideal
• Use properly installed PL-259 connectors with full-coverage solder joints
• Ensure adequate voltage standoff from nearby grounded metal surfaces
• Route away from control lines and station wiring to minimise coupling^[1][9]

Ladder Line Selection and Routing

• 450-Ω or 600-Ω windowed ladder line preferred
• Route with minimum proximity to metal structures, wiring, and building materials
• Maintain at least several conductor-spacings clearance from grounded objects
• Twist 180° every 3–5 metres when running alongside any conductive surface
• Use standoff insulators at building entry; seal weatherproof at both ends^[1][9][15]

7. HF Antennas Suited to This Configuration

80-Metre Full-Wave Loop (Primary Application)

Configuration: Square or rectangular loop, approximately 84 metres (275 feet) total wire, fed at a corner or centre of one side with 450-Ω ladder line.

• Fundamental resonance on 80m with ~100–150 Ω resistive feedpoint impedance
• Multiband coverage on 80m, 40m, 20m, 15m, and 10m
• Four-lobe broadside pattern on 80m transitions to low-angle multi-lobe pattern on higher bands
• Typical gain: 1–2 dBd on 80m, increasing to 3–4 dBd on 20m and above^[11][12]

The 4:1 balun is particularly beneficial here: the 80m loop presents 150–400 Ω on most useful bands, and the transformation brings these into the 40–100 Ω range where the 180S-1 tunes easily with low component stress.^[2][10]

40-Metre Full-Wave Loop

Configuration: Square or rectangular, approximately 42 metres (138 feet) total wire. Covers 40m, 20m, 15m, and 10m (1λ, 2λ, 3λ, 4λ respectively). More compact than the 80m loop; excellent DX characteristics on 20m and above. Will not operate effectively on 80m.^[12][16]

Delta Loop (80m or 160m)

Configuration: Triangular loop with apex at the highest support point, fed at one lower corner. Three support points simplify installation on some properties. Corner-fed delta loops tend toward slightly higher feedpoint impedances than centre-fed rectangular loops — the 4:1 ratio accommodates this effectively. The delta configuration may provide a more omnidirectional pattern on higher bands compared with square or rectangular loops.^[12]

Large Multiband Doublet (Centre or Off-Centre Fed)

Configuration: Approximately 40–41 metres (130–135 feet) of wire, fed at centre or approximately one-third from one end. Long doublets present very high impedances — often 500–2,000 Ω on some bands when operated as multiple half-wavelengths. The 4:1 transformation brings these into a range the 180S-1 can accommodate. Doublets require only two supports, making them the simplest installation option.^[1][9][15]

8. Common-Mode Suppression with the Model 1115d

The Common-Mode Problem

Even with a properly configured current balun at the tuner output, ladder line feeding a multiband antenna can develop common-mode currents that radiate from the feedline itself, couple RF into station equipment, raise the received noise floor, and produce SWR readings that change when the feedline routing is altered.^[3][17][18]

Common-mode current flows as an in-phase component on balanced transmission lines, distinct from the differential-mode signal carrying useful RF power. Suppressing it requires a dedicated common-mode choke — ideally positioned between the transmitter chain and the balun.

Balun Designs Model 1115d

The Model 1115d is a dual-core 1:1 isolation balun (common-mode choke) designed for maximum common-mode suppression across the HF spectrum. Building on the original 1:1 design of Dr. Jerry Sevick (W2FMI, SK), it uses two low-permeability ferrite cores in a stacked configuration, with windings of Mil-Spec 50-ohm coaxial cable.^[3][19]

Installation Locations

The 1115d can be placed at multiple points in the system. The three principal options are:

Primary (recommended) — tuner output:

Alternative — transmitter output:

Dual-choke (maximum isolation):

RFI Reduction Performance

At 1 kW transmitter output, even 1% common-mode current represents 10 W radiating from the feedline and coupling into station equipment. The 1115d’s >10,000 Ω choking impedance at HF reduces this to milliwatt levels, effectively eliminating the problem. Typical results reported by operators include common-mode reduction of 30–40 dB, elimination of RF on station equipment and microphone cases, and 1–2 S-unit reductions in received noise floor on 40m and 80m.^[3][17][18]

9. Complete System Design and Installation

Recommended Component Order

1. Transmitter / transceiver — 50 Ω unbalanced output
2. Collins 180S-1 Antenna Tuner
3. Balun Designs Model 1115d (1:1 choke) — strongly recommended
4. Short coaxial jumper: 1–2 metres LMR-400 or equivalent
5. Balun Designs Model 4114 (4:1 current balun) — at tuner output or outdoor transition
6. 450-Ω or 600-Ω ladder line — routed to antenna feedpoint
7. Multiband loop antenna at appropriate height

Physical Mounting — Model 4114

• Mount securely using stainless steel hardware
• Maintain 10–15 cm (4–6 inches) clearance from grounded metal surfaces to prevent arcing at high-voltage points
• Outdoor installation preferred; if indoor, provide adequate ventilation
• Orient enclosure for water drainage if mounted outdoors
• Apply proper strain relief for ladder line connections to prevent mechanical stress on the binding studs^[2]

RF Grounding and Bonding

• Bond Collins 180S-1 chassis to station RF ground bus using wide copper strap or braid
• Connect all balun enclosure ground lugs to the same RF ground bus
• Use single-point grounding topology to avoid ground loops
• Ground conductors should be as short as practical; wide copper strap (50mm minimum) preferred over wire^[17]

Tuning Procedure

1. Set transmitter to 10–25 W for initial tuning
2. Select desired operating frequency
3. Key transmitter and adjust 180S-1 capacitor and inductor controls for minimum reflected power
4. Target SWR < 2:1 at the tuner input — achievable on most bands with this system
5. Increase power gradually while monitoring SWR and tuner operation
6. Record tuner settings per band for future reference^[7]

10. Measured Performance Examples

Case Study: 80-Metre Square Loop

Configuration: 84-metre square loop (21m per side), mounted at 4.5m average height, fed at centre of one side with 450-Ω ladder line; 18m of ladder line to shack; Collins 180S-1 + Model 1115d + Model 4114; 500 W continuous.

RFI observations: Without Model 1115d — RF on microphone, computer speaker clicks, SWR readings varied with cable routing. With Model 1115d installed — no RFI detected, SWR readings stable and repeatable. Noise floor reduction of 1–2 S-units on 40m and 80m versus previous dipole installation.^[17][18]

Case Study: 40-Metre Delta Loop

Configuration: 42-metre delta loop, fed at lower corner; apex at 10.5m, base corners at 2.5m; 450-Ω ladder line, 14m to shack; same tuner and balun configuration; 1,000 W (legal limit).

11. Troubleshooting and Optimisation

High SWR on Specific Bands

• Adjust ladder line length by 0.5–1.5 metres to shift impedance transformation on problematic bands
• Verify all connections — corrosion at ladder-line-to-balun stud connections is a common culprit
• Check ladder line for damage, water intrusion, or contamination
• Confirm antenna wire length and geometry match design specifications^[1][9]

RFI Persists Despite Model 1115d

• Add a second 1115d at the transmitter output for a dual-choke configuration
• Verify the station RF ground system is adequate — multiple ground rods bonded with wide copper strap
• Check for ground loops between station equipment
• Confirm ladder line routing maintains adequate clearance from metal structures^[17][18]
• If low-band (160m/80m) common-mode is the primary problem, replace the 1115d with a Model 1116d

Tuner Difficult to Adjust on Some Bands

• Adjust ladder line length slightly to improve impedance presentation to the balun
• Verify short coax between tuner and balun is high quality with secure, properly soldered connectors
• Ensure balun is mounted with adequate clearance from grounded metal surfaces^[1][7]

Balun Enclosure Runs Warm

• Slight warmth is normal at high power — the ferrite core dissipates a small fraction of transmitted power
• Excessive heat indicates SWR or power level is beyond the balun’s rated range for that configuration
• Ensure adequate ventilation if mounted indoors; outdoor mounting is preferred for natural convection^[2]

Optimisation: Ladder Line Length

The electrical length of ladder line between balun and antenna significantly affects the impedance presented to the tuner. Adjusting physical length by 0.5–3 metres can dramatically improve matching on problematic bands. Add length if very high impedances dominate; reduce length if very low impedances are the issue. Changes of even 0.5–1 metre can shift the impedance picture significantly on higher HF bands.^[9][15]

12. Conclusion

The integration of multiband horizontal loop antennas with the Collins 180S-1 Antenna Tuner, using the Balun Designs Model 4114 4:1 current balun and Model 1115d common-mode choke, represents a well-proven, high-performance approach to multiband HF operation from a single wire antenna. This configuration provides efficient impedance transformation across 160m through 10m, exceptional common-mode suppression, and stable repeatable tuning characteristics well-matched to the 180S-1’s design operating envelope.

The Model 4114’s Guanella-topology, Thermaleze-wound single-core design — refined by Dr. Jerry Sevick (W2FMI, SK) from the original Guanella architecture — addresses the unique demands of elevated-SWR multiband operation. The Model 1115d’s Mil-Spec coax windings deliver choking impedances exceeding 10,000 Ω across the HF band, eliminating the common-mode problems that typically plague balanced-feedline stations.

For amateur stations prioritising multiband capability, installation simplicity, and a clean noise floor, this combination of proven Collins hardware and modern Balun Designs components remains one of the most effective approaches available.