Magnetic loop without capacitor

Field Day 2026: R&D in the Field With another ARRL                                 Yuriy,   AC6A

The high-density RF environment of ARRL Field Day 2026—featuring 11 co-located transmitters—provided an ideal testing ground for evaluating alternative receive (RX) and transmit (TX) antenna geometries. Our focus was centered on high-efficiency, low-cost engineering implementations. Over the next few logs, I will share the empirical data, deployment strategies, and practical lessons learned from these field tests. To initiate this series, this first entry details the design and performance analysis of a low-profile, high-Q magnetic loop built from utility-grade hardware components.

The $0-Capacitor Field Day Experiment: A High-Efficiency PEX Magnetic Loop for 15m Digital

Introduction 

     Deploying effective antennas for digital stations during fieldoperations often presents logistical challenges involving mast height and structural weight. Small magnetic loops offer a compact footprint alternative. However, a common assumption within the amateur radio community is that high efficiency requires large-diameter solid copper pipe and costly vacuum variable capacitors. This project evaluates that assumption by using a composite PEX-AL-PEX tubing loop. PEX-AL-PEX is physically flexible and lightweight, simplifying builds, transport and deployment compared to rigid copper tube. Over the field day period, this design operated stably alongside CW and phone stations on the same band without triggering mutual receiver desensitization.

Sizing at the Self-Resonance Limit

   The main loop was constructed with a 1.2-meter diameter using PEX-AL-PEX tubing marked as 1-1/4 inches. Antenna parameters were evaluated using the VK3CPU online magnetic loop model to approach the self-resonant limit near 0.26 lambda. At 21.074 MHz, factoring in a calculated internal aluminum layer loss of 0.0573 Ohm and an environmental proximity loss of 0.051 Ohm, the model indicates that 9.8 pF of capacitance is required for resonance. Because the loop perimeter is 3.77 meters (0.265 lambda), the calculated radiation resistance rises to 0.974 Ohm. This high radiation resistance relative to the total loss resistance yields a system efficiency of approximately 90 percent compared to an ideal dipole, with a usable operating half-power bandwidth of 60 kHz.

Ground Independence: No High Towers Required

    Ground Proximity Performance. Magnetic loops respond primarily to the magnetic component of the electromagnetic wave, rendering them less susceptible to near-field ground losses than voltage-fed vertical or horizontal wire antennas. Consequently, this antenna does not require elevated mounting heights to achieve its calculated performance. During deployment, the loop operated reliably mounted less than 2 meters above ground level, eliminating the requirement for guyed masts or tall support structures.

RF Current Distribution in Coaxial Composite Tubing 

    The skin effect dictates that at high frequencies, alternating current concentrates on the outermost boundaries of a conductor. At 21.074 MHz, the skin depth of RF current in aluminum is 17.85 micrometers. The PEX tubing features an internal aluminum core thickness of 0.3 mm (300 micrometers). Because this layer is nearly 17 times thicker than the skin depth, the RF current flows entirely within the aluminum jacket boundary without penetrating the outer plastic sheathing or experiencing dielectric attenuation from the inner PEX layers.

Technical Sidebar: Breaking Down the PEX-AL-PEX Math 

To calculate the skin depth of aluminum at the operating frequency, we use the standard electromagnetic equation: 

delta = square root of ( rho / ( pi * f * mu_0 * mu_r ) )  Where: 

rho = Resistivity of aluminum (2.65 x 10^-8 Ohm*m) 

f = Operating frequency (21.074 x 10^6 Hz) 

mu_0 = Permeability of free space (4 * pi x 10^-7 H/m) 

mu_r = Relative permeability of aluminum (1.0) 

    Plugging in 21.074 MHz yields an RF skin depth of 17.85 micrometers. To determine the final AC resistance (Rac) of the 1.2-meter diameter loop (total conductor length L = 3.77 m, measured aluminum conduction path diameter d = 0.0311 m), we use: 

Rac = ( rho * L ) / ( pi * d * delta ) 

Rac = ( 2.65 x 10^-8 * 3.77 ) / ( pi * 0.0311 * 1.785 x 10^-5 ) = 0.0573 Ohm 

 Because a single-turn loop layout spreads the conductor far apart, the proximity effect multiplier remains essentially 1.0. This leaves us with a total physical loop resistance of just 0.056 Ω—proving that budget-friendly PEX delivers low-loss performance that rivals traditional solid copper. 

(References: ARRL Antenna Book for Radio Communications, Chapter 5 - Conductor Losses.)

Co-Location and Mutual Interference Mitigation 

   Operating three stations (Digital, CW, and Phone) simultaneously on the 15-meter band required careful power management. The digital station power was limited to 10W QRP to prevent desensitizing the adjacent CW receiver. The high-Q characteristic of the magnetic loop provided narrow bandpass filtering on both transmit and receive paths, which rejected out-of-passband signals and eliminated the need for external coaxial bandpass filters. This sharp selectivity, rather than the construction method of the loop itself, provided the necessary isolation to operate without cross-station interference. Additionally, small loops exhibit distinct directional nulls perpendicular to the plane of the loop. This pattern allows operators to null out localized interference sources by physically rotating the loop assembly, though ambient noise levels during this test did not require active mitigation.

The "No-Capacitor" Tuning Trick & Hardware Store Engineering

     Integrated Tuning Mechanism.   Rather than employing an external vacuum variable capacitor, tuning was accomplished by creating a coaxial capacitor out of the PEX pipe itself. The ends of the tube were overlapped without making direct electrical contact with the internal aluminum core. Tuning is adjusted by sliding the overlapping section to alter the capacitive area. To maintain a uniform dielectric gap, standard U-shaped PVC pipe clamps matching the 32.1 mm outer diameter were snapped over the overlap joint. Once resonance was established, the assembly was secured using heavy-duty cable ties, preventing frequency drift from mechanical stress or thermal expansion.

Feed Method and Mechanical Layout 

    The coupling loop was constructed from a section of 1/4 inch semi-rigid hardline coaxial cable formed into a shielded Faraday loop. While standard documentation suggests sizing the coupling loop at 1/5 of the main loop diameter, this deployment utilized an oversized loop at roughly 1/4 diameter. This broader coupling profile simplified impedance matching across the digital operating window, allowing shifts between the FT8 (21.074 MHz) and FT4 (21.140 MHz) frequencies via small rotational adjustments of the coupling loop relative to the main loop plane. The 1.2-meter loop assembly was supported by a structural mast built from standard PVC piping and fittings. The integrated PEX-overlap capacitor demonstrated minimal frequency drift under changing ambient outdoor temperatures, indicating stable mechanical and electrical performance.

⚠️ Critical RF Safety Considerations 

High-Q resonant loops generate high voltages and high localizednear-field electromagnetic energy. Modeling indicates that a 10W input produces approximately 1,130 Volts RMS across the capacitive overlap section. If the station power is increased to a standard 100W level, the voltage across the capacitor scales up to approximately 3,580 Volts RMS, creating a peak voltage of over 5,000 Volts. Adequate insulation thickness at the overlap joint is mandatory to prevent dielectric breakdown or arcing at these higher power thresholds, and direct contact during transmission will cause severe RF burns. Because the antenna was positioned less than 2 meters above ground level, near-field radiation boundaries must be evaluated. For a 1.2-meter loop operating at 21 MHz, operators should compute the controlled and uncontrolled Maximum Permissible Exposure (MPE) zones. Exact exposure boundaries can be evaluated via the ARRL RF Exposure Calculator in compliance with FCC regulations. For specialized loop proximity guidelines, refer to the ARRL Antenna Book, Chapter 2 regarding RF Environmental Safety. As a standard safety practice, the antenna must be isolated from operators, foot traffic, and observers. In this configuration, the loop was located 80 feet from the operating tent, which satisfies regulatory safety criteria for a 10W and 100W exposure boundary.

RF Safety References & Calculators:

 To evaluate exact exposure distances based on your environment's duty cycle, use the ARRL RF Exposure Calculator, which complies with the updated FCC RF exposure rules. For dedicated magnetic loop proximity safety layouts, consult the guidelines outlined in Chapter 2 of the ARRL Antenna Book (RF Environmental Safety). The Golden Rule: Keep the antenna fenced off or placed well away from the operating position, foot traffic, and spectators. [1] Our Setup: In our Field Day configuration, the antenna was placed roughly 80 feet away from the operator's position. This distance far exceeds all regulatory safety criteria for 10W exposure limits, ensuring completely safe operation for the team and spectators.

Field Measurements and Log Analysis 

     The antenna system performed reliably over the operating period. Operating at a moderate pace, the station completed over 130 digital contacts across the FT8 and FT4 modes. Running 10W into a loop constructed from standard utility tubing and surplus feedline confirms that effective digital field stations can be implemented without specialized structural components.

Document Control Data:
Prepared By: Yuriy Fuchs, AC6A
Contact: as listed on QRZ page Copyright Notice:
Copyright (C) 2026 by the Author.

This engineering log is the intellectual property of the author. Selected data sets from this document may be reproduced for non-commercial amateur radio experimentation provided explicit attribution to the author and call sign is maintained.Technical Reference Manuals Silver, H. W. (Ed.). (2023). The ARRL Antenna Book for Radio Communications (26th ed.).

American Radio Relay League. Available from the ARRL Antenna Book Official Portal. Silver, H. W. (Ed.). (2024). The ARRL Handbook for Radio Communications American Radio

Relay League. Available via ARRL Handbook Product Details. Digital Modeling Utilities Vaca, M. (VK3CPU). (2025). Small Transmitting Magnetic Loop Antenna Calculator (Version 10.6) [Computer software]. Available from the VK3CPU Magloop Calculator Web Tool. https://miguelvaca.github.io/vk3cpu/magloop.html?loop_diameter=1.2&conductor_diameter=31.2&loop_turns=1&loop_spacing=8.46&transmit_power=10&external_losses=53&unit=metric&metal=Al&shape=circle 

 Vaca, M. (VK3CPU). (2025). Small Transmitting Magnetic Loop Calculator Design Equations [Technical documentation]. Available from the VK3CPU Mathematical Equations List. https://miguelvaca.github.io/vk3cpu/magloop_equations.html

Regulatory and Safety Compliance Utilities American Radio Relay League. (2021). RF Exposure Calculator [Online application]. 

Available from the ARRL RF Exposure Calculator Portal. American Radio Relay League. (2021). RF Exposure Evaluation and Maximum Permissible Exposure (MPE) Regulations [Technical briefing]. Available from ARRL RF Exposure Instructions. Materials Standards ASTM International. (2023). ASTM F1281-17: Standard Specification for Crosslinked Polyethylene/Aluminum/Crosslinked Polyethylene (PEX-AL-PEX) Pressure Pipe. West Conshohocken, PA; ASTM International. (Dimensional core constraints and aluminum cladding thickness baselines).