Introduction
Ham radio offers the joy of not just operating but also building your own gear. Every step in the build is a learning experience—and antennas, in particular, always pique my interest. Whether the project is large or small, simple or intricate, I’m drawn to antenna projects even if I’m as thrifty as they come.
While browsing through an ARRL magazine, I encountered a higher-priced, field‐deployable magnetic loop antenna. With commercial mag-loops retailing for over $500, I naturally began to wonder:
- Why do they cost so much?
- What makes them “magnetic”?
- And do they really work as advertised?
My curiosity led me on a research journey through books, videos, magazine articles, and blogs until the mystery started to clear—I finally understood enough to say, “I can build that!” In this article, I share that journey and the insights I gained along the way.
The Magnetic Loop Concept
Technical Explanation
Magnetic loop antennas operate as a resonant LC circuit—one where the loop acts as an inductor (L) and an adjustable, usually air-variable, capacitor (C) tunes the circuit to resonance. When the capacitor is set just right, the inductive reactance of the loop and the capacitive reactance cancel each other out. This results in a sharp resonance where the loop’s impedance is dominated by a very small radiation resistance.
This resonant condition means that the antenna is exceptionally selective in frequency—a high-quality factor (Q) leads to narrow bandwidth performance but, when tuned correctly, efficient signal pickup. Instead of predominantly interacting with the incident electric field, a magnetic loop is designed to respond to the magnetic component of incoming electromagnetic waves. The inherent design can also help mitigate radio frequency interference (RFI) because the loop can be rotated to place its null in the direction of unwanted signals. The performance is sensitive to small physical or environmental changes, which is why a well-adjusted, robust tuning mechanism is key.
Key Points Summarized
- LC Resonance: The loop inductance and capacitor form a resonating circuit that cancels out reactive components at the tuned frequency.
- Magnetic Field Interaction: Unlike most antennas that focus on the electric field, the mag-loop couples with the magnetic field component, offering unique performance characteristics.
- High Q Factor: A high quality factor means excellent selectivity and efficiency at resonance—though it also results in a narrow bandwidth that requires precise tuning.
- RFI Mitigation: Its ability to be rotated for nulling out undesired signals gives the mag-loop an edge in crowded RF environments.
Design Overview
Inspired by what I saw in the magazine, here’s a breakdown of the design:
Core Components
- Small Inner Loop:
- Made from a 1″ x 1/8″ aluminum section (roughly 7″ in diameter).
- Equipped with a J-Pole connector for the coax feed line from the transceiver.
- This loop couples directly to the transceiver.
- Large Outer Loop:
- Constructed with a 32″ diameter segment of LMR-400 coaxial cable.
- Electrically isolated from the inner loop and transceiver, its sole role is to resonate with the inner loop.
- Variable Capacitor & Project Box:
- An oversized dual- (or optionally triple-) gang variable capacitor is housed in a project box equipped with a vernier reduction tuning dial for fine adjustments.
- Support Structure:
- A non-metallic support pole (I opted for a carbon fiber monopod, though PVC is an alternative) mounts the inner loop, project box, and tuning mechanism to a tripod.
(Detailed design specifications and a complete parts list are provided in the next section.)
The Build Process
- Project Box & Air Variable Capacitor:
The capacitor, pre-wired with coax connectors, sits housed in the project box. I verified the wiring on all three sections with a meter (special thanks to WB8OOK for advice on the wiring). - Large Outer Loop:
The LMR-400 cable is already shaped into the loop. I intentionally shorted its ends for continuity (marked them with red tape to avoid mix-ups). - Small Aluminum Loop:
This component is structurally complete; the only pending step is attaching the J-Pole connector.
- Mounting Hardware:
I used zip ties to secure the project box to the monopod. For now, zip-ties will suffice until a more permanent mounting solution is sourced. - Tripod or Alternative Support:
I’m settled for a carbon fiber camera tripod found at Goodwill (Any suitable used alternative can be found cheaply at a yard sale, Goodwill, or eBay).
Parts List & Resources
Below is a detailed parts list including supplier information, links, and approximate prices (prices may vary by supplier and region):
| Component | Description & Link | Approximate Price |
| Aluminum Flat Bar | 1″ x 1/8″, 30″ aluminum flat bar (hardware/home improvement stores) | ~$10–$20 |
| SO-239 x 3/8 x 24 Stud Connector | Available from American Radio Supply(Two additional SO-239 and PL-239 connectors are required for the main loop and tuning unit) | ~$5–$10 each |
| LMR-400 Coaxial Cable | 15′ length | $2 per ft ~$30 |
| Dual Gang Variable Capacitor | At least 365 pF per section with >0.5KV spacing. Part #C-V365-X3 from Antique Radio Supply or the triple-section version from Tubes and More | ~$40–$60 |
| Enclosure | Plastic or carbon fiber enclosure to house the variable capacitor | ~$20–$50 (depending on material) |
| RG-58U Coax Cable | 10′ length with PL-239 connectors from DX Engineering | ~$15–$25 |
| Ferrite Chokes & Heat Shrink Tubing | Four ferrite chokes plus enough 7/8″ heat shrink tubing; e.g., from Amazon | ~$10–$15 for a set |
| Mounting & Supporting Hardware | Includes PVC fittings, brass screw inserts, etc. | ~$10–$20 |
| Optional: Vernier Reduction Gear Unit | A vernier knob assembly for fine tuning; see:MGS4U, MFJ Enterprises, or Amazon Philmore S38 | ~$15–$30 |
Additional Resources
- Tuning Techniques & Field Analysis:
Eham.net Magazine Article – Features an in-depth look at adding a Vernier dial with an antenna analyser for quick field tuning. - Mag-Loop Calculator:
DL0HST’s Windows Mag-loop Calculator – A nifty tool for calculating optimal dimensions and parameters.
Field Performance & Initial Tests
While this article focuses mainly on the design and build process, here are some early test results that hint at the potential of this antenna design:
- Band Suitability:
The current 7″ inner loop and 32″ outer loop combination perform well on 30 and 40 meters, with measured SWR values of approximately 1.3 on 30 meters and 1.2 on 40 meters. - Tuning by Listening:
One of the most fascinating aspects is auditory tuning—the S-meter jumps several bars when resonance is reached, accompanied by a surge in background noise. This phenomenon allows for quick fine-tuning in under 10 seconds. - Signal Reception:
Despite its compact size, the antenna delivers solid reception indoors, picking up both regional signals and DX (with contacts over 1,200 miles possible on modes like JS8Call and WSPR, using only 6 watts). - Directional Nulling:
The ability to rotate the antenna and create an 8° null to cancel out interference from nearby stations is a bonus feature for crowded frequency environments.
Enhancements & Upgrades
No homebrew project is ever truly finished—there’s always room for improvement. So far, these are some of the enhancements I’ve implemented:
- Loop Extension:
I added extra LMR-400 cable via a barrel connector to extend the outer loop. This effectively brings the antenna into resonance with the 60 and 80-meter bands. - Fine-Tuning Module:
In addition to the large variable capacitor, I integrated a small trimmer air capacitor. This allows for finer control when zeroing in on the resonance “sweet spot.”
Project Reflections & Future Improvements
Was this project worth it?
Absolutely. I managed to build a working magnetic loop for roughly one-third the cost of commercial alternatives—and incidentally, some of my SWR readings rival those of well-known brands.
Future improvements might include:
- Downsizing the inner loop to extend the antenna’s range to the 20-meter band.
- Utilizing a smaller, single-gang variable capacitor coupled with a more compact project box for enhanced portability.
- Substituting copper for aluminum in the inner loop for potential performance gains.
The beauty of homebrewing is that every project, no matter how successful, leaves room for learning and future tinkering.
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