Build a Portable Magnetic Loop Antenna
A portable magnetic loop antenna collapses into a daypack, deploys in under ten minutes, and makes genuine HF contacts from a park bench, a summit, or a hotel balcony. Unlike a portable vertical or wire dipole, it needs no radials, no elevated support, and no horizontal space — the entire antenna fits within a 3-foot square when deployed and a 24-inch tube when packed. This guide builds a field-deployable magnetic loop optimised for POTA, SOTA, and travel operation on 20m and 40m at QRP to 20W power levels, with all design decisions weighted toward pack weight, deployment speed, and mechanical durability in the field.
What Changes for a Portable Build
A portable magnetic loop makes deliberate trade-offs compared to a fixed-station loop. Every design decision involves a tension between performance and portability — understanding these trade-offs helps set realistic expectations and make good build choices:
- Conductor size: the fixed-station loop uses heavy copper pipe for maximum efficiency. The portable loop uses lighter conductor — LMF-400 coax, RG-8X, copper tape, or thin-wall copper tubing — accepting 2–5 dB of efficiency reduction in exchange for a loop that weighs under 1 lb and packs flat.
- Power limit: QRP to 20W is the practical portable limit. At 10W the capacitor voltage stress is 3× lower than at 100W — a much simpler, lighter capacitor suffices. At QRP (5W) almost any good-quality variable capacitor handles the voltage safely.
- Frame and support: the fixed-station loop uses a rigid copper pipe frame. The portable loop uses a collapsible frame — PVC segments, fiberglass spreaders, or a packable rigid hoop — that assembles and disassembles quickly without tools.
- Tuning mechanism: the fixed-station loop uses a quality air variable with a vernier drive. The portable loop uses a smaller, lighter variable capacitor — often a 365 pF AM radio-type variable for 40m, or a small 100 pF variable for 20m. Tuning feel is less precise but adequate for field use.
- Connection quality: the fixed-station loop uses silver solder throughout. The portable loop uses silver solder at critical joints but accepts good-quality crimp connectors at the field-demountable connections — a practical compromise for a loop that is assembled and disassembled repeatedly.
Conductor Options for Portable Loops
Three conductor types make practical portable loops, each with different performance and portability profiles:
Power Level and Capacitor Voltage
Limiting operating power to QRP or low power dramatically simplifies the capacitor requirement — and the capacitor is the heaviest and most expensive component in the loop:
Frame Options — Collapsible and Packable
The frame supports the loop conductor in its circular shape during operation and collapses for transport. Three approaches cover most portable use cases:
- Rigid hoop from thin-wall copper: a pre-bent 3-ft diameter hoop of 1/2-inch thin-wall copper tubing — not collapsible but packs as a single rigid ring. Fits in a large duffel or straps to a pack frame. Fastest deployment — just stand it up. Best for vehicle-portable or base-camp use.
- PVC segmented frame: four 90° sections of 1/2-inch PVC pipe with push-fit joints, forming a square with rounded corners that approximates a circle. Packs to four 14-inch segments. Conductor clips to the outside of the frame with nylon cable ties. Assembly time ~3 minutes. Best all-around portable frame.
- Fiberglass spreader cross: two fiberglass tent-pole sections cross at the center, held by a plastic hub. Conductor runs around the perimeter supported by the spreader tips. Collapses to a 24-inch bundle of spreader sections weighing under 200g. Fastest to pack; slightly less stable in wind. Best for ultralight backpack use.
| Component | Material choice | Weight | Notes |
|---|---|---|---|
| Main loop conductor | LMR-400, 10 ft | ~1.1 lbs (500g) | 9.4 ft used; 0.6 ft spare |
| Frame | 4 × PVC 90° segments | ~0.35 lbs (160g) | Four 14-inch segments of 1/2-inch PVC |
| Tuning capacitor | 365 pF AM variable | ~0.09 lbs (40g) | For 10W QRP operation |
| Capacitor enclosure | ABS project box, small | ~0.07 lbs (30g) | Weatherproofs capacitor in field |
| Coupling loop | RG-8X, 2 ft | ~0.04 lbs (20g) | 6-inch diameter; SO-239 end |
| Support mast | Camera monopod, 24-inch | ~0.22 lbs (100g) | Extends to 48 inches; folds for pack |
| Coax to radio | RG-8X, 10 ft | ~0.22 lbs (100g) | Wrapped on small card spool |
| Hardware and clips | Nylon zip ties, connectors | ~0.04 lbs (20g) | BNC connector at radio end |
| Total packed weight | — | ~2.1 lbs (970g) | Well within daypack capacity |
Mag Loop Portable Calculator
Materials for a packable 3-ft diameter magnetic loop covering 20m and 40m at up to 10W
Building the Portable Magnetic Loop
This guide builds a PVC-framed, LMR-400 conductor loop for field use at QRP to 10W. The build is done at home — only deployment is done in the field. All connections are soldered and all components are tested before any field activation.
Prepare the LMR-400 Loop Conductor
Cut 9.75 feet of LMR-400. At each end, strip back 4 inches of the outer jacket and fold the exposed braid back over the jacket — this exposes both the braid and the center conductor, which are then connected together to form a solid loop. The LMR-400 coax itself is the loop conductor — the center conductor and braid are shorted at each end so the full conductor cross-section carries the RF current.
Build the Capacitor Box
The capacitor box houses the 365 pF air variable, the two loop-end connections, and the feedpoint SO-239. It is the nerve center of the loop — every connection here is in the high-current circuit and must be clean and secure.
Drill holes in the ABS project box: two holes sized to pass the LMR-400 jacket (approximately 7/16 inch) on opposite sides of the box for the loop ends to enter; one hole sized for the capacitor shaft to exit through the front face; one hole for the SO-239 on the side or bottom face. Mount the SO-239 chassis connector and the 365 pF air variable inside the box before making any connections.
Secure the LMR-400 where it enters the box with a nylon cable tie cinched tightly around the coax jacket just inside the box — this strain relief prevents tugging on the feedline from stressing the internal solder joints. Apply a small bead of silicone sealant around each cable entry hole after final assembly to weatherproof the box.
Build the PVC Frame
Cut four 14-inch sections of 1/2-inch PVC pipe — these will form the sides of the frame square. Do not glue the 90° elbows to the pipe sections — push-fit joints allow the frame to be assembled and disassembled in the field without tools. The friction of a well-fitting push joint is adequate to hold the frame together during operation; a slight twist locks the joint more securely.
Attach Conductor to Frame
With the frame assembled into its square shape and the capacitor box installed at the top corner, route the LMR-400 conductor around the frame perimeter. Secure it to the PVC pipe every 4–6 inches using small nylon zip ties — pull snug but not so tight that the zip tie bites into the coax jacket. The conductor must remain at a consistent distance from the PVC — direct contact is fine, since PVC is non-conductive.
At the three non-capacitor corners where the conductor must turn 90°, form a gentle bend in the LMR-400 around the outside of the elbow. The minimum bend radius for LMR-400 is approximately 1.5 inches — the outside of a 1/2-inch PVC 90° elbow is approximately 1.25 inches radius. For a permanent installation this would be marginal, but for a portable loop that is assembled and disassembled frequently, this bend is acceptable. If the bend seems tight, use a slightly larger elbow (3/4-inch PVC elbows) at the three conductor corners.
Build and Attach the Coupling Loop
For a portable loop at QRP power, a simple coax coupling loop made from RG-8X is ideal — lightweight, flexible, and durable. Form a 6-inch diameter circle from 24 inches of RG-8X. Short the far end (connect braid to center conductor). Connect an SO-239 at the near end — either a chassis SO-239 on a small L-bracket, or a PL-259 directly if using BNC connectors at the radio.
Attach the coupling loop to the frame at the bottom-center of the square — the side directly opposite the capacitor box. Use nylon zip ties to secure the coupling loop to the PVC frame, positioning it parallel to the main loop conductor with approximately 1–1.5 inches of spacing. For a portable loop the coupling loop position is slightly less critical than for a fixed-station build — some SWR variation from the ideal is accepted in exchange for a simpler, faster setup.
Home Testing — Resonance and Coupling Verification
Before any field activation, fully assemble the loop at home and verify operation with the NanoVNA. This step catches wiring errors and sets the baseline for field operation:
The lower Q of the LMR-400 portable loop compared to a copper pipe fixed loop is actually an operational advantage in the field — the 2:1 SWR bandwidth is 30–60 kHz on 20m rather than 10–20 kHz, which means less frequent retuning when working across a wider frequency range during an activation.
Pack for Field Use
The fully tested loop packs into two containers: a 30-inch tube bag for the frame segments and conductor, and a small stuff sack or padded pouch for the capacitor box, coupling loop, NanoVNA, and coax to radio.
Field Operation — Tuning and Band Changes
In the field, tune the loop using either the NanoVNA (connected temporarily to the coupling loop SO-239 before attaching the radio) or the radio's built-in SWR indicator. The pre-marked capacitor positions give a starting point that should be within one small adjustment of resonance at each operating frequency:
- Rotate capacitor to the marked position for the target frequency
- Key the radio briefly at low power (1–2W) and check the SWR meter — should show below 2:1
- Adjust the capacitor in small increments while keying briefly — find the minimum SWR position
- Once minimum SWR is found, operate normally. Re-tune if you move more than ~30 kHz on 20m or ~15 kHz on 40m
Ultralight Backpack Version (<1 lb)
For summit activations (SOTA) where every ounce matters, a further-simplified loop achieves sub-1-pound total weight:
- Conductor: RG-8X coax instead of LMR-400 — lighter but lower efficiency (68–74% on 20m vs 72–78%). At QRP (5W) the difference is less than 1 dB and fully acceptable.
- Frame: two fiberglass tent-pole spreaders (or hiking pole sections) crossed at center with a plastic hub — the coax conductor runs around the tips of the four spreader arms supported by small nylon loops tied at each tip. Collapses to a 20-inch bundle under 4 oz.
- Capacitor: a 100 pF trimmer capacitor for 20m-only operation, or two in parallel for 20m+17m coverage. At 5W the voltage stress is under 250V — any quality trimmer handles this safely.
- Power limit: strictly QRP (5W). At 10W the trimmer capacitor voltage approaches its limit — stick to 5W with this configuration.
- Total weight: approximately 0.75 lbs including a 6-ft coax pigtail to the radio — fits in a belt pouch.
Vehicle-Portable High-Performance Version
When operating from a vehicle or base camp where weight is not a constraint but packability still matters, an upgraded version significantly improves performance:
- Conductor: 1/2-inch thin-wall copper tubing bent into a rigid 3-ft hoop — 80% efficiency on 20m. Carries as a 3-ft diameter ring strapped to the vehicle roof or in the back seat. No frame assembly required.
- Capacitor: a 150 pF air variable with 1.5 kV rating — covers 20m through 40m at 50W. Housed in a weatherproof enclosure mounted at the loop gap.
- Power: up to 50W at 20m — extends from QRP to mid-power operation, enabling SSB contacts under marginal propagation conditions.
- Support: a photographer's light stand or collapsible fishing rod holder — stable in wind, adjustable height.
- Performance: within 2 dB of the fixed-station copper pipe loop on 20m. For a mobile or camping activation, this is an exceptionally capable antenna that packs into a 4-ft bag.
| Symptom in field | Most likely cause | Quick fix |
|---|---|---|
| No resonance visible anywhere in band sweep | Loose connection at capacitor box or coupling loop short failed | Check both LMR-400 ends are firmly seated in capacitor box terminals; verify coupling loop is properly shorted at far end |
| SWR at resonance is 5:1 — cannot match | Coupling loop has shifted or is incorrectly positioned | Slide coupling loop closer to main loop by 0.5 inches; re-check SWR |
| Resonance present but will not reach 40m — stops at 10–12 MHz | 365 pF capacitor not reaching full capacitance — dirty plates or mechanical stop | Rotate capacitor to fully meshed position; blow any debris from between plates; if mechanical — remove one plate section |
| SWR jumps erratically while transmitting | Loose connection in capacitor box vibrating from RF current | Open box and re-tighten all bolts at capacitor terminals and loop end connections |
| Resonance shifts as you move near the loop | Normal body capacitance effect — move hands away during transmit | Use non-conductive stick to adjust capacitor; step back 3 feet before keying |
| Capacitor makes crackling sound during transmit | Plates arcing — power too high for this capacitor at this band | Reduce power immediately; 365 pF capacitor is QRP-only — do not exceed 10W on 40m or 20W on 20m |
| Frame collapses in wind | PVC push-fit joints loosening | Wrap one layer of electrical tape around each pipe end before inserting into elbow — increases friction significantly |
Can I make POTA contacts with a portable magnetic loop at QRP?
Yes — portable magnetic loops are among the most popular antennas for POTA activations. On 20m at 10W FT8, a well-built portable loop regularly generates contacts across North America and occasionally transatlantic during band openings. Many POTA operators report that the loop's low noise receive (due to its magnetic aperture rejecting electric field noise from nearby infrastructure) makes received signals cleaner than a wire antenna at the same site — an important advantage in urban parks with high RFI. CW contacts at 5W are routine on 20m with a portable loop. SSB at QRP on 40m is more challenging but achievable during good conditions.
How does a portable mag loop compare to a portable vertical for POTA?
For 20m, the comparison is close. A portable vertical (fishing pole with wire) is slightly more efficient (85–90% vs 70–78% for the loop) and has a lower radiation angle — advantageous for DX. The loop wins on setup time (no radials, no elevated support needed), on noise performance in noisy urban parks, and on the ability to set up anywhere including on a picnic table or inside a vehicle with windows cracked. For 40m, the portable vertical is clearly better — its efficiency advantage over the loop is larger at lower frequencies where the loop's radiation resistance is very small. Most serious POTA operators carry both: a portable loop for 20m urban parks and a lightweight vertical for 40m rural activations.
Can I use a portable magnetic loop indoors at a hotel?
Yes — this is one of the most compelling use cases for a portable magnetic loop. The loop operates without a ground plane, without any outdoor antenna, and without HOA or hotel policy concerns since it stays entirely inside the room. Set it up near a window (keeping it 2–3 feet from metal window frames), tune to 20m FT8, and operate. At 10W the electric field inside the room is very low — far lower than a dipole strung in the same space. Many travel operators run FT8 on 20m and make 50–100 contacts per session from a hotel room with a portable loop and a 10W radio — a complete and satisfying operating experience from a suitcase-size kit.
How do I tune the loop quickly during a POTA activation?
Speed comes from preparation: calibrate the capacitor positions for all common operating frequencies before the activation. During the activation, rotate to the marked position, key briefly at 1–2W, and tweak for minimum SWR — this takes 10–15 seconds once you know the loop. For FT8 operation where you stay on one frequency, tune once and operate for the full session without retuning. For SSB where you move frequently, accept that band changes require 15–30 seconds of retuning and plan operating time accordingly. After 3–4 activations with the same loop, the tuning positions become intuitive and the process is no longer a conscious thought.
What is the minimum height above ground for the loop to work well?
Unlike a vertical antenna, a magnetic loop's performance is not strongly dependent on height above ground. The loop's radiation pattern is determined by its orientation (vertical vs horizontal) rather than by ground height. A loop sitting on a picnic table at 2.5 feet above ground performs nearly as well as the same loop at 6 feet — the difference is typically less than 1 dB. Keep the loop conductor at least 12 inches above any metal surface (a metal picnic table top, a car roof) — closer than this causes measurable efficiency reduction as the metal surface couples to the loop's near-field. On grass, concrete, or wood, the loop can be as low as 1 foot above the surface without significant performance degradation.
Can I use a portable magnetic loop for SOTA summits?
Yes — the loop's self-contained nature (no radials, no supports other than its own frame and a simple monopod) makes it attractive for SOTA where every gram and every minute of setup matters. The key consideration for SOTA is wind — a 3-ft frame on a monopod in high summit winds requires guying or ballasting the monopod base (a small stuff sack filled with summit rocks works well). The ultralight tent-pole spreader design described in the alternative builds section is particularly suitable for SOTA — it collapses to a small bundle, the spreader arms deflect in wind rather than presenting a rigid sail, and the whole assembly weighs well under a pound. SOTA operators using portable loops primarily work 20m CW at 5W, where the loop's performance is fully adequate for the typical SOTA contact profile.