Build a 20 Meter Magnetic Loop Antenna
A dedicated 20m magnetic loop is the single most efficient small transmitting loop you can build — because a loop optimized for one band can be sized exactly right for that frequency, uses the minimum required capacitance range, and can be constructed from heavier conductor material without the weight penalty becoming unmanageable. On 20m, a well-built 3-foot copper pipe loop reaches 80–87% efficiency — performance that puts it within 1–2 dB of a full-size outdoor dipole. This guide covers a 20m-dedicated build that prioritizes maximum efficiency through conductor selection, connection quality, and a correctly sized coupling arrangement for the 14 MHz band.
The 20m Advantage — Loop Sizing
A multi-band loop must be sized to cover the lowest band (typically 40m) — which means it is oversized for 20m and the capacitor operates near minimum capacitance on 20m, making tuning very sensitive. A dedicated 20m loop can be sized specifically for 14 MHz, placing the capacitor at mid-range for 20m operation where tuning is smoother and the loop geometry is optimised for this frequency:
20m Loop vs Multi-Band Loop — Performance Comparison
The dedicated 20m loop has measurable advantages over a multi-band loop operated on 20m:
- Higher Q on 20m: a loop sized for 20m operates at the frequency where its circumference-to-wavelength ratio is ideal. A multi-band loop optimised for 40m coverage has a circumference that is electrically smaller on 20m than the optimal — slightly lower Q and efficiency.
- Lighter construction possible: a 20m-only loop does not need to survive the extreme voltages seen at 40m operation. A 2 kV capacitor is adequate for 100W on 20m — lighter and less expensive than the 5 kV capacitor needed for 40m operation from the same loop.
- Simpler capacitor: the 2 pF required tuning range for the full 20m band means a small, precise capacitor suffices. A 20–60 pF air variable with fine thread or vernier drive makes tuning smooth and repeatable — no hunting across a wide range.
- Better bandwidth: a loop sized larger (as a fraction of wavelength) on its target band has somewhat lower Q and therefore slightly wider usable bandwidth. The 3-ft loop at 20m covers the full 350 kHz 20m band with SWR below 2:1 after only 2–3 capacitor adjustments, versus a multi-band loop which may need 5–6 adjustments to cross the same span.
- Known and repeatable performance: a single-band loop is easier to characterise, document, and operate repeatably. Capacitor position for each common frequency can be marked precisely and returned to quickly.
Conductor Choice for Maximum 20m Efficiency
At 14 MHz, the skin depth in copper is approximately 17.7 µm. All RF current flows in this thin surface layer — conductor efficiency depends entirely on the surface area of the conductor, not its cross-sectional area:
20m Loop Bandwidth and Operating Implications
Understanding the 20m loop's bandwidth helps plan operating strategy and avoids frustration:
Most 20m mag loop operators develop a comfortable routine of quick retuning as part of normal operating — similar to tuning an old-style VFO radio. Fixed operating frequencies (FT8 at 14.074, CW calling at 14.025, SSB calling at 14.225) can be marked on the capacitor dial for instant return without hunting.
| Capacitor type | Capacitance range | Voltage rating needed (100W) | Tuning feel | Cost | Verdict |
|---|---|---|---|---|---|
| Air variable — single section | 15–65 pF | ≥2 kV | Good — linear rotation | $15–40 surplus | Good — widely available, proven |
| Air variable — vernier drive | 15–65 pF | ≥2 kV | Excellent — fine control | $30–80 | Best for manual tuning — precise |
| Butterfly (split-stator) | 10–50 pF | ≥2 kV | Excellent — balanced | $40–100 | Best overall — no sliding contact |
| Vacuum variable | 5–100 pF | 5–15 kV | Very smooth | $80–200 surplus | Overkill for 20m — better for 40m builds |
| Silver mica fixed | Fixed value | 500V | No tuning possible | $1–3 each | QRP only — fixed single frequency |
| NP0/C0G ceramic variable | 5–30 pF | 200–500V | Fair | $5–15 | QRP only — insufficient voltage at 100W |
Mag Loop 20m Calculator
Materials for a 3-ft diameter 20m magnetic loop using 1.5-inch copper pipe
Building the Dedicated 20m Magnetic Loop
This guide builds a 3-ft diameter, 1.5-inch copper pipe loop optimised for 14 MHz. The construction is identical in method to the HF multi-band loop but with adjustments specific to the 20m band — lighter capacitor voltage requirements, smaller coupling loop, and higher-Q construction goals throughout.
Bend the 1.5-Inch Copper Pipe Loop
Cut 1.5-inch copper pipe to 9.75 feet — 4 inches longer than the target 9.42-foot circumference to allow for the capacitor gap and any trimming. A 1.5-inch pipe bender is required — this pipe diameter cannot be hand-bent without kinking. Work in the same progressive bending technique as the 1-inch loop: 10–12 overlapping bends around the circumference, each gentle, building up the curve incrementally.
The 1.5-inch pipe is noticeably heavier and stiffer than 1-inch — it requires more force at the bender but produces a very rigid, stable loop that holds its shape better under thermal cycling and handling. The completed 3-ft loop in 1.5-inch copper pipe weighs approximately 1.8 lbs — heavy enough to require a stable support but light enough for a desk tripod or mic stand base.
Prepare and Silver-Solder All Connection Points
Prepare the capacitor gap ends by cutting clean and square with a tubing cutter. Clean the outer surface of each pipe end with 220-grit sandpaper until bright copper — work quickly and apply flux immediately. Silver-solder a copper strap to each pipe end:
After soldering, allow joints to cool fully before handling. Do not quench with water — thermal shock can crack the solder joint. Inspect each joint: it should be smooth, shiny silver-colored, and fully bonded with no voids or gaps visible at the edges. A dull or granular joint indicates a cold solder — reheat until it flows again.
Select and Mount the 20m Tuning Capacitor
For a dedicated 20m loop at 100W, a 2 kV rated air variable in the 15–65 pF range is the correct specification. This is a lighter and less expensive component than the 5 kV capacitor required for 40m work — a meaningful advantage of the single-band design. A surplus broadcast variable or a new-production Jackson Brothers type fits this specification well.
If a vernier drive capacitor is available, use it — the 20m band requires only ~2 pF of total tuning range, which means a standard direct-drive variable has very high angular sensitivity (a small rotation causes a large frequency jump). A vernier reduction drive (typically 6:1 or 10:1) spreads that 2 pF over a larger rotation angle and makes tuning smooth and precise.
Mount the capacitor on a short non-conductive PVC or Delrin bracket between the two pipe-end straps. The bracket spans the 3–4-inch capacitor gap, holding the capacitor body securely while keeping all metal parts away from the high-voltage plates. Bolt the capacitor terminals to the copper straps using stainless steel bolts — apply a thin layer of Noalox at the metal-to-metal contact surfaces before bolting.
Build and Mount the 20m Coupling Loop
For a dedicated 20m loop, the coupling loop diameter is sized to the 20m band only — no compromise with other bands required:
Mount the coupling loop to the support structure — not to the main loop itself. The coupling loop must be fixed in position; any movement changes the coupling coefficient and shifts the SWR at resonance. A small bracket from the PVC support mast holds the coupling loop securely at the correct position.
Build the Support Structure
A dedicated 20m loop benefits from a stable, purpose-built support that holds it at a fixed height and orientation. Recommended approach: a 1-inch PVC vertical mast, 4 feet tall, secured to a heavy wooden base (a 12×12-inch square of 3/4-inch plywood weighted with a concrete paver works well). The loop attaches to the top of the mast with two nylon hose clamps that clamp the pipe without making electrical contact.
Position the bottom of the loop approximately 18–24 inches above the floor — high enough to clear carpet and furniture, low enough for easy capacitor access. The coupling loop connects to the feedline from its position at the bottom center of the main loop. Route the coax along the PVC mast and out along the floor to the operating position — keep the coax path away from the plane of the loop for the first 3 feet to minimise coupling between the feedline and the antenna field.
Initial Resonance Check and Q Measurement
Connect the NanoVNA to the coupling loop SO-239. Sweep 13.5–15.5 MHz. The resonance appears as a sharp SWR dip — on a high-Q 20m loop it is extremely narrow and easy to miss if the sweep step size is too coarse. Use a sweep step of 5 kHz or less when first searching for the resonance.
Optimise Coupling Loop for 50 Ω Match
Adjust the coupling loop position to achieve SWR as close to 1:1 as possible at resonance. With the 20m-only loop, the coupling optimisation is done once for 14 MHz and does not need to be compromised for other bands:
- Start with the coupling loop centered at the bottom of the main loop, 2 inches from the main conductor
- Measure SWR at resonance — aim for below 1.3:1
- If SWR at resonance is above 1.5:1: move the coupling loop 0.25 inches closer to the main loop and re-measure
- If two SWR dips appear instead of one: move the coupling loop 0.5 inches further from the main loop
- Repeat in 0.25-inch increments until SWR at resonance is minimised
Calibrate the Capacitor Dial and Verify Full Band Coverage
Once coupling is optimised, calibrate the capacitor position scale for key 20m frequencies. Tune to each frequency listed below, record the capacitor dial position, and mark the scale:
Transfer the calibration marks to a permanent laminated label attached to the base of the capacitor. With this calibration, routine band changes require no test equipment — simply rotate to the marked position and operate. Reserve the NanoVNA for periodic verification and after any physical changes to the loop.
On-Air Verification with WSPR
Run WSPR for 24 hours on 20m at 5W to characterise actual on-air performance. Compare the SNR of spots received by distant stations (1000+ miles) against published median SNR values for well-performing 20m antennas at similar power levels. A well-built 3-ft 1.5-inch copper loop at 5W typically generates spots across North America and transatlantic during normal 20m propagation conditions.
Also use WSPR to verify the loop's directional null — rotate the loop 90° during a WSPR session and compare the spot map before and after. Stations that were strong in one orientation should become weaker when they are broadside to the loop face (in the null direction). Confirming this null behavior verifies that the loop is functioning correctly as a magnetic loop and not as a leaky electric field antenna due to a connection problem.
| Antenna | Space required | Efficiency (20m) | DX capability | Band switching | Best suited for |
|---|---|---|---|---|---|
| 3-ft dedicated 20m loop (this build) | 3 ft × 3 ft floor space | 80–87% | Good — 1–2 dB below dipole | 20m only — very fast within band | HOA/apartment, indoor, fixed station |
| 3-ft HF multi-band loop (40m–15m) | 3 ft × 3 ft floor space | 75–83% on 20m | Good — 2–3 dB below dipole | 40m–15m with retuning | HOA/apartment — multi-band in same space |
| 20m dipole at 30 ft outdoor | 33 ft horizontal span | ~95% | Excellent | 20m only — direct feed | Unrestricted outdoor installation |
| 20m vertical at ground level | 17 ft height + radials | ~88% | Excellent DX (low angle) | 20m direct; 10m natural harmonic | Good outdoor lot with room for radials |
| EFHW 20m portable | 16.5 ft wire + support | ~90% | Good — especially elevated | 20/10m harmonics | Portable, POTA, temporary |
| Attic dipole at 15 ft indoor | 33 ft attic run | ~70–80% | Fair — high radiation angle | 20m only or with ATU | Attic installation in non-metal-roof home |
How does a dedicated 20m loop compare to a 20m dipole?
A well-built dedicated 20m magnetic loop is within 1–2 dB of a 20m dipole at 30 feet on the transmitted signal — close enough that the difference is barely perceptible in most operating situations. On receive, the loop has a significant advantage in noise-limited environments because its narrow aperture rejects electric field noise while responding to the magnetic component of the incoming wave — this often produces a cleaner, quieter receive than a dipole in an urban or suburban setting with high local RFI. For operators constrained to indoor or balcony antennas, the 20m loop is the closest alternative to a full-size outdoor dipole that can be built within a 3-foot footprint.
Can I operate FT8 continuously with a magnetic loop?
Yes, with one important consideration: FT8 uses a 50% duty cycle (transmitting 15 seconds, receiving 15 seconds). The loop conductor and particularly the capacitor experience significant heating during the transmit periods due to the high circulating currents. At 100W this heating is manageable for a well-built loop — the copper pipe acts as its own heat sink. At 50W or below, thermal issues are essentially non-existent for continuous FT8 operation. Monitor the capacitor temperature during early FT8 sessions at full power — if the capacitor housing becomes too hot to touch comfortably, reduce power to 50W. A warm capacitor is normal; a hot capacitor indicates either high contact resistance or a power level beyond the component's thermal rating.
Does the loop need to be exactly 3 feet in diameter?
No — the 3-foot diameter is a practical recommendation, not a requirement. A larger loop (3.5 or 4 ft) on the same 20m band is more efficient because the larger circumference-to-wavelength ratio raises the radiation resistance, reducing the impact of conductor losses. A 4-ft diameter loop on 20m with 1.5-inch copper pipe would achieve approximately 88–91% efficiency — a meaningful improvement over the 3-ft version. The trade-off is physical size and the larger, heavier structure. Conversely, a 2-ft loop on 20m is smaller and lighter but less efficient (~65–70%). Size the loop to the largest diameter that the installation space comfortably accommodates.
What happens if metal objects are near the loop during operation?
Metal objects within the loop's near-field (approximately 1–2 loop diameters) affect the antenna in two ways: they add loss (nearby conductors couple to the loop field and absorb energy) and they detune the antenna (nearby conductors change the effective inductance of the loop, shifting the resonant frequency). Ferrous metal (steel, iron) is worse than non-ferrous (aluminum, copper) because ferrous materials have magnetic losses in addition to resistive losses. Keep metal structures at least 1 loop diameter (3 feet for a 3-ft loop) from the conductor. If a metal object must be near the loop, position it along the loop axis (in the null direction) where coupling is weakest, rather than broadside to the loop face where coupling is strongest.
Is it worth upgrading to a 2-inch copper pipe for the 20m loop?
For a permanent fixed-station installation where maximum 20m performance is the goal, yes — the upgrade from 1.5-inch to 2-inch copper pipe improves efficiency from approximately 86% to 90% on 20m, a gain of about 0.2 dB. This is a real but small improvement. The practical trade-offs: 2-inch pipe is significantly heavier (the loop weighs approximately 2.8 lbs), requires a stronger support structure, is harder to bend cleanly without professional tooling, and costs roughly twice as much per foot. For a portable or occasionally relocated loop, 1-inch or 1.5-inch is the better practical choice. For a permanent desktop or wall-mounted loop that will never move, 2-inch is worth considering if maximum 20m efficiency is the priority.
Can this loop cover 17m and 15m as well as 20m?
Yes — a 3-ft loop sized for 20m also covers 17m (18.068–18.168 MHz) and 15m (21.000–21.450 MHz) by reducing the capacitor to smaller values. On 17m the efficiency improves slightly relative to 20m (higher radiation resistance at higher frequency). On 15m efficiency is higher still — the 3-ft circumference is closer to λ/5 at 21 MHz, which raises radiation resistance significantly and pushes efficiency toward 90–93%. The capacitance required for 15m is approximately 15–20 pF, which is within range of most 20–65 pF air variable capacitors. The voltage stress on the capacitor is also reduced at 15m and 17m compared to 20m — the same 2 kV capacitor that is marginal at 100W on 20m is very comfortable at 100W on 17m and 15m.