Ham Radio Loop Antennas — Complete Guide

Why a Loop Outperforms a Dipole

A full-wave loop antenna is electrically equivalent to two half-wave dipoles fed in phase — one horizontal section stacked above the other, producing approximately 2 dBd of gain over a single dipole. This is not magic — it is the same principle behind any stacked antenna system. The loop achieves the stacking effect in a single closed conductor.

The gain advantage manifests differently depending on loop orientation and feed position:

• Horizontal loop (flat, looking down): gain is broadside to the loop plane — like a large horizontal dipole with improved low-angle radiation
• Vertical loop (standing up): gain is broadside to the loop plane — perpendicular to the wire
• Delta loop fed at apex: predominantly vertical polarization, low-angle radiation — competitive with a vertical for DX
• Delta loop fed at base: predominantly horizontal polarization — similar pattern to a high horizontal dipole

The second major advantage is receive noise rejection. A loop antenna responds primarily to the magnetic component of arriving electromagnetic waves and is less sensitive to near-field electric-field noise from power lines, switching supplies, and other local noise sources. In noisy suburban environments this can make the difference between a readable signal and a QRM nightmare.

Loop Shape — Square, Delta, or Circle?

All three shapes enclose the same wire circumference (one full wavelength) and therefore resonate at the same frequency. Their performance differences are small in practice:

• Square loop — most symmetrical, cleanest pattern, easiest to model accurately. Feed impedance ~100Ω at the bottom center. Requires 4 support points.
• Delta loop (equilateral triangle) — most popular for practical installations. Apex at the top needs only two high supports; base corners can be low. Feed at apex gives ~50Ω; feed at base corner gives ~100Ω.
• Right-triangle delta — one vertical side, one horizontal top, one sloping hypotenuse. Useful when one tall support and one horizontal span are available. Feed characteristics vary with configuration.
• Circular loop — theoretically highest efficiency (highest radiation resistance, lowest loss) but impractical to support and align. Performance improvement over a square loop is less than 0.5 dB in practice — rarely worth the support difficulty.

The delta loop is the practical winner for most installations — two high supports plus two low anchor points is achievable on most properties, and the 50Ω apex feed eliminates the need for a matching transformer.

Feed Systems and Impedance

Feed impedance for a full-wave loop varies significantly with shape and feed position. Getting the right impedance at the feedpoint is important for efficient power transfer and accurate SWR measurements:

Matching options for each impedance:

• 50Ω feedpoint (delta apex): direct 50Ω coax with 1:1 current choke — simplest option
• 100Ω feedpoint: use a 2:1 balun, or a 75Ω coax quarter-wave matching section between the antenna and 50Ω coax
• For multi-band operation with a tuner: use ladder line (300–600Ω) from the feedpoint to a balanced tuner at the radio — the high-impedance ladder line handles SWR variation across bands with minimal loss

The multi-band ladder line approach is by far the most effective for covering 40m through 10m (or 80m through 10m for larger loops) from a single antenna installation. The low-loss characteristics of ladder line at high SWR make it far superior to coax for this application.

Multi-Band Operation with Ladder Line

A full-wave loop fed with ladder line and a balanced tuner is one of the most effective multi-band antenna systems available at any price. The key reason: ladder line has a matched-line loss of approximately 0.1 dB per 100 feet at 30 MHz regardless of SWR — compared to RG-8X which shows 1.7 dB/100ft matched and substantially more at high SWR.

How the system works:

• Build the loop for the lowest desired band (e.g., 40m → 134 ft total wire)
• Feed with 300Ω or 450Ω ladder line from the feedpoint to the shack
• At the shack, connect to a balanced (link-coupled) tuner or a 4:1 balun followed by an unbalanced tuner
• The tuner presents 50Ω to the radio on each band regardless of antenna impedance
• A 40m loop works on 40m, 20m, 15m, 10m, 17m, 12m, and 30m with appropriate tuner settings
• A 80m loop works on all bands from 80m upward

The balanced tuner option (Z-match, MFJ balanced, or commercial balanced tuner) is preferable because it keeps the feedline balanced throughout, reducing common-mode current issues. A 4:1 balun at the unbalanced tuner works but can introduce common-mode problems if the balun is not high-quality.

Building and Supporting a Square Loop

A square full-wave loop has four equal sides, fed at the center of one side. The feedpoint is typically at the bottom center, allowing the coax or ladder line to drop straight down from the feed. Four support points are needed — one at each corner.

Practical support options:

• Four tree branches or trunks at roughly equal distances from center — ideal if available
• Four lightweight fiberglass poles (Jackite, Spiderbeam, or similar) at the corners
• Two masts supporting the top corners, with the bottom corners pegged or staked to the ground
• The loop does not need to be perfectly square — a rectangle within 20% of square dimensions performs nearly identically
• The plane of the loop should be as close to vertical as possible for maximum gain broadside to the loop

At the bottom-center feedpoint, the impedance is approximately 100Ω. A 2:1 current balun transforms this to 50Ω for direct coax connection. Alternatively, a 75Ω coax quarter-wave matching section provides the same transformation with less complexity.

Radiation Pattern of a Vertical Square Loop

A vertical square loop fed at the bottom center radiates broadside to the loop plane — strongest perpendicular to the wire plane, with deep nulls in the plane of the loop (off the edges). This produces a bidirectional pattern similar to a dipole but rotated 90°.

Unlike a dipole, where the nulls are off the wire ends, the loop's nulls are in the plane of the loop. This gives the loop an interesting interference-rejection capability: rotating the loop by 90° brings the null to bear on an unwanted signal while maintaining gain toward the desired station.

• Peak gain: ~2 dBd broadside to the loop plane
• Null depth: 20–30 dB in the loop plane — effective for interference rejection
• Elevation angle: similar to a dipole at the same height — varies with loop height above ground
• Polarization: vertical when fed at the bottom center; mix of polarizations when fed at a corner

For fixed stations, orienting the loop broadside toward the primary target DX direction maximizes gain. The nulls fall naturally toward local noise sources if the loop is oriented thoughtfully.

How Magnetic Loops Work

A magnetic loop (also called a small transmitting loop or STL) is fundamentally different from a full-wave loop. It is a highly resonant LC circuit where the loop conductor forms the inductance and a variable capacitor provides the tuning capacitance. The loop circumference is typically λ/10 or less at the operating frequency — far shorter than a resonant full-wave loop.

Because the loop is electrically small, it responds primarily to the magnetic component of arriving electromagnetic waves rather than the electric component. This gives it a unique property: it is relatively insensitive to near-field electric-field noise from power supplies, computers, and power lines — noise that overwhelms most wire antennas in urban environments.

• Operating principle: high-Q resonant LC circuit with the loop as L and a variable cap as C
• Very high Q factor (300–1000) — sharp resonance, requires retuning every 10–50 kHz
• Efficiency: 10–20 dB below a full-size antenna — real contacts are possible but challenging at QRO
• Maximum operating power is limited by voltage across the capacitor — typically 10–150W depending on construction
• Best size: loop diameter of λ/8 to λ/10 for best efficiency compromise

Building a Magnetic Loop — Key Variables

The efficiency of a magnetic loop is determined by three factors, in order of importance:

• Main loop conductor quality — the larger the diameter and better the conductivity, the lower the loss resistance. Copper pipe (3/4" or 1") is the best accessible conductor for most builders. Large-diameter coax (RG-213) is a popular alternative. The conductor must have very low resistance — any high-resistance joint in the loop dramatically reduces efficiency.
• Capacitor Q factor — the variable capacitor must have very low loss (high Q). A standard air-variable capacitor works but needs to handle high RF voltages — at 100W on 40m, the voltage across the capacitor can exceed 2000V. Butterfly or split-stator capacitors are preferred. Cheap capacitors burn out or arc over at elevated power levels.
• Loop diameter — larger diameter = higher efficiency. A 1-meter diameter loop on 40m is noticeably more efficient than a 0.5-meter loop. Make the loop as large as the available space permits.

Coupling to the feedline: a small coupling loop of approximately λ/5 the diameter of the main loop is mounted concentrically inside it. This provides a close-to-50Ω feedpoint without any matching network — the transformer coupling between the two loops handles the impedance transformation.