Delta Loop
A triangle of wire, one wavelength around. Hang it from one tall pole, point up. Fed near a bottom corner, it's vertically polarized: low-angle signals for DX from one pole, with no radials.
KENTUCKY PORTABLE CLUB • ANTENNA GUIDES
LOOPS & DELTA LOOPS
Full-wave loops are quiet, simple and work great from a single fiberglass pole. Small magnetic loops fit on a picnic table. Here's how to size, feed and tune both.
TWO FAMILIES
Big Loops and Small Loops
"Loop" means two very different antennas. Big loops are about one wavelength of wire. Small "magnetic" loops are a tiny fraction of a wavelength and need a tuning capacitor.
Quad / Square Loop
Same idea, four sides. A little more gain than a delta, but it needs two supports or spreaders. It's the building block of the cubical quad beam.
Horizontal Loop ("Skywire")
A big loop strung flat through trees, a wavelength or more around on your lowest band. Fed with ladder line and a tuner, it works on almost every band and is quiet on receive. Home-station favorite.
Magnetic Loop
A 3–4 ft hoop of copper or coax with a tuning capacitor. Tiny, quiet, and great for balconies and picnic tables. Very narrow bandwidth: you retune every few kHz. Watch the high voltage.
CALCULATOR
Full-Wave Loop Calculator
Wire lengths, side lengths, height and the matching section, for any band.
Fiberglass poles: 20, 26, 31 and 40 ft are common. Check your coax's spec sheet for the velocity factor (VF).
PORTABLE FAVORITE
A Delta Loop on One Pole
A 20 m delta is about 71 ft of wire, so it fits on a 31 ft fiberglass pole with room to spare. Where you feed it decides the polarization.
Three Feedpoints
| A · BOTTOM MIDDLE | Horizontal polarization. Higher-angle signals. Good for regional contacts (in-state hunters). |
| B · LOWER CORNER | A mix of both. Easy to reach. A good all-round choice. |
| C · SIDE, ¼ λ FROM TOP | Vertical polarization, low angle. Best for DX from a low loop. That's a short way up from the lower corner. |
Steps
- Cut the wire to the length from the calculator, plus a foot for connections and trimming.
- Put a small ring or insulator at the top. It slides over the pole tip or ties to the top section.
- Pick your feedpoint (A, B or C) and break the wire there with a small feedpoint box or insulator.
- Tie off the two bottom corners with cord to stakes, 3–7 ft off the ground. Keep the bottom wire out of reach of people.
- Feed it with a quarter-wave 75 Ω matching section, or a 2:1 unun, then a choke. See feeding ↓
- Tune by trimming or adding wire at the corners. Longer wire moves the resonance lower in frequency.
MATCHING
Feeding a Full-Wave Loop
A full-wave loop is usually about 100–130 Ω at a good height. Mounted low, it can drop toward 70 Ω. That's too high for 50 Ω coax straight in, so match it one of these ways.
Quarter-Wave 75 Ω Section
A ¼-wavelength piece of 75 Ω coax (RG-6, RG-59, RG-11) between the loop and your 50 Ω coax turns ~110 Ω into ~50 Ω. The calculator gives the length, including the velocity factor. It's single-band, plus odd harmonics (a 40 m section also works on 15 m).
2:1 Unun
A 2:1 transformer takes 100 Ω down to 50 Ω on any band. Build it like a tapped autotransformer on a mix 43 or 61 core, or buy one. Add a 1:1 choke after it, because the loop is balanced.
Ladder Line + Tuner
Run 300 or 450 Ω ladder line to a 4:1 balun and tuner. Now a 40 m loop also works on 30, 20, 17, 15 and 10 m. This is how most horizontal "skywire" loops are fed.
SMALL LOOPS
Magnetic Loops
A small hoop (usually 3–4 ft across) tuned with a capacitor. It works because the loop and capacitor resonate together. That makes it efficient for its size, but very sharply tuned.
The Parts
The main loop and capacitor make a tuned circuit. The little coupling loop at the bottom matches it to 50 Ω. Make the coupling loop about ⅕ the diameter of the main loop, from coax or stiff wire.
Mag Loop Facts
- Fatter is better. Loss is all about conductor resistance. ½–1" copper tubing beats wire, and LMR-400 shield beats RG-58.
- Bigger is better on the lower bands. A 3 ft loop on 40 m is only a few percent efficient. On 20 m it can be over half.
- Narrow bandwidth. Often 10–30 kHz on 20 m. Retune when you move. A motor or long tuning shaft helps keep your hand out of the field.
- The capacitor sees thousands of volts at 100 W, and still hundreds at QRP. Use a wide-spaced air variable, a butterfly cap or a vacuum variable rated for it. Never touch the loop or capacitor while transmitting.
- Stay back. The field close to a mag loop is intense. Check your distance with the RF exposure tool, and keep people away while you transmit.
- No radials, no ground. That's why they're popular for balconies, HOAs and picnic tables.
CALCULATOR
Magnetic Loop Calculator
Inductance, tuning capacitance, efficiency, bandwidth and capacitor voltage for a round loop.
Estimates for a single-turn round loop with good joints and a low-loss capacitor. Real loops usually do a little worse.
FIELD NOTES
Loop Tips
- Loops are quiet. A closed loop hears less local noise than a dipole or vertical. Many hams keep a loop just for receive.
- Higher is better for horizontal loops, but a vertically polarized delta works well with its bottom only a few feet up.
- Keep the bottom wire safe. Run it above head height, or flag it and keep people out of the triangle. At 100 W the wire carries a real voltage.
- Insulated wire runs a bit slow. Plan on cutting 2–4% shorter than the formula, or just cut long and trim.
- Choke the feedline right at the feedpoint. A loop is balanced and coax isn't. Choke recipes
- Second harmonic is a mismatch. A 40 m loop on 20 m is high impedance (~200 Ω+). Use ladder line and a tuner if you want more than one band.
- Mag loop tuning: listen for the noise to peak sharply while you turn the capacitor. That's resonance. Then fine-tune for lowest SWR at low power.
- Mag loops on the ground lose efficiency. Get the bottom at least one loop diameter up if you can.
Get Outside. Get On The Air.
CLOSE THE LOOP
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