Build a Spiral Loop Antenna
A Spiral Loop takes the same full wavelength of wire a large single-turn loop would use and winds it into a tight multi-turn spiral instead, shrinking the physical footprint dramatically for balconies, small yards, or portable use. This is a genuine and honest size-for-performance tradeoff, not a free lunch: the tighter the spiral, the more efficiency and bandwidth you give up compared to a full-size single-turn loop. This guide covers the complete build from wire length and turn count through feedpoint assembly and tuning, with the tradeoffs stated plainly so you can decide if the compromise is worth it for your situation.
Same Total Wire Length, Much Smaller Footprint
The starting point is the same full-wave loop perimeter formula used by this site's delta loop, quad, and Bisquare designs — but instead of stringing that length of wire around one large single-turn loop, the Spiral Loop winds it into several smaller-diameter turns stacked close together, dramatically shrinking the space the antenna occupies.
Why Compacting a Loop Costs Efficiency
As turns are wound closer together to shrink the antenna, adjacent turns increasingly cancel each other's radiated field in the near zone — the same basic mechanism that makes a loaded/coiled mobile whip less efficient than a full-size vertical, or a small transmitting loop's tuning capacitor more critical than a full-size loop's feedpoint. The tighter the spiral, the more radiation resistance drops relative to loss resistance, and the lower the antenna's overall efficiency becomes.
- Fewer turns, larger diameter: closer to full-size loop performance, less size reduction.
- More turns, smaller diameter: much smaller footprint, but a genuinely less efficient antenna with narrower usable bandwidth.
Where This Design Makes Sense
The Spiral Loop is honestly a compromise antenna for situations where a full-size loop, dipole, or vertical genuinely won't fit — an apartment balcony, a small urban lot, or a portable kit with strict size limits. Where you do have room for a full-size loop or a simple dipole, those designs will outperform a tightly wound Spiral Loop on the same band, often by a meaningful margin.
Feedpoint and Matching
Feedpoint impedance for a spiral loop varies considerably with turn count and spacing, generally trending lower than a comparable single-turn loop as more turns are added — expect to need an antenna tuner rather than a direct 50Ω match, and expect the SWR bandwidth to be narrower than a full-size loop of the same wire length.
| Band | Full wire length (ft) | Approx. diameter at 6 turns (ft) | Notes |
|---|---|---|---|
| 40m (7.15 MHz) | 140.6 ft | ~7.5 ft | Still a fairly large spiral even compacted |
| 20m (14.15 MHz) | 71.0 ft | ~3.8 ft | A practical balcony/small-yard size at 6 turns |
| 15m (21.2 MHz) | 47.4 ft | ~2.5 ft | |
| 10m (28.4 MHz) | 35.4 ft | ~1.9 ft | Small enough for a compact portable frame |
Spiral Loop Calculator
Materials for a complete Spiral Loop build
Building the Spiral Loop Antenna
Choose your turn count first — more turns means a smaller footprint but more efficiency lost, so pick the smallest turn count your space will actually accommodate.
Build the Frame
Construct or assemble a non-conductive frame sized to the spiral diameter from the calculator, with mounting points for standoff insulators at each turn's radius.
Cut the Full Wire Length
Cut the total wire length from the calculator, plus a small trim allowance.
Wind the Spiral
Starting from the outermost or innermost point (your choice, consistent with your feed plan), wind the wire around the frame at consistent spacing for each turn, securing it to standoff insulators as you go.
Install the Feedpoint
Connect your feedline at one end of the completed spiral, with the other end left open (unterminated) unless your specific design calls for a closed-loop configuration.
Mount the Completed Spiral
Mount the frame in your intended location — balcony rail, small yard stand, or portable support — keeping it clear of large metal objects that would detune it further.
Connect to Your Tuner and Verify SWR
Connect the coax to your antenna tuner and find a workable match at your target frequency. Given the design's inherently narrow bandwidth, expect to re-tune more often when moving across a band than you would with a full-size antenna.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Tuner can't find a match anywhere | Shorted turns, or feedpoint connection fault | Check for accidental contact between adjacent turns and inspect the feedpoint | Separate any touching turns with additional standoffs; re-solder the feedpoint |
| Signal reports much weaker than expected for the power used | Normal reduced efficiency for a tightly wound spiral | Compare turn count and diameter against the size/performance tradeoff described in this guide | Not necessarily a fault — this is the expected cost of the compact size; reduce turn count if you can accept a larger footprint |
| Extremely narrow usable bandwidth | Normal high-Q behavior for a tightly wound compact loop | Sweep SWR across the band and note how quickly it rises away from the tuned point | Expect to re-tune more often; this is inherent to the design, not a build error |
| Resonance drifts over time | Turn spacing shifting due to wind, temperature, or a loose frame | Check physical turn spacing against the original build | Re-secure loose standoffs; add additional support points if the frame flexes |
| SWR changes significantly with nearby objects | Compact loops are more sensitive to nearby detuning objects than full-size antennas | Check for metal objects, gutters, or railings near the spiral | Relocate away from nearby metal where possible |
Is this as good as a full-size loop on the same band?
No, and this guide doesn't claim otherwise — a full-size single-turn loop will outperform a Spiral Loop of the same total wire length, both in efficiency and in usable bandwidth. The Spiral Loop is a deliberate size-for-performance tradeoff for situations where the full-size version genuinely won't fit.
How many turns should I use?
As few as your available space allows — fewer turns (larger diameter) keeps you closer to full-size loop performance, while more turns (smaller diameter) shrinks the footprint further at a real efficiency cost. Don't add more turns than you actually need to fit your space.
Why is my bandwidth so narrow?
Compact spiral-wound loops are inherently higher-Q than full-size loops of the same wire length, meaning SWR rises faster as you move away from the tuned frequency. This is expected behavior, not a fault — plan on re-tuning more often within a band.
Can I use this for portable/field operation?
Yes — its compact size makes it a reasonable option for space-constrained portable setups, understanding the same efficiency tradeoffs apply as they would for a fixed installation.
What wire gauge should I use?
#16-18 AWG insulated stranded wire is a reasonable choice — thinner than what a full-power transmit loop might use elsewhere on this site, appropriate given this antenna's typical use at modest power levels in space-constrained settings.
Does the spiral need to be flat, or can it be a cylindrical coil instead?
Either geometry works in principle — a flat spiral (like a coiled garden hose) or a cylindrical helix (like a spring) both compact the same total wire length into a smaller footprint, with similar efficiency tradeoffs. Choose whichever shape fits your available mounting space better.