Build a 40m Delta Loop Antenna
The delta loop is a full-wavelength loop antenna stretched into a triangular shape — combining the efficiency advantages of a full-wave loop (approximately 2 dB gain over a dipole in the favored direction) with a support structure that needs only one tall apex point rather than the two tall supports a horizontal loop requires. A 40m delta loop covers 140 feet of wire in a triangle that fits most residential properties, delivers competitive DX performance on 40m, and provides useful multi-band operation on 20m and 15m from a single installation. This guide covers every feed position option, the geometry trade-offs, and a complete step-by-step build process.
Full-Wave Loop Physics
A delta loop is a closed full-wavelength wire loop in the shape of a triangle. Because the circumference equals one full wavelength at the operating frequency, the current distribution around the loop creates constructive reinforcement in specific directions — producing gain over a simple dipole. The theoretical gain of a full-wave loop is approximately 1.8–2.2 dBd (decibels over a dipole), depending on orientation, height, and the presence of ground effects.
The delta loop's behavior at harmonics is also valuable:
- At the fundamental (40m): the loop is one full wavelength and presents its characteristic radiation pattern
- At 2× the fundamental (20m): the loop is two full wavelengths — produces a different pattern with more lobes and somewhat higher gain in some directions
- At 3× the fundamental (15m): three full wavelengths — additional pattern complexity but still useful radiation
- With a tuner, usable on 80m, 10m, 17m, 12m as well — the complete loop wire with ladder line feed covers all HF bands
Delta vs Horizontal Loop vs Vertical Loop
A full-wave loop can be oriented in three ways — each producing a different radiation pattern and requiring different support structures:
- Delta loop (this guide): triangular, apex up. One tall support (the apex) plus two low anchors. The horizontal base wire near the ground gives the antenna some low-angle radiation mixed with higher-angle radiation. Practical for most properties — only one tall support needed.
- Horizontal loop (sky loop): flat, all wire at the same height. Requires supports at all corners. On 40m, a horizontal loop at 30+ feet radiates primarily straight up (NVIS) — excellent for regional 40m contacts, limited DX performance. Needs 4+ supports at significant height.
- Vertical loop: the loop plane is vertical, like a large rectangular or circular frame standing upright. Requires a very tall support (70+ feet for 40m). Produces low-angle radiation ideal for DX — the best orientation for DX performance. Most difficult to install.
- Delta loop tilted: apex side high, base side low — this guide's configuration. A compromise between the full vertical loop (best DX) and the horizontal loop (best NVIS). Very practical for most installations.
The apex-up delta loop in this guide produces a mixed pattern suitable for both regional and DX contacts — a practical choice for most operators.
| Feed Position | Location on Loop | Feedpoint Impedance | Matching Needed | Polarization | Best For |
|---|---|---|---|---|---|
| Bottom center | Midpoint of the base wire | ~100–120Ω | 2:1 or 4:1 balun to 50Ω | Horizontal | Simplest install; horizontal pattern; regional contacts |
| Bottom corner | One lower corner of the triangle | ~50–75Ω | 1:1 current choke to 50Ω | Mixed | Near-direct coax feed; convenient corner access |
| 1/3 up one side | One-third of the way up a vertical side | ~50Ω | 1:1 current choke | Vertical (dominant) | Low-angle DX radiation; best for DX performance |
| Apex | Top of the triangle | ~100–130Ω | 2:1 or 4:1 balun to 50Ω | Vertical | Best vertical polarization; difficult access at height |
| Ladder line (any position) | Any — routed to shack tuner | Variable | Balanced tuner in shack | Both | All-band operation; maximum flexibility |
For most builders: bottom corner feed is the most practical — near-direct 50Ω feed, easy ground-level access, and good mixed polarization for both regional and DX contacts. The 1/3-up-one-side position is the best choice if DX performance is the primary goal.
Complete materials for the standard bottom-corner-fed delta loop
Building the 40m Delta Loop
Survey the installation site first, then build the feedpoint assembly, cut the wire, and raise the loop. Allow 3–4 hours for the complete build and SWR verification session.
Plan the Triangle Geometry
A delta loop can take many triangular shapes — equilateral (all sides equal) or isosceles (two sides equal, one different). The equilateral triangle is the most common choice and the easiest to plan. For a 40m delta loop with 140 feet of wire, an equilateral triangle has sides of approximately 46.7 feet each.
Walk the site and identify: a tall tree or mast at the desired apex height (minimum 35 feet, 47 feet for an equilateral triangle with base at ground level), and two base anchor points approximately 47 feet from the apex horizontally and approximately 47 feet apart from each other.
Build the Current Choke
Wind 8 turns of RG-8X coax through the FT-240-31 toroid and mount in the Hammond enclosure. Install SO-239 on the coax output side and two wire terminals (stainless binding posts) on the antenna side — these two terminals connect to the two wire ends of the loop at the feed corner. The current choke prevents RF from flowing on the coax outer shield and keeps the loop's radiation pattern clean.
A current choke is particularly important on loop antennas because the loop's asymmetric current distribution at harmonics creates strong common-mode tendencies. Without a choke, the coax becomes a radiating element on 20m and 15m where the loop's current distribution is more complex.
Cut the Wire and Plan the Loop
Cut a single continuous piece of wire to 144.8 feet (140.6 ft + 3% for trimming room). The delta loop is made from one unbroken piece of wire — both ends of this wire come together at the feed corner and connect to the two binding posts on the choke enclosure. The wire leaves the feedpoint, travels up one side of the triangle to the apex, across the top wire or down the other slanted side and then along the base to complete the loop back to the feedpoint.
Wait to cut the wire until you know the exact perimeter of your planned triangle — measure the actual distances on site using a measuring tape rather than relying on calculated geometry alone. Site geometry is rarely perfectly equilateral and actual ground conditions often require adjustments from the theoretical plan.
Prepare the Feedpoint Corner
The feedpoint corner is one of the two base corners of the triangle — typically the one closest to the shack for convenient coax routing. At this corner, both ends of the single wire loop come together and connect to the two binding post terminals on the current choke enclosure.
Strip 1.5 inches of insulation from each end of the loop wire. Form loops with round-nose pliers. Connect one end to one binding post and the other end to the other binding post — the polarity does not matter for a symmetric delta loop at the fundamental frequency. Apply No-Ox-Id to both connections and tighten securely. The feedpoint connection must be both electrically reliable and mechanically strong — it carries the tension of the entire loop perimeter.
Install Corner Insulators on the Non-Feed Corners
At the apex and the second base corner, install egg insulators in the loop wire. These insulators provide the mechanical attachment points to the support ropes while keeping the wire electrically continuous through the loop. To install a mid-wire insulator: at the planned corner position, cut the wire, thread each end through the insulator holes, and solder a mechanical wrap — doubling back each end 3 inches and wrapping 4 times before soldering. The result is an insulator with the continuous loop wire running through it, held mechanically at each corner.
Raise the Apex Support
Get a rope over the target apex branch or mast point at the planned height. Use a throw weight and line for tree installations — a 50g throw weight on 40m of thin braided line reaches most suitable branches. Pull a support rope through the throw line. Tie the apex insulator to the support rope.
For a mast installation: attach the apex insulator to the mast at the highest practical point using a weatherproof eyebolt or mast ring. Ensure the apex attachment hardware is rated for the weight of the wire loop plus wind loading — a 140-foot wire loop in 30 mph wind generates significant uplift and lateral forces at the apex.
Raise the Apex to Height
With the apex insulator attached to the support rope, haul the apex to the target height. As the apex rises, the two wire sides of the triangle begin to take shape. Have a helper manage the wire to prevent tangling as the apex is raised — a 140-foot wire loop hoisted from the apex creates significant tangle risk if not carefully managed from the ground.
Once the apex is at height, pull both base corners of the triangle outward to their planned ground anchor positions. Secure the feedpoint corner enclosure to its anchor point — this is typically a fence post, garden stake, or small PVC mast driven into the ground. Secure the far base corner insulator to its anchor point.
Tension and Shape the Loop
With all three corners secured, adjust the tension on each side of the triangle. The goal is a wire loop with visible tension — not tight enough to risk breaking the wire or overstressing the support points, but taut enough that the wire is not sagging significantly. Some sag is normal and acceptable, especially in the base wire which may have up to 2–3 feet of sag at the mid-point on a 47-foot base span.
Verify that the loop wire is not touching the mast, tree trunk, or any metal structure along its length. Even a brief contact point with a metal structure introduces a ground path that can dramatically alter the antenna's electrical performance. Check all three sides carefully for contact points.
Initial SWR Sweep on 40m
Connect the NanoVNA at the shack end of the coax. Sweep 6.8 to 7.5 MHz. A full-wave loop presents a very clean, narrow SWR dip at its resonant frequency — narrower and sharper than a dipole's SWR curve. Look for the dip and note its frequency and minimum SWR value.
With the loop wire cut to 144.8 feet for a 7.150 MHz target, expect resonance around 7.0–7.1 MHz before trimming. Typical minimum SWR at a bottom corner feedpoint with 1:1 current choke: 1.2–1.8:1 (the feedpoint impedance at a loop corner is naturally close to 50Ω).
Trim to Target Frequency
Trim the loop wire in increments to raise 40m resonance to 7.150 MHz. Since the loop is a single continuous wire, trimming can be done at any convenient access point — the far base corner is usually the easiest to lower temporarily for trimming. Cut equal amounts from both wire ends at that corner to keep the geometry symmetric. Re-attach after each trim and re-sweep before cutting further.
Work conservatively — 3-inch increments once you are within 50 kHz of the target. The loop's resonance changes somewhat with the exact triangle geometry, so the trim rate is an approximation. Final trim amounts will depend on the specific installation geometry.
Check 20m and 15m SWR
After 40m is tuned, sweep 20m and 15m. The delta loop does not resonate as cleanly on 20m and 15m as it does on 40m — the harmonic behavior of a loop is more complex than a dipole's simple harmonic series. Typical results:
- 20m sweep (13.5–15 MHz): expect SWR of 1.5–3.5:1. The loop at 2× its fundamental frequency may or may not show a clean dip near 14.200 MHz depending on the exact geometry and installation height. A tuner handles 20m comfortably if SWR is above 2.5:1.
- 15m sweep (20.5–22 MHz): expect SWR of 1.5–4:1. Similar variability to 20m. Some installations show excellent 15m SWR; others are more marginal.
If 20m or 15m SWR is unacceptably high and a tuner is not available, feed the loop with 450Ω ladder line to a balanced tuner for full multi-band coverage at maximum efficiency. Coax-fed loops accept the constraint that 20m and 15m may need a tuner.
Weatherproof and Document
Apply self-amalgamating tape to the feedpoint enclosure SO-239 connector and the two wire terminal entry points. Wrap from the enclosure outward onto the wire/coax for 2 inches on each connection. Apply PVC tape over the self-amalgamating tape for UV protection.
Record the final loop wire length (total after all trimming), the triangle dimensions (side lengths and apex height), the feedpoint location (which corner), the resonant frequency and minimum SWR on 40m, the SWR on 20m and 15m, and the installation date. Photograph the complete loop from multiple angles — the triangular shape of a delta loop is difficult to visualize from a single image; multiple photos capture the geometry. Inspect the apex attachment annually for rope wear and chafing.
Delta Loop vs 40m Dipole — Honest Comparison
The delta loop is often promoted as dramatically superior to a dipole, which overstates the real-world difference. An honest comparison for a typical residential installation:
- Gain: approximately 1.5–2.0 dBd advantage for the loop in the favored direction — equivalent to roughly doubling the transmitter power. Real but modest in on-air use.
- Bandwidth: the delta loop is narrower than a dipole — SWR rises more steeply away from resonance. The dipole wins for wide-band coverage without a tuner.
- Noise: loops generally have lower ambient noise pickup than dipoles of the same height — a real advantage on receive, particularly on 40m where local interference is common.
- Height requirement: the loop needs a 47-foot apex for a proper equilateral triangle; a 40m dipole needs only 30 feet for its center. The loop's tall support requirement is a genuine disadvantage for many installations.
- Multi-band: both work on harmonics with a tuner. Neither is dramatically better than the other for multi-band use with a tuner and ladder line.
- Construction: the delta loop is more complex to build and install than a dipole — three support points, a continuous wire loop, and more careful geometry planning required.
The Delta Loop as a 2-Element Beam System
A 40m delta loop can be elevated into a 2-element beam system by adding a second parasitic element — either a reflector or director loop — at a spacing of approximately 0.2λ (28 feet for 40m). Two delta loops in this configuration produce 4–5 dBd of gain in the forward direction with a front-to-back ratio of 10–15 dB — competitive with a simple 2-element Yagi on 40m.
This "bi-square" or "two-element loop beam" configuration requires two apex supports 28 feet apart and four base anchors. While complex to build and requiring a phasing or switching network, it represents one of the most powerful fixed-station low-band antenna systems achievable with wire. For operators who are serious about 40m DX and have the space, two delta loops with a 10-meter element spacing is worth serious consideration.
Phased vertical arrays →Feeding with Ladder Line for All-Band Use
For operators who want the delta loop to cover all HF bands — not just 40m, 20m, and 15m — feeding the loop with 450Ω ladder line to a balanced antenna tuner provides complete HF band coverage with low feedline loss regardless of SWR:
- Connect ladder line directly to the feed corner of the loop — no balun or choke needed at the antenna
- Route the ladder line to the shack maintaining 6 inches clearance from all metal structures
- Connect to a balanced (Z-match, Johnson Matchbox, or commercial balanced tuner) in the shack
- The tuner handles 80m, 30m, 17m, 12m, and 10m in addition to the primary 40m, 20m, and 15m bands
- Feedline loss with ladder line is very low even at high SWR — acceptable on all HF bands
- This all-band approach makes the 40m delta loop one of the most versatile HF installations possible from a single wire structure
Installing the Loop in Tight Spaces
The delta loop's triangular geometry can be adapted for limited properties where a perfect equilateral triangle is not achievable:
- Tall narrow triangle: apex at 60 feet, base only 25 feet wide — the two slanted sides are longer (approximately 60 feet each), the base shorter (approximately 20 feet). Increases the vertical-wire fraction and lowers the radiation angle — actually improves DX performance over an equilateral triangle.
- Bent base: the base wire can have a bend to avoid obstacles — a slight bend in the base has minimal effect on performance. A 90° angle in the base is acceptable if necessary.
- Tilted plane: the entire triangle can be tilted from the vertical plane — an apex high on one side and both base corners at different heights. This produces an asymmetric radiation pattern but still works well as a loop antenna.
- Inverted delta: apex at the bottom (low), base at the top (high between two high supports). Requires two tall supports instead of one. Produces a different radiation pattern with more horizontal polarization — a sky loop variant.
Does the delta loop need a ground or radial system?
No — this is one of the delta loop's significant advantages over vertical antennas. The full-wave loop is a balanced antenna that does not rely on ground for the return current path. All the current flows within the loop wire itself. No radials, no ground rod connection to the antenna, no ground system required at the feedpoint. A current choke at the feedpoint prevents the coax from becoming a ground path, but this is a common-mode management concern rather than a ground system requirement. The delta loop is an excellent antenna for sites with poor ground conductivity where verticals perform badly.
Why is my 40m SWR good but 20m SWR high?
A delta loop's harmonic behavior is more complex than a dipole's because the loop's current distribution at harmonic frequencies creates multiple radiation lobes and complex impedance patterns. The feedpoint impedance at 20m (twice the fundamental) depends on where around the loop the feedpoint is located relative to the current peaks at that frequency. If the feedpoint is near a voltage maximum at 20m, SWR will be high. Moving the feedpoint to a different position (such as feeding 1/3 of the way up one side rather than at a base corner) can dramatically improve 20m SWR. Alternatively, feed the loop with ladder line to a balanced tuner for a solution that works on all bands without this concern.
What is the gain of a 40m delta loop at typical residential heights?
At an apex height of 40–50 feet with a bottom corner feed, the 40m delta loop produces approximately 1.5–2.0 dBd of gain compared to a dipole at the same height. This gain is in specific directions determined by the loop orientation and height — broadside to the loop plane for horizontal polarization components, and in the direction of current flow for vertical polarization components. The gain figure that matters for DX is at low takeoff angles, where the loop at 47 feet apex height shows approximately 1.5 dBd improvement over a dipole at the same height. Raising the apex to 65+ feet noticeably improves low-angle performance.
Can I use the delta loop on 80m?
Yes, with a tuner. The 140-foot wire loop is approximately half a wavelength on 80m — it works as an antenna but SWR without a tuner is high (typically 5:1 or more). With a tuner, the delta loop makes an effective 80m antenna with better efficiency than a shortened antenna. The radiation pattern on 80m when used as a half-wave antenna is similar to a horizontal dipole at the installation height, which provides NVIS regional coverage at typical residential heights. For regular 80m DX from a delta loop, feeding with ladder line and a balanced tuner gives better results than trying to match the high SWR through a shack tuner via coax.
How do I orient the delta loop for best DX on 40m?
For a bottom-corner-fed apex-up delta loop, the maximum radiation is broadside to the plane of the triangle — perpendicular to the direction the loop faces. Orient the loop so the plane faces your primary DX target direction. For US East Coast operators targeting Europe: orient the loop plane east-west (the plane "faces" north-south, but broadside radiation goes east-west). The effect is modest — the loop is not as directional as a Yagi — but consistent orientation toward the target DX region extracts the antenna's natural directional advantage. For operators whose primary DX interest is in multiple directions, the loop's modest directivity is not a significant limitation.
Is a rectangular or square loop better than the triangular delta loop?
A square or rectangular full-wave loop has marginally more gain than an equilateral delta loop (approximately 0.3–0.5 dB) because the square distributes more wire at maximum current amplitude. However, a square loop requires four support points at significant height — a much more demanding installation. The delta loop's practical advantage is requiring only one tall support point. Most operators who could manage the four supports needed for a square loop at useful height on 40m would be better served by that additional effort going into a 2-element Yagi, which provides far more gain than the 0.5 dB difference between a delta and a square loop. For wire antenna installations, the delta loop is the most practical loop choice for most operators.