A single triangular wire loop with one coaxial feedline covers three complete HF bands without a tuner, without band-switching relays, and without separate antennas crowding the same mast:
- 80 metres(3.5–4 MHz) — the fundamental band, determined by the physical loop perimeter
- 40 metres (7–7.3 MHz) — derived by the 40M traps in the bottom element
- 30 metres (10.1–10.15 MHz) — derived by the 30M traps sitting inboard of the 40M traps
Understanding this antenna requires holding all three bands in mind simultaneously. Every component — the 80-foot slant legs, the trap sequence, the matching section — is doing a specific job for at least one of those bands, and nothing in this design is accidental.
For foundational reading on full-wave loop behaviour, the ARRL Antenna Handbook and ON4UN’s Low Band DXing both cover delta loop radiation patterns and feedpoint impedances with measured data across multiple configurations.
Trapped Delta Loop Antenna Geometry
The antenna is a triangle. A single apex hangs from one elevated support — a mast, tall tree, or tower — and two slant legs descend outward to anchor points at each side. The bottom of the triangle is a horizontal wire element carrying all four traps and the centre feedpoint insulator.
The slant legs are each approximately 80 feet long and completely untapped. That length is deliberate. A full-wave loop on 80 metres needs a total perimeter of roughly 270 feet at the low end of the band. The two slant legs contribute 160 feet of that total, with the remaining wire coming from the bottom horizontal element between the two corner insulators. The legs are dimensioned to make 80M the fundamental resonance of this loop — 40M and 30M are then carved out of that framework by the traps.
On 80 metres, everything radiates: both slant legs carry RF, the full bottom element including the traps carries RF, and the feedpoint sees the impedance of a complete one-wavelength loop. This is the band the physical geometry was built around.
How the Traps Create 40M and 30M Operation
The bottom horizontal element is where the antenna earns its tri-band capability. From the centre insulator outward on each side, the wire passes through a 30M trap first, then a short wire section, then a 40M trap, and terminates at the corner insulator connecting to the base of the slant leg.
On 40 metres, the 40M traps resonate at approximately 7.1–7.15 MHz and present very high impedance at that frequency, effectively opening the circuit at that point. RF cannot travel beyond them toward the corner. The active loop on 40M consists of the inner bottom wire sections from centre to each 40M trap, plus both complete 80-foot slant legs — a combination that forms one electrical wavelength on 40 metres. The 30M traps, sitting below their resonant frequency on this band, appear inductive and electrically lengthen the inner wire sections to compensate for their shorter physical measurement.
On 30 metres, the 30M traps take over. At 10.1 MHz they block RF from reaching the wire between the 30M and 40M traps. The active loop shrinks to the short inner bottom sections on each side plus the two full slant legs — one electrical wavelength on 30 metres.
The two long slant legs participate in radiation on all three bands. Only the bottom element changes its effective electrical length as you change bands. This is precisely why the legs are left entirely plain — they are the common radiating backbone of the whole system across 80M, 40M, and 30M alike.
30M Traps — Construction Details
Each 30M trap sits 23 feet outward from the centre insulator along the bottom horizontal element. This is the innermost trap on each side, and it is wound to resonate at 10.1 MHz — the CW end of the 30 metre allocation.
At 10.1 MHz the trap’s parallel LC circuit hits resonance and presents very high impedance, blocking further RF travel toward the outer wire. Below that frequency — on both 40M and 80M — the same trap appears inductive and adds electrical length to the wire it is inserted in. This inductive contribution is part of why the physical inner wire section from centre to the 30M trap is shorter than a straightforward wavelength calculation for 30M would suggest.
A few practical points worth noting for the 30M trap build:
- Wind on a 1 to 1.5 inch PVC former using heavier gauge wire — coil Q directly affects efficiency, so fewer turns of thicker wire outperforms many turns of thin wire
- Use silver mica or ceramic disc capacitors for temperature stability at the resonant frequency
- Verify resonance at exactly 10.1 MHz with an antenna analyser before sealing the trap body
- Seal with self-amalgamating tape and enclose in UV-resistant PVC tube — the 30M trap carries both tensile load and RF current simultaneously
The trap resonance calculator is a practical online tool for working out coil dimensions and capacitor values for your chosen former diameter.
40M Traps — Dimensions and Behaviour
From each 30M trap, a 7-foot section of wire leads to the 40M trap. Beyond each 40M trap, a further 5-foot stub extends to the corner insulator at the base of the slant leg. These short, precisely cut sections determine where the 40M resonance falls within the band.
The 40M traps resonate at approximately 7.1–7.15 MHz. At this frequency they isolate the outer 5-foot stubs from the active loop circuit, and the antenna’s effective perimeter becomes one wavelength on 40 metres. On 30M, the 40M traps sit above their resonant frequency and appear capacitive — but since the 30M traps have already blocked RF before it reaches them, their behaviour on 30M has no practical consequence on the antenna’s 30M performance.
On 80M, both the 30M and 40M traps are well below their respective resonant frequencies and both appear inductive. The combined inductive loading from all four traps means the physical loop perimeter can be somewhat shorter than a plain 80M delta loop while still achieving resonance at 3.5–4 MHz — a genuine practical benefit for operators with limited real estate.
Choosing the 40M trap resonant frequency is a deliberate decision rather than a default:
- 7.0 MHz — better SWR at the CW low end, steeper rise toward the top of the band
- 7.15 MHz— centred curve, suits mixed SSB and CW operation across the full allocation
- 7.2 MHz — optimised for the voice segment, less ideal for CW
Quarter-Wave Matching Section for 80 Metres
The diagram labels this explicitly: “RG-59/U Quarter-Wave Matching Section for 75 Metres (40.1 ft, including choke coil).” In amateur radio convention, 75 metres and 80 metres refer toe main RG-9X feedline takes over.
For verification, VE3SQB’s online antenna design tools include a coaxial matching section calculator that confirms these values cleanly.
Choke Coil
Five turns of RG-9X wound into a coil of approximately one foot diameter form the common-mode choke immediately below the matching section. Its job is to prevent RF current from travelling back along the outer braid of the coaxial feedline toward the shack.
On a loop antenna this is particularly important. The loop is inherently a balanced structure, while the coaxial feedline is unbalanced. Without choking at the feedpoint, the feedline shield becomes part of the radiating circuit, the antenna’s symmetry breaks down, and the symptoms become quickly familiar to any HF operator — RF in the shack at higher power, SWR that varies depending on how the cable is routed across the floor, and elevated receive noise because the feedline is picking up local interference alongside the antenna signal.
Five turns at one foot diameter provides adequate choking reactance across 30M and 40M. On 80M the choking impedance of an air-wound coil is somewhat lower, but sufficient to suppress common-mode current at typical amateur power levels without the insertion loss that ferrite cores can introduce at higher powers.
A full-wave delta loop fed at the midpoint of the bottom element presents a feedpoint impedance in the range of 100 to 120 ohms. A direct connection to 50-ohm coax produces an SWR of roughly 2.2:1 to 2.4:1 — workable, but not clean, and worse at band edges. The quarter-wave section of RG-59/U at 75-ohm characteristic impedance transforms that feedpoint impedance down toward what the main RG-9X coaxial feedline expects. Using the quarter-wave transformer relationship — Zm² = Z1 × Z2 — gives 75² ÷ 50 = 112.5 ohms as the transformed load, squarely in the expected feedpoint range of this antenna on 80M.
RG-59/U with solid polyethylene dielectric has a velocity factor of approximately 0.66. Applied to a quarter-wave at 3.75 MHz, this produces a physical length of around 40 feet — matching exactly what the diagram specifies. The matching section connects at the centre insulator and runs down to the choke coil junction before thun legal power on 80M.
Support System and Mechanical Rigging
At each of the two bottom corners, the wire terminates through an egg insulator into the support rope assembly. The critical detail here is the screen door spring inserted in series with the support rope, combined with a fixed strain-limit loop. The spring absorbs dynamic wind loads — 80-foot wire legs in a moderate wind generate significant tension variation — while the strain-limit loop prevents the spring from being extended to its elastic limit under extreme load. This small assembly is the difference between an antenna that survives monsoon season intact and one that needs full rebuilding afterward.
The apex hangs from a loop and ring arrangement that lets the wire swing slightly without chafing at the top support. The centre insulator carries the combined pull of the full bottom element under tension and should be a rated strain type — commercial fibreglass or PTFE works well. Standard egg insulators are adequate at the corners.
Build Sequence
Getting the order of operations right saves significant effort on this antenna. A logical sequence:
- Wind all four traps and confirm resonance — 30M traps at 10.1 MHz, 40M traps at your chosen centre frequency — before cutting any antenna wire
- Assemble the bottom element outward from the centre insulator using the diagram dimensions, pre-sealing all trap connections at the bench before installation
- Cut the two slant legs to 80 feet and connect at the corners
- Haul the apex to its support first, then tension the two bottom corners until the loop approximates an equilateral triangle
- Attach the RG-59/U matching section at the centre insulator, wind the five-turn choke coil, and connect the RG-9X main feedline
Take a full SWR sweep across all three bands before any trimming. Start with 80M — the fundamental resonance the physical geometry was designed around — then check 40M and 30M. Trap loops are forgiving on first assembly and initial readings are usually close enough to operate without adjustment.
This tri-band trapped delta loop antenna is a serious antenna for a modest footprint. Three bands — 80 metres for evening regional contacts and low-band DX, 40 metres for the backbone of everyday HF activity, and 30 metres for the quietest and most interference-free WARC band experience available — all from one wire triangle, one feedline, and one mast. The 80-foot slant legs carry the fundamental loop resonance, the trap sequence hands off cleanly to 40M and 30M, and the quarter-wave matching section keeps the 80M feedpoint impedance honest without any additional components.
Build the traps carefully, verify each one before assembly, and this trapped delta loop antenna will reward you across three bands for years.
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