TermiLoop — Technical Overview
The TermiLoop is a broadband terminated HF loop engineered as a smarter alternative to the classic T2FD/BBTD. By folding the wire into an asymmetric end-fed loop and placing the resistive termination at a low-current, high-voltage point, it avoids the large waste losses of traditional designs while preserving smooth impedance and stable SWR across 160–6 m.
Ready to deploy
Complete kit available: 4:1 UNUN, resistor box, 20 cm spacers, hardware.
Specs at a Glance
| Spec | Details |
|---|---|
| Coverage | 160–6 m continuous (with tuner) |
| Total Wire | 26 m perimeter; <13 m span when folded with 20 cm spacers |
| Termination | ≈500 Ω non-inductive resistor at HV/low-current node |
| Feedpoint | 4:1 UNUN; cold side linked to resistor box via ~55 cm insulated wire |
| Enclosures | UNUN and resistor boxes mounted ~20 cm apart for compact layout |
| Power | 100 W standard; scalable to QRO with resistor upgrade |
| Typical SWR | <3:1 across HF (tuner-friendly) |
| RX Behavior | Lower noise than random wires; smooth broadband impedance |
| Anchoring | Eyebolts on ends; 2x carabiners, 2x thimbles, 2x bulldogs + for the far end a plastic egg insulator + bulldog clip, all included (bring rope) |
Maintenance Tip — Corrosion-Proof Connections
Apply AL-1100 aluminum paste on all aluminum joints during assembly to prevent galvanic corrosion and keep contact resistance low.
How often? One thorough application at install is typically sufficient. In coastal/polluted environments, reapply every 2–3 years during inspection.
Apply AL-1100 aluminum paste on all aluminum joints during assembly to prevent galvanic corrosion and keep contact resistance low.
How often? One thorough application at install is typically sufficient. In coastal/polluted environments, reapply every 2–3 years during inspection.
Physical Layout & Dimensions
- Total wire: 26 m. When folded with 12x 20 cm spacers, installed flattop span is <13 m, suitable for small gardens.
- Spreaders: 12x 20 cm spacing maintains conductor separation; strain-relief eyebolts ≈20 cm apart.
- Feedpoint: 4:1 UNUN; cold side linked to the resistor box (~55 cm insulated lead).
- Boxes: UNUN & resistor enclosures mounted ~15 cm apart; resistor box has ~45 cm vertical connector spacing.
- Termination: ≈500 Ω non-inductive load at the HV/low-current node.
Why This Geometry Works
- Lower loss than T2FD: Typical resistive dissipation ~15% (160–80 m) and ~20–25% (higher HF), versus 40–50% for classic T2FDs.
- Traveling-wave behavior: Load placement suppresses strong standing-wave resonances → stable SWR and predictable tuning.
- Impedance control: Native 100–300 Ω loop feed, transformed via 4:1 UNUN to ≈25–75 Ω — easy for internal tuners.
Why it outperforms classic terminated dipoles for NVIS
On 40–160 m the TermiLoop delivers stronger NVIS signals than traditional T2FD/BBTD-style antennas of the same size. The reason isn’t “more perimeter radiation” — it’s smarter geometry. By placing the ~500 Ω load at a low-current, high-voltage point, far less RF is wasted as heat compared to classic designs. That means more of your power actually goes into the wire and launches upward for reliable NVIS coverage, while the loop form keeps the impedance smooth and tuner-friendly across all HF bands.
On 40–160 m the TermiLoop delivers stronger NVIS signals than traditional T2FD/BBTD-style antennas of the same size. The reason isn’t “more perimeter radiation” — it’s smarter geometry. By placing the ~500 Ω load at a low-current, high-voltage point, far less RF is wasted as heat compared to classic designs. That means more of your power actually goes into the wire and launches upward for reliable NVIS coverage, while the loop form keeps the impedance smooth and tuner-friendly across all HF bands.
Quiet Receive Characteristics
- Loop advantage: Reduced E-field pickup → lower RX noise than random wires.
- Common-mode damping: The termination damps CM currents, further lowering noise ingress.
- Broadband RX: Smooth impedance and pattern suit SDR/skimmer monitoring.
Role of the 20 cm Spacers
- EM control: Preserves folded-loop effect without excessive wind load.
- Mechanical order: Keeps conductors parallel, preventing twist/sag.
- Deployment: Easier handling; resists tangling during setup.
Design Clarifications (quick answers)
- Why not put the resistor after ~13 m? The load sits at a low-current, high-voltage node to minimize resistive heating and maximize radiated power; placing it where current is high wastes RF as heat.
- Why a loop, not a straight wire? The folded loop establishes a traveling-wave path that smooths impedance and suppresses strong resonances, keeping SWR stable across 160–6 m and cutting common-mode pickup.
- Why parallel wires (20 cm spacing)? Controlled spacing preserves the folded-loop effect for predictable current distribution (flatter SWR) and locks in the geometry mechanically so performance stays consistent.
Band Behavior
- 160–80 m: NVIS at modest heights with solid coverage for the footprint.
- 60–40 m: NVIS + low-angle DX mix.
- 30–10 m: Forward bias in the “hot-leg” direction; works well as a sloper.
Feedline & Common-Mode Control
- 100W add 8 m coax + ferrite choke (~100 W) to suppress return currents.
- QRO: add a high-power line isolator: 2025-F-010-30 or 9 kW +45 dB CMR.
- Coax routing: Leave the loop at right angles for the first meters; avoid long parallel runs along metalwork. At least 48m of coax before the final shack choke !
Choke & Coax Length Requirement
For stable SWR and proper common-mode suppression, install the first high-CMR choke at about 0.05 λ from the feedpoint (≈8 m reference). After this, run at least 48 m of coax before the final shack choke. This spacing preserves balanced current distribution and prevents feedline resonances that can skew SWR on 60 m and below.
For stable SWR and proper common-mode suppression, install the first high-CMR choke at about 0.05 λ from the feedpoint (≈8 m reference). After this, run at least 48 m of coax before the final shack choke. This spacing preserves balanced current distribution and prevents feedline resonances that can skew SWR on 60 m and below.
Installation Notes
- Height: 5–6 m favors NVIS; 7–8 m balanced; 10–12 m more DX.
- Sloper trick: Feedpoint low, far end high → NVIS + DX combined.
- Trimming: Not required — the SWR remains unchanged thanks to the resistor and loop design.
VSWR Sweep
(Indicative) Expect small shifts with soil, height, and nearby structures.
Radiation Patterns
Patterns shown are simplified models for clarity — see why we use simplified models and not NEC. Real-world results vary with soil, height, and surroundings.
160 m
80 m
60 m
40 m
30 m
20 m
17 m
15 m
12 m
10 m
Install with confidence
Use the included choke, keep the 20 cm spacer separation consistent, route coax cleanly, and add a high-power isolator for QRO. Expect stable, tuner-friendly SWR on every HF band.
Mini-FAQ
- Does it need a tuner? — Yes, but SWR is under 3:1, so any rig tuner will handle it.
- Power handling? — 100 W as standard; upgrade the resistor for QRO.
- Receive-only? — No. TermiLoop is efficient for TX and excellent for broadband SDR/skimmer RX.
- Directional? — Omni on 160/80 m; develops forward bias from 40 m up, stronger as a sloper.
- Chokes & coax length? — Place the first choke at ~0.05 λ from the feedpoint (≈8 m reference), then run at least 48 m of coax before the shack choke to avoid feedline resonances.
- Why is NVIS stronger than classic T2FD/BBTD? — Load at a low-current, high-voltage node wastes less RF as heat, so more power is launched upward.
Interested in more technical content? Subscribe to our updates for deep-dive RF articles and lab notes.
Questions or experiences to share? Contact RF.Guru.