T2LT Antenna – The End-Fed Dipole That Looks Like a Dipole, But Isn't

The T2LT (Tuned Transmission Line Trap) antenna often sparks more debate than it deserves. At first glance, it appears to be a center-fed dipole built from coax — yet electrically, it’s a sleeve (coaxial) dipole, also known as a Resonant Feedline Dipole (RFD) or “Flower-Pot” antenna. The design uses the outer surface of the coax shield as one radiating leg and the continued center conductor as the other, while a high-impedance choke defines where the antenna ends and the feedline begins.

How It Works

Inside the coax, RF travels in TEM (differential) mode between the center conductor and the inside of the shield. Above the choke, RF current intentionally flows on the outer surface of the shield—that’s one λ/4 leg of the dipole—while the extended center conductor forms the other λ/4 leg. Below the choke, any current on the outside of the braid becomes unwanted common-mode current on the feedline. The choke’s job is to stop it there.

Physical Construction

• Top leg: Expose the coax center conductor and extend it with wire to roughly λ/4 (free-space start, trim for resonance).
• Lower leg (sleeve): The outer surface of the coax shield from feedpoint to choke forms the lower λ/4 radiator.
• Choke location: Place a high-impedance common-mode choke ≈ λ/4 below the feedpoint (measured along the coax). This defines where the antenna ends.

(Start slightly long and trim both legs for resonance; jacket proximity and nearby objects shift electrical length by several percent.)

The Choke That Makes It a T2LT

• Target impedance: ≥ 5 kΩ (resistive) at the operating band for reliable HF performance, especially QRO.
• Build options: Two FT240-31 toroids with 7–10 turns of RG-316/PTFE or RG-400 coax provide several-kΩ choking impedance on 20–10 m. Impedance scales approximately with turns².
• Bead strings: W2DU-style bead chokes can work, but you’ll need a long chain; eight snap-ons seldom exceed 3 kΩ on HF.
• Why resistive matters: A choke with high resistive impedance damps the system. If it’s mostly reactive, it may resonate with the feedline and worsen common-mode current.

Electrical Behavior

• Feedpoint impedance: Around 50–70 Ω when built correctly — no 49:1 transformer needed.
• Current maximum: At the electrical center of the half-wave (mid-span), as in any dipole.
• Voltage maximum: At the ends, not at the feedpoint — unlike a true high-Z EFHW.
• Without an effective choke: The feedline radiates, SWR varies with cable length, and noise increases.

Radiation Pattern & Mounting

• Vertical orientation: Near-omnidirectional pattern with low takeoff angles — excellent for DX.
• Horizontal orientation: Broadside lobes with end nulls, same as a half-wave dipole.
• Feedline routing: Keep metal and mast sections away for the first λ/4, or run the coax at right angles before descent.

Design Tips

• Use the simple half-wave formula L ≈ 143 / f(MHz) for the total radiator length (e.g., 10.07 m at 14.2 MHz), split roughly equally between upper wire and coax sleeve.
• Start long, then trim both legs while monitoring SWR or return loss.
• Add a second choke near the station if RF noise or common-mode coupling persists.
• For portable builds, PTFE coax and weatherproof chokes ensure long-term reliability.

Common Misconceptions

• "It’s a disguised EFHW." → Better described as an end-fed dipole. The T2LT is physically end-fed but electrically a center-fed sleeve dipole with a low-to-moderate feed impedance. It doesn’t behave like the high-impedance EFHW that needs a 49:1 transformer. In ham terminology, “end-fed” often implies a transformer-fed high-Z point, but here it simply means the feedpoint is located at the physical end of the coax radiator.
• "The braid isn’t part of the antenna." → Incorrect. The outer surface of the braid above the choke is one radiating leg of the dipole.
• "No choke needed — it’s balanced." → Wrong. Without a proper choke the feedline radiates, pattern distorts, and noise increases.
• "The choke must be exactly λ/4 from the tip." → Approximate. Use λ/4 as a design center and trim empirically; jacket and surroundings alter the electrical length.

Summary

The T2LT — or Resonant Feedline Dipole — is a physically end-fed but electrically center-fed half-wave antenna. The outer shield above a λ/4-placed choke forms one leg, and the continued center conductor forms the other. The choke, providing ≥ 5 kΩ resistive impedance, defines the feedpoint and prevents the feedline from radiating. When tuned correctly, it delivers the same clean pattern and efficiency as a dipole in an elegantly simple coax-only form — ideal for portable and field operations.

Because sometimes, looking balanced is just good disguise — the real trick is knowing where the current stops.

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