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

The T2LT (Tuned Transmission Line Trap) antenna is one of the most misunderstood antennas in amateur radio. At first glance, it appears to be a center-fed dipole fed with coax — but electrically, it is not. The T2LT is actually a cleverly constructed end-fed half-wave (EFHW) antenna disguised in dipole clothing. Its performance depends heavily on the suppression of common-mode currents using a tuned choke, which serves as the defining element that gives the T2LT its name.

Physical Construction

  • The T2LT uses coaxial cable as both the feedline and part of the radiating structure.
  • The center conductor of the coax extends to the top of the antenna and acts as the top half of the radiator.
  • The braid (outer shield) of the coax acts as the lower half of the radiator up to a point where a tuned sleeve choke is inserted.
  • The choke is placed roughly 1/4 wavelength from the tip of the antenna (i.e., 1/4 λ from the end, or 1/4 λ from the feedpoint).

Beyond the choke, the coax continues to the radio and must be properly decoupled to prevent the rest of the feedline from acting as a radiator.

Electrical Behavior

Despite the dipole-like layout, the T2LT behaves electrically as an end-fed λ/2 antenna:

  • The current maximum is at the center of the radiator, just like any half-wave antenna.
  • The voltage maximum is at the feedpoint, which is typical of EFHW designs.
  • Without a choke, the coax shield will continue to carry RF current, leading to severe common-mode radiation, feedline pickup, distorted radiation pattern, and noise.
  • The sleeve choke (sometimes made from a tubular ferrite sleeve, or a multi-turn toroidal choke) blocks RF current from continuing down the coaxial shield past the designated point.

Thus, the choke is not just optional; it is functionally mandatory to preserve pattern, symmetry, and reduce RFI.

It is important to understand that the coax braid in a T2LT radiates in differential mode, not common mode. This contradicts the widespread myth that any current on the coax is necessarily common mode. In the T2LT, the coax itself is part of the antenna's intended radiating structure. The ferrite choke forces the return current to follow a controlled path and prevents the feedline from continuing as part of the radiator. Since nearly half the RF power is handled by the braid portion of the radiator, this makes proper suppression of unwanted current beyond the choke even more critical. For reliable operation at high power (QRO) on 10 to 20 meters, a choke built from at least 8 sleeves of Type 31 material is recommended.

Radiation Pattern

If constructed and choked correctly, the T2LT has the same far-field pattern as a half-wave dipole:

  • When hung vertically, the pattern is omnidirectional in azimuth, with maximum radiation at low angles (good for DX).
  • When hung horizontally, the pattern shows broadside lobes and deep nulls off the ends, similar to a standard dipole.
  • The pattern becomes distorted if the choke fails to block common-mode current, causing asymmetry and undesired lobe formation.

Advantages

  • Simple construction from coaxial cable
  • No need for a center insulator or external balun
  • Low SWR at resonance, as it's a resonant antenna
  • Stealthy and compact for portable ops

Design Considerations

  • The sleeve choke must provide at least 3–5 kΩ impedance on the operating frequency to be effective. Using Type 31 ferrite cores with 6–8 turns of coax on 6–8 stacked toroids is ideal.
  • Tuning is typically done by adjusting the overall length of the exposed center conductor.
  • The length from the feedpoint to the choke must be exactly λ/4 for proper sleeve action.

Misconceptions

  • "It’s just a dipole fed with coax." → False. It is an end-fed antenna that mimics the geometry of a dipole but not the feed symmetry.
  • "The braid isn’t part of the antenna." → False. The braid is actively radiating RF up to the choke.
  • "No choke needed — it's balanced." → False. It is highly unbalanced without the choke.

Summary

The T2LT is a disguised EFHW — an end-fed antenna with a clever use of coaxial cable and a tuned choke to suppress common-mode current. Its appearance may resemble a center-fed dipole, but electrically it is not. Its effectiveness hinges entirely on the performance of the choke. When designed correctly, it offers all the benefits of a resonant dipole in a simple, lightweight, and easy-to-deploy form, making it an excellent choice 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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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.