What is T2FD Antenna in Amateur Radio?

The T2FD (Terminated Tilted Folded Dipole), often just called a Terminated Folded Dipole, is a broadband wire antenna originally developed by the US Navy in the 1940s. In amateur radio, it is highly valued for its ability to operate across a massive frequency range (often an entire HF spectrum like 80m through 10m) without requiring an antenna tuner.

How It Works

Physically, the T2FD looks like a standard folded dipole, but with a critical modification: the loop is broken at the center of the top wire and bridged by a non-inductive terminating resistor.

• The Balun: The feedpoint is at the center of the bottom wire. Because the antenna naturally presents a high impedance, a voltage balun is used to match it to standard 50-ohm coaxial cable.
• The Resistor: The resistor absorbs RF energy that would otherwise reflect back down the feedline as standing waves.
• The Ratio: The balun ratio and resistor value must match. A common pairing is a 450Ω resistor with a 9:1 balun (50Ω × 9 = 450Ω), or a 600Ω resistor with a 12:1 balun.
• The Tilt: While it can be mounted horizontally or as an inverted-V, it is traditionally installed as a “sloper” (tilted at a 20 to 40-degree angle), which gives it a slightly omnidirectional radiation pattern.

Performance Trade-offs

The T2FD is the ultimate broadband antenna, but it relies on a specific compromise to achieve its broad bandwidth.

Feature	The Advantage	The Compromise
Bandwidth	Extremely flat SWR across multiple HF bands; allows frequency hopping or scanning.	Lossy transmission. The terminating resistor dissipates up to 30% of your TX power as pure heat.
Receive Noise	The closed-loop design makes it a very “quiet” antenna, rejecting atmospheric static and local QRM.	–
Equipment Safety	Modern solid-state rigs won’t fold back power due to high SWR.	Requires high-wattage components.

Sizing and Power Considerations

To work efficiently on lower HF frequencies like the 80m or 160m bands, a T2FD still requires significant real estate. The total end-to-end length of the antenna should ideally be about 1/3 of the wavelength of the lowest planned operating frequency. For 80m (3.5 MHz), this means the antenna needs to be roughly 90 feet (27.5 meters) long.

When transmitting, especially if pushing output from a 100W linear amplifier, component selection becomes critical. The terminating resistor must be strictly non-inductive (carbon composition or thick film, never wire-wound, as the coil will act as a choke) and rated to safely dissipate at least 30 to 50 watts of continuous heat to avoid burning out during heavy duty-cycle modes.