What is an LFA Yagi Antenna?

An LFA (Loop Fed Array) Yagi is a highly specialized directional antenna designed specifically to deliver an extremely low noise floor. Developed by antenna designer G0KSC, it is primarily used in amateur radio for weak-signal work. While it looks largely similar to a standard Yagi-Uda antenna, its defining feature is its driven element (the part where the transmission line connects). Instead of a simple straight rod (dipole), the LFA uses a flat, closed rectangular loop.

How the Loop Kills Noise

The magic of the LFA lies in the specific shape and phasing of that rectangular loop:

• The Broadsides: The long sections of the loop run parallel to the other parasitic elements. These do the actual radiating and receiving in the forward direction.
• The End Sections: The short sides of the loop run parallel to the central boom. Because of the loop’s precise dimensions, the electrical currents flowing through these two short end sections are exactly 180 degrees out of phase with one another.
• Phase Cancellation: When currents are out of phase, their electromagnetic fields cancel each other out. This effectively acts as a spatial filter, drastically shrinking the antenna’s side lobes (its sensitivity to signals coming from the sides, top, and bottom).

Key Advantages

1. Exceptional Noise Rejection: By suppressing side and rear lobes, the LFA Yagi ignores much of the man-made electrical noise (QRM) common in urban environments, such as power line noise and electronic interference.
2. Better Signal-to-Noise Ratio (SNR): Because the background “noise floor” is physically blocked from entering the receiver, incredibly faint signals that would normally be buried in static become perfectly readable.
3. Direct 50-Ohm Match: Many LFA designs naturally present a 50-ohm impedance at the feed point. This means they can be connected directly to standard coaxial cable without needing complex or lossy matching networks (like hairpin matches or gamma matches) that standard Yagis often require.

Because of their noise-rejecting properties, LFA Yagis are highly prized on VHF and UHF bands (like 6 meters and 2 meters) for extreme weak-signal propagation modes, including Earth-Moon-Earth (moonbounce), meteor scatter, and tropospheric ducting.