That antenna is a highly effective, simple design known by several names, most commonly the Bi-Square Antenna, the Butterfly Antenna, or simply the Double-Square Loop Antenna. It is a variation of the classic Bi-Quad Antenna but is often built without the backing reflector, which is why it achieves the desirable bidirectional radiation pattern.
Among all homebrew amateur radio antennas, the Square Loop Antenna—often referred to as the double diamond, butterfly antenna, or even infinity antenna—has earned a reputation for being one of the most effective simple antennas ever created. Its elegant geometry provides excellent gain, a stable impedance, and a remarkably broad bandwidth, making it a favorite for both VHF and UHF enthusiasts.
One of the biggest advantages of this design is its broadband nature: unlike many resonant antennas, it requires no fine tuning and remains efficient across a wide frequency range. Whether it is used for FM broadcast reception, 2-meter amateur radio, UHF listening, weather channels, or general monitoring, the square loop performs consistently well.
The double-square geometry creates two connected loops with a large effective aperture, resulting in better signal capture than a thin wire dipole. Being a loop antenna, it naturally exhibits a low noise floor, making weak signals easier to hear.
Despite its simplicity, the antenna offers:
- Decent forward gain
- Stable impedance around 50–75 ohms
- Broadband performance without trimming
- Bidirectional operation when used without a reflector
- Good suppression of electrical noise
This makes it exceptionally useful in urban or noisy environments, where dipoles and whips struggle.
Butterfly antenna – Dimensions
One of the strengths of the square loop antenna is that scaling is easy. You simply adjust the side length of each square.
For the VHF 2-meter band (144–146 MHz)
Side length of each square: 51 cm
This size gives reliable operation across the entire 2-meter band without needing adjustments.
For the UHF 70-cm band (430–440 MHz)
Side length of each square: 17.3 cm
This compact version is excellent for handheld radio improvement, UHF repeaters, and UHF scanners. No other adjustments are required. The antenna remains broadband and stable across the entire band once the square size is correct.
Bidirectional Operation
Unlike Yagi antennas, which require multiple elements and careful spacing, the square loop antenna works well without a reflector or director. This gives it a natural bidirectional pattern, strongest front and back relative to the plane of the squares.
Gain
While a single-element quad has good gain, combining two loops yields a further improvement. This simple antenna can provide several dB of gain over a standard dipole, giving you a noticeable advantage in signal strength.
Butterfly antenna – Feed Point
The feed point of the Butterfly antenna is located exactly at the junction where the two loops meet. This is the small 5 cm gap shown in the construction diagram, and it plays a vital role in achieving proper impedance and efficient radiation.
At this point:
- The center conductor of the coaxial cable is soldered to one side of the gap.
- The shield (braid) of the coaxial cable is soldered to the other side.
This center-fed configuration forms a balanced-to-unbalanced transition that naturally brings the antenna’s feed impedance close to 50 Ω or 75 Ω, depending on the build and loop size. Because the loops are symmetrical, the feed behaves almost like a built-in matching section, eliminating the need for extra baluns or tuning components.
This simple feed arrangement is one of the reasons the butterfly antenna is so broadband, easy to match, and forgiving in real-world installations.
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