For radio amateurs looking to boost their signal without a massive tower or expensive rotators, the Bi-Square antennais a top-tier choice. While it looks like a large loop, it is technically an array of four half-wave elements configured as two full-wave radiators fed in phase. This geometry results in a bidirectional pattern with significant gain—often outperforming a standard dipole by nearly 4 dB in free space.
Originally a favorite for the 10-meter band, this antenna is equally effective for higher HF frequencies (12m, 15m) and VHF (6m, 2m) when mounted at the correct height.
Bi-square antenna : Design and Characteristics
Unlike a closed-loop antenna (like a Delta or Quad), the Bi-Square antenna is open at the top. The ends of the two wavelengths of wire do not meet; instead, they are held in place by an insulator at the apex.
- Polarization: Horizontally polarized when mounted in the traditional diamond shape.
- Radiation Pattern: Bidirectional, broadside to the plane of the wire.
- Impedance: The feed-point impedance is naturally high—ranging from 300 to 3000 ohms depending on the specific configuration.
- Height Requirement: To truly outperform a dipole, the bottom of the antenna should be at least 1/2 wavelength above the ground, requiring a total mast height of roughly 1.25 wavelength.
Feeding and Matching
Because of the high impedance at the feed point, a direct 50-ohm coax connection will result in a high SWR. There are two primary ways to match this antenna:
- The Matching Stub (Monoband): Use a 1/4 wavelength section of 300-ohm or 450-ohm ladder line as a transformer. For 10 meters, a 2.65m section of ladder line (adjusted for velocity factor) will provide a near-perfect match to 50-ohm coax.
- Multiband with Tuner: Feed the antenna directly with ladder line into a balanced tuner in your shack. This allows the antenna to be used on bands higher than its design frequency.
What Makes the Bi-Square antenna Different
The name confuses many builders at first. Despite containing the word square, this antenna technically functions as an array of four half-wave elements rather than a closed loop. The top wires remain disconnected, creating the distinctive electrical characteristics that produce gain.
The total perimeter measures exactly 2 wavelengths. Each of the four sides spans one-half wavelength. This specific proportion creates current distributions that align in phase, concentrating radiation perpendicular to the antenna plane. Maximum signal strength appears broadside to the structure.
Traditional installations use a diamond orientation. This shape simplifies support from a single mast and makes the current flow patterns more obvious. However, the antenna performs equally well configured as a square or delta shape. Choose the geometry that fits your available supports.
Real-World Performance Numbers
Free space modeling shows the bi-square antenna producing approximately 4dB better signal strength than a dipole. This figure represents significant improvement that translates directly into better communication range. However, ground effects modify the actual advantage you’ll experience.
The fair comparison uses a dipole mounted at the average height of the bi-square, not at the same top height. When both antennas share identical maximum heights, the performance gap narrows. The bi-square still wins, but expect 2 to 3dB improvement rather than the full 4dB theoretical gain.
Height matters tremendously for this design. The bottom edge should clear the ground by at least one-half wavelength to achieve meaningful gain over simpler antennas. This rule places the total required height around 1.25 wavelengths from ground to the top support point.
Critical Dimensions for Higher HF Bands
10 Meter Band
The 10 meter version represents the most popular configuration. Each side measures 5 meters or 16.4 feet. Total wire requirement reaches 20 meters before accounting for connections and the feed arrangement.
At 28.5 MHz center frequency:
Wavelength = 300 / 28.5 = 10.53 meters
Side length = 10.53 / 2 = 5.26 meters
Apply velocity factor: 5.26 × 0.95 = 5.0 meters per side
Minimum height for good performance: 1.25 × 10.53 = 13.2 meters or 43 feet
The quarter-wave matching section requires 2.65 meters of 300 to 450 ohm transmission line, assuming a velocity factor of 0.95 for ladder line.
15 Meter Band Bi-Square antenna
Scaling to 15 meters increases the physical dimensions while maintaining the same electrical properties:
At 21.2 MHz:
Wavelength = 300 / 21.2 = 14.15 meters
Side length = (14.15 / 2) × 0.95 = 6.72 meters or 22 feet
Minimum installation height = 1.25 × 14.15 = 17.7 meters or 58 feet
Quarter-wave matching section = 3.54 meters
6 Meter Band
The 6 meter band offers the most compact bi-square configuration, making it practical for space-limited installations:
At 50.5 MHz:
Wavelength = 300 / 50.5 = 5.94 meters
Side length = (5.94 / 2) × 0.95 = 2.82 meters or 9.25 feet
Minimum installation height = 1.25 × 5.94 = 7.43 meters or 24.4 feet
Quarter-wave matching section = 1.49 meters or 4.9 feet
Total wire needed = 2.82 × 4 = 11.28 meters or 37 feet
The compact dimensions make 6 meters ideal for rooftop installations or yard-mounted systems where taller structures prove impractical. A modest 25-foot support provides adequate height clearance. The short matching section simplifies construction and reduces loss.
Six meter propagation characteristics benefit significantly from the bi-square’s gain and directivity. During band openings, the ability to concentrate signal toward specific directions improves weak-signal contacts. The horizontal polarization matches standard 6 meter operation practices.
Height requirements explain why most operators choose 10 meters or 6 meters for their first bi-square project. The 6 meter version requires the least vertical space while still delivering the full performance advantage.
Bi-Square antenna :Solving the High Impedance Challenge
The feed point impedance measures around 3000 ohms. This extremely high value prevents direct connection to standard 50 ohm coaxial cable. Two practical matching approaches solve this problem effectively.
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Quarter-Wave Matching Section Method
A quarter-wavelength section of 300 to 450 ohm ladder line transforms the impedance down to usable levels. This elegant solution requires no additional components. The transformation occurs naturally through transmission line theory.
Connect the matching section directly to the antenna feed point. Run this section exactly one-quarter wavelength at the operating frequency, accounting for the velocity factor of your chosen line. At the far end of this section, attach 50 ohm coaxial cable for the run to your station.
For 10 meters using ladder line with 0.95 velocity factor, the matching section measures 2.65 meters. This method typically produces SWR readings below 2:1 across the band without additional tuning.
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Balanced Line with Tuner Approach
Running ladder line directly from the feed point to an antenna tuner provides maximum flexibility. This configuration supports multiband operation and eliminates concerns about matching section length precision. Quality balanced line maintains low loss even when operating with high SWR.
Use a tuner designed for balanced line input. Many modern tuners include this capability. Position the tuner near your operating position, keeping the balanced line run as direct as possible to minimize coupling with nearby objects
Multiband Operation Strategies
The traditional bi-square serves a single band. Clever modifications extend operation across multiple frequencies without compromising performance on the primary band.
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The Half-Wave Stub Technique
Adding a half-wavelength open-circuited stub of ladder line across the open top converts the Bi-Square antenna into a dual-band performer. This stub appears as an open circuit at the design frequency, typically 10 meters. The antenna operates normally.
At exactly half the frequency (20 meters), the stub measures one-quarter wavelength. Quarter-wave open stubs transform into short circuits. The top connection effectively closes, transforming the antenna into a full-wave loop for 20 meters.
Practical experience shows this configuration also works on 15 meters, though the current distribution departs from the ideal pattern. The impedance drops significantly on 20 meters compared to 10 meters, making balanced line to a tuner the preferred feed method.
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Lazy H Configuration
Reconfiguring the bi-square antenna geometry into a Lazy H arrangement provides another path to multiband capability. This variation maintains the diamond or square outline but changes the wire routing and feed point location.
The Lazy H version doesn’t demand precise element lengths. All elements should match each other, but exact half-wavelength dimensions become less critical. This flexibility allows operation across all bands from 20 through 10 meters with a single installation.
Balanced feed line to an antenna tuner becomes essential for this approach. The impedance varies significantly across the operating range. Corner connections can be either shorted or left open without affecting performance.
Wire Selection and Preparation
Choose 12 or 14 AWG copper wire for permanent installations. These gauges balance durability against weight and cost. Heavier wire survives wind stress better but requires stronger support structures. Lighter gauges work fine for temporary or portable setups.
Measure carefully and cut four equal lengths. Add an extra 15 to 20 centimeters per section for connections and adjustments. Precise initial measurements reduce frustration during tuning. Mark the calculated dimensions before cutting.
Support Structure Design
The diamond orientation needs just one tall mast. Position this support at the top apex. Guy ropes or separate supports anchor the side corners. Non-conductive rope works best for the guys. Include rope breaks or insulators if using conductive materials.
For portable operations, telescoping fiberglass poles provide lightweight solutions. These compress for transport and extend to full height quickly. Secure the base properly because the antenna catches considerable wind.
Permanent installations benefit from robust mast construction. Consider galvanized steel pipe or commercial antenna masts. Foundation depth should reach one-tenth of the above-ground height at minimum. Concrete bases prevent movement in storms.
Bi-Square antenna :Assembly and Installation Sequence
Connect the four wire sections into a continuous path using quality mechanical connectors or soldered joints. Install insulators at the top apex and bottom feed point. Position additional insulators at the side corners if using guy rope attachment points there.
The top wires remain disconnected in the traditional bi-square configuration. Use a quality insulator at this junction to maintain the gap while providing mechanical support. Some builders prefer installing an adjustable shorting link here for experimentation.
Attach the feed line at the bottom point. If using the quarter-wave matching section method, connect this precisely measured section first. Seal all connections thoroughly against weather. Coax seal or quality self-amalgamating tape provides reliable protection.
Raise the antenna gradually. Check the geometry continuously during the lifting process. Adjust guy rope tensions to achieve proper shape. The structure should form clean straight lines between corners without sagging or distortion.
Tuning for Peak Performance
Connect an antenna analyzer at the feed point before raising to full height. Sweep the target band and note the frequency showing minimum SWR. This measurement reveals whether the antenna runs long or short electrically.
Resonance below the target frequency indicates excessive length. Remove wire in small increments. Work at the corners or feed point where adjustments prove easiest. Test after each change. Resonance above target means insufficient length. Adding wire becomes necessary but complicates matters compared to initial accuracy.
For the quarter-wave matching section approach, verify the matching line length carefully. Physical measurement matters less than electrical length. Account for the velocity factor of your specific ladder line. Small adjustments to this section produce large impedance changes at the transition point.
Acceptable SWR readings fall below 2:1 for most applications. Some installations achieve 1.5:1 or better across significant portions of the band. Perfect 1:1 readings rarely matter for practical communication. Focus on keeping SWR below 2:1 where you operate most frequently.
Bi-Square antenna : Radiation Characteristics and Directivity
The radiation pattern shows strong bidirectional characteristics. Maximum signal appears perpendicular to the plane containing the antenna. Imagine a line passing through the mast. The pattern shows deep nulls along this axis while producing strong signals at right angles.
This directivity helps reject interference arriving from unwanted directions. Stations off the sides of the antenna receive and transmit poorly. The pattern works excellently for point-to-point communication when you can orient the broadside toward your target area.
Horizontal polarization dominates the radiated signal. This matches most HF communication requirements where horizontal antennas produce the best long-distance results. The low radiation angle benefits DX work when the antenna achieves proper height above ground.
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