Build a Cubical Quad Antenna for HF
The cubical quad has fascinated radio amateurs since Clarence Moore W9LZX first described it from HCJB in Quito, Ecuador in 1942, where standard Yagi antennas were suffering corona discharge in the thin high-altitude air. The full-wave loop elements of a quad avoid the voltage stress problems at element tips while delivering gain comparable to a 3-element Yagi, broader bandwidth, lower angle radiation at moderate heights, and a feed point impedance that sits conveniently close to 50 Ω. This guide builds a two-element 20 m quad and covers multi-band extension to a 5-band quad.
Full-Wave Loop Elements
A cubical quad element is a full-wave closed loop — a square or diamond-shaped loop whose total wire circumference equals one wavelength at the operating frequency. Unlike a Yagi element (which is a half-wave dipole), the quad element is a complete loop where current flows around the full circumference. This current distribution creates a different radiation pattern than a Yagi: the quad's main lobe is broader in the vertical plane (encompassing more elevation angles), which can be advantageous for propagation that arrives at variable angles.
A two-element quad consists of a driven element (DE) and a reflector, separated by approximately 0.12–0.18λ. The driven element is a square loop fed at the midpoint of one side. The reflector is a slightly larger loop (longer total circumference), left unconnected at the equivalent position. Mutual coupling between the two loops causes the reflector current to lag the driven element current in phase, producing a cardioid-like radiation pattern with the maximum radiation in the direction from the reflector toward the driven element — the same mechanism as a Yagi, but with the larger radiating area of a full-wave loop.
Quad Element Dimensions
The key formulas for quad element sizes are among the simplest in antenna engineering. The driven element is a full-wave loop — a square with each side equal to λ/4. The reflector is approximately 3–5% longer in total circumference, shifting its resonance slightly below the operating frequency so it acts as a current-lagging parasitic element.
Spreader Arm Hub Construction
The spreader arm system is the mechanical heart of the quad. Each element has four spreader arms radiating from a central hub at 90° intervals, holding the element wire in a square (or diamond) shape. The hub attaches to the boom at the element's position. Two competing approaches are widely used:
- Single plate hub — simplest build: a 150 mm square piece of 3 mm aluminium plate with four holes drilled at 90° positions near the corners. Each fibreglass spreader arm bolts to the plate through one of these holes using stainless U-bolts or clamps. The plate then U-bolts to the boom. In the diamond orientation, two arms point up and sideways and two arms point down and sideways — all at 45° to the boom.
- Cross-boom hub — stronger, heavier: a short cross-arm of aluminium tube perpendicular to the boom, with two arms each side, allows the spreader arms to be attached without the plate twisting under wind load. This approach is stronger under mechanical stress but heavier and more complex to fabricate. Commercial quad kits (K4KIO, Cubex, Gem Quad) typically use this approach for their central hubs.
- Spreader arm material: fibreglass rod is preferred — non-conductive, relatively light, resists UV well if gel-coated. Avoid hollow fibreglass tube for spreaders over 3 m — it can crack along the wall at the hub clamp under sustained wind load. At 20 m, 10 mm solid rod is adequate; at 40 m, 12–16 mm is recommended.
Multi-Band Quad Construction
One of the cubical quad's great advantages over the Yagi is how naturally it extends to multi-band operation. Because each element is a separate closed loop with its own resonant frequency, multiple bands can share the same spreader arms by running concentric loops of different sizes — all four spreader arms support loops for every band. A 5-band quad (10, 12, 15, 17, 20 m) uses five nested loops on four shared spreader arms per element, with all driven element loops connected to their respective feed points and all reflector loops closed.
Each band's driven element feed point requires its own coaxial connection — a multi-band quad has five feed points on the driven element, one per band, typically distributed around the perimeter of the driven element frame. Each feed point has its own short coax run to a band selector box at the boom. Alternatively, all driven elements can be fed through a common feed point using a single wire that makes multiple loops of different sizes — this sacrifices some flexibility in optimising each band's reflector spacing but simplifies the feed system to a single coax.
| Band | Freq (MHz) | DE side (m) | REF side (m) | Spacing (m) | Arm length (m) | Boom length (m) |
|---|---|---|---|---|---|---|
| 40 m | 7.150 | 10.57 | 10.91 | 5.87 | 7.48 | ~6.5 |
| 20 m | 14.175 | 5.33 | 5.50 | 2.96 | 3.77 | ~3.5 |
| 17 m | 18.118 | 4.17 | 4.30 | 2.32 | 2.95 | ~2.8 |
| 15 m | 21.225 | 3.56 | 3.68 | 1.98 | 2.52 | ~2.4 |
| 10 m | 28.500 | 2.65 | 2.74 | 1.47 | 1.87 | ~1.8 |
Diamond vs. square orientation: the dimensions above assume a diamond (rotated 45°) orientation — the preferred shape for most multi-band quads because the spreader arms radiate outward from the boom at 45° and the element wire hangs between adjacent arm tips. Electrical performance is identical to square orientation; diamond is mechanically more elegant and standard for commercial quad kits.
Cubical Quad Dimension Calculator
Materials for a 2-element 20m quad (14.175 MHz)
Building the Cubical Quad
Fabricate the hub plates and boom first, then install the spreader arms, string the wire elements, connect the feed point, and tune. Allow 8–14 hours for the complete build.
Fabricate the hub plates and drill spreader arm holes
Cut two 150×150 mm hub plates from 3 mm aluminium. For diamond orientation, mark four hole positions at 45° angles from the plate centre, each 60 mm from centre — these locate the spreader arm U-bolt saddles. Drill 8 mm holes at each location. Drill two 10 mm holes through the plate centre for the boom U-bolt. Deburr all holes. The hole spacing must match your U-bolt saddle size — verify against your hardware before drilling.
Cut the boom and mark element positions
Cut the 50 mm boom to 3.7 m. Mark the driven element position 0.85 m from the front of the boom, and the reflector position 0.85 + 2.96 m = 3.81 m from the front — or equivalently, 0.85 m from the rear. (These positions balance the centre of gravity approximately over the mast mounting point at boom centre.) Clamp the hub plates at these positions using boom U-bolts before fully tightening — you will need to adjust positions during first SWR check.
Cut and install the spreader arms
Cut eight spreader arms from 10 mm fibreglass rod, each 4.0 m long (slightly longer than the calculated arm length of 3.77 m to allow for attachment at the hub). The wire will be positioned at the tips at the correct distance from the hub centre during the wire stringing step. Attach all eight arms to their respective hub plates using stainless U-bolt clamps. Arms should extend outward at 45° from the boom (diamond orientation) and be firmly clamped — any arm movement under wind load will shift element resonance.
Cut the element wires and install corner insulators
Cut the driven element wire: total circumference = 4 × 5.33 m = 21.32 m. For the side midpoint feed, cut this as two equal pieces of 10.66 m each — one from the feed point to the corner, around the element, and back to the feed point on the other side. Alternatively, cut as a single piece with the feed point cut at the midpoint of one side. Install small corner insulators at each arm tip — loop the wire around a small pulley or through a fibreglass eyelet at each corner. Keep the wire under light tension to prevent sag between spreader tips.
String the reflector wire
The reflector is a continuous loop with no feed gap — total circumference 4 × 5.50 m = 22.0 m. Cut a single piece of wire to this length and join the ends securely with a solder joint inside a weatherproof strain-relief sleeve. String the reflector over its four spreader arm tips in the same manner as the driven element. The reflector loop should hang freely without any electrical connection to the hub plate or boom — verify with an ohmmeter that the reflector is isolated from any metal structure.
Install the feed point
At the midpoint of one side of the driven element, connect the coaxial cable. The two halves of the driven element connect to inner conductor and outer braid respectively. Install a 1:1 current balun (for side-midpoint feed) or a 2:1 voltage balun (for bottom-corner feed) at this junction. Mount the feed assembly in a small ABS weatherproof box cable-tied to the nearest spreader arm for mechanical support. Tape the coax to the spreader arm and then to the boom as it runs back to the mast.
Raise and perform initial SWR test
Mount the boom on the mast or rotator at the intended height. Connect an antenna analyser at the shack end of the feedline. Sweep 13.5–15.0 MHz. A properly built 20 m quad should show SWR minimum of 1.0–1.5:1 within 200 kHz of 14.175 MHz, with SWR below 2:1 across the whole 20 m band. If the minimum SWR is too high at all frequencies (say, always above 2.5:1), the most likely cause is the feed point balun — verify the balun connection and check for any short between coax inner and outer at the connector. If resonance is shifted significantly, adjust wire length at the easily accessible bottom segment.
Check F/B ratio and optimise reflector
With the beam pointed at a known station, compare signal strength to the front vs. rotating 180°. A correctly spaced and sized reflector should give 15–20 dB F/B. If F/B is poor (under 10 dB), the reflector circumference may need adjustment. Slightly lengthening the reflector wire (add 50–100 mm to total circumference) typically improves F/B on the low side of the band. Alternatively, some builders add a small variable capacitor across the reflector's virtual feed point (the point opposite the DE feed) to allow fine-tuning of reflector resonance without physically changing the wire length.
| Antenna | Gain (dBi) | F/B (dB) | SWR BW | Feed Z | Boom |
|---|---|---|---|---|---|
| 2-el quad, 20m | 7.5 | 15–20 | ~700 kHz | 50–70 Ω | 3.5 m |
| 3-el Yagi, 20m | 9.7 | 20–25 | ~600 kHz | 25 Ω + match | 7.0 m |
| 2-el Yagi, 20m | 7.2 | 10–12 | ~1,000 kHz | 25 Ω + match | 3.0 m |
| Moxon, 20m | 7.65 | 30–35 | ~800 kHz | 50 Ω direct | 3.0 m depth |
| Single quad loop | 5.1 | none | ~1,000 kHz | 100–130 Ω | n/a |
The quad's real-world advantage: while NEC2 models show the 2-element quad gain as approximately equal to a 2-element Yagi, many operators report that the quad consistently receives and transmits better than a 2-element Yagi at the same height in comparable conditions. The most accepted explanation is the quad's lower radiation angle — because the full-wave loop occupies more vertical aperture than a half-wave element, it concentrates radiation at lower elevation angles. This advantage is most pronounced at heights below 0.5λ, where most amateur stations operate.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| High SWR on all frequencies | Feed point wiring error | Verify the balun is connected correctly and that neither the inner nor outer of the coax is accidentally shorted to any part of the hub plate or boom through the feed box mounting hardware | For a side-midpoint feed, confirm the wire is actually cut at the midpoint of the chosen side and not at a corner — this easy assembly error changes the feed impedance dramatically |
| Minimum SWR too high (above 2:1) but at correct frequency | Feed point impedance not matching the coax | Side-midpoint feed on a 2-element quad can range from 50 Ω to 75 Ω depending on element spacing — if spacing is tight (less than 0.12λ) the feed impedance rises | Try a short section of 75 Ω coax (approximately λ/4) between the feed point and the 50 Ω main feedline as an impedance transformer. Bottom-corner feed presents ~120 Ω — always use a 2:1 or 4:1 balun for this feed position |
| Poor F/B despite correct element spacing | Reflector wire length incorrect | Measure the reflector's total circumference precisely — it should be approximately 3% more than the driven element. If the reflector is exactly the same length as the DE, it acts as a director and the beam direction may be reversed | Add wire to the reflector at a splice point if it is too short. Ensure the reflector loop is not accidentally touching the hub plate or boom — even intermittent contact from wire sag significantly disrupts the reflector's current phase relationship |
Is a quad better than a Yagi?
At comparable heights, a 2-element quad produces similar forward gain to a 2-element Yagi but with lower take-off angle and better bandwidth. A 3-element Yagi provides significantly more gain than a 2-element quad. The quad's advantage is its compact boom length, direct 50 Ω feed, and superior performance at low mounting heights. Many operators report that a quad at 10 m height "feels" like a Yagi at 15 m height in actual DX operating.
What is the best feed point position for DX?
For maximum low-angle DX radiation, feed the driven element at the midpoint of one side (side-midpoint feed). This produces a radiation pattern with lower peak elevation angle compared to bottom-corner (horizontal polarisation) feed. The side-midpoint feed also offers a more convenient ~50–70 Ω impedance. Bottom-corner feed is used when strictly horizontal polarisation is desired (matching a horizontally-polarised station on the other end).
Can I build a quad on a non-rotating mount?
Yes — a fixed-direction quad pointed toward your primary DX direction works well if that direction covers your main operating targets. Alternatively, a bidirectional quad (with two driven elements on opposite sides of the same reflector) covers two directions simultaneously, with each driven element activated by a switch at the feed point. For a UK station, this could cover North America and Asia from a single fixed installation.
How does wind loading compare to a Yagi?
A 2-element 20 m quad presents significantly more frontal wind area than a 2-element Yagi — approximately 4 m² of loop area vs 0.5 m² of Yagi element area. However, most of the quad's area is wire rather than solid tube, so the actual wind force is substantially less than the frontal area implies. The spreader arms are the main structural concern — they must be rated for the cantilever load at the wind speeds expected at your site. A 40 m quad in high-wind environments requires engineering analysis of the spreader arm bending moments.
Why use fibreglass spreader arms rather than aluminium?
Aluminium spreader arms would be electrically conductive and would interact with the element wire running near them — effectively creating a partial short-circuit at each arm tip, detuning the element. Fibreglass is non-conductive and transparent to RF, allowing the element wire to pass close to the arm without electrical interaction. PVC pipe can substitute for fibreglass at lower cost but has inferior UV resistance and greater wind deflection over long arm lengths.
Does the quad work better in rain or bad weather?
Unlike Yagi elements with their high voltage at the tips, quad loops have distributed voltage around the circumference — the tips are not a high-voltage point. This reduces the corona discharge and precipitation static problems that affect Yagis. Wet wire detuning still occurs but is less pronounced than precipitation static noise on a Yagi. In stormy conditions the quad often has a clear receive advantage over a Yagi of equivalent gain.