Ham Radio Cubical Quad Antennas — Complete Guide

Full-Wave Loop Elements vs Half-Wave Dipole Elements

The fundamental difference between a quad and a Yagi is the element type. A Yagi uses half-wave dipole elements — straight conductors. A quad uses full-wave loop elements — closed wire squares (or other loop shapes). This difference in element type produces several performance advantages:

• A full-wave loop has approximately 2 dBd gain over a half-wave dipole — it is inherently a stronger radiator per element
• The loop element produces lower-angle radiation at a given antenna height compared to a dipole at the same height — better for DX
• The loop geometry reduces sensitivity to nearby conducting objects — quads are less detuned by rain, ice, and wet foliage than Yagi dipole elements
• Loop elements are less susceptible to static charge buildup — quads are somewhat self-draining via the closed loop conductor
• The closed loop provides DC continuity between all parts of the element — simplifying lightning protection

The quad's gain advantage over a Yagi comes entirely from the superior efficiency of loop elements versus dipole elements. A 2-element quad (driven + reflector) delivers approximately the same gain as a 3-element Yagi on a comparable boom length — the extra element in the Yagi compensates for the dipole's inherently lower gain per element.

The Quad vs Yagi Debate — What the Measurements Say

Few topics generate more discussion in amateur radio than whether a quad outperforms a Yagi. The honest answer, based on NEC2 modeling and practical measurements:

• A 2-element quad delivers approximately 7–8 dBd — slightly more than a 3-element Yagi (~7 dBd) on similar boom length
• A 3-element quad delivers approximately 9–10 dBd — comparable to a 5-element Yagi
• The gain advantage of the quad is real but modest — typically 0.5 to 1.5 dBd over a comparable Yagi
• The quad's lower takeoff angle at the same physical height is a genuine advantage for DX propagation
• The quad's quieter receive characteristic is real — fewer operators report the noise increase when switching to a quad that they do when switching to a Yagi
• The quad's mechanical disadvantage — fiberglass spreaders, heavier wire loops, more complex construction — is also real

Bottom line: the quad is a genuinely superior HF DX antenna when built well. The mechanical challenge is the real barrier to wider adoption — not any theoretical performance limitation.

Element Dimensions — The Loop Formula

Quad loop elements use a different length formula from dipoles. A full-wave loop is approximately 2–3% longer than the theoretical full wavelength — the loop conductor's end-effect is opposite to a dipole's, adding electrical length rather than subtracting it:

The reflector is longer than the driven element — opposite to a Yagi where the reflector is longer than a dipole but the comparison is to the driven element. For a quad, the driven element is already a full-wave loop; the reflector needs to be slightly longer to provide the correct phase relationship for the reflector action.

Feed Systems and Impedance

The feedpoint impedance of a quad loop depends on the feed position. A square loop fed at the bottom center presents approximately 100–130Ω. This requires a matching network for direct 50Ω coax connection:

• λ/4 coax matching section — a quarter-wave section of 75Ω coax transforms 100Ω to 56Ω (close to 50Ω). The simplest matching approach for a single-band quad.
• 2:1 current balun — a 2:1 (or 1.5:1) current balun at the feedpoint steps the impedance down to 50Ω. More expensive but frequency-independent.
• Corner feed — feeding at a corner of the square loop rather than the bottom center changes the impedance toward 50–75Ω depending on the exact position, potentially eliminating the need for a matching transformer.
• Delta loop feed — feeding a triangular loop at the apex typically presents ~50Ω, simplifying the match.

A current choke (1:1 current balun) at the feedpoint is always recommended to prevent common-mode current from flowing on the coax shield — the quad's geometry makes it slightly more prone to this issue than a center-fed dipole in a Yagi.

Adding a Director to the Quad

A 3-element quad — driven element, reflector, and one director — delivers approximately 9–10 dBd of gain with a front-to-back ratio of 25–30 dB. This is comparable to a 5-element Yagi on a similar boom length and represents a serious DX and contest antenna for the operator who can manage the mechanical challenge of quad construction.

The director loop is shorter than the driven element — approximately 3% shorter in circumference. Its spacing from the driven element is typically 0.12–0.18 wavelengths in the forward direction. The interaction between reflector, driven, and director in a quad is similar to a Yagi but the loop elements produce lower mutual coupling, which means the element spacings and lengths are somewhat less critical than in a tightly-coupled Yagi design.

• 3-element quad boom length: approximately 0.35–0.40λ total
• For 20m at 14.2 MHz: boom length approximately 24–28 feet
• Gain advantage over 2-element: approximately 2 dBd additional
• F/B improvement: from ~20 dB (2-element) to ~28 dB (3-element)
• Adding more directors continues to increase gain — each additional director adds ~1 dBd

3-Element Quad Dimensions — 20m Example

A complete 3-element quad for 14.200 MHz using the standard design ratios:

Spreader Materials and Construction

The spreader arms that hold the quad loop wire in its square shape are the most mechanically demanding aspect of quad construction. They must be non-conductive (so they don't detune the loops), strong enough to support the wire tension and wind loading, and light enough not to overload the rotator and mast. The main material options:

• Fiberglass tubing (most common) — available from kite shops, antenna suppliers, and composites dealers. UV-resistant fiberglass is the standard choice. Hollow tube is lighter than solid rod but less rigid over long spans. Typical OD: 3/4" to 1" for HF quads.
• Bamboo — historically popular, still used by homebrewers. Lightweight, strong, readily available. Must be sealed against moisture absorption (varnish or epoxy). Degrades over 3–5 years outdoors — needs periodic replacement.
• PVC pipe (schedule 40) — heavy and UV-degrades over time (becomes brittle). Acceptable for shorter spreader arms on higher bands (15m, 10m) but not ideal for 20m where arms exceed 12 feet.
• Carbon fiber — strongest and lightest option. Excellent mechanical properties. One significant caveat: carbon fiber is electrically conductive — it will significantly detune the loops if the spreaders contact the wire. Require careful electrical isolation at all contact points.
• HDPE or polycarbonate rod — excellent UV resistance, low weight, easy to machine. More expensive than fiberglass but very clean to work with.

Hub Design and Spreader Mounting

The central hub connects the spreader arms to the boom and mast. Four spreader arms radiate outward at 90° to each other, angled downward at approximately 5–10° from horizontal to create a slight droop that helps the wire loops maintain their shape. Hub construction options:

• Commercial fiberglass quad hubs are available from several antenna suppliers — these are the most reliable option for a permanent installation
• PVC pipe fittings (cross tee) provide a simple homebrew hub — drill holes for the spreader arms and secure with set screws or bolts through the PVC wall
• Aluminum plate hub — a piece of aluminum flat bar or plate with holes drilled for each spreader arm. Strong and durable but heavier than PVC or fiberglass options
• Each spreader arm connects to the hub with a set screw, clamping bolt, or through-bolt plus epoxy for permanent installations
• The wire loop attaches to the spreader arm tips via small insulators — commercial end caps or homemade holes through the fiberglass tip
• The corner where the wire reaches each spreader tip should be supported by a small loop of UV-resistant cord through the insulator — the wire tension should be borne by the cord, not by a sharp corner that could cut the wire over time

How Multi-Band Quads Work

A multi-band quad uses multiple sets of loops on the same spreader structure — one set of loops (driven + reflector) per band, all mounted on the same four spreader arms. The outer loops (larger circumference) resonate on the lowest frequency band; progressively smaller inner loops resonate on higher frequency bands. At any given operating frequency, only the loops for that band are resonant — the other loops present a high impedance and do not significantly load or detune the active band's loops.

The most common commercial configuration covers 20m, 15m, and 10m from one set of spreader arms:

• Outermost loops: 20m driven and reflector (~17.7 ft each side)
• Middle loops: 15m driven and reflector (~11.9 ft each side)
• Innermost loops: 10m driven and reflector (~8.8 ft each side)
• All six loops share the same four spreader arms — the 20m loop runs near the spreader tips, 15m at mid-arm, 10m near the hub
• Each band has its own feedline — three separate coax runs to the operating position, or a relay box for remote band selection

Multi-Band Quad Challenges

Multi-band quad construction is significantly more complex than a single-band quad. The additional loops interact with each other electromagnetically, and the mechanical challenge of routing multiple wire loops on the same spreader arms while maintaining proper spacing and tension is considerable. Key issues to manage:

• Loop interaction — the 15m and 10m loops affect the 20m loop's resonance slightly and vice versa. Final tuning requires iteration — adjusting one band's loop affects adjacent bands. Model the complete multi-band structure in 4NEC2 before cutting wire.
• Wire routing — six loops of wire on four spreader arms creates a complex web that must be organized to prevent tangling, chafing, and electrical contact between loops of different bands.
• Feedline routing — three separate feedlines must run from the three driven element feedpoints down the boom to the mast without coupling to each other or to the other antenna loops.
• Weight and wind loading — three sets of loops plus three feedlines adds significant weight and wind area compared to a single-band quad. Verify the rotator and mast load ratings before finalizing the design.

What Is a Moxon Rectangle?

The Moxon rectangle (developed by Les Moxon G6XN) is a 2-element beam antenna where the element tips are folded back toward each other, forming a compact rectangular shape. It is essentially a 2-element Yagi with the element ends bent inward to reduce the antenna's footprint by approximately 30% compared to a full-size 2-element Yagi on the same band.

The folded tips create a capacitive coupling between the driven element and reflector that improves the front-to-back ratio beyond what a conventional 2-element Yagi achieves — typically 25–35 dB F/B at the design frequency. Forward gain is approximately 4.5–5.5 dBd — slightly less than a full-size 2-element Yagi but with a dramatically smaller footprint.

• Footprint: approximately 70% of a full-size 2-element Yagi width
• Forward gain: 4.5–5.5 dBd (less than standard 2-element Yagi's ~5.5 dBd)
• F/B ratio: 25–35 dB at design frequency (significantly better than standard 2-element)
• Feed impedance: ~50Ω — direct coax feed with a 1:1 current choke
• Construction: wire on a simple PVC or fiberglass cross frame — no aluminum tubing required
• Popular for: 10m, 15m, 20m portable and stealth installations, balcony beams

Moxon Dimensions — 10m and 20m

The Moxon rectangle dimensions are more complex than a simple dipole or loop because the tip gap and element spacing are critical to the F/B ratio and impedance. The Moxon Antenna Project calculator (moxonantennaproject.com) provides precise dimensions for any frequency and wire diameter — always use a calculator or NEC2 model rather than estimating.