Build a 4-Square Vertical Array
The 4-square phased vertical array is the most effective directional HF antenna that can be built without a tower or beam — four quarter-wave verticals arranged in a square, fed with carefully controlled phase relationships that steer the combined radiation pattern in any of four directions. On 40m and 80m, a well-built 4-square delivers 5–7 dB of forward gain over a single vertical and 20–25 dB of front-to-back rejection — performance that transforms DX pile-up operation and dramatically reduces interference from unwanted directions. This guide covers the complete design, construction, phasing network, feed system, and switching for a 40m 4-square array.
Phased Array Fundamentals
When two or more antennas are fed with signals of controlled amplitude and phase, their individual radiation patterns combine — adding in some directions and cancelling in others. The 4-square exploits this by feeding four identical verticals arranged in a square with specific phase relationships that create a cardioid (heart-shaped) pattern pointing in the chosen direction:
Why the 4-Square Excels on Low Bands
The 4-square is most commonly built for 40m and 80m — the bands where it delivers the most unique advantage over other antenna types:
- No tower required: four 33-foot verticals are far more manageable than a tower-mounted beam at equivalent height. The 4-square's gain and directivity are achieved entirely at ground level.
- Low radiation angle: like a single vertical, the 4-square radiates at low elevation angles ideal for DX. The phasing adds gain at those same low angles — unlike a horizontal beam, which adds gain at whatever elevation its height provides.
- Four directions: a single relay switching box routes the phased feed to any of four 90° sectors. Rotating the pattern covers the entire compass in four switch positions.
- Receive advantage: the 20–25 dB front-to-back rejection makes the 4-square an excellent receive antenna — dramatically reducing local interference and noise from behind the selected direction. This receive improvement is often more practically valuable than the forward gain.
- Scales to any low band: the same design principles apply to 80m (element spacing ~65 ft, elements ~66 ft tall) and 160m (element spacing ~130 ft). Physical size grows, but the design is identical.
The Christman Feed System
Several phasing network designs exist for the 4-square. This guide uses the Christman method — the most widely replicated homebrew approach, using coaxial delay lines to achieve the required phase shifts:
Ground System Requirements — More Critical Than for Single Verticals
The radial system for a 4-square is more demanding than for a single vertical because four elements share a common ground plane area and the mutual impedances between elements depend on a consistent, symmetrical ground return:
- Minimum 16 radials per element: each element needs its own set of radials. With 4 elements, that is a minimum of 64 radials total. In practice, radials from adjacent elements overlap in the center of the square — this is acceptable and the shared copper improves the ground return for the inner portions of the system.
- Interconnect element ground points: bond the radial hubs of all four elements together with bare copper wire running along the ground between them. This inter-element ground bonding is important — it ensures all four ground systems share a common reference, which is essential for consistent mutual impedances and correct pattern formation.
- Symmetry matters: unlike a single vertical where asymmetric radials cause only minor pattern distortion, the 4-square's pattern depends on symmetry between elements. Try to install similar radial counts and lengths for all four elements. Large asymmetries degrade the front-to-back ratio.
- Target 32 radials per element: for the best 4-square performance, 32 radials per element (128 total) produces near-ideal ground return symmetry. Even 16 per element works well — the system still produces impressive gain and front-to-back ratios.
| Parameter | Formula | 40m (7.150 MHz) | 80m (3.750 MHz) | Notes |
|---|---|---|---|---|
| Element height (λ/4) | 234 / f(MHz) | 32.7 ft (9.97 m) | 62.4 ft (19.0 m) | Cut 34 ft; trim to resonance |
| Element spacing (λ/4) | 246 / f(MHz) | 34.4 ft (10.5 m) | 65.6 ft (20.0 m) | Square side length — use 33–35 ft for 40m |
| Square diagonal | spacing × √2 | 48.6 ft (14.8 m) | 92.8 ft (28.3 m) | Distance between diagonally opposite elements |
| Total footprint | spacing² | ~1185 sq ft | ~4300 sq ft | Clear ground area required inside square |
| Delay line length (75Ω, VF 0.66) | (983.6 × VF) / (4 × f) | 22.7 ft (6.92 m) | 43.3 ft (13.2 m) | λ/4 delay for rear elements |
| Feed coax length (equal runs) | any λ/2 multiple | 22.7 ft or 45.4 ft | 43.3 ft or 86.6 ft | All four runs must be equal length |
| Radial length per element | 234 / f(MHz) | 32.7 ft (9.97 m) | 62.4 ft (19.0 m) | Same as element height — cut 34 ft and trim |
4 Square Calculator
Materials for a 40m 4-square with Christman phasing and 4-direction switching
How the Christman Network Works
The Christman feed system uses coaxial delay lines of specific electrical length to provide the correct phase shift between front and rear element pairs. The key insight is that a λ/4 length of coax introduces exactly 90° of phase shift — which is precisely what the 4-square requires between front and rear elements:
Direction Switching
Four directions are available from the 4-square by switching which pair of elements receives the 0° (front) feed and which pair receives the −90° (rear, delayed) feed. A relay matrix inside the phasing box routes the delay lines to the correct elements for each direction:
- North: NW and NE elements at 0°; SW and SE at −90°
- South: SW and SE elements at 0°; NW and NE at −90°
- East: NE and SE elements at 0°; NW and SW at −90°
- West: NW and SW elements at 0°; NE and SE at −90°
Four DPDT RF relays inside the weatherproof phasing box implement this switching. A 4-conductor control cable from the shack carries the 12V DC switching signals. Each relay handles one element's feed selection — switching between the direct (0°) and delayed (−90°) feed leg. A simple 4-position rotary switch or pushbutton panel in the shack controls the relay states via the control cable.
Representative of a Finished 4 Square Antenna
Building the 40m 4-Square Array
The 4-square build proceeds in three phases: (1) install and tune all four elements individually as single verticals, (2) build and install the phasing network, (3) connect the phasing network and verify pattern with on-air tests. Do not skip phase 1 — each element must be independently verified before the phasing network is connected.
Survey and Mark the Array Layout
Select a site with at least 100 × 100 feet of clear, relatively flat ground. The array square measures approximately 34 × 34 feet, but the radial systems for each element extend 33 feet beyond the element — the full installation footprint is approximately 100 × 100 feet. Mark the four element positions:
Install All Four Radial Systems
Install the radial hub and ground plane for each element before raising any elements. Each element gets its own radial hub with 16 radials at 34 feet each. Install all 64 radials first — working with radials on the ground is far easier without four 33-foot elements already standing in the way.
After all individual radial systems are installed, bond all four radial hubs together with bare #14 AWG copper wire running along the ground between adjacent elements. Run wire hub-to-hub along the perimeter of the square and across both diagonals — this creates a fully interconnected ground system that ensures all four elements share a common ground reference.
Build and Raise All Four Elements
Build all four elements identically following the same aluminum tubing construction used for the single-band 40m vertical — 1.0-inch OD lower section telescoped with 0.75-inch OD upper section, stainless hose clamps and lock bolts at each joint, Noalox at all joints. Cut all four upper sections to the same starting length of 14 feet before raising.
Install and raise all four elements. Install the feedpoint SO-239 and current choke (FT-240-31, 5–6 turns of coax) at each element base. Run a short coax stub (2–3 feet) from each feedpoint to a labeled weatherproof enclosure at the element base — this stub will connect to the phasing network coax runs later.
Tune Each Element Individually
Before connecting the phasing network, tune each element to resonance individually. Temporarily connect the NanoVNA to each element feedpoint one at a time, with the other three elements' feedpoints terminated in 50Ω loads (or left open — open is acceptable for this measurement). Sweep 6.8–7.5 MHz and trim the upper section of each element to resonance at 7.150 MHz.
Cut and Verify the Phasing Coax Delay Lines
Cut the 75Ω phasing coax delay lines to exact electrical length. The physical length depends on the coax velocity factor — measure it accurately for the specific coax you are using, as velocity factor varies between manufacturers and even between batches of the same product:
Build the Phasing Box and Relay Switching Network
The phasing box is a weatherproof enclosure mounted at the array center point — equidistant from all four elements. Inside the box: four DPDT RF relays, the coax interconnects for each direction selection, and the control cable terminal block. The box sits at ground level at the center of the square.
Wire the relay matrix to implement the four-direction switching. For each direction, two relays connect their respective elements to the direct (0°) feed and two connect to the delayed (−90°) feed. The specific wiring for all four directions:
Route all coax connections inside the box keeping runs short and dress the coax away from the relay control wiring. Install weatherproof SO-239 bulkhead connectors on the outside of the box for each of the four element feed runs and the single coax run to the shack.
Run Coax from Phasing Box to Each Element
Cut and run the four coax legs from the phasing box to each element feedpoint. For the front elements (for North direction: NW and NE), run the direct equal-length coax: 22.7 feet of 75Ω coax from the phasing box relay output to the element SO-239. For the rear elements (SW and SE), run the same equal-length 22.7 feet of 75Ω coax from the relay output to the element, but with the 22.7-foot delay line inserted in series — total rear leg coax is 45.4 feet.
Since the array is switched to four directions, all four elements must be wired with both direct and delayed feed capability — the relay in the phasing box selects which feed each element receives on each band. Wire each element's feedpoint to two relay contacts: one for 0° (direct) and one for −90° (delayed). The delay line is permanently installed between the relay contact and the element for each leg that may become a rear element.
Run Control Cable and Build Shack Switching Panel
Run the 4-conductor control cable from the phasing box to the operating position. Bury or protect the cable along the same route as the main coax run to the shack. At the shack end, wire a simple switching panel: a 4-position rotary switch (or four momentary pushbuttons with LED direction indicators) that applies 12V DC to the appropriate relay control conductors for each direction.
Initial On-Air Testing and Pattern Verification
With everything connected, connect the NanoVNA to the main coax at the shack. Switch to each direction in turn and sweep 6.8–7.5 MHz. The SWR curve should show a resonance near 7.150 MHz in all four switch positions — the resonant frequency and SWR value should be consistent across all four directions (within 50 kHz and 0.3:1). Inconsistency between directions indicates a wiring error in the phasing box relay matrix.
Fine-Tune and Document
If front-to-back ratio on-air is lower than expected (less than 15 dB), the most common causes are: element resonance mismatch between elements (re-verify each element individually), delay line length error (re-measure and trim delay lines), or ground system asymmetry (add radials to elements with fewer radials or check inter-element bonding). Work through these systematically — a 4-square that achieves only 15 dB front-to-back is still a significant improvement over a single vertical, but the theoretical 20–25 dB is achievable with careful construction.
Document: all four element resonant frequencies, delay line physical lengths and measured VF, relay wiring diagram, SWR on each direction, and front-to-back measurements on each of the four headings. Photograph the phasing box interior before sealing. Weatherproof all outdoor connections with self-amalgamating tape and RTV sealant. Install lightning protection: ground rod at each element base bonded to the radial hub, and a coax surge protector at the shack entry.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Poor front-to-back ratio in all directions (less than 10 dB) | Elements not identical — resonant frequency mismatch | Disconnect phasing network; measure each element resonance individually | Trim longer elements until all four resonate within 25 kHz of each other |
| Good front-to-back in two directions, poor in the other two | Relay wiring error — one relay switching incorrectly | Check relay states with a multimeter in each switch position | Correct relay wiring per the switching matrix diagram |
| Pattern works on one direction pair but not the perpendicular pair | Delay lines connected to wrong element pair | Verify delay line routing — trace each coax from relay to element | Swap delay line connections to correct element pair |
| SWR varies significantly between switch positions | Element radial systems not symmetrical | Count radials on each element — unequal counts cause impedance mismatch | Add radials to under-equipped elements; verify inter-element ground bonding |
| System works initially then degrades over weeks | Coax connector corrosion at outdoor connections | Inspect all outdoor PL-259/SO-239 connections for green oxidation | Strip, clean, and re-weatherproof all outdoor coax connections; apply self-amalgamating tape |
| Relay switching erratic — directions change without command | Control cable picking up RF from antenna | Add ferrite clamp to control cable at phasing box entry | Install snap-on type 31 ferrite on control cable near phasing box; bypass relay coil with 0.01 µF capacitor |
| No SWR resonance visible — SWR flat and high across band | Open circuit in main feed coax or phasing box | Disconnect at phasing box and check each element individually | Trace the feed path; check all SO-239/PL-259 connections; verify relay contacts are making |
How much space does a 40m 4-square actually need?
The four element positions define a 34 × 34-foot square — about 1200 square feet. Adding the radial systems for each element, which extend 33 feet beyond each element in all directions, the total required clear ground footprint is approximately 100 × 100 feet. This is the area needed for a full installation with 16 radials per element. In practice, radials from adjacent elements overlap in the interior of the square, so the effective total area is somewhat less than four independent 33-foot-radius circles. A property with at least 120 × 120 feet of clear open ground can accommodate a 40m 4-square with full radial systems.
Can I build a 4-square for 80m?
Yes — an 80m 4-square is the same design scaled up. Element height is ~62 feet per element, element spacing is ~65 feet (the square side), the total footprint is approximately 200 × 200 feet, and the delay lines are ~43 feet of 75Ω coax. The performance advantage of the 80m 4-square is even more dramatic than at 40m because the competition (horizontal dipoles and beams) is even worse at 80m due to the impractically large height required for a low-angle horizontal antenna at 3.7 MHz. Many serious 80m DX operators consider the 4-square the definitive 80m antenna for fixed installations. The required footprint — about an acre of clear ground — is the limiting factor for most operators.
Can I use the 4-square for receiving only — without transmitting through it?
Yes — the 4-square is an outstanding receive antenna even at QRP power levels, and some operators build one specifically for receive while using a different antenna for transmit. On receive, the front-to-back ratio and directional pattern are just as valuable as on transmit — often more so, because the ability to null out a noise source or interfering station from a specific direction transforms the receiving environment dramatically. A receive-only 4-square can be built with lighter materials (no heavy aluminum tubing required — fiberglass fishing poles and wire elements work perfectly) and does not need weatherproof high-power RF connectors. The phasing network and switching are identical.
Does the 4-square work on multiple bands?
A 4-square optimized for one band does not perform correctly on other bands. The element spacing, element length, and delay line lengths are all designed for a specific frequency. On other bands, the phasing relationships are incorrect and the pattern degrades — the array may produce gain in an unexpected direction or show poor front-to-back. Some operators build separate 4-square arrays for 40m and 80m at the same location, with the two arrays interleaved (the 40m array's elements placed between the 80m elements). This is a significant undertaking but the result — switchable directional 4-square coverage on both low bands — is among the most effective fixed HF station configurations possible.
What is the difference between the Christman and Lewallen phasing methods?
Both produce the same 4-square pattern — the difference is in the network implementation. The Christman method (used in this guide) uses only coaxial delay lines — simple, all-coax construction with no discrete components. Its limitation: the element feedpoint impedances in the array (approximately 25 Ω front, 100 Ω rear) are not perfectly matched by the 75Ω feed lines, producing a small but real efficiency loss. The Lewallen method uses a hybrid coupler network with lumped components (capacitors and transmission line sections) that provides true impedance matching to each element and theoretically slightly better efficiency. The Lewallen network is more complex to build but produces marginally better results. For a first 4-square build, the Christman method's simplicity makes it the better starting point — the performance difference in practice is small compared to the installation accuracy of the elements and radial systems.
How do I verify the pattern without sophisticated test equipment?
Three practical methods for on-air pattern verification without an antenna range: use WSPR — transmit at a fixed power for 24 hours in each direction and compare the average SNR of spots received from distant stations in the target direction versus the opposite direction; use a cooperative local operator at a known bearing as a reference — switch directions while transmitting and have them report signal levels in each position; or use a distant beacon on 40m (WWV and NIST signals are on 10 MHz, too high for a 40m array, but various ARRL and other beacons operate on 40m) and compare S-meter readings in each direction. A well-functioning 4-square produces a 3 S-unit or greater difference between the front and rear directions — easy to measure with any calibrated S-meter.