Build an HF Receive Loop Antenna — Complete Construction Guide
A well-built receive loop can transform your HF listening experience. Where a full-size transmit antenna picks up every nearby noise source equally, a shielded receive loop rejects electric-field interference — QRM from switch-mode power supplies, LED drivers, and solar inverters — while still responding well to the magnetic component of distant radio signals. This guide builds a practical 1-metre shielded loop covering 1–30 MHz with a broadband JFET preamp.
Why a Dedicated Receive Loop?
Modern suburban and urban amateur radio environments are plagued by locally generated electrical noise — switch-mode power supplies in computers and phone chargers, LED driver circuits in lighting, solar panel inverters, and plasma televisions. All of these devices radiate electrical noise primarily as electric-field (E-field) interference. A standard wire antenna or vertical is equally sensitive to both the electric and magnetic components of a radio wave, which means it cannot discriminate between wanted distant signals and unwanted local noise sources.
A shielded magnetic loop antenna responds predominantly to the magnetic component (H-field) of the electromagnetic wave. Local noise sources — being electrically close to the antenna — have a near-field structure dominated by the E-field. By electrostatically shielding the loop conductor and breaking the shield at one point to prevent it from becoming a shorted turn, you create an antenna that is inherently less sensitive to E-field noise while retaining full sensitivity to the H-field of distant sky-wave signals. The practical result can be a 10–20 dB improvement in signal-to-noise ratio on difficult bands like 40 m, 80 m, and 160 m in urban locations.
Electrostatic Shielding Effect
The outer copper or aluminium shield intercepts E-field interference and conducts it to ground, preventing it from reaching the inner conductor. The gap in the shield — at the top of the loop — breaks any shorted-turn current that would prevent the H-field from inducing a signal in the inner conductor.
Figure-8 Radiation Pattern
A loop antenna has a classic figure-8 (bidirectional null) pattern with deep nulls perpendicular to the loop plane. Rotating the loop to place its null toward a noise source can further reduce local interference by 20–40 dB. This directionality is a significant practical advantage in tight band conditions.
Receive Only — Not for Transmit
This design is for receive use only. The small loop has extremely low radiation resistance and high reactance — making it a very inefficient transmitter. A JFET preamp at the feed point compensates for the loop's inherently low output impedance and raises the signal level for the coax run to the receiver.
Design Overview & Theory
The antenna consists of three main subsystems: the shielded loop element, the impedance-matching preamp at the feed point, and the DC power injection system that delivers 12 V to the preamp through the coaxial feedline. The loop is a single-turn inductor approximately 1 metre in diameter, made from 15–22 mm copper or aluminium tube or RG-213 coaxial cable with the outer jacket stripped and the shield left intact but broken at the top.
At HF frequencies below 30 MHz, the 1 m diameter loop has a circumference of approximately π metres — about 3.14 m total. At 14 MHz, this is approximately 0.15λ. At 3.5 MHz it is 0.037λ. The loop is therefore electrically small across the entire HF range, which means it has extremely low radiation resistance (milliohms) and high reactance. The output voltage across the feed point gap is proportional to the loop area and the frequency of the incident wave.
Voc = 2π × f × μ₀ × N × A × Hfield
where N = turns (1), A = loop area (m²), f = frequency, H = magnetic field strength
L (µH) ≈ 0.001257 × r × (ln(8r/a) − 2) — r = loop radius (m), a = conductor radius (m)
For a 1 m diameter loop (r = 0.5 m) made from 15 mm copper tube (a = 7.5 mm): L ≈ 0.001257 × 0.5 × (ln(8×0.5/0.0075) − 2) ≈ 0.001257 × 0.5 × (ln(533) − 2) ≈ 0.001257 × 0.5 × 4.28 ≈ 2.69 µH. At 14 MHz the reactance is 2π × 14 × 2.69 = 237 Ω — the loop output must be buffered by a low-noise preamp to drive a 50 Ω feedline without severe impedance mismatch loss.
Materials required — 1 m shielded loop with JFET preamp
JFET Preamp Circuit
The preamp is the heart of the receive loop system. It buffers the high-impedance loop output, provides 10–20 dB of gain to overcome coax feedline loss, and presents a 50 Ω output to the receiver. The JFET source-follower configuration is the most widely used design for receive loop preamps because it is inherently low-noise, draws minimal current, and is robust against signal overload from strong local broadcast stations.
Component selection notes
- J310 JFETThe classic choice for HF receive preamps. Low noise figure (1–2 dB), high transconductance, available from Mouser/Digi-Key. The 2N5484 is a direct substitute with very similar specifications. The MPF102 is slightly noisier but widely available and works acceptably for this application.
- 10 kΩ gate resistorProvides a DC path to ground for the JFET gate and prevents static charge buildup. Does not affect RF performance significantly at HF due to the JFET's high input impedance.
- 100 Ω source resistorSets the drain current operating point. The JFET self-biases through this resistor — no negative supply rail required. Adjust between 68 Ω and 220 Ω if the J310 is substituted with a different part number to optimise noise figure vs. gain.
- 47 Ω output resistorProvides a 50 Ω match to the coax. A small mismatch here is acceptable — the receiver's front end will see near-50 Ω and function correctly across the HF range.
- Ferrite chokes on supply railThread the supply lead through four ferrite beads to prevent RF from travelling back into the power supply and causing interference or instability.
Receive Loop Inductance & Impedance Calculator
Copper Tube Loop — Construction Sequence
Build the loop conductor, create the electrostatic shield gap, assemble the JFET preamp, then mount and feed the antenna. Allow 4–6 hours for the complete build.
Choose loop geometry: circular vs. square
A circular loop maximises area for a given circumference — the best geometry theoretically. In practice, a square loop (with four 90° corners) using push-fit copper plumbing fittings is far easier to build and performs nearly as well. For a 1 m diameter equivalent, use four sides of 785 mm each (total circumference 3.14 m matches the 1 m circular diameter). The gain difference between a circular and square loop of equal area is negligible — less than 0.1 dB.
Cut and assemble the copper tube loop
Cut four lengths of 22 mm copper tube, each 785 mm long, using a pipe cutter for clean square ends. Join with four 22 mm push-fit 90° elbows to form a square. If using solder-type fittings, clean and flux all joints before soldering. Leave one side un-joined — this will become the two ends that connect to the preamp enclosure at the bottom of the loop. The gap between the two bottom ends is where the shield break (for electrostatic shielding) will be implemented. Total loop perimeter: 4 × 785 mm = 3.14 m.
Create the electrostatic shield gap at the top
The electrostatic shield gap is a physical break in the loop conductor at the point diametrically opposite the feed point — the top of the loop. For a copper tube loop, this is achieved by cutting the top horizontal section at its midpoint and inserting a 20–30 mm gap filled with a short section of PVC tube or a push-fit fitting with a 2 mm plastic shim. The shield break prevents E-field-induced currents from circulating around the loop, while the H-field induction (which acts across the whole loop area) is unaffected. Without this gap, the loop would be a shorted turn with very low impedance that would reduce H-field sensitivity.
Mount the preamp enclosure at the feed gap
The preamp enclosure attaches to the bottom of the loop where the two free ends of the copper tube terminate. Drill two 22 mm holes in the top face of the ABS enclosure, sized to accept the copper tube ends with approximately 20 mm of insertion depth. Apply epoxy or silicone to seal the tube-to-enclosure joint weathertight. Inside the enclosure, the two copper tube ends connect to the loop terminals of the JFET preamp PCB via short lengths of heavy copper wire or bus bar. The SO-239 coax connector mounts on the bottom face of the enclosure.
Build and install the JFET preamp PCB
Assemble the JFET source-follower preamp on a small piece of stripboard or a custom PCB. Keep all leads as short as possible — long component leads introduce inductance that can cause instability above 20 MHz. The JFET gate connects directly to one loop terminal. The source connects through the 100 Ω resistor to the coax centre conductor. The drain connects through the 100 nF decoupling capacitor to the 12 V supply rail. Install ferrite beads on the supply lead before it enters the enclosure.
Mount on a non-conductive mast
The loop must be mounted on a non-conductive mast or support — PVC pipe, fibreglass rod, or wood. Any conductive mast within 500 mm of the loop will distort its pattern and reduce its E-field rejection capability. A 1.5–2 m length of 40 mm PVC conduit makes an excellent mast, supporting the loop at head height for rotation. Fix the loop to the mast with two 22 mm P-clips. For permanent outdoor installation, ensure all copper-to-fitting joints are fully soldered (not just push-fit) and seal the enclosure with marine-grade silicone.
Run the coaxial feedline and DC power
Run RG-58 or RG-213 coaxial cable from the SO-239 on the preamp enclosure to the receiver. At the receiver end, use a bias tee to inject 12 V DC onto the coax centre conductor — this power travels back up the coax to the preamp. Alternatively, run a separate shielded DC cable to the preamp if a bias tee is not available. Tape the coax to the mast with UV-resistant cable ties, leaving a drip loop at the enclosure to prevent water from running down the coax into the connector.
RG-213 Coax Cable Loop — Construction Sequence
An excellent and lower-cost alternative to the copper tube loop is to use a length of RG-213 coaxial cable itself as the shielded loop element. The outer braid of the coax acts as the electrostatic shield, and the inner conductor is connected at both ends to create a single-turn loop. The shield break at the top is achieved simply by cutting the outer braid and jacket at the top midpoint and insulating the cut with self-amalgamating tape — without disturbing the inner conductor, which runs continuously from one feed end to the other.
Cut 3.3 m of RG-213
Cut a 3.3 m length of RG-213. This will form a circular loop approximately 1 m in diameter. At the midpoint (1.65 m from each end), carefully cut through the outer PVC jacket and braid only — do not cut the inner dielectric or centre conductor. Remove a 20–30 mm section of braid and jacket at this midpoint, creating the shield gap. Insulate the exposed inner conductor at this point with self-amalgamating tape, leaving only the gap in the outer shield.
Connect inner conductors at the feed point
At each free end of the RG-213, strip back the jacket, braid, and dielectric to expose 30 mm of centre conductor. Twist or solder both centre conductors together — this connects the inner conductor at both ends of the loop, completing the single-turn loop formed by the inner conductor running the full circumference. The two braid/shield ends are NOT connected together — they connect separately to the preamp's differential input terminals.
Form the loop shape
Shape the RG-213 into a circle approximately 1 m in diameter and secure it with a few cable ties to a lightweight fibreglass or PVC support frame. The coax's natural stiffness holds a rough circular shape; a hula-hoop-size PVC ring provides a more elegant support. The feed point is at the bottom where both free ends terminate into the preamp enclosure.
Loop Orientation & Null Steering
The receive loop's figure-8 pattern has two deep nulls — one on each side of the loop plane (perpendicular to the plane of the loop). These nulls are typically 20–40 dB deep for a well-constructed shielded loop. Rotating the loop to align a null toward a nearby noise source — a neighbour's solar inverter, a street light controller, a plasma TV — can dramatically improve the signal-to-noise ratio on affected frequencies. Mount the loop on a rotating base (a lazy-Susan bearing on the PVC mast works well) for easy null steering.
Multiple Turn Loops for Lower Frequencies
For improved sensitivity on 160 m and below, a multi-turn loop (3–5 turns) increases the output voltage by the number of turns N. However, multiple turns also increase the loop inductance by N², which raises the reactance and makes the preamp design more challenging. A 3-turn loop on 160 m may provide useful improvement; above 10 MHz multi-turn loops become difficult to use due to inter-winding capacitance effects. For a broadband 1–30 MHz receive loop, a single turn is the best compromise.
Reducing Common-Mode Noise
Even a well-shielded loop can pick up common-mode noise that travels up the outer surface of the feedline coax. Installing a choke balun — 10 turns of the feedline coax wound on an FT-240-31 toroid immediately below the preamp enclosure — suppresses this common-mode path effectively. This single step can recover 10–15 dB of noise rejection that would otherwise be lost to coax shield noise.
| Band | Frequency | Signal sensitivity | Noise improvement vs wire | Notes |
|---|---|---|---|---|
| 160 m | 1.8–2.0 MHz | Good | 10–20 dB typical | Excellent for noisy environments; most dramatic improvement here |
| 80 m | 3.5–4.0 MHz | Very good | 8–15 dB typical | Often transforms 80 m from unusable to workable in urban areas |
| 40 m | 7.0–7.3 MHz | Very good | 5–12 dB typical | Strong improvement; good sensitivity |
| 30 m | 10.1 MHz | Excellent | 3–8 dB typical | Loop sensitivity peaks in this range |
| 20 m | 14 MHz | Excellent | 2–6 dB typical | Good performance; sky noise begins to dominate |
| 15–10 m | 21–28 MHz | Good | 1–4 dB typical | Atmospheric noise dominates; loop advantage reduces |
Sky noise floor: above 14–20 MHz, the sky noise (galactic background + atmospheric noise) increasingly dominates the noise floor rather than local man-made interference. Above this crossover frequency, the receive loop's noise reduction advantage diminishes because the noise floor is already set by the propagation medium rather than local sources. Below 10 MHz in urban areas, the receive loop is often transformative.
Can I transmit through this receive loop?
No — this design is strictly receive-only. The JFET preamp would be destroyed by even 1 W of transmit power. The loop itself has extremely low radiation resistance (milliohms) and extremely high reactance, making it a very inefficient transmitter. Always switch the receiver to a separate transmit antenna or use a receive/transmit switching relay before transmitting.
How does this compare to a commercially built loop like the Wellbrook ALA1530?
The Wellbrook ALA1530 uses a similar shielded loop concept with a professionally designed low-noise amplifier. A carefully built home-brew loop with a quality JFET preamp performs comparably for most purposes — within 3–6 dB of commercial units. The commercial advantage is in the preamp's noise figure optimisation, PCB quality, and weatherproofing. The home-brew advantage is cost (roughly 10–20% of the Wellbrook price) and the ability to customise loop size.
Does the loop need to be exactly 1 metre in diameter?
No — loop size is a practical trade-off. Larger loops collect more signal (sensitivity scales with area) but are physically harder to manage and rotate. A 500 mm loop is easier to handle indoors; a 1.5 m loop provides better sensitivity on 160 m and 80 m. For most HF amateur use, 800 mm to 1.2 m diameter is the practical optimum. The JFET preamp design works identically across this range.
Can I use aluminium tube instead of copper?
Yes — aluminium tube is lighter and cheaper than copper and works well for the loop element. Use anodised aluminium for corrosion resistance. The key difference is that aluminium cannot be soldered with standard solder — use stainless hardware bolted connections at all joints, or use copper compression fittings. The electrical performance difference between copper and aluminium for a receive-only loop is negligible.
What JFET should I use if J310 is unavailable?
The 2N5484 is the closest substitute — identical pinout, similar noise figure and transconductance. The MPF102 is slightly noisier (NF ≈ 4–5 dB vs 1–2 dB for J310) but is widely available and gives acceptable results. The BF245C and BF256B are European equivalents with similar specifications. Avoid using bipolar transistors (BJTs) as substitutes — their base current noise is significantly higher than a JFET's gate noise, degrading the noise figure noticeably.
How do I use the directional nulls effectively?
Identify the noise source direction if possible — a nearby street, a neighbour's house, or a known industrial location. Rotate the loop so the plane of the loop points toward the noise source (the null is perpendicular to the plane). In practice, slowly rotate the loop while listening to a quiet spot on the band and find the orientation that minimises the noise. A 20–40 dB null is a dramatic improvement that transforms a noisy band into a usable one.