ON6URE Visits SNW: innovative 80-meter ARDF receiver

ON6WJ aka Jos invited ON6URE (Joeri) to attend a presentation by Kurt (ON4CHE), hosted at UBA Section SNW (Sint-Niklaas). The event showcased an innovative 80-meter ARDF receiver designed by Kurt in collaboration with Marc (ON4FOX) and Jean-Marie (ON7EN).

The design features a compact, shielded H-loop (magnetic field sensor) paired with a small E-probe (electric field sensor). Both antennas are co-located to minimize spatial phase error and are connected to a single HF receiver based on a patched SI4732 chipset. These Silicon Labs chips—such as the SI4735 or SI4732—can be patched to enable additional demodulation modes like SSB and CW. While the standard firmware only supports AM/FM, community-discovered patches allow USB/LSB reception. However, these patched SI4732 chips do not offer automatic gain control (AGC) in SSB mode, so manual gain handling is required.

The core concept uses phasing at the RF level—not audio—to create a directional pattern. Specifically, the E-field probe provides an omnidirectional signal (cos(θ) component), while the H-loop provides a null-based directional signal (sin(θ) component). A 90° RF phase shift is applied to one of these paths (typically the H-loop), and the two signals are then combined. This combination produces a cardioid pattern, with clear front/back discrimination and a single signal peak. This mono signal can be used effectively without requiring stereo decoding or multiple receivers.

ON6URE proposes a refinement that simplifies both the electrical and mechanical design: a fully passive system using a balanced H-loop with a center-tap, combined via a Wilkinson combiner. The E-probe is injected asymmetrically into only one leg of the loop. The loop itself is differential, with its outputs:

V_H(+) = A · sin(θ), V_H(−) = −A · sin(θ)

With a standard differential input, these signals cancel. However, injecting the E-probe (B · cos(θ)) into one side results in:

V_out = (1/√2) · (A · sin(θ) + B · cos(θ))

This resolves into a cardioid vector:
V_out ∝ √(A² + B²) · cos(θ − φ), with φ = arctangent(A / B)

This means front-back and left-right discrimination are both embedded in the RF domain — no audio phasing, no DSP, no stereo headphone tricks. Everything is resolved before demodulation.

Because the H-loop is center-tapped and fully balanced, the system achieves excellent common-mode rejection (CMRR). Unlike traditional designs where the E-probe is referenced to ground, ON6URE’s approach references it to the differential center of the H-loop. This eliminates variable field distortion due to proximity to the ground or human body. Any influence from the operator becomes purely parasitic and has little to no effect on the directionality.

Mechanically, this also simplifies things dramatically. With the balanced loop no longer needing isolation from ground, the frontend becomes compact and reproducible. A simple 3D enclosure with two PCB layers acting as shields is sufficient, especially at 80 meters where even mesh or stainless spacers provide adequate shielding.

No active switching or AGC is needed. A fixed or stepped attenuator can be used for range control, or a push-pull post-stage amplifier can be added downstream of the vector node if extra gain or buffering is needed. A simple overload clamp (diodes) can be added to protect the passive frontend.

In summary, the system is:

• Fully passive
• Mechanically minimalistic
• Electrically clean with galvanic isolation
• Immune to ground coupling and operator proximity
• Outputting a mono, cardioid RF signal directly to a single-chip HF receiver like the SI4732

This approach results in a robust, intuitive ARDF solution with minimal complexity, excellent precision, and no software-side signal processing. More test results and PCB layouts will follow soon.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.