ON6URE Visits SNW: A Mono-Output 80 m ARDF Receiver
ON6URE Visits SNW: A Mono-Output 80 m ARDF Receiver
Jos ON6WJ invited me to Sint-Niklaas for Kurt ON4CHE's presentation of an 80-metre ARDF receiver developed with Marc ON4FOX and Jean-Marie ON7EN. The intriguing part was not another S-meter with headphones. It was the attempt to resolve direction in RF and deliver one useful mono signal to one receiver.
RF.Guru working definition: Common-mode current is the non-cancelling phasor-sum current in a specified set of conductors, evaluated at a defined cross-section and using a declared current-direction convention. In the intended differential transmission-line mode, the outgoing and return currents are equal and opposite, so their phasor sum is zero. When they do not cancel, the remaining current must close through another reference or return path—such as the outside of a coax shield, a mast, equipment chassis, station wiring, nearby structures, earth, the operator, or distributed coupling through the environment.
This broader working definition is especially useful in practical antenna systems. On transmit, non-cancelling current on the outside of the coax can make the feedline and connected structures part of the radiating antenna system unless that path is intentional, clearly defined and properly controlled—for example by providing the required return path and placing a suitable common-mode choke at the correct boundary.
ARDF rewards an unambiguous bearing, controlled gain and a pattern the operator can trust. Kurt's receiver brings those requirements together with a compact loop, a sense element, RF-domain combining and a single demodulator.
The People Behind the Receiver
Jos, ON6WJ, brought me—Joeri, ON6URE—to the SNW presentation by Kurt, ON4CHE. Kurt developed the receiver with Marc, ON4FOX, and Jean-Marie, ON7EN. That origin matters: this article is about their practical direction-finding problem and the engineering discussion it triggered, not a generic catalogue of loop antennas.
The receiver concept paired a compact shielded H-field loop with a small E-field sense element. Both were placed close together and fed one HF receiver. The prototype used an SI4732-family receiver with community SSB-patch support, a practical route to compact CW/SSB reception when firmware, gain control and overload behaviour are verified for the exact device and patch.
Why a Loop Alone Leaves an Ambiguity
An electrically small loop has a bidirectional figure-eight response in its useful plane. Its two nulls can give a sharp bearing line, but the loop alone cannot tell which end of that line points toward the transmitter. Rotating through a null gives precision without front/back identity.
A co-located sense element can remove that ambiguity. In the ideal azimuth model, let the loop contribution be proportional to cos θ and let the sense contribution be approximately omnidirectional. After their amplitudes and electrical phases are aligned, adding them produces a cardioid-like response:
Vout(θ) = A + B cos θ
When A and B are equal at the combining reference plane, one direction adds and the opposite direction cancels.
The required phase correction is not a decorative “90-degree” label. A loop and an electric-field sense element have different transduction phase, impedance and frequency response. The network must bring their measured voltages into the intended relative phase at the combiner. Cable delay, loading, enclosure coupling and the receiver input all belong in that calibration.
Direction Is Resolved Before Demodulation
Kurt's attractive architectural choice was RF-domain combining. Once the two sensor paths have the correct amplitude and phase relationship, their sum already contains the front/back discrimination. One receiver can then demodulate one mono signal. No stereo audio comparison is required for the basic bearing indication.
That simplicity is operationally valuable on foot. The operator can concentrate on attenuation, bearing and terrain instead of mentally comparing two audio channels. It does not make the RF network self-calibrating: the combined pattern still has to be measured across the intended 80-metre frequency range and at the signal levels encountered near a transmitter.
The Passive Refinement We Discussed
The visit led me to explore a more symmetrical passive front end: a balanced loop interface, a separately defined sense injection path and a passive combining node ahead of the receiver. The aim is worth keeping—reduce cable count and active circuitry near the sensors while giving the receiver one controlled output.
Symmetry can help control common-mode pickup, but a centre tap, shield or Wilkinson label does not prove immunity to the operator or the ground. A Wilkinson combiner is designed around stated port impedances and phase relationships; it does not by itself transform two arbitrary antenna sources into a calibrated cardioid. The loop and sense paths need defined source impedances, isolation, amplitude tracking and phase tracking. Passive loss also consumes signal margin.
The right public conclusion is therefore constructive and bounded: the passive topology is a promising way to simplify the handheld receiver, while its cardioid depth, common-mode response and body/ground sensitivity remain measured quantities rather than consequences of the schematic alone.
What the Receiver Must Prove
| Question | Useful measurement |
|---|---|
| Does the sum form a cardioid? | Azimuth response of loop alone, sense alone and combined output at several 80 m frequencies |
| Is front/back identity stable? | Bearing reversal and null-depth checks after cable, enclosure and hand-position changes |
| Does the receiver remain usable near a fox? | Compression, blocking and attenuation range with the exact SI4732 patch and gain settings |
| Is the loop really balanced? | Differential and common-mode transfer measurements with stated source impedances and cable routing |
| Does passive combining cost too much margin? | Insertion loss, sensor noise margin and minimum readable signal at the receiver reference plane |
IARU Region 1 allows competitors to use receivers and antennas of any type, while also limiting unwanted receiver radiation. That leaves room for this kind of architecture: compact, quiet, directionally useful and judged by field performance rather than by how many channels it contains.
What stayed with me after SNW: a good ARDF receiver does not need to make its signal path complicated. It needs to make direction unambiguous. Kurt, Marc and Jean-Marie pursued that in RF, and the mono-output result is exactly why the design deserves careful measurement rather than being flattened into another loop-receiver recipe.
Sources and project references
Mini-FAQ
- Why does a loop need a sense element? The loop's figure-eight pattern gives two opposite nulls. A sense element can turn that bidirectional response into a cardioid-like pattern with front/back identity.
- Why combine the signals at RF? Correctly calibrated RF combining produces one directional mono signal before demodulation, so one receiver and one audio channel can be used.
- Is a fixed 90-degree network always correct? No. The required phase correction follows the measured sensor and network transfer functions at the combining reference plane.
- Does a balanced loop eliminate operator coupling? Not automatically. Balance can reduce one coupling mode, but enclosure, cable, ground and body sensitivity still require measurement.
- Can a Wilkinson combiner connect the two sensors directly? Only when its port impedances, isolation, amplitude and phase conditions are satisfied. Arbitrary sensor sources need appropriate interfaces.
- What made the SNW design memorable? Kurt ON4CHE, Marc ON4FOX and Jean-Marie ON7EN pursued front/back discrimination in RF and delivered one practical mono output for portable ARDF.