Silent Scout: Why a Passive, Balanced ARDF Probe Beats “LNA-on-Probe”
SilentScout: Why a Passive, Balanced ARDF Probe Beats “LNA-on-Probe”
(Indicative field notes included; your mileage will vary with terrain, groundwave, and beacon height.)
The Goal in ARDF: Null Integrity, Not Raw Gain
For bearings, the quality and repeatability of the cardioid null determine success far more than absolute sensitivity. A balanced loop + E-probe that keeps symmetry intact will out-perform a “more gain” approach almost every time.
Why Bolting an LNA to a Balanced Probe Backfires
- Balance gets broken: Most LNAs are single-ended. Tying one leg to ground injects common-mode paths, flattening nulls and making the probe hand-sensitive.
- Mismatched impedance: Small ARDF probes are not 50 Ω; a 50 Ω-optimized LNA sees a poor match and squanders any theoretical NF advantage.
- Headroom shrinks: ARDF beacons are strong. Front-end gain before filtering simply pushes the receiver toward IMD/overload faster.
- Resonance reliance: Using resonance to “fix” phase locks performance to a narrow peak; hands, detuning, or nearby objects then skew both amplitude and phase.
E vs H Changes with Distance (near-field vs far-field)
Close to the beacon, the E-field contribution dominates; farther away the H-field balance improves. If you sum E and H asymmetrically (e.g., inject E on one loop leg via a Wilkinson path), you can maintain a usable cardioid without risking complete destructive cancellation. The ratio shifts with distance, but the null stays useful when the structure remains fully balanced.
Resonance for Phase Control: Tempting but Brittle
- Amplitude swings: By forcing resonance on the loop or E-probe to “freeze” phase, you also create a steep amplitude peak. Small detuning then wrecks your balance.
- Cardioid drift: Phase “fixed” by resonance can still wander as the environment shifts the tune point. The cardioid null moves with it.
- Better approach: Keep the antenna broadband and control phase with passive vectoring and damping; let the combiner/phasing define directivity, not a razor-thin LC peak.
Preselection Beats Gain: Protect Receiver Headroom
Adding gain raises wanted and unwanted energy equally. If the receiver has limited headroom, a preselector (even a modest-Q low-pass or band-pass) helps more than an LNA. On 80 m ARDF (≈3.5–3.6 MHz), a simple passive filter in front of the radio often nets 10–15 dB less out-of-band crud — that’s real dynamic-range you can feel.
Free-space/groundwave path loss is logarithmic. Roughly every +6 dB is ~×2 range (very approximate on 80 m groundwave). If your reference receiver reaches 6 km, being ~20 dB less sensitive points you nearer ~2 km. You don’t fix that deficit with a front-end LNA — you fix it by filtering (headroom) or using a better receiver.
Q: High vs Low — and How to Measure It Properly
- High-Q: sharp peak, narrow bandwidth, high detuning sensitivity — risky for handheld ARDF.
- Low-to-moderate Q: wider bandwidth, stable nulls, less sensitive to hands/nearby objects.
- Loaded vs unloaded Q: Many “Q meters” actually measure the loaded system (source + balun + DUT). For antennas, a VNA bandwidth-based Q estimate is usually more meaningful.
If a balun “changes Q by ×3”, that’s a clue the method is seeing impedance transform and source loading, not just the loop itself.
Implementation Notes
- Loop pair (H-field): Orthogonal 19 cm loops form a differential pickup; a broadband damping resistor smooths nulls and controls peaking.
- E-probe (15 cm): Capacitive injection provides the E-vector. Damping is switchable so you can soften/boost E contribution for the cleanest cardioid.
- Wilkinson + summing on one leg: Summing E onto a single loop leg via a Wilkinson maintains isolation/phase while preventing fully destructive cancellation as the E/H ratio changes with distance.
- Isolation & CMRR: Keep everything balanced up to a galvanic isolator (e.g., a 1:1 transformer) and use a proper output choke to kill common-mode backflow.
- Protection: Clamp diodes and RF DC-blocks handle ESD/lightning transients and improve low-frequency stability.
- Attenuator pad: For close-range hunting, a 20–50 dB switchable pad at the receiver is more practical than “detuning” the probe.
Field Notes That Map to the Theory
- Beacon power/height matters: Typical tests around 2–3 W, ~6 m antenna height. Target “long range” ≈ 3–6 km on 80 m depends strongly on terrain and ground conductivity.
- Receiver sensitivity is king: If a handheld’s SINAD spec is ~25 µV (≈-115 to -119 dBm), it will set your ceiling. Better radios instantly extend range.
- Attenuation near the fox: With 50 dB attenuation, peiling was possible almost against the beacon antenna. This is the clean way to cope with close-in overload.
- Resonance “helps” range but hurts stability: Forcing resonance increased distance in tests, but made close-in peiling worse and nulls touchy. The passive broadband version remained consistent.
Bottom Line
Keep the probe balanced, passive, and broadband. Use damping to shape Q and a Wilkinson/transformer path to manage vectoring without breaking symmetry. If you need more reach, add a preselector or use a better receiver — not an LNA at the antenna.
Mini-FAQ
- Should I add an LNA? — No. It reduces headroom, invites IMD, and compromises balance. Filter first.
- Why do my nulls drift when I tune for resonance? — Because small detuning shifts both amplitude and phase; broadband vectoring is more stable.
- How big should the loops be? — Around 19 cm diameter worked well in tests; keep geometry rigid and balanced.
- Why sum E on one loop leg? — It preserves a cardioid as E/H ratios change with distance and avoids complete cancellation.
- Best “more range” upgrade? — A better receiver or a simple front-end preselector (LPF/BPF), not extra gain.
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