Build the KD9SV Receive Loop — Pennant Loop for 160m & 80m
The KD9SV pennant receive loop is one of the most effective small directional receive antennas for the low HF bands. Developed by Robye Lahlum W1MK and popularised by Bruce Claflin KD9SV, the pennant combines a terminated small loop with a cardioid radiation pattern to reject noise from unwanted directions by 20–25 dB — transforming a noisy 160 m or 80 m band from a frustrating cacophony into a listenable DX path.
The Pennant Loop Concept
The pennant loop is a terminated triangular or rectangular small loop antenna — a close relative of the Flag antenna and the K9AY loop. Like all terminated loop designs, it achieves its directional cardioid pattern through a combination of the loop's magnetic response (H-field) and the terminating resistor's effect on the current distribution. The termination at one corner of the loop damps the resonant Q of the antenna, converting what would be a bidirectional small loop (figure-8 pattern) into a unidirectional cardioid with 20–25 dB front-to-back ratio over a useful frequency range without mechanical rotation.
The KD9SV design specifically refers to a compact pennant (roughly triangular flag shape) with careful attention to the terminating resistance value, preamp matching, and the optional phasing unit that allows two pennants to be combined for improved F/B performance or switched directionality. The antenna is electrically small on all HF bands up to approximately 10–14 MHz, making the terminating resistance the dominant factor in its pattern rather than element resonance.
Why the Pennant Works — Pattern Theory
The pennant (and related Flag, EWE, and K9AY loop antennas) achieves directional reception through the superposition of two electromagnetic responses. The loop itself responds to the magnetic component of the incoming wave — a small loop has a figure-8 (bidirectional) pattern. The terminating resistor at the opposite corner from the feed point creates a resistive unbalance that introduces a component of electric-field (E-field) response. When the H-field and E-field responses are combined in the correct phase relationship, they add constructively in one direction and cancel in the opposite direction, producing a cardioid pattern.
The resulting pattern has its maximum (forward) gain in the direction away from the terminating resistor — toward the feed point. With the optimal terminating resistance (typically around 870 Ω), null depths of 20–30 dB are achievable. In practice, 20–25 dB is the most common measured result for well-built examples.
The Two-Pennant Phasing System
A single pennant provides a fixed cardioid pattern — maximum gain in one direction, null in the opposite. For operators who need to receive from multiple directions, two pennants facing opposite directions can be combined using a phasing box to steer the combined pattern. With two pennants oriented 180° from each other and combined in the correct phase, the system provides a steerable null that can be placed in any direction by adjusting the phasing controls.
The KD9SV phasing unit uses a simple hybrid coupler — two resistors in a cross configuration — to combine the two pennant signals. By adjusting the amplitude and phase of the signal from each pennant before combining, the null can be steered through approximately ±90° from the antenna's mechanical null direction.
Phasing Unit Construction
The KD9SV phasing unit is built on a small piece of double-sided copper-clad board or stripboard. It contains a variable attenuator (a potentiometer and some resistors) on one of the two signal paths, and a 90° phase shift network (a small capacitor or inductor) switchable in or out. The two signals, after amplitude and phase adjustment, pass through a hybrid combiner (two 75 Ω resistors) to the receiver. The controls — labelled "amplitude" and "phase" — are adjusted while listening to the rear-arriving noise source until it disappears into the noise floor.
| Dimension | Standard 160m pennant | Compact variant | Notes |
|---|---|---|---|
| Width (horizontal top wire) | 2.74 m (9 ft) | 1.83 m (6 ft) | Primary dimension affecting gain |
| Height (right side vertical wire) | 1.52 m (5 ft) | 1.07 m (3.5 ft) | Proportional to width |
| Hypotenuse (sloping wire) | 3.14 m (10.3 ft) | 2.13 m (7 ft) | Calculated from above |
| Total wire circumference | 7.40 m (24.3 ft) | 5.03 m (16.5 ft) | Sum of three sides |
| Terminating resistance | 820–910 Ω | 820–910 Ω | Same for all sizes |
| Feed point impedance | ~450–600 Ω | ~400–500 Ω | Requires 9:1 unun |
The terminating resistance is the most important single value. The pattern null depth depends critically on R_term — the correct value cancels the magnetic and electric field components at the back of the antenna. A 10% error in R_term typically reduces the F/B ratio by 5–8 dB. Use precision ±1% metal film resistors and measure with an accurate ohmmeter before installation.
KD9SV Pennant Loop Calculator
Materials for standard KD9SV pennant (160m primary)
Building the Pennant
Build the unun and preamp first, then the frame and terminating resistor assembly, then string the wire and optimise the null. Allow 4–6 hours for the complete build.
Build the 9:1 unun transformer
Wind 9 bifilar turns of 1.0 mm enamelled copper wire on an FT-140-43 toroid — the same construction as described in the W3EWP and receive loop guides. Connect the primary (50 Ω) side to the SO-239 output connector. The secondary (450 Ω) side connects directly to the loop wire at the feed point. Mount the toroid inside the weatherproof feed point enclosure. Verify with an ohmmeter that there is no DC short between primary and secondary.
Build the JFET preamp (at feed point)
The pennant's electrically small size means it has very low output voltage — far less than a resonant antenna at the same frequency. A JFET preamp providing 10–15 dB of gain at the feed point compensates for this and overcomes coax feedline loss. Use the same J310 source-follower circuit described in the receive loop guide, mounted on a small PCB inside the feed point enclosure alongside the 9:1 unun. Power the preamp via a DC bias tee at the receiver end of the coaxial feedline.
Prepare the terminating resistor assembly
Solder three resistors — 820 Ω, 860 Ω, and 910 Ω metal film, all ±1% — to a small three-position rotary switch or three-way toggle switch. Mount the switch assembly inside the small terminator weatherproof box. This switched resistor bank allows fine-tuning of the null depth without physical modification of the antenna. If you prefer a fixed value, measure all three resistors with an accurate meter and choose the one closest to 870 Ω.
Build the triangular frame and string the wire
Construct three corner supports from 10 mm fibreglass rod or PVC pipe, each approximately 200 mm long, bent at 90° or fitted with T-fittings. At the top-left corner (feed point), mount the feed point enclosure on one corner support. At the bottom-right corner (terminator), mount the terminator box on another support. String the wire between the three corner assemblies: top wire (width, 2.74 m), right vertical wire (height, 1.52 m), and sloping hypotenuse wire (3.14 m). All three wires are continuous — do not cut between corners.
Mount vertically facing the primary DX direction
The pennant must be mounted vertically — the plane of the triangle vertical, the forward direction (away from the terminator) pointing toward your primary DX target. Mount using UV-resistant rope between the two top corners to a horizontal support rope or single mast. The antenna should be elevated at least 2–3 m above ground for 160 m; higher is better. Keep the antenna well away from metal structures within 3 m — these distort the cardioid pattern and degrade the null depth.
Connect the coax and power the preamp
Run RG-58 or RG-213 coaxial cable from the SO-239 on the feed point enclosure down to the shack. At the receiver end, install a bias tee to inject 12 V DC back up the coax to power the JFET preamp. Verify the preamp is drawing current (typically 15–25 mA). If no current flows, check the bias tee polarity and the coax continuity.
Optimise the terminating resistance
Tune to a strong, consistent signal arriving from the rear of the antenna (approximately 180° from the forward direction). Switch between the 820 Ω, 860 Ω, and 910 Ω terminating resistors and listen for the minimum signal on the rear-arriving station. The resistor value that produces the deepest null is the optimum for your installation. Record the optimum value and, if a fixed resistor is acceptable, replace the switch assembly with the single optimum value resistor.
| Band | F/B ratio (typical) | Gain vs dipole | Noise improvement (urban) | Notes |
|---|---|---|---|---|
| 160 m (1.8 MHz) | 20–28 dB | −12 to −18 dBd | 15–25 dB | Best band — loop most effective here |
| 80 m (3.5 MHz) | 18–25 dB | −8 to −14 dBd | 12–20 dB | Excellent noise reduction |
| 40 m (7 MHz) | 15–22 dB | −4 to −10 dBd | 8–15 dB | Still very useful in noisy environments |
| 30 m (10 MHz) | 12–18 dB | −2 to −6 dBd | 5–10 dB | Useful at upper end of range |
The gain deficit is not a problem: the pennant has substantially lower gain than a resonant antenna — typically 10–18 dB below a dipole on 160 m. On the low bands, man-made noise routinely exceeds the galactic and atmospheric noise floor by 20–40 dB in suburban environments. Reducing the noise by 20 dB with the pennant's directional null, while accepting −15 dBd gain, results in a net signal-to-noise improvement of 5–25 dB.
How does the KD9SV pennant differ from the Flag or EWE?
The Flag is a rectangular version of the same terminated loop concept — same operating principle, rectangular geometry. The EWE (named for its shape) uses a ground stake as one side of the loop. The pennant uses a triangular geometry that reduces the physical size while maintaining the characteristic cardioid pattern. All three achieve similar F/B ratios; the pennant's compact size makes it practical for constrained sites. The KD9SV design adds specific attention to preamp design and the phasing unit for the two-loop system.
Can I use the pennant for transmitting?
No — the pennant is a receive-only antenna. The terminating resistor dissipates approximately half the transmitter power, dramatically reducing efficiency (similar to the terminated folded dipole). The preamp would be destroyed by even 1 W of transmit power. Use a separate transmit antenna and switch the receiver to the pennant during receive periods.
Does the terminating resistor need to be exactly 870 Ω?
No — the optimum value varies between installations depending on the exact loop geometry and ground conditions. The 870 Ω value is a starting point; values between 820 Ω and 910 Ω all produce good results. Building a switched resistor bank (three values) and selecting empirically by listening is the most reliable approach. A 5% error from the optimum typically reduces F/B by only a few dB.
How far from the transmit antenna must the pennant be?
At least 10–20 m from any transmit antenna during transmit, to prevent transmit RF from coupling into the receive loop and potentially damaging the preamp. Direct near-field coupling from a close transmit antenna can produce dangerous voltage at the preamp input. Use an antenna switch with enough isolation (typically 40+ dB) or a TR relay to disconnect the pennant feedline during transmit.
What height above ground does the pennant need to be?
The pennant should be at least 2–3 m above ground for 160 m use to reduce ground loss effects. Higher is marginally better — at 5–10 m the pattern becomes more consistent. The important constraint is vertical orientation. Keep the pennant at least 2 m from any metal fencing, guttering, or building structure.
Can I build a pennant for 40m and 20m instead?
Yes — on 40 m and 20 m the pennant works but is less dramatically effective because the atmospheric noise floor on those bands is lower and man-made interference is less dominant. On 20 m the loop circumference is approximately 25% of a wavelength — no longer electrically very small — which means the F/B pattern becomes more frequency-dependent. For 40 m and above, a shielded receive loop is often a better choice due to its broader frequency coverage.