Table of Contents
CHAPTER 1 — GENERAL INFORMATION
1-1. SCOPE
This manual covers the Terminated Sloper Receiving Antenna for low-noise directional reception on 160M, 80M, 40M (1.8–7.3 MHz). This is a receive-only antenna. The primary performance metrics are front-to-back ratio, noise figure, and signal-to-noise ratio improvement over a reference dipole. Efficiency in the conventional sense does not apply to receiving antennas; sensitivity and directivity are the figures of merit.
1-2. APPLICABLE REFERENCES
- ON4UN’s Low Band DXing — Receiving Antenna Chapters
- K9AY Loop Documentation (Gary Breed, QST October 1997)
- NEC2 model: terminated_sloper_receiving.nec (in antenna directory)
- Beverage Antennas for Amateur Radio (ARRL Antenna Book)
1-3. SAFETY PRECAUTIONS
CHAPTER 2 — THEORY OF OPERATION
2-1. DIRECTIVITY AND NOISE REJECTION
Long sloped wire with resistive termination at far end producing traveling-wave cardioid directional receive pattern. Receive-only antennas exploit directivity (front-to-back ratio) and aperture to separate signals from noise. A terminated long-wire or loop antenna achieves a cardioid or kidney-shaped pattern with deep null in one direction: F/B ratio typically 15–25 dB. This null can be steered toward interference sources (power-line noise, broadcast QRM) to dramatically improve signal-to-noise ratio on weak HF signals.
2-2. NOISE FIGURE AND SNR
At HF below 30 MHz, external noise (atmospheric, man-made) dominates over receiver noise figure. A receiving antenna with poor efficiency but good directivity can outperform a high-gain antenna pointed at a noise source. The key metric is signal-to-noise ratio (SNR) improvement over the reference antenna (typically a 40M dipole), not absolute signal level. An SNR improvement of 10–20 dB (1–2 S-units) on a target station makes marginal copy copy into solid copy.
2-3. TERMINATION RESISTANCE
Most directional receive antennas use a termination resistor (typically 500–900 Ω) to absorb backward-traveling waves and prevent re-radiation from the far end. This resistance determines the F/B ratio and the wave velocity factor along the antenna. 60–120 ft wire. The termination must be a non-inductive resistor (carbon composition or metal film; not wirewound) mounted in a weatherproof housing.
CHAPTER 3 — MATERIALS AND CONSTRUCTION
3-1. BILL OF MATERIALS
| Qty | Item | Specification |
|---|---|---|
| 1 length | Antenna wire | #22–#18 AWG; length per design |
| 1 | Termination resistor | 560–1000 Ω non-inductive (carbon film or metal film); weatherproofed; ¼ W sufficient for receive |
| 1 | Feed transformer | 9:1 or step-up per design; FT-82-43 or FT-140-43 core; wound for low insertion loss |
| 1 | Support rope/stake | Non-conductive; anchor at wire ends |
| 1 | Coax feed line | RG-174 or RG-58 sufficient for receive; minimize length and bends |
3-2. KEY DIMENSIONS
CHAPTER 4 — ASSEMBLY PROCEDURES
- Deploy antenna wire from feed end in desired direction of maximum received signal. For Beverage: wire runs toward the desired signal direction. For K9AY: loop lies in vertical plane; null is broadside to loop axis.
- Install termination resistor enclosure at far end of wire. Terminate wire to top of resistor; bottom of resistor to ground stake. Verify resistor value with ohmmeter before sealing enclosure.
- Install feed transformer at near end. Wire terminal to antenna; ground to earth stake (Beverage: ground at feed end). Connect output to coax.
- Route coax toward receiver. Ensure at least 5 m horizontal separation from any transmitting antenna to avoid overloading the receive preamp during transmit.
- At receiver end, install T/R relay or receive antenna switch for TX/RX switching (this antenna must be disconnected from receiver before transmitting).
CHAPTER 5 — CALIBRATION PROCEDURE
- Connect receiving antenna to receiver. Set receiver to target band (40M or 160M). Use CW or AM mode with narrow filter to minimize noise bandwidth.
- Listen for a strong reference station on a known bearing. Note S-meter reading.
- Rotate antenna (K9AY/frame loop) or note signal versus antenna direction. Record signal level in each direction.
- Adjust termination resistance: vary RT by ±20% and note effect on F/B ratio. Optimize for maximum ratio, not maximum signal level.
- Use TinySA as signal tracer: inject a −40 dBm signal at antenna terminal; verify output at receiver connector is ≥−80 dBm (insertion loss ≤40 dB is acceptable for receive-only).
- Record: insertion loss, F/B at 40M, 80M, 160M; SNR vs. reference antenna on noise floor test (no signals, 40M, 2100 UTC).
CHAPTER 6 — TUNING AND ADJUSTMENT
Adjust termination resistance for maximum F/B ratio using the on-air method: rotate array toward a known noise source (AC power line, broadcast station in null direction). Vary the termination resistance in small steps while monitoring noise level. Minimum noise in the null direction corresponds to optimal termination resistance. Fine-tune the feed transformer coupling for minimum insertion loss while maintaining impedance match.
CHAPTER 7 — VERIFICATION
| Parameter | Requirement | Pass/Fail |
|---|---|---|
| SWR at receiver input | < 1.5:1 | ____ |
| Front-to-back ratio (measured) | ≥15 dB (minimum useful) | ____ |
| Insertion loss (antenna term. to coax) | ≤40 dB | ____ |
| SNR vs. reference dipole on target band | Improvement ≥3 dB on noise floor | ____ |
| T/R isolation (if switching) | ≥60 dB during transmit | ____ |
APPENDIX A — CALCULATIONS AND FORMULAS
APPENDIX B — EXAMPLE RESULTS
| Band | F/B Ratio (dB) | SWR | SNR Improvement | Notes |
|---|---|---|---|---|
| 160M | 18–22 | <2:1 | +10 dB vs dipole | Best at night, 1.835 MHz |
| 80M | 16–20 | <2:1 | +8 dB vs dipole | 3.5 MHz region |
| 40M | 14–18 | <2:1 | +6 dB vs dipole | 7.0 MHz DX window |