Table of Contents
CHAPTER 1 — GENERAL INFORMATION
1-1. SCOPE
This manual covers design, construction, and calibration of the End-Fed Quarter-Wave (Efqw) Antenna for amateur radio operation on 80M, 40M, 20M (separate antennas). End-fed wire antennas are popular for portable and emergency use because they require only one support point and can be deployed rapidly.
1-2. APPLICABLE REFERENCES
- ARRL Antenna Book — End-Fed Antennas and Matching Networks
- NEC2 model: end_fed_quarter_wave.nec (in antenna directory)
- SOTA Antenna Notes — End-fed systems
- FCC OET Bulletin 65 — RF Exposure Evaluation
1-3. SAFETY PRECAUTIONS
CHAPTER 2 — THEORY OF OPERATION
2-1. END-FED MATCHING
Quarter-wave wire element fed at high-impedance end via 9:1 unun with short counterpoise. An end-fed wire presents a high impedance at its end: ~2500–5000 Ω for a half-wave wire; ~1000–2000 Ω for a quarter-wave wire. This high impedance is transformed down to 50 Ω by the matching transformer or unun. Design values: ~1000–2000 Ω → 50 Ω via 9:1 unun. The transformation ratio n² = Zwire/Zcoax; for a 49:1 unun, n = 7, so the wire impedance appears as 49×50 = 2450 Ω from the wire side.
2-2. RADIATION PATTERN
An end-fed wire radiates in a pattern that varies with length and frequency. A half-wave wire at 10 ft height produces a bidirectional broadside pattern similar to a dipole. At harmonic frequencies the pattern has multiple lobes and nulls. Gain per design: 0 dBi (vertical, omnidirectional). The pattern is affected by the run of the feed line if common-mode current is not controlled.
2-3. COUNTERPOISE REQUIREMENTS
A counterpoise (short wire or coax braid) is required as a reference conductor for the feed voltage to work against. Without a counterpoise the coax braid serves as the counterpoise, causing RF current on the braid. Recommended counterpoise: 0.05λ minimum length wire from the unun ground terminal, or a 1:1 choke isolating the antenna current from the coax outer.
CHAPTER 3 — MATERIALS AND CONSTRUCTION
3-1. BILL OF MATERIALS
| Qty | Item | Specification |
|---|---|---|
| 1 | Matching transformer (unun) | 49:1 unun for EFHW (binocular core BN-43-2402 or FT-140-43); 9:1 unun for random wire (FT-140-43) |
| 1 length | Antenna wire | #22–#18 AWG stranded copper; length per design |
| 1 | End insulator | UV-stable; rated for wire tension |
| 1 | Counterpoise wire | #22 AWG; minimum 0.05λ at lowest operating frequency |
| 1 | Enclosure for transformer | UV-stable ABS; seal with silicone for waterproofing |
| 1 | SO-239 connector | Mount on enclosure; connect to coax feed line |
3-2. WIRE LENGTHS
CHAPTER 4 — ASSEMBLY PROCEDURES
- Wind matching transformer per design (49:1 unun: 3 trifilar turns primary on BN-43-2402 binocular core, or 2 turns primary and 14 turns secondary on FT-140-43 single core).
- Seal transformer in weatherproof enclosure. Route wire terminal out one end, coax SO-239 on side.
- Attach antenna wire to transformer wire terminal. Cut wire 5% long for trimming.
- Attach counterpoise wire (0.05λ minimum) to transformer ground/coax-braid terminal. Tape counterpoise away from antenna wire.
- Deploy antenna wire: raise far end as high as possible, at least 20 ft for HF operation. Attach end insulator and support halyard.
- Run coax from SO-239 to station. Install common-mode choke (Mix-31 toroid, 8 turns) at shack entry.
- Perform SWR sweep per Chapter 5.
CHAPTER 5 — CALIBRATION PROCEDURE
- SOLT calibrate NanoVNA at end of feed line (shack end) with the antenna deployed at operating height.
- Set sweep: target band ±15%.
- Connect NanoVNA. Record SWR across band. Resonance appears as dip in SWR (may be very sharp for EFHW).
- If SWR dip is at wrong frequency: trim wire end for higher frequency (too long), or add wire for lower frequency (too short). Change ~6 in for ~50 kHz shift at 40M.
- Verify SWR on harmonics for multiband use (EFHW is harmonically resonant at 2f, 3f, ...).
- Check common-mode current: touch coax braid — if RF is felt, add choke at feedpoint.
CHAPTER 6 — TUNING AND ADJUSTMENT
Trim wire for resonance. For multiband EFHW, the 49:1 unun presents high impedance at the design frequency; SWR on harmonics is naturally low if the wire is properly trimmed at the fundamental. If SWR is acceptable at fundamental but poor at harmonics, adjust ATU or add a series capacitor (50–100 pF) in the counterpoise for harmonic correction.
CHAPTER 7 — VERIFICATION
| Parameter | Requirement | Pass/Fail |
|---|---|---|
| SWR at design frequency | < 2.0:1 | ____ |
| Common-mode current on coax | Not perceptible at feed end | ____ |
| Gain (NEC2) | 0 dBi (vertical, omnidirectional) | ____ |
| Efficiency | 80–92% | ____ |
APPENDIX A — CALCULATIONS AND FORMULAS
APPENDIX B — EXAMPLE RESULTS
| Band | SWR | R (Ω) | X (Ω) | Notes |
|---|---|---|---|---|
| 40M (design) | <2.0:1 | 40–65 | ±20 | After wire trimming |
| 20M (2nd harmonic) | <2.5:1 | 30–80 | ±30 | Harmonically resonant |
| 15M (3rd harmonic) | <3:1 | 25–90 | ±40 | May need ATU |