UNCLASSIFIED
TM-ANT-058
SLINKY ANTENNA — 20M PORTABLE
Spring-Steel Slinky Helical Loading Element for 20M Compact Dipole
Prepared by: Mervyn Martin, KO6NNH
Merced, California  •  26 May 2026
Amateur Radio / Antenna Engineering — Not for commercial use

Table of Contents

  1. Chapter 1 — General Information
  2. Chapter 2 — Theory of Operation
  3. Chapter 3 — Materials and Construction
  4. Chapter 4 — Assembly Procedures
  5. Chapter 5 — Calibration Procedure
  6. Chapter 6 — Tuning and Adjustment
  7. Chapter 7 — Verification
  8. Appendix A — Calculations and Formulas
  9. Appendix B — Example Results

CHAPTER 1 — GENERAL INFORMATION

1-1. SCOPE

This manual covers design, construction, calibration, and field operation of the Slinky Antenna — 20M Portable for amateur radio use. The antenna is intended for operation on 20M (14.0–14.35 MHz). All procedures are written for the tools available at the field station: NanoVNA (vector network analyzer), TinySA (spectrum analyzer), and standard hand tools.

1-2. APPLICABLE REFERENCES

  • ARRL Antenna Book, 25th Edition, Chapter on Dipoles and Doublets
  • NEC2 model: slinky_antenna_20m_portable_spec.nec (in antenna directory)
  • FCC OET Bulletin 65 — RF Exposure Evaluation
  • ITU-R P.533 — HF propagation prediction methods

1-3. SAFETY PRECAUTIONS

CAUTION — RF EXPOSURE Maintain minimum safe distance from all energized antenna elements during transmission. At QRP power levels (≤5 W) the MPE boundary is typically <1 m for HF antennas. At 100 W the controlled exposure limit for HF antennas requires maintaining ≥3–10 m distance depending on frequency (per FCC OET Bulletin 65). Never touch feed-point hardware or support structures while transmitting. Verify PTT key is open before antenna work.
NOTE Ensure antenna ends and feed point are clear of personnel and metal structures by at least 2 m before applying RF power.

CHAPTER 2 — THEORY OF OPERATION

2-1. RADIATION PHYSICS

Spring-steel slinky toys deployed as loading coils in each arm of a compact dipole, providing electrical length in minimal physical length. A dipole radiates because time-varying current in the conductor produces a time-varying magnetic field; the resulting displacement current produces a time-varying electric field; together these fields propagate outward as an electromagnetic wave. For a half-wave dipole at resonance the radiation resistance Rr = 73 Ω and reactance X = 0, producing maximum current amplitude for a given drive voltage.

2-2. RADIATION PATTERN

A horizontal half-wave dipole at height h above ground produces a bidirectional pattern broadside to the wire axis. At height ≥0.5λ the main lobe elevation angle θ ≈ 15–30°, ideal for DX paths. At height <0.15λ (NVIS regime) the main lobe is near-vertical. The E-plane pattern is a figure-eight; H-plane is omnidirectional. Gain broadside is 2.1 dBi relative to isotropic.

2-3. IMPEDANCE AND BANDWIDTH

Feed impedance: 50 Ω. A longer or cage construction widens bandwidth because the increased effective conductor diameter reduces the Q of the resonance. For a simple wire dipole, the 2:1 SWR bandwidth at 40M is typically 100–200 kHz; a cage construction widens this to 300–500 kHz or the full band.

2-4. POLARIZATION AND PROPAGATION

This antenna is linearly polarized. At HF, ionospheric propagation rotates polarization (Faraday rotation), so polarization match at the far end is largely unpredictable. Cross-polarization loss of 3–10 dB is common on skywave paths. For NVIS and groundwave propagation, horizontal polarization is preferred for local coverage.

CHAPTER 3 — MATERIALS AND CONSTRUCTION

3-1. BILL OF MATERIALS

Typical Materials for Slinky Antenna — 20M Portable
QtyItemSpecification
Per designCopper wire (antenna elements)#14 AWG solid or stranded copper, PVC-jacketed preferred for weather resistance
1Feed-point insulator / center connectorSO-239 or UHF-F, weatherproof, UV-resistant housing
2End insulatorsEgg insulators or equivalent; rated for wire tension at operating temperature
As neededHalyard / support ropeDacron or polypropylene, non-conductive, UV-resistant
1Choke balun (1:1 current balun)Mix-31 ferrite cores, 8–12 turns of RG-142 or RG-8X through core; FB-31-6873 or equivalent
1Coaxial feed lineRG-8X or RG-213 to station; minimize length for efficiency

3-2. DIMENSION FORMULAS

Half-wave dipole element length (each arm, feet)Larm = 234 / fMHz
Full dipole length (feet)Ltotal = 468 / fMHz
Example: 40M at 7.150 MHzLarm = 234 / 7.150 = 32.7 ft — each arm

Note: The 468 constant assumes wire of #12–#14 AWG copper at ambient temperature; adjust down 1–2% for thick conductors or cage construction (velocity factor <1).

CHAPTER 4 — ASSEMBLY PROCEDURES

CAUTION — RF EXPOSURE Maintain minimum safe distance from all energized antenna elements during transmission. At QRP power levels (≤5 W) the MPE boundary is typically <1 m for HF antennas. At 100 W the controlled exposure limit for HF antennas requires maintaining ≥3–10 m distance depending on frequency (per FCC OET Bulletin 65). Never touch feed-point hardware or support structures while transmitting. Verify PTT key is open before antenna work.
  1. Calculate element lengths using the formula in Chapter 3. Add 5% extra wire for trimming (do not cut to final length until resonance is verified).
  2. Solder or crimp conductors to center feedpoint connector. Orient SO-239 with center pin to one arm and braid to the other. Install 1:1 current balun at feedpoint.
  3. Attach end insulators and support ropes to both wire ends. Tie off-load with a bowline knot — not a slip knot.
  4. Raise antenna to operating height. Orient element perpendicular to desired direction of maximum radiation (broadside direction).
  5. Connect RG-8X or RG-213 feed line from balun to shack. Route cable away from element to avoid coupling. Secure with UV-resistant cable ties.
  6. Perform initial SWR measurement per Chapter 5 before first transmission.

CHAPTER 5 — CALIBRATION PROCEDURE

5-1. NANOVNA SWR AND IMPEDANCE SWEEP

NOTE The NEC2 model file for this antenna is included in the antenna directory. Run it with xnec2c, 4nec2, or any NEC2-compatible engine to generate polar plots, impedance data, and gain figures. The NanoVNA measurements in Chapter 5 should be compared against NEC2 predictions — deviations >3 dB or >20% impedance indicate a construction error.
  1. Perform SOLT calibration on NanoVNA using the SOL (Short-Open-Load) kit at the antenna end of the feed line.
  2. Set NanoVNA sweep range to cover ±10% of target center frequency (example: 40M → 6.5–7.8 MHz).
  3. Connect NanoVNA to feedpoint. Navigate to CH0 S11 display. Select Smith Chart and SWR graphs.
  4. Record: frequency of minimum SWR (fres), SWR at fres, SWR at band edges, R + jX at fres.
  5. Resonance is confirmed when X ≈ 0 and R ≈ 73 Ω (simple dipole) or per design (matched system).
  6. Compare measured fres to design frequency. If fres is too high, the element is short — lengthen each arm 1–2 in. If fres is too low, the element is long — trim each arm 1 in. Repeat until fres is within ±0.5% of design frequency.

CHAPTER 6 — TUNING AND ADJUSTMENT

6-1. RESONANCE ADJUSTMENT

Trim or extend element arms symmetrically to shift resonance. Each 1-inch change in total length shifts resonance by approximately fMHz/468 × 12 kHz for a 40M dipole. Adjust in 2-inch increments and re-measure SWR between adjustments. Finalize element length when SWR at design frequency is <1.5:1 or as specified.

6-2. IMPEDANCE MATCHING

If impedance at resonance differs from 50 Ω, adjust element height (increases ground effect), add a matching network (L-network, λ/4 transformer, or series capacitor), or use a 4:1 balun for designs with higher feed-point impedance such as folded dipoles (50 Ω).

CHAPTER 7 — VERIFICATION

7-1. ACCEPTANCE CRITERIA

Verification Criteria
ParameterRequirementPass/Fail
SWR at resonance< 2.0:1____
Resonant frequencyWithin ±1% of design____
Feed impedance (R)As designed (±15%)____
Feed reactance (X)<±10 Ω at resonance____
Gain (NEC2 model)0–1 dBi (loading reduces radiation resistance)____
Efficiency60–80% (slinky resistance loss)____
  1. Confirm SWR <2.0:1 at center frequency.
  2. Verify resonance frequency within ±1% of design center.
  3. Confirm impedance real part within 15% of design value.
  4. Record results in station log with date, antenna height, and feed line length.

APPENDIX A — CALCULATIONS AND FORMULAS

Half-wave dipole element length (each arm, feet)Larm = 234 / fMHz
Velocity factor correction (for insulated wire)Ladj = Larm × Vf (Vf ≈ 0.95 for #14 AWG PVC)
Radiation resistance (half-wave dipole, free space)Rr = 73.1 Ω
Gain over isotropic (free space)G = 2.14 dBi
Height for DX angle (degrees elevation for max gain)θ ≈ arcsin(1 / (2h/λ)) for h > 0.25λ

APPENDIX B — EXAMPLE RESULTS

Typical NanoVNA Measurements — Slinky Antenna — 20M Portable
Bandfres (MHz)SWRR (Ω)X (Ω)Notes
Per designDesign center<1.5:150–73−5 to +5After trimming to resonance
Band edge lowflow<2.0:140–65−30 to 0Capacitive below resonance
Band edge highfhigh<2.0:140–650 to +30Inductive above resonance
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