UNCLASSIFIED
TM-ANT-060
SLOT ANTENNA — UHF 70cm
PCB Slot Antenna for 430–440 MHz Full-Band Coverage
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 the Slot Antenna — Uhf 70Cm, a specialized antenna for operation on 70cm (430–440 MHz). Design approach: Slot antenna cut in PCB ground plane; complementary to dipole by Babinet's principle; wideband design.

1-2. APPLICABLE REFERENCES

  • ARRL Antenna Book — Specialized and Experimental Antennas
  • NEC2 model: slot_antenna_uhf.nec (in antenna directory)
  • FCC OET Bulletin 65 — RF Exposure Evaluation

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.

CHAPTER 2 — THEORY OF OPERATION

2-1. DESIGN PRINCIPLES

Slot antenna cut in pcb ground plane; complementary to dipole by babinet's principle; wideband design. Key parameters: 36 mm slot on PCB ground plane; 50 Ω microstrip feed at slot center; wideband for full 70cm band coverage.

2-2. RADIATION CHARACTERISTICS

Gain: 2 dBi omnidirectional in plane. Efficiency: 80–92%. Feed impedance: 50 Ω (microstrip inset or coax probe). The pattern and polarization depend on the specific antenna geometry; consult the NEC2 model for accurate polar plots and gain/elevation data.

2-3. PROPAGATION APPLICATION

This antenna is optimized for its specific application (see subtitle). Operating it outside the designed frequency range or in a different orientation from the NEC2 model will result in degraded performance — consult the simulation before making substitutions.

CHAPTER 3 — MATERIALS AND CONSTRUCTION

3-1. BILL OF MATERIALS

Materials — Slot Antenna — Uhf 70Cm
QtyItemSpecification
Per NEC modelConductor elementsDimensions from NEC2 wire list; use copper or aluminum per design frequency
1Feed connectorSO-239 or N-type; 50 Ω (microstrip inset or coax probe) system
1Support hardwareUV-stable; sized for operating environment (wind, ice load)
As neededFeed lineRG-213 or LMR-400; match to feed impedance 50 Ω (microstrip inset or coax probe)

3-2. CRITICAL DIMENSIONS

Primary dimension formula (wavelengths)λ (m) = 300 / fMHz
Design-specific parameters36 mm slot on PCB ground plane; 50 Ω microstrip feed at slot center; wideband for full 70cm band coverage

All critical dimensions are embedded in the NEC2 model wire card (GW) definitions. Extract from slot_antenna_uhf.nec for precise construction reference.

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. Extract all element lengths and spacings from NEC2 model file slot_antenna_uhf.nec (GW card dimensions). Convert from meters to inches or feet for construction.
  2. Fabricate each element to ±1 mm tolerance (VHF/UHF) or ±0.5 in (HF). Label elements before assembly.
  3. Assemble support structure. Verify element positions match NEC model geometry. Use non-conductive fasteners where element isolation is required.
  4. Install feed connector at designed feedpoint location. Install any required matching network (gamma match, balun, transformer).
  5. Connect feed line. Install common-mode choke (Mix-31 toroid) at feedpoint. Route feed line to minimize coupling with antenna elements.
  6. Perform SWR measurement per Chapter 5 before installing at full height.

CHAPTER 5 — CALIBRATION PROCEDURE

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. SOLT calibrate NanoVNA at feedpoint (antenna connector).
  2. Set sweep range: ±10% of design center frequency.
  3. Connect NanoVNA to antenna. Record SWR minimum and frequency, R + jX at resonance.
  4. Compare measured impedance to NEC2 predicted values. Investigate discrepancies >20%.
  5. Verify SWR ≤2.0:1 across 430–440 MHz at operating frequency. If not, adjust matching per Chapter 6.
  6. For gain verification: compare received signal strength (using TinySA as receiver) from a known beacon vs. reference antenna.

CHAPTER 6 — TUNING AND ADJUSTMENT

Element lengths can be adjusted by ±2% of design length to shift resonance and optimize impedance. For matching network adjustment: follow gamma-match, delta-match, or transformer-tap procedure per the specific implementation. Refer to NEC2 model for sensitivity analysis — the model can be re-run at ±2% element length to determine which element most affects resonance and F/B ratio.

CHAPTER 7 — VERIFICATION

Acceptance Criteria — Slot Antenna — Uhf 70Cm
ParameterRequirementPass/Fail
SWR at design frequency< 2.0:1 across 430–440 MHz____
Resonant frequencyWithin ±1% of design____
Gain (NEC2 or field test)2 dBi omnidirectional in plane____
Efficiency80–92%____

APPENDIX A — CALCULATIONS AND FORMULAS

Wavelength in free spaceλ (m) = 300 / fMHz
Effective gain formula (over isotropic)GdBi = 10 log10(4π × Aeff / λ²) where Aeff = effective aperture (m²)
Special parameters36 mm slot on PCB ground plane; 50 Ω microstrip feed at slot center; wideband for full 70cm band coverage

APPENDIX B — EXAMPLE RESULTS

Expected Measurements — Slot Antenna — Uhf 70Cm
FreqSWRR (Ω)X (Ω)Notes
Design center< 2.0:1 across 430–440 MHz~50~0After full match/tune
Band edge low<2.5:1~40≤−20Capacitive below resonance
Band edge high<2.5:1~40≥+20Inductive above resonance
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