Dual-Band 10m / 60m Inverted-L with NRT: A Collaborative Design with F1QM

This design article is the result of an engaging technical exchange between Jean-Claude Ducasse (F1QM) and Joeri (ON6URE). It started with a discussion around antenna tuners and baluns, and quickly evolved into the idea of building an efficient dual-band antenna for the 10m and 60m amateur bands using a single wire and a Non-Resonant Trap (NRT).

The Goal
To construct a compact antenna that:

  • Radiates efficiently on both 28.5 MHz (10m) and 5.3 MHz (60m)
  • Uses a single wire in an inverted-L configuration
  • Uses a 4:1 UNUN for broadband matching
  • Offers manageable SWR across both bands

Antenna Configuration

  • Feedpoint: 70 cm above ground in a waterproof PVC box
  • Transformer: RF.Guru 4:1 UNUN (1 kW PEP, N connector output)
  • Vertical section: ~3.45 m from feedpoint to NRT
  • Horizontal/sloping section: ~12.3 m beyond the NRT
  • Total wire length: 15.8 m
  • Ground system: 15 m earth copper wire going to a borehole + optional short counterpoise

NRT Placement Logic
In dipoles, NRTs are often placed using the geometric mean between two bands. But here, the configuration behaves more like a vertical, so we apply the arithmetic mean approach:

f_NRT = (f_10m + f_60m) / 2
       = (28.5 + 5.3) / 2 = 16.9 MHz

This means the NRT should present high impedance at 10m and remain transparent at 60m.

NRT LC Calculation
Targeting a resonance at 16.9 MHz:

Using the standard LC resonance formula:
f = 1 / (2 * pi * sqrt(L * C))

For C = 68 pF:
L = 0.427 µH

This can be wound as 5-6 turns on an air-core former or use a low-loss toroid. The trap should not be too sharp; a moderate-Q trap ensures broadband blocking of 10m while remaining invisible to 60m.

Wire Length Calculation

  • 10m band: λ/4 = 2.63 m × 0.95 (VF) ≈ 2.5 m
  • 60m band: λ/4 = 14.15 m × 0.95 (VF) ≈ 13.45 m

Final geometry:

  • Trap placed at 2.5 m from the feedpoint
  • Total wire length: 13.4 to 15.8 m (to allow trimming)

Expected SWR
With a 4:1 UNUN:

  • 10m: typically 1.5:1 to 2.5:1
  • 60m: around 2.0:1 to 3.0:1

A basic tuner will comfortably handle the residual mismatch on both bands.

Field Testing and Final Adjustment
As with any antenna using reactive elements like NRTs, real-world testing is essential. While the calculated wire lengths provide a solid starting point, the presence of the trap introduces some reactance that makes the antenna behave electrically longer on both bands. Therefore, the actual wire lengths may need to be trimmed slightly during tuning. Fortunately, this effect is less pronounced than with high-Q resonant traps, and the broadband nature of the NRT helps preserve a stable SWR across each band.

Conclusion
This configuration is elegant, compact, and practical. It avoids the high losses of wideband EFHWs and provides targeted performance on two popular bands. With minimal hardware and careful planning, operators like Jean-Claude can integrate such designs into existing installations and terrain constraints.

Additional resources:

Interested in more technical content like this? Subscribe to our notification list — we only send updates when new articles or blogs are published: https://listmonk.rf.guru/subscription/form

Questions or experiences to share? Feel free to contact RF.Guru or join our feedback group!

Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.