Nextgen Non-Resonant Traps: Engineering Performance through Frequency Means and Symmetry
In the world of multiband HF antenna design, non-resonant traps are rapidly gaining popularity. Unlike classic LC traps, these smart components do not block or isolate bands by resonance, but instead shift the current distribution across the antenna. This enables more broadband and efficient designs without the usual limitations of narrowband traps.
But here's the hidden art: where you place your non-resonant trap—and which frequency you center it on—can make or break your antenna's performance.
The Secret Lies in the "Mean Frequency"
When covering two bands (like 80m and 40m, or 40m and 20m), you can optimize your trap position and electrical behavior by choosing the right mean frequency between those bands. There are different ways to calculate this, and each has its ideal use case:
| Mean Type | Formula | Use When |
|---|---|---|
| Arithmetic Mean | (f1 + f2)/2 | Physical symmetry, e.g. two equal pole segments |
| Geometric Mean | sqrt(f1 * f2) | Logarithmic band spacing, especially octave-based systems |
| Harmonic Mean | 2 / (1/f1 + 1/f2) | Impedance-focused systems, less common for traps |
| Log Midpoint | exp[(ln(f1) + ln(f2))/2] | Same as geometric mean in effect, better for log plots |
Octaves vs. Physical Symmetry
Most amateur band pairs are close to being octaves:
- 160m/80m
- 80m/40m
- 40m/20m
- 20m/10m
For these, the geometric mean gives excellent current symmetry when designing dipoles or loops that use non-resonant traps. However, in verticals where you want two equal-length physical elements—say two 4.5 m aluminum poles stacked—the arithmetic mean ensures that the current distribution is balanced across the physical center of the antenna.
This is especially useful when you’re designing a low-profile vertical that still performs on 80m to 10m. By placing a non-resonant trap at the arithmetic mean between 3.6 MHz and 28.5 MHz (i.e., 16.05 MHz), you ensure a symmetrical split and optimized performance for the entire HF range.
Why It Matters
Different means give you control over:
- Where the current shifts between bands
- How symmetrical the antenna behaves across multiple bands
- Feedpoint impedance consistency
- Physical balance, which affects durability and mechanical simplicity
When to Use Each Mean
| Antenna Type | Best Mean to Use | Why |
|---|---|---|
| Vertical with two equal segments | Arithmetic Mean | Ensures physical and electrical symmetry |
| Dual-band dipole (e.g., 40/20m) | Geometric Mean | Optimized current symmetry across octaves |
| Broad log-spaced receive loops | Log Midpoint | Aligns performance with log band spacing |
| Matching or coupling circuits ? | Harmonic Mean | Still figuring out where this could be useful for ... |
Applications That Benefit Most:
- Dual-band or tri-band verticals with symmetrical sections
- Non-resonant delta loops or dipoles with shared elements
- Non-resonant verticals
- Phased arrays or traps in multiband receiving systems
- Hybrid antennas (vertical + capacity hat + trap)
Final Thoughts
Nextgen non-resonant traps open the door to smarter, more scalable antenna designs. But to get the most from them, stop thinking in terms of just band edges or physical dimensions. Instead, start designing with frequency means—and choose the one that aligns with your mechanical and RF symmetry goals.
Your traps will thank you.
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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.