Trapped in a Trap: Coaxial Traps in Multiband Antennas
At RF.Guru, we’ve seen a lot of antenna designs come and go — but one recurring topic in the ham radio community is the use of coaxial traps in multiband antennas. While they offer a neat way to make a single radiator resonate on multiple bands, they also come with serious trade-offs. Here’s why we approach them with caution.
What Are Coaxial Traps?
A coaxial trap is a resonant LC circuit made from a short length of coax wound into a coil. At its design frequency, it acts as an RF choke, blocking current and isolating parts of the wire. This lets a long antenna behave like multiple shorter resonant sections on higher bands.
Why We Are Wary of Traps
- Reduced Bandwidth: The sharp resonance of a trap narrows usable bandwidth. Digital modes and wide SSB segments suffer most.
- Insertion Loss: Every trap introduces resistive and dielectric losses. Over time, water ingress or coax aging increases inefficiency.
- Extra Reactance: Outside its design band, the trap behaves capacitively, altering resonance and complicating tuning.
When We Do Use Traps
If traps are unavoidable, we keep it to one per leg, maximum. More than that and losses, detuning, and mismatch issues pile up quickly. One well-designed trap can be a reasonable compromise when space is extremely limited.
- Minimized Reactance: Fewer traps mean less capacitive loading at non-resonant frequencies.
- Simplicity: Easier to build, more rugged in the field, and easier to maintain.
- Performance First: Efficiency and bandwidth remain usable with just one trap — especially if tuned smartly.
Smarter Trap Placement: Sub-Resonant Tuning
Classic trap dipoles place the LC circuit right at the higher band’s resonance. We prefer a sub-resonant approach: tuning the trap slightly below the higher band (e.g., 3.4 MHz for an 80m trap). This softens its effect, reduces arcing stress, and broadens impedance behavior.
– Smoother impedance transition across both bands
– Lower voltage stress and reduced arc risk
– Trap L and C share the load evenly, improving reliability
– Outer wire still contributes to radiation on higher bands
The Geometric Mean Method (NRT Dipole)
ON7WP pioneered using the geometric mean of two target bands as the trap frequency. For example, for 160/80m (1.875 MHz & 3.7 MHz):
√(1.875 × 3.7) ≈ 2.63 MHz
This ensures both bands see a balanced impedance transition. Unlike resonant traps that favor one band, mean-frequency traps spread current more evenly, improving bandwidth and radiation efficiency.
Practical Example
160/80m Trap Dipole:
- Legs ~28 m each
- Trap placed ~10 m from feedpoint
- Trap tuned sub-resonant (~3.4 MHz) or at mean (~2.6 MHz)
- Outer segment contributes radiation on both bands
Final Thoughts
Traps aren’t evil — but they are compromises. At RF.Guru we avoid them where possible, limit them when necessary, and favor smarter non-resonant or mean-frequency approaches for efficiency and bandwidth.
Mini-FAQ
- Do traps always kill efficiency? — Not always. One carefully tuned trap per leg can work, but multiple traps quickly add loss and narrow bandwidth.
- What’s better than a coaxial trap? — Non-resonant (mean-frequency) traps maintain symmetry, improve radiation, and reduce voltage stress.
- Can I homebrew coaxial traps? — Yes, but weatherproofing is critical. Moisture ingress is the #1 killer of coax traps.
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