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 of discussion in the ham radio community is the use of coaxial traps in multiband antennas. While these components offer a simple and elegant way to make a single radiator resonate on multiple bands, we tend to approach them with a healthy dose of caution. Why? Let's dive into it.

What Are (Coaxial) Traps?

A coaxial trap is a resonant LC circuit made from a short length of coaxial cable wound into a coil. It acts as an RF choke at a particular frequency, blocking current and effectively isolating sections of the antenna above that frequency. This allows the antenna to behave like a shorter resonant element on higher bands.

Why RF.Guru Is Wary of Traps

While traps make antenna design more compact and reduce the need for multiple wires or separate antennas, they come with trade-offs:

• Reduced Bandwidth: A trap introduces a sharp resonance, which limits usable bandwidth on the band it’s designed for. This is especially problematic for wideband modes or digital operation where SWR tolerance is more critical.
• Insertion Loss: Traps add small but noticeable losses, especially if not perfectly constructed or weather-protected. Over time, water ingress or coax degradation can introduce losses or detuning.
• Capacitive Reactance: Each trap adds capacitive reactance to the system, which can shift the resonance and complicate tuning, especially on bands where the trap is not active.

When We Do Use Traps—Less Is More

At RF.Guru, if we must use traps, we keep it to a maximum of one trap per leg, and ideally just one in the entire system. Why?

• Minimized Reactance: Each additional trap adds to the antenna’s overall capacitive behavior, which can worsen matching and create unwanted impedance transformations. For example, if two traps are added in series, each contributes its own capacitive reactance at non-resonant frequencies, causing the antenna to appear too short electrically. This leads to a higher voltage standing wave ratio (VSWR), increased mismatch losses, and difficulty achieving a good match without a tuner.
• Simplicity: One trap keeps the build simple, repeatable, and more rugged for field use.
• Performance First: We prioritize efficiency, bandwidth, and durability. A single well-designed trap can be a good compromise when size or mounting height constraints make other solutions impractical.

Final Thoughts

Traps are not inherently bad—they're clever tools when used with care. But as with all things in RF engineering, every component is a compromise. That’s why at RF.Guru, our philosophy is clear:

Avoid traps when you can. Use just one when you must. And always design for efficiency first.

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.