Coax-Wound Trap Construction
A coax-wound trap builds the same parallel LC resonant circuit as any other antenna trap, but from a single coiled length of ordinary coax cable instead of a separate coil and capacitor — the cable's own center conductor and shield already form both a coil and a distributed capacitor, self-contained inside one weatherproof jacket. It trades a little Q for a genuinely faster build and a component that's already sealed against the weather. This guide covers how the self-contained LC circuit works, choosing cable and length, a worked example, a calculator, and troubleshooting.
Why a piece of coax already contains both parts of the circuit
Every coax cable has a center conductor and a shield separated by a dielectric, and that arrangement is, electrically, already a capacitor — manufacturers publish it as a per-foot capacitance rating (commonly in the 20-30 pF/ft range, though it varies by cable type and construction) because it matters for other RF design purposes too. Coil the cable into a loop, and the same length also forms an inductor from its physical geometry, exactly like a coil of ordinary wire would. A coax-wound trap simply puts both effects to work at once: the coax's built-in per-foot capacitance becomes the trap's C, and the coiled shape becomes the trap's L, all inside a single piece of cable with no separate parts to source or assemble.
How the two conductors are connected
At one end of the coax length (the end that stays internal to the coil, sometimes called the "far" or "dead" end), the center conductor is soldered directly to the shield, shorting them together. That short is what lets the coiled cable behave inductively, the same way a shorted turn of ordinary wire would. At the other end (the "open" end), the center conductor and shield are left separate — these two leads become the trap's actual connection points, splicing into the antenna wire on either side. The distributed capacitance between the two conductors, present along the entire coiled length, sits in parallel with that inductance, completing the resonant circuit.
Why Q is lower than a discrete coil-and-capacitor trap
The coax's shield and center conductor were designed to be a low-loss transmission line, not an optimized high-Q capacitor — and using both the shield and center conductor together as the "wire" of the coil (rather than a single heavy-gauge conductor) adds more resistive loss per turn than a purpose-built air-wound coil would have. The combined effect typically lands a coax-wound trap's Q somewhere in the 50-100 range, noticeably below the 150-300 range a well-built coil-and-capacitor trap reaches. For most home-station multiband antennas this difference is a modest efficiency trade, not a dealbreaker — see the Coil-and-Capacitor Trap Construction guide if maximum Q matters more to your build than simplicity.
The weatherproofing advantage
Because both conductors of a coax-wound trap already live inside the cable's own jacket, the trap's core LC circuit is inherently protected from moisture in a way a discrete capacitor never is — there's no exposed capacitor lead or solder joint sitting directly in the RF path waiting to corrode. The only real exposure points are the two shorted-end connections and the two open-end leads, which is a much smaller weatherproofing job than sealing an entire discrete-component trap body.
- Best fit: builders who want a working multiband antenna quickly, or who find sourcing high-voltage, high-Q capacitors inconvenient.
- Trade-off: somewhat lower Q and therefore somewhat more loss than a discrete coil-and-capacitor trap at the same power level.
| Method | Construction | Typical Q | Best For |
|---|---|---|---|
| Coil-and-Capacitor Trap | Discrete air-wound coil plus a separate high-Q capacitor (silver mica or NPO ceramic), wired in parallel and potted | 150-300 | Maximum efficiency on high-power, competition-grade, or DX-focused multiband dipoles and verticals — see the Coil-and-Capacitor Trap Construction guide |
| Coax-Wound Trap | A single coiled length of coax cable; the cable's own distributed capacitance forms C, the coil forms L | 50-100 | Faster, simpler, inherently weatherproof builds where a small efficiency trade-off is acceptable |
Coax Trap Length Calculator
Materials for one coax-wound trap
A coax-wound trap on its PVC form, with the coiled coax lashed in place and the shorted far end and open-end leads visible.
Building a Coax-Wound Trap
Confirm your specific cable's pF/ft rating before calculating length — this value varies enough between coax types that a generic assumption can throw off the whole design.
Choose the target frequency, coil inductance, and cable
Pick a trap resonant frequency the same way you would for any trap — at or slightly above the top of the band it should block, or at band center. Pick a target coil inductance that's practical for your chosen form diameter, and look up your specific coax's capacitance-per-foot rating from its datasheet.
Calculate the required coax length
Use the calculator above to find the required capacitance from your target frequency and inductance, then convert that to a coax length using your cable's pF/ft rating.
Cut the coax and prepare both ends
Cut the coax a few inches longer than calculated to leave room for trimming and connections. At the end that will stay internal to the coil (the far end), strip back the jacket and shield enough to solder the center conductor directly to the shield, shorting them together. At the other end (the open end), strip and separate the center conductor and shield as two distinct leads without shorting them.
Wind the coax onto the form and verify resonance
Coil the cable around the form with the shorted end innermost, securing turns with cable ties or lashing cord — snug enough to hold shape, not so tight that the cable's round cross-section gets pinched, which can shift its capacitance. Bring the two open-end leads out to accessible connection points, then sweep the trap alone with a NanoVNA to confirm resonance.
Trim and seal
If resonance is too high, add a small amount of extra coax length (increasing capacitance); if too low, the coil needs slightly fewer turns or a shorter length, which means starting over with a shorter cut — plan for this by cutting generously long in step 3. Once resonance is confirmed, seal the shorted end and both open-end leads with self-amalgamating tape over heat-shrink; the coiled body itself needs no further sealing since the coax jacket already protects it.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Measured resonance is noticeably off from the calculated target | Actual cable capacitance-per-foot differs from the assumed value, or the coil was wound tighter/looser than planned | Confirm the cable's actual pF/ft spec from its datasheet rather than a generic assumption, and re-sweep with a NanoVNA | Add or trim coax length to adjust capacitance, or adjust the coil's turn count for a small inductance correction |
| Resonance shifts after final lashing/securing of the coil | Lashing pinched the coax, deforming the dielectric and changing its capacitance | Compare resonance before and after final securing; a shift appearing only after tightening points to pinching | Loosen and redo the lashing with less tension, using wider ties or cord to spread the pressure |
| Trap resonance is correct but SWR performance seems worse than expected on the pass-through band | Lower Q than a discrete coil-and-capacitor trap is producing more loss than anticipated for this installation | Compare against the expected Q range for coax-wound traps (50-100) rather than assuming a fault | This is inherent to the method; if the loss is unacceptable for your use case, consider a coil-and-capacitor trap instead |
| Shorted end connection runs warm or shows discoloration | Under-soldered or mechanically weak joint at the shorted far end, which carries real circulating current | Inspect the joint for a dull, cracked, or incomplete solder connection | Re-solder with a hotter iron and adequate flux, then re-seal with fresh heat-shrink |
| SWR degrades in wet weather | Moisture has reached one of the two exposed connection points (shorted end or open-end leads), since the coiled body itself is normally protected by the coax jacket | Inspect specifically at the shorted joint and the open-end leads, not the coil body | Re-seal those two exposure points with fresh self-amalgamating tape over heat-shrink |
Which coax type is best for a coax-wound trap?
Smaller-diameter coax like RG-58 or RG-8X is generally easier to coil tightly on a compact form and is the most common choice. Any coax works electrically as long as you use its actual, correct pF/ft rating in your calculation — larger-diameter cable simply needs a larger form to coil comfortably.
Why do I short the center conductor to the shield at one end?
That short is what makes the coiled cable behave as an inductor in the first place — without it, you'd just have a length of transmission line, not a coil. The short stays fully internal to the trap; it is not connected to the antenna wire on either side, only the two open-end leads are.
Is a coax-wound trap actually weatherproof, or does it still need sealing?
The coiled body is inherently well protected since both conductors already live inside the cable's own jacket, but the two connection points — the shorted far end and the two open-end leads — are cut, stripped, and soldered, which removes that built-in protection right at those spots. Sealing just those two points with heat-shrink and self-amalgamating tape is enough; the rest of the coil needs no additional enclosure.
How much less efficient is a coax-wound trap compared to a coil-and-capacitor trap?
The Q difference (roughly 50-100 for coax-wound versus 150-300 for a well-built coil-and-capacitor trap) translates to a modest efficiency loss — noticeable in careful measurement, but rarely a deciding factor for casual or general home-station use. It becomes more relevant for competition-grade or serious DX-focused installations, where the Coil-and-Capacitor Trap Construction method is the better choice.
Can I use this method for a trap vertical, not just a trap dipole?
Yes — the trap itself doesn't care whether it's spliced into a horizontal dipole leg or a vertical radiator; the same LC resonance principle and construction method apply either way. See the Multi-Band Trap Vertical guide for a worked real-world example of traps used in a vertical antenna.