W3DZZ Trap Dipole
The W3DZZ is the original coax-trap dipole design that put resonant traps on the map for HF hams: one wire, one coax feedline, and two traps that give you 80m and 40m coverage from a single antenna, plus incidental usable operation on several higher bands as a long-wire/random-length radiator through a tuner. It's a two-trap, two-band design specifically — simpler than the 3-band 80/40/20m trap dipole already covered elsewhere on this site — and it's the design most hams mean when they say "trap dipole" in a DX Engineering or eham.net thread. This guide covers building your own traps from PVC coil forms and door-knob capacitors, or substituting purchased commercially made trap kits, plus full wire lengths and the tuning sequence.
A trap is a parallel LC circuit at the 40m break point
Each trap is a coil and capacitor wired in parallel, cut to resonate right around 7.2 MHz. At resonance, a parallel LC circuit presents a very high impedance — electrically, it acts almost like an open circuit, isolating everything beyond it. On 40m, the antenna sees only the wire between the feedpoint and the traps: a 40m half-wave dipole. On 80m, the traps are well below their own resonant frequency, so they look mostly inductive and simply add a small amount of loading inductance in series with the rest of the wire running out past them to the tips.
At 40m: trap = high-Z, isolates outer wire (antenna = 40m half-wave)
At 80m: trap = inductive reactance, adds to the outer wire's electrical length
The specific values that make this "the" W3DZZ
The commercially sold and widely home-built W3DZZ design uses two identical traps, each roughly 13-15 µH in parallel with a 60-64 pF mica or ceramic doorknob capacitor, resonating close to 7.15-7.20 MHz. Slight variation between published versions comes down to wire gauge, coil form diameter, and which exact capacitor value the builder had on hand — the resonant frequency of the trap matters far more than the exact L/C split used to get there.
Why this differs from the 3-band trap dipole already on this site
The general 3-band trap dipole guide uses two pairs of traps (four total) to add a clean 20m section inside the 40m section. The W3DZZ uses exactly one pair of traps and stops at two bands by design — it trades the extra band for a simpler, lower-loss build with one less set of traps to wind, weatherproof, and maintain.
- W3DZZ (this guide): 2 traps, 80m/40m no-tuner, incidental multi-band coverage via tuner on the higher bands as a random-length wire.
- 3-band trap dipole: 4 traps, clean 80m/40m/20m no-tuner coverage, more construction and more potential trap loss.
The "bonus bands" reputation, and why it's overstated
The W3DZZ has a long-standing reputation as a "multiband" antenna because the full length of wire, fed through a tuner, presents a workable match on 20m, 17m, 15m, 12m, and 10m as an odd-multiple long-wire. That's genuinely useful, but it isn't the same as the clean no-tuner 80m/40m dipole performance the traps are actually providing — treat the higher bands as a tuner-assisted bonus, not a core spec.
Installation options
- Inverted-V from a single center mast: the most common install — one center support carries the feedpoint, with both legs sloping down to lower end supports, keeping the traps at a manageable working height.
- Flat-top between two supports: if you have two trees or masts far enough apart, running the antenna flat and level improves the pattern symmetry slightly, at the cost of needing more real estate.
- Sloped single-support install: for a single support with no second high point, one leg can run higher than the other — expect a somewhat asymmetric, more directional pattern than a symmetric inverted-V or flat-top.
| Section | Length (ft) | Length (m) | Notes |
|---|---|---|---|
| Feedpoint to trap (each side) | ~33 ft | ~10.1 m | Sets 40m resonance; total inner section ~66 ft, close to a 40m half-wave |
| Trap to end insulator (each side) | ~33 ft | ~10.1 m | Adds the electrical length needed to bring the full wire to 80m resonance |
| Total wire, tip to tip | ~132 ft | ~40.2 m | Starting point for a center design frequency near 3.75/7.15 MHz; trim per your target |
| Trap coil | 13-15 µH | — | ~10-12 turns, close-wound, on a 2.5-3 in diameter PVC form, typically #12-14 AWG |
| Trap capacitor | 60-64 pF | — | Mica or ceramic doorknob type, rated 3 kV or higher for legal-limit power |
| Recommended feed height | 35-60 ft | 10.7-18.3 m | Inverted-V or flat-top, same siting guidance as any full-size HF dipole |
W3DZZ Trap Dipole Dimension Calculator
Materials for W3DZZ Trap Dipole
Building the W3DZZ Trap Dipole
Winding and checking the two traps before you cut a foot of the outer wire saves the most rework — get the traps resonant on the bench first, then build the wire lengths around them.
Wind the trap coils
Close-wind #12-14 AWG wire around each PVC form to get roughly 13-15 µH — start with 10-12 turns on a 2.5-3 in form and expect to trim turns during bench testing.
Wire the doorknob capacitor across each coil
Solder a 60-64 pF doorknob capacitor directly across the leads of each finished coil, keeping lead lengths short to avoid adding stray inductance to the trap.
Bench-test each trap's resonant frequency
Using a NanoVNA or grid-dip meter, confirm each trap resonates close to 7.15-7.20 MHz before mounting it in the antenna. Add or remove a turn to bring it on frequency.
Cut the inner (feedpoint-to-trap) wire sections
Cut two lengths of antenna wire, about 33 ft each, running from the center feedpoint to each trap.
Cut the outer (trap-to-tip) wire sections
Cut two additional lengths, about 33 ft each, running from each trap out to the end insulators.
Assemble the center feedpoint
Connect both inner wire sections to the center insulator's SO-239 connector, install the choke balun, and attach your coax feedline.
Mount the traps in-line
Splice each trap between its inner and outer wire sections, weatherproofing the connections and the trap enclosure against rain intrusion.
Attach end insulators and raise the antenna
Tie off the outer wire ends to egg insulators, then hoist the antenna as a flat-top or inverted-V, keeping both legs' traps roughly symmetric in height.
Sweep and check 40m first
With the antenna in its final position, sweep the SWR across 40m — this section is the simplest to verify since it's isolated by the traps.
Sweep and trim 80m
Sweep 80m and trim the outer wire tips evenly (both sides, same amount) to move the resonant dip to your target frequency.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| One band's resonance is off, the other looks fine | Traps isolate the two sections, so each needs its own adjustment | Note whether 40m or 80m is the one off frequency | If 40m is off, recheck the trap's bench resonance; if 80m is off, trim the outer tips evenly |
| High SWR on both bands | Common-mode current or a loose connection at the center feedpoint | Check the choke balun and center insulator solder joints | Re-solder connections and confirm the choke balun is intact |
| Traps arc or fail under power | Undersized capacitor voltage rating for your power level | Check the doorknob capacitor voltage rating against your operating power | Use capacitors rated 3 kV or higher, or reduce power until upgraded parts are installed |
| SWR rises noticeably toward the edges of 80m or 40m | Normal bandwidth roll-off away from the trimmed resonant point | Compare SWR at the band edges against the design frequency | Expected behavior for a two-trap dipole; re-trim to shift the center frequency if a different part of the band is your priority |
Is the W3DZZ the same as the trap dipole already on this site?
No — that guide covers a 4-trap, 3-band (80/40/20m) design. The W3DZZ uses exactly 2 traps for a simpler 80m/40m no-tuner antenna, with higher bands available only through a tuner as a long-wire.
Can I use purchased trap kits instead of winding my own?
Yes — commercially made trap kits built to the same 13-15 µH / 60-64 pF target values are a drop-in substitute for the homebrew traps in this guide, and skip the bench-testing step since they arrive pre-tuned.
Does the W3DZZ really work on 20m and higher?
The full length of wire, fed through an antenna tuner, presents a usable match on several higher bands as a random-length long-wire. It's a real bonus, but it isn't clean resonant dipole performance the way 80m and 40m are.
How much power can the traps handle?
With properly rated doorknob capacitors (3 kV or higher) and adequately sized coil wire, homebrew W3DZZ traps handle legal-limit power reliably — undersized capacitors are the most common failure point, not the coil.
Why use traps instead of just building two separate dipoles?
Traps let you run one wire and one feedline instead of two separate antennas (like a fan dipole), at the cost of some trap loss and a more involved build — a real tradeoff, not a free win either way.
What length should I start with if I want a different center frequency?
Use the calculator above with your target 80m and 40m frequencies — it scales both the inner and outer wire sections from the standard published design ratios.