Extended Double Zepp (EDZ) Antenna
The Zepp gets its name from Zeppelin airships, which trailed an end-fed half-wave wire behind the gondola and ran the feeder up through an insulator to the radio operator inside — one of the earliest practical HF antenna feed methods. The Extended Double Zepp is the modern, center-fed descendant hams actually build today: instead of stopping each leg at a plain quarter-wave like an ordinary dipole, each leg runs out to about 0.64 wavelength, and that extra length reshapes the current distribution to add real gain broadside to the wire — commonly cited at roughly 3 dB over a simple half-wave dipole. It's fed with open-wire ladder line into a balanced tuner, which is also what makes it naturally multiband.
Where the name actually comes from
The original Zepp antenna was end-fed: a half-wave wire trailed out from a Zeppelin airship, with an open-wire feeder running from a high-impedance point near the wire's end down to the transmitter. That end-fed, high-impedance feed approach is genuinely where "Zepp" comes from in ham radio vocabulary, even though almost nobody builds a true single-ended airship-style Zepp today.
What "extended" and "double" mean here
"Double" means this version is center-fed and symmetrical — effectively two Zepp half-antennas back to back, sharing one feedpoint. "Extended" means each leg is stretched past a plain quarter-wave to about 0.64 wavelength, so the whole antenna runs about 1.28 wavelengths tip to tip instead of the 0.5 wavelength of an ordinary dipole.
Extended Double Zepp: 1.28 wavelength total, current lobes reshaped to reinforce broadside
Why the extra length adds gain instead of just adding loss
Stretching a dipole's legs past a quarter-wave doesn't just make it longer — it changes how current is distributed along the wire. At 0.64 wavelength per leg, the current lobes line up so that more of the antenna's radiation reinforces in the broadside direction rather than spreading it evenly in all directions, which is where the roughly 3 dB gain over a plain dipole comes from. Push the leg length further and the lobes start splitting into additional minor lobes off the ends, which is why 0.64 wavelength is the commonly published sweet spot rather than "longer is always better."
Why ladder line and a balanced tuner, not coax direct
Because each leg is longer than a plain quarter-wave, the feedpoint impedance is high and swings widely with frequency — not a clean 50 ohms anywhere close to what coax wants. Open-wire or ladder line has very low loss even at these high, reactive impedances, and a balanced (or unun/balun-fed) antenna tuner at the shack end handles the matching — the same reason doublets and G5RV-family antennas use the same feed approach.
- This design: genuinely multiband when fed this way, since the tuner absorbs the impedance swings band to band.
- Coax-direct designs (like a resonant dipole): single-band without a remote matching network.
Installation options
- Flat-top between two supports: the standard install — level, as high as practical, running broadside toward your priority direction.
- Inverted-V from a single center mast: workable with one high support, though the pattern and gain advantage both soften somewhat compared to a flat, level run.
- Sloped single-support install: usable in tighter yards, trading some gain and pattern symmetry for a smaller footprint.
| Parameter | 20m (14.15 MHz) | 40m (7.15 MHz) | Notes |
|---|---|---|---|
| Each leg (0.64 wavelength) | ~42.3 ft | ~83.6 ft | Scales directly with your chosen design frequency |
| Total wire, tip to tip | ~84.5 ft | ~167.2 ft | 1.28 wavelength overall |
| Ladder line feeder | 450-600 ohm window line | Same | Length is not critical for tuning as long as a balanced tuner is used |
| Recommended feed height | 30-50 ft | 35-60 ft | Higher feed improves the broadside gain advantage this design is built for |
Extended Double Zepp Dimension Calculator
Materials for Extended Double Zepp
Building the Extended Double Zepp
This build is mechanically similar to a doublet — the difference from a plain dipole is entirely in the leg length and the fact that the feed impedance is intentionally not 50 ohms.
Choose your design frequency and cut both legs
Use the calculator above to get each leg's length at 0.64 wavelength, and cut two equal lengths of antenna wire with a few extra inches for tie-off at each end.
Assemble the center feedpoint
Attach both wire legs to opposite sides of a center insulator/spreader, and connect the ladder line's two conductors to the same two lugs.
Attach end insulators
Tie off the far end of each leg to an egg insulator, ready for support rope.
Raise the antenna flat-top
Hoist the antenna between two supports as level and as high as your site allows — this design's gain advantage depends more on getting it up high than most simple dipoles.
Route the ladder line into the shack
Bring the ladder line in through a dedicated feed-through panel or standoffs, keeping it away from metal siding, gutters, or other conductive surfaces along the way.
Connect to a balanced tuner
Connect the ladder line directly to a balanced antenna tuner, or to an unbalanced tuner through a 4:1 balun/unun rated for the impedance swings this design presents.
Tune each band from the shack
Because matching happens at the tuner rather than at the antenna itself, there's no trimming step at the wire — tune for lowest SWR at the transmitter on each band you plan to use.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Tuner won't find a match on some bands | Feedpoint impedance lands outside the tuner's matching range at that frequency | Check which band or frequency fails to match | Try switching the tuner's balanced-line taps, or add/remove a fixed length of extra ladder line |
| RF feels present in the shack | Common-mode current on the ladder line entering the building | Check the feed-through routing and the balun/unun rating at the tuner | Route through a proper feed-through panel clear of household wiring and metal; confirm the balun/unun is rated for these impedances |
| Expected gain isn't noticeable on the air | Antenna height or orientation doesn't match the design assumptions | Compare against a dipole at the same height and orientation | Not a fault — raise the antenna or reorient it toward your priority path for the gain to show up |
| Match or gain is inconsistent on harmonic bands | Current distribution no longer matches the 0.64-wavelength condition off the design band | Sweep SWR and compare the design band against other bands used via the tuner | Expected; treat the design frequency as the band this length is optimized for |
Do I need to build the true single-ended Zeppelin-style version?
No — almost nobody builds the original end-fed airship version today. The Extended Double Zepp is the practical, center-fed descendant that actually gets built, and it's what this guide covers.
Can I feed this with coax instead of ladder line?
Not directly with good results — the feedpoint impedance is high and reactive, which coax handles poorly over any real run length. Ladder line into a balanced tuner is the standard approach for a reason.
Is this really 3 dB better than a dipole?
Roughly, broadside, at the design frequency and with comparable height — it's a real, commonly cited figure from standard antenna modeling, not a marketing exaggeration, but it's a directional gain, not a free improvement in every direction.
Does this work on multiple bands?
Yes — the ladder-line-and-tuner feed approach is inherently multiband, the same reason doublets and G5RV-style antennas cover several bands from one wire.
Why 0.64 wavelength per leg specifically?
That's the commonly published length where the current distribution lines up for maximum broadside reinforcement before minor lobes start splitting off the ends — shorter gives less gain, longer starts giving some of it back to extra lobes.
How is this different from a plain doublet?
A doublet is any center-fed, ladder-line-fed wire antenna regardless of length. This is a doublet cut to a specific length (1.28 wavelength total) chosen deliberately for the broadside gain benefit.