Build an All-Band Doublet Antenna
The doublet is the purest expression of the all-band HF wire antenna — a centre-fed dipole of any convenient length, fed with open-wire or ladder-line feedline all the way to a balanced antenna tuner in the shack. Unlike the G5RV or ZS6BKW, the doublet makes no attempt to present a specific impedance at a junction point — it deliberately uses low-loss balanced feedline to deliver whatever impedance the antenna presents on each band directly to an ATU that can handle it. The result is a single antenna that covers every HF band from 160m through 10m with low feedline loss and genuine efficiency, limited only by the ATU's matching range. This guide covers doublet theory, wire length selection, open-wire feedline construction, ATU requirements, feedline routing, and installation for a permanent station doublet.
The Doublet Concept — Why It Works on Every Band
A doublet is simply a centre-fed wire antenna — electrically identical to a dipole — fed with high-impedance balanced transmission line rather than coax. The key insight is that a centre-fed wire of any length will radiate on any frequency if the feedline can deliver power to it efficiently and the ATU can match the resulting impedance. The open-wire feedline makes this possible by keeping feedline loss low even at very high SWR:
Wire Length — What to Avoid and Why
The doublet has no critical wire length — any length works in principle. However, certain lengths create extreme impedances on specific bands that even a good ATU cannot match, or create feedline imbalance problems. There are lengths to prefer and lengths to avoid:
Open-Wire Feedline — Types and Characteristics
The feedline is the doublet's most important component after the wire itself. The choice of open-wire feedline type determines loss, power handling, and ease of installation:
- 450 Ω ladder line (window line): the standard choice for most doublet installations. Pre-made, inexpensive, flexible, and handles 1500W comfortably. Velocity factor approximately 0.91. Must be kept 6 inches from metal surfaces. Davis RF and similar suppliers sell quality ladder line by the foot.
- 300 Ω twin-lead (TV ribbon): lower impedance, more loss than ladder line, absorbs moisture over time — not recommended for a permanent installation. Usable as a temporary feed but ladder line is worth the modest additional cost.
- 600 Ω open-wire (homebrew): the lowest-loss option. Made from two parallel wires held apart by ceramic or PTFE spacers every 6–12 inches. Velocity factor approximately 0.975. Handles any power level. More work to build and route than commercial ladder line but produces genuinely excellent results. Suitable for a permanent fixed-station doublet at any power level.
- Feedline length: any length works — the doublet is not sensitive to feedline length as long as the ATU can handle the resulting impedance. Practical consideration: keep the feedline run as straight and vertical as possible, and avoid sharp bends.
ATU Requirements — the Critical Link
The doublet's all-band capability depends entirely on having an ATU that can match the impedances presented by the open-wire feedline. Not all ATUs are suitable — the doublet requires a balanced ATU with a wide impedance matching range:
- Balanced input required: the open-wire feedline connects to the balanced (antenna) terminals of the ATU — not to a coax SO-239. An ATU with a balanced output (typically accessed via binding posts or a balanced output switch) is required. A coax-input ATU can be used with a 4:1 balun added externally, but a true balanced ATU is preferred.
- Wide matching range: the doublet presents impedances from as low as 10 Ω to as high as several thousand ohms depending on band and feedline length. The ATU must handle this range. Recommended ATUs: LDG AT-100Pro (with external balun), MFJ-949E (has built-in balanced output), Palstar AT-Auto, Elecraft T1 (with external balun). Avoid narrow-range internal rig tuners — they typically handle 3:1 SWR, not the 50:1 or higher that a doublet can present.
- Power rating: match ATU power rating to your transceiver. At 100W, most commercial ATUs handle the doublet without issue. At 500W or 1500W, verify the ATU's balanced-output power rating — some ATU balanced terminals are rated lower than the coax input.
- Johnson Matchbox and homebrew T-match: the classic doublet ATU is a balanced T-match or L-match. A homebrew T-match using two air variable capacitors and an air-core inductor with roller or tapped connections handles any impedance the doublet presents and costs $30–50 to build from surplus parts.
| Wire length | Each side | 160m | 80m | 40m | 30m | 20m–10m | Notes |
|---|---|---|---|---|---|---|---|
| 88 ft | 44 ft | Marginal | Good | Good | Good | Excellent | Most popular all-band doublet length; avoids worst impedance extremes |
| 102 ft | 51 ft | Marginal | Good | Good | Awkward | Excellent | G5RV flat-top length; 30m can be difficult for some ATUs |
| 130 ft | 65 ft | Good | Excellent | Good | Good | Excellent | Excellent 80m doublet; needs 140+ ft span |
| 135 ft | 67.5 ft | Good | Avoid — λ/2 | Good | Good | Excellent | Avoid this length — exact half-wave on 80m causes problems |
| 267 ft | 133.5 ft | Excellent | Excellent | Excellent | Excellent | Excellent | Full half-wave on 160m; the ultimate all-band doublet if space permits |
| 44 ft | 22 ft | Poor | Marginal | Good | Awkward | Good | Compact doublet for 40m–10m; limited on lower bands |
| Band | Wire in wavelengths | Feedpoint impedance range | ATU difficulty | Radiation pattern | Notes |
|---|---|---|---|---|---|
| 160m (1.85 MHz) | ~0.17 λ | Very low — 5–20 Ω + reactance | Difficult — needs wide-range ATU | Broad, near-omnidirectional | Very short electrically — marginal but possible with good ATU |
| 80m (3.7 MHz) | ~0.33 λ | 50–200 Ω typical | Easy | Broadside, moderate gain | Good performer; near half-wave — practical 80m antenna |
| 40m (7.1 MHz) | ~0.65 λ | 100–500 Ω typical | Easy–moderate | Multi-lobe | Excellent 40m antenna with multiple radiation lobes |
| 30m (10.1 MHz) | ~0.92 λ | 200–2000 Ω | Moderate — depends on feedline length | Multi-lobe | Near full-wave — impedance varies widely; most ATUs handle it |
| 20m (14.15 MHz) | ~1.28 λ | 50–300 Ω typical | Easy | Multi-lobe, moderate gain | Excellent performer; multiple radiation directions |
| 17m (18.1 MHz) | ~1.65 λ | 50–400 Ω typical | Easy | Multi-lobe | Good all-round performer |
| 15m (21.1 MHz) | ~1.92 λ | 100–800 Ω | Easy–moderate | Multi-lobe, higher gain lobes | Good; near 2× half-wave |
| 12m (24.9 MHz) | ~2.27 λ | 50–300 Ω typical | Easy | Multi-lobe | Reliable performer |
| 10m (28.5 MHz) | ~2.59 λ | 50–400 Ω typical | Easy | Multi-lobe, good gain | Strong performer when band is open |
Materials for an 88-ft all-band doublet with 450 Ω ladder-line feedline
Building the 88-ft All-Band Doublet
This guide builds an 88-ft doublet fed with 450 Ω ladder line to a balanced ATU. The doublet is the simplest HF wire antenna to build — there are no traps, no matching sections, and no critical dimensions. The feedline routing and ATU selection are the details that matter most.
Cut and Prepare the Flat-Top Wire
Cut two wire legs to 45 feet each — 44 feet nominal plus one foot for the centre and end insulator connections. Stretch each wire on the ground and mark before cutting. At the centre end of each wire, strip 2 inches of insulation and form a tight loop by folding back and twisting 4–5 turns; solder the twist. At the far end, form a similar loop through the end insulator hole and solder. The doublet has no resonant length requirement — cutting to exactly 44 feet is not critical. Anywhere from 43 to 45 feet per leg works identically.
Assemble the Centre Insulator
The doublet centre insulator connects the two flat-top wires directly to the two conductors of the ladder-line feedline — nothing more. There is no balun, no coax connector, and no SO-239 at the centre of a doublet. The entire system from wire to ATU is balanced. A simple centre insulator can be made from a 6-inch length of 3/4-inch PVC pipe with stainless steel machine screws and nuts as the terminal posts, or from a commercial antenna centre insulator that has balanced solder lugs.
Connect one wire leg to one terminal, and the other wire leg to the other terminal. Separate the two conductors of the ladder line at the top end by 3 inches and connect one conductor to each terminal — one conductor per wire leg. Solder all connections, allow to cool, then weatherproof with self-amalgamating tape. Attach the Dacron halyard rope to the top of the centre insulator through its strain-relief loop.
Plan the Feedline Route
Before raising the antenna, plan the complete feedline route from the antenna centre to the ATU. This is the most important installation planning step for a doublet — a poorly routed ladder-line feedline degrades performance on all bands. The route must:
- Run vertically as far as possible from the antenna centre toward the ground — avoid horizontal sections of ladder line near the antenna, which can interact with the flat-top and cause imbalance
- Maintain 6-inch clearance from all metal surfaces (gutters, conduit, window frames, metal siding) for its entire run — use standoff insulators mounted on wooden battens or PVC to achieve this clearance along walls
- Enter the shack through a dedicated ladder-line feedthrough insulator — not a coax feedthrough, which would crush the conductor spacing. Dedicated ladder-line feedthroughs are available from antenna suppliers or can be made from a 1-inch-thick wooden board with two ceramic standoff insulators drilled through it
- Avoid sharp bends — any bend in ladder line that reduces the conductor spacing at the bend point changes the line impedance at that point and causes a reflection. Smooth, gentle curves only.
Raise the Antenna
Raise the centre insulator to the desired height using the halyard, then pull each wire leg out to its end support and anchor. The doublet installation procedure is identical to a flat-top dipole — attach halyards to end insulators, haul both ends to height simultaneously, and tie off. The flat-top should be as straight and horizontal as practical, with both ends at approximately equal height.
Once the flat-top is up, run the ladder-line feedline down from the centre insulator along the planned route. Attach the feedline to standoff insulators at each mounting point using UV-resistant zip ties or stainless steel cable ties — do not let the ladder line flap freely in the wind along a wall run, as repeated flexing at contact points damages the insulation over time.
Connect the Feedline to the ATU
Route the ladder line through the wall feedthrough insulator and connect to the balanced terminals of the ATU. The balanced terminals on most ATUs are a pair of binding posts or wing nuts — one per conductor. Connect one ladder-line conductor to each binding post. Polarity does not matter for a balanced system — either conductor can go to either post.
Do not coil excess ladder line inside the shack. If you have more feedline than needed, cut it to length — a coil of ladder line inside the shack at HF frequencies acts as an inductor that interacts with the ATU and can cause unpredictable matching behaviour. If length adjustment is needed for testing, lay excess feedline out straight on a non-conductive surface rather than coiling it.
Verify ATU Tune on All Intended Bands
With the antenna connected, verify the ATU achieves a match on each band. Transmit a low-power carrier (5–10W) and tune the ATU for minimum SWR on the radio's meter. Work through each band from lowest to highest — 160m (if applicable), 80m, 40m, 30m, 20m, 17m, 15m, 12m, 10m.
Homebrew 600 Ω Open-Wire Feedline
For a permanent fixed-station doublet, homebrew 600 Ω open-wire feedline is worth considering — it has the lowest loss of any practical feedline type and handles any power level without concern:
- Construction: two parallel #14 AWG bare copper wires held 6 inches apart by ceramic or PTFE spacers every 12 inches. The spacers can be commercial ceramic standoffs, PTFE rod with holes drilled, or even short lengths of fibreglass rod.
- Velocity factor: approximately 0.975 — very close to free-space velocity. Feedline length is not critical for the doublet, so the exact VF does not matter.
- Loss: under 0.1 dB per 100 feet at HF frequencies — effectively lossless even at very high SWR.
- Power handling: limited only by conductor current rating — #14 AWG copper handles several kilowatts at HF without heating.
- Disadvantage: more work to build than buying commercial ladder line; the spacers must be UV-resistant and must maintain consistent spacing; wind causes the wires to sway, which is acceptable as long as the spacing stays consistent.
- Best use case: a permanent installation where the feedline route is clear and straight, and where maximum performance at all power levels is desired.
Dealing with Feedline Routing Challenges
The most common practical challenge with an open-wire fed doublet is routing the feedline into the shack without compromising its balanced nature. Several solutions are used by doublet operators:
- Dedicated ladder-line wall passthrough: best solution — two ceramic standoffs through a wooden board sealed into the wall. Maintains spacing and provides weather seal. Available from W4RT, Palomar, and other antenna suppliers.
- Window entry: route the ladder line through a slightly open window with foam weatherstripping around it — not ideal for permanent installation but acceptable as a temporary measure. Use a ladder-line strain relief at the window frame.
- Balun at the wall entry: some operators prefer to terminate the open-wire line at the exterior wall with a 4:1 balun, then run coax from the balun into the shack. This sacrifices some of the open-wire feedline's advantage (introduces additional component loss) but simplifies the shack entry and allows use of a coax-input ATU inside.
- ATU outside the shack: for maximum performance, mount a weatherproof ATU at the base of the mast and run coax from there into the shack. This eliminates all high-SWR coax runs entirely — the ATU matches the antenna right at the feedpoint and the coax carries only the matched 50 Ω signal. This configuration is used by serious DX stations and contest operators.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| ATU cannot tune on one specific band | Feedline length presenting extreme impedance on that band to the ATU | Try adding or removing 2–3 ft of feedline — impedance at ATU shifts with feedline length | Adjust feedline length in 1-ft increments until ATU can tune that band; 2 ft of change often resolves the problem |
| ATU cannot tune on any band | Open circuit in feedline or wire; feedline conductors shorted together | Check DC continuity across each feedline conductor from ATU to antenna; check for shorted conductors at feedline connections | Re-solder centre insulator connections; inspect feedline along full length for damage, staples, or contact with metal |
| RF feedback in shack on all bands | Feedline imbalance — ladder line too close to metal surfaces along its run | Touch shack equipment chassis — RF tingle confirms common-mode current; check feedline route for proximity to metal | Install or reposition standoff insulators to maintain 6-inch clearance from all metal; add ferrite choke at ATU balanced terminals |
| SWR high even after ATU tune — signal reports poor | High loss joint in flat-top wire or centre insulator connection | Use NanoVNA to check impedance at ATU terminals — compare to expected range for that band | Inspect and re-solder all connections at centre insulator; check for corroded wire ends at end insulators |
| ATU arcs or overheats on certain bands | Very high impedance at ATU balanced terminals; ATU not rated for the power and impedance combination | Listen for arcing in ATU during transmit; note which bands cause the problem | Adjust feedline length to shift impedance at ATU to a more manageable value; use an ATU with higher voltage-rated components |
| Feedline swings in wind — SWR changes continuously | Ladder line unsecured along wall or mast run | Observe SWR meter during windy conditions — continuous variation confirms wind movement | Secure feedline to standoff insulators with cable ties at 2–3 ft intervals; prevent free movement along the entire feedline route |
| Performance much worse after rain | Water on or between ladder-line conductors temporarily changes line impedance and loss | Compare SWR and signal reports in wet vs dry conditions | This is normal for open-wire feedline and resolves when dry; for 300 Ω twin-lead, replace with 450 Ω ladder line which handles moisture better |
Is the doublet better than the G5RV?
For multi-band all-HF operation, the doublet with open-wire feedline to a balanced ATU is generally superior to the G5RV. The G5RV uses a specific matching section intended to present a workable impedance to coax — with mixed results across bands, and coax loss at elevated SWR on several bands. The doublet makes no such compromise — it uses low-loss balanced feedline on every band and relies on the ATU to handle whatever impedance is presented. The trade-off is that the doublet requires a quality balanced ATU and careful feedline routing, while the G5RV works with any ATU and coax. For a permanent fixed-station installation where maximum performance on all bands is the goal, the doublet is the superior choice.
Does feedline length matter for the doublet?
Not critically — any feedline length works with a doublet. However, the impedance the ATU sees on each band does change with feedline length, and certain lengths can present impedances outside a specific ATU's matching range on specific bands. If the ATU cannot tune a particular band, adding or removing 2–3 feet of feedline often resolves the problem by shifting the impedance at the ATU terminals to a more manageable value. As a general starting point, a feedline length that is not a multiple of a quarter-wave on your most important band is slightly preferable, but in practice the ATU handles the variation and feedline length is a secondary concern.
Can I use the doublet on 160m?
An 88-ft doublet can be used on 160m with a wide-range ATU — the wire is roughly 0.17 wavelengths, presenting a very low impedance with significant reactance. Some ATUs handle this; others cannot. A 267-ft doublet (half-wave on 160m) works excellently on 160m and all higher bands and is the preferred choice when space allows. If 160m is a priority and only an 88-ft span is available, a dedicated 160m antenna (inverted-L or loaded vertical) is more practical than trying to force the short doublet to work on the band.
Do I need a balun with a doublet?
Not at the antenna feedpoint — the doublet is fully balanced from wire to ATU and no balun is needed at the centre insulator. At the ATU, a balun may or may not be needed depending on whether the ATU has balanced output terminals. If your ATU has dedicated balanced terminals (binding posts), connect the ladder line directly — no balun needed. If your ATU has only a coax input, use a 4:1 current balun between the ladder line and the ATU coax input to provide the balanced-to-unbalanced transition. Some operators use a 1:1 balun in this position — either works; the 4:1 ratio is a closer match to the typical impedance transformation needed.
What is the best doublet length for a typical suburban lot?
For a typical suburban lot with 80–100 feet of available span, the 88-ft doublet is the most practical choice — it fits the available space with a modest catenary, avoids the worst impedance extremes on the common HF bands, and provides genuine all-band capability from 80m through 10m. If the lot is smaller and only 60–70 feet of span is available, a 60-ft doublet still works on 40m through 10m with a good ATU, though 80m performance will be limited. The doublet's strength is that it works at whatever length fits your space — there is no wrong length, only lengths that are slightly more or less convenient for the ATU to match on specific bands.
Can I run a doublet at legal limit (1500W)?
Yes — the doublet's wire (#14 AWG copper) and 450 Ω ladder-line feedline both handle 1500W without concern for conductor heating. The limiting factor at high power is the ATU — verify your ATU's balanced-output power rating before running high power. Many commercial ATUs are rated 300W or 1000W on their balanced terminals even if the coax port is rated higher. At 1500W with a doublet, a robust ATU such as the Palstar AT-AUTO (1500W), a commercial Johnson Matchbox, or a well-built homebrew T-match is required. The feedline and antenna wire are not the weak link.