Build an Inverted-V Dipole Antenna
The inverted-V dipole is one of the most practical and widely used HF antennas in amateur radio. It requires only a single centre support — a mast, tree, or tower — with the two wire legs sloping downward at an angle, making it an ideal choice when only one suitable high support is available. Compared to a flat-top dipole, the inverted-V uses less horizontal space, is easier to raise and tension, and performs nearly as well across all HF bands. This guide covers the theory, wire length calculations, apex height requirements, leg angle effects on radiation pattern, feedline selection, and complete installation procedure for a single-band or multi-band inverted-V dipole.
Inverted-V vs Flat-Top Dipole
The inverted-V is electrically a centre-fed half-wave dipole — identical in principle to a flat-top dipole. The difference is physical geometry: the flat-top dipole runs horizontally between two supports at the same height, while the inverted-V hangs from a single centre support with the legs drooping at an angle toward ground anchors or low supports. This change in geometry has measurable but modest effects on antenna performance:
Leg Angle and Its Effect on Performance
The angle between the two legs of an inverted-V — measured at the apex — is the most important variable in the design. It affects feedpoint impedance, radiation pattern, and the required horizontal space:
In practice, the leg angle is determined by the apex height and the distance between the end anchor points. If you have a 40-foot apex and can anchor the ends 47 feet apart horizontally, you achieve approximately 120° — the ideal configuration.
Wire Length Calculations
The inverted-V uses the same half-wave wire length formula as a flat-top dipole, with a small correction factor for the end effect of sloping wires. The standard formula gives a good starting point; final trimming with an antenna analyser is always required:
Apex Height and Performance
For an inverted-V, apex height is the single most important installation variable. The apex is the highest point and where most of the radiation originates — raising the apex improves low-angle DX radiation and increases the useful leg angle:
- 40m inverted-V at 25 ft apex: predominantly high-angle radiation — excellent for regional contacts (500–1500 miles), limited DX. Leg angle with 47-ft span: ~75° — marginal.
- 40m inverted-V at 35 ft apex: good mix of high and low-angle radiation — works well for both regional and DX operation. Leg angle with 47-ft span: ~95° — good.
- 40m inverted-V at 50 ft apex: low-angle radiation dominant — competitive DX performance approaching that of a flat-top at the same height. Leg angle with 47-ft span: ~115° — excellent.
- 80m inverted-V: needs 50–70 ft apex for reasonable DX performance. At 35 ft it is primarily an NVIS/regional antenna — still very useful for regional nets and emcomm.
- General rule: get the apex as high as your support allows. Every 10 feet of additional apex height on 40m and 80m produces a meaningful improvement in low-angle radiation and DX capability.
- End height: keep the wire ends at least 8 feet above ground for safety — the ends of a half-wave dipole carry high voltage and must not be touchable by people or animals during transmission.
| Band | Total wire length | Each leg | Apex height | Horizontal span (120° angle) | Horizontal span (90° angle) | End height at 90° |
|---|---|---|---|---|---|---|
| 80m | 128 ft | 64 ft | 60 ft | 111 ft | 91 ft | ~15 ft |
| 80m | 128 ft | 64 ft | 50 ft | 111 ft | 91 ft | ~5 ft — raise ends |
| 40m | 66 ft | 33 ft | 45 ft | 57 ft | 47 ft | ~22 ft |
| 40m | 66 ft | 33 ft | 35 ft | 57 ft | 47 ft | ~12 ft |
| 20m | 33 ft | 16.5 ft | 25 ft | 29 ft | 23 ft | ~14 ft |
| 20m | 33 ft | 16.5 ft | 20 ft | 29 ft | 23 ft | ~9 ft |
| 15m | 22 ft | 11 ft | 18 ft | 19 ft | 16 ft | ~10 ft |
| 10m | 16.5 ft | 8.25 ft | 15 ft | 14 ft | 12 ft | ~9 ft |
Inverted V Dipole Calculator
Materials for a single-band 40m inverted-V dipole — scales directly to any HF band
Building the 40m Inverted-V Dipole
This guide builds a single-band 40m inverted-V. The same procedure scales directly to any HF band — substitute the wire lengths from the dimensions table above. Work top-down: prepare the feedpoint and wire legs on the ground, then raise to the support.
Cut and Prepare the Wire Legs
Cut two wire legs to 33.7 feet each — the formula length of 32.7 feet plus 3% for trimming allowance. Stretch each wire out on the ground and mark the cut length with tape before cutting. At one end of each wire, strip 2 inches of insulation (if using insulated wire) and form a small loop by folding back and twisting 4–5 turns. This loop attaches to the centre insulator. Solder the twist for permanence.
At the far end of each wire, strip 2 inches and form a similar loop through the hole in the end insulator, fold back, twist, and solder. The end insulator terminates the wire mechanically and electrically isolates the wire end from the rope and anchor system.
Assemble the Centre Feedpoint
The centre insulator is the heart of the inverted-V — it supports the mechanical load of both wire legs and connects to the coax feedline. For most HF inverted-V antennas, a commercial centre insulator with solder lugs and a strain-relief loop for the halyard rope is the most reliable option. A homebrew version using a 6-inch length of 3/4-inch PVC pipe and stainless steel hardware is also entirely adequate.
Connect the loop from each wire leg to the solder lugs on opposite sides of the centre insulator. Mount the 1:1 current choke balun directly at the centre insulator — the balun's balanced terminals connect to the solder lugs (one per wire leg), and the coax connects to the balun's unbalanced (coax) terminal. This places the balun exactly at the feedpoint, which is the correct location for maximum common-mode rejection.
Attach the Centre Halyard and Prepare to Raise
Attach a Dacron rope halyard to the strain-relief loop on the top of the centre insulator. This halyard will bear the full weight of both wire legs plus the coax pull — use at least 3/16-inch Dacron rope and tie with a bowline knot, not a slip knot. Lay out the antenna on the ground in its final inverted-V shape: centre insulator in the middle, both wire legs extending outward and slightly downward to simulate the installed position.
Run the coax from the balun toward the shack entry point. Coil any excess and tie off with cable ties — do not leave the coax dragging on the ground under the antenna where it can be damaged during raising. Attach the coax to the halyard rope with a UV-resistant cable tie every 2–3 feet so the coax hangs alongside the rope rather than putting strain on the balun connector.
Raise the Centre to the Support
Throw the halyard over the support point — a tree branch, mast pulley, or push-up mast top — and haul the centre insulator to the desired apex height. For a tree, use a throw bag and fishing line to place a rope over a suitable branch, then pull the halyard rope up with the fishing line. For a push-up mast, attach the halyard to the mast top before raising the mast.
Once the centre is at height, tie off the halyard securely but leave the wire ends free on the ground. The antenna will hang vertically at this point — the wire legs will trail down from the centre. This is normal; the legs will be pulled out to their anchor positions in the next step.
Anchor the Wire Ends
Pull each wire leg out from the base of the support to its anchor point. The anchor can be a tent stake, an eye bolt in a fence post, a tree branch at low height, or any other secure, non-conductive point. Attach a short Dacron rope from the end insulator to the anchor — the rope takes the mechanical tension; the wire carries only its own weight between the centre and the anchor rope.
Adjust the tension on each leg so the wire is taut but not under excessive strain. The wire should describe a smooth, shallow arc from the apex downward to the end anchor — not a sharp angle. Check that both wire ends are at least 8 feet above ground level. If an end is too low with your available apex height and anchor distance, move the anchor farther from the mast to increase the leg angle and raise the end height.
Measure SWR and Trim to Resonance
With the antenna fully installed and the coax run to the shack or NanoVNA, measure SWR across the target band. Connect the NanoVNA to the coax at the shack end and sweep ±500 kHz around the target frequency. The SWR minimum (resonance point) will appear as a sharp dip in the SWR display.
Weatherproof and Finalise
Once resonance is confirmed, weatherproof all outdoor connections. Wrap each solder joint and coax connector with self-amalgamating tape — apply with 50% overlap and stretch the tape slightly as it goes on for a waterproof seal. Apply a second layer of standard electrical tape over the self-amalgamating tape to protect it from UV degradation.
Make a final visual inspection: check that no wire is in contact with metal surfaces, tree branches, or gutters; verify that all rope connections are secure with no slipping knots; confirm that the coax drip loop is present at the entry point where the coax enters the shack (a downward loop just before the wall entry prevents water running down the coax into the building).
Trap Inverted-V
Adding traps to the inverted-V legs creates a multi-band antenna from a single feedpoint and a single support. Each pair of traps resonates on one band and electrically shortens the antenna to that band's length, while allowing the full wire beyond the trap to be active on lower bands:
- Typical configuration: 20m traps at 16.5 ft from centre, then wire continues to 40m length — gives 40m and 20m from one antenna
- Three-band version: 15m traps at 11 ft, 20m traps at 16.5 ft, wire continues to 40m — covers 40m, 20m, 15m
- Trap loss: each pair of traps introduces approximately 0.5–1 dB of insertion loss on the lower bands — acceptable for most operation
- Commercial traps: MFJ, Unadilla, and Cushcraft traps are widely available; homebrew traps from coax coil designs also work well
- Tuning: a trap inverted-V requires more careful trimming than a single-band version — each band must be tuned independently, starting from the highest band and working down
Fan Inverted-V
The fan inverted-V uses multiple wire pairs from a single centre insulator and feedpoint, each pair cut to a different band. The wires fan outward from the apex at slightly different angles, creating a separate resonant antenna for each band without traps or switching:
- Typical configuration: 40m, 20m, and 15m wire pairs all connected to the same feedpoint — three inverted-Vs stacked from the same apex
- Interaction: the wires interact slightly when in close proximity — space them at least 12 inches apart at the ends to minimise coupling and maintain independent resonance on each band
- SWR: each pair is trimmed to resonance independently; the combined SWR on each band is typically slightly higher than a single-band inverted-V due to interaction — usually under 2:1 at resonance
- Advantage over traps: no trap loss on any band; simpler construction; easier to adjust individual bands
- Disadvantage: more wire to manage; the apex must support the weight of multiple wire pairs plus the coax; visual impact is greater
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| SWR high across entire band — no clear minimum | Open circuit in wire leg or coax connection fault | Check DC continuity from centre insulator to each wire end; check coax connector and balun connections | Re-solder any suspect joint; replace coax connector if corroded; verify balun terminal connections |
| Resonance minimum shifted far above target band | Wire legs too short — trimmed too aggressively or measured incorrectly | Measure actual leg length with tape measure; compare to formula result | Splice additional wire to each leg end using a barrel connector or solder splice; re-measure SWR |
| Resonance minimum shifted far below target band | Wire legs too long — common on first builds | Sweep with NanoVNA to find actual resonance frequency; calculate how much to trim | Trim 3 inches per leg at a time; re-raise and re-measure; repeat until resonance is on target |
| SWR at resonance is 3:1 or higher — poor match | Leg angle too steep (below 60°) or missing balun causing pattern distortion | Measure leg angle; check that balun is installed at feedpoint | Increase horizontal span to widen leg angle toward 90–120°; install choke balun at feedpoint if missing |
| RF feedback in shack — buzzing in audio | Common-mode current on coax — no feedpoint balun or inadequate balun | Touch coax connector body — RF tingle confirms common-mode current | Install 1:1 current choke balun at feedpoint; add ferrite choke on coax at shack entry (5 turns through FT-240-31) |
| SWR changes when touching the coax | Coax acting as part of antenna — common-mode current problem | Confirmed by SWR shift when hand approaches or touches coax run | Install feedpoint balun; ensure balun is a current choke type not a voltage balun |
| Antenna resonance shifts significantly with weather | Normal — wire length changes with temperature and humidity | Measure SWR on hot vs cold days to quantify shift | Accept shift if SWR stays below 2:1 across operating range; or re-trim to compromise resonance point |
| Wire end too low — safety concern | Apex too low or anchor too close to mast base | Measure end height above ground | Move anchor farther from mast to increase leg angle and raise the end; or raise the apex if possible |
Is an inverted-V as good as a flat-top dipole?
For most practical installations, yes — the performance difference between an inverted-V and a flat-top dipole at the same apex height is 1–3 dB, which is barely perceptible in actual operation. The inverted-V's ability to use a single support and its slightly lower feedpoint impedance (better match to 50 Ω coax) make it the better practical choice for the majority of amateur installations. A flat-top dipole is theoretically superior for DX because of its flatter, lower-angle radiation pattern, but the real-world advantage is small and usually outweighed by the inverted-V's installation practicality.
Can I use the inverted-V on multiple bands without a tuner?
A single-band inverted-V will present high SWR on all bands except its design band — it requires an ATU for multi-band use from one feedpoint. The exception is bands at harmonic relationships: a 40m inverted-V will show usable SWR on 15m (the third harmonic of 40m) and sometimes 20m. For reliable multi-band no-tuner operation, use a fan inverted-V (multiple wire pairs) or a trap inverted-V (traps at each band boundary). Either approach provides low SWR on multiple bands from a single feedpoint without requiring an ATU.
What is the minimum apex height for a 40m inverted-V?
A 40m inverted-V can be installed with an apex as low as 25 feet, but performance is primarily high-angle (NVIS/regional) at that height. For a useful mix of regional and DX performance, 35 feet is a practical minimum apex height for 40m. At 25 feet the leg angle with a 47-foot horizontal span is approximately 70°, which starts to degrade both the radiation pattern and the feedpoint impedance match. If you are limited to 25 feet, the antenna still works — particularly for regional contacts and digital modes — but raising to 35 feet or higher delivers a meaningful improvement.
Do the wire legs need to be equal length?
Yes — both legs must be equal length for the antenna to be a balanced, centre-fed half-wave dipole. Unequal legs shift the feedpoint impedance and can create an asymmetric radiation pattern with some common-mode current on the coax. If your installation geometry forces one leg to be shorter (an obstacle in the way, for example), trim the longer leg to match the shorter one — you will end up with a shorter overall antenna resonant at a slightly higher frequency, which is acceptable. Alternatively, install the shorter leg vertically downward if space allows, making an L-shaped configuration that still works reasonably well.
Can I build an 80m inverted-V in a small garden?
Yes, with compromises. An 80m inverted-V with a 50-foot apex and a 90° leg angle requires approximately 91 feet of horizontal span. If your garden is shorter, you can steepen the legs — a 60° leg angle requires only 64 feet of span, at the cost of higher-angle radiation and a lower feedpoint impedance around 25–30 Ω, which the ATU can handle. You can also fold the ends of the wire legs horizontally or downward if the ends reach the ground — the folded section reduces the resonant frequency slightly and the SWR will require checking. In very small spaces, a magnetic loop or loaded vertical may be more practical than an 80m wire antenna.
Which direction should I orient the inverted-V?
Orient the antenna so the broadside direction (perpendicular to the wire) points toward your most-wanted contacts. A 40m inverted-V oriented north-south radiates primarily east and west. Maximum radiation is broadside to the wire at low radiation angles, with nulls off the wire ends. For a US station wanting European contacts on 40m, orient the wire north-south so the broadside faces east toward Europe. If DX in multiple directions is desired, a north-south and east-west fan inverted-V combination — two antennas from the same apex — covers all directions with broadside efficiency.