Inverted V Antenna: Height, Angle and Length for HF
A half-wave dipole is the classic first antenna, but a straight horizontal one needs two supports at the right height, and most gardens have only one tree or mast that is tall enough. The inverted V antenna solves that by hanging the dipole from a single high point, so that the two legs slope down on either side. The result is the same half-wave wire, fed in the centre, with a bend in the middle. It is cheaper, easier to raise and usually more forgiving than a flat dipole, and it has a handy side effect: its feed impedance is close to the 50 ohms that your coax and transceiver expect. This guide explains how it works, how the apex height and the angle between the legs change its behaviour, and how to work out the sizes for your own site.
What an Inverted V Is
An inverted V is a balanced antenna with two symmetric elements angled towards the ground. In the words of the general description of the dipole antenna, it is a half-wave dipole with a bend in the middle. Like a sloper, it has the practical advantage of elevating the antenna while needing only a single support. The feed point is at the apex, where the two halves meet, and the far ends are supported low down by rope, usually tied to stakes or fence posts.
The antenna keeps the main properties of a dipole. It is resonant when each leg is about a quarter of a wavelength long, it has a figure-of-eight pattern in the horizontal plane, and it does not need a ground system. If you are new to dipoles, our guide to a first HF antenna compares the basic types and explains why the dipole is such a good starting point.
Why It Feeds Directly With 50 Ohm Coax
A half-wave dipole in free space has a feed impedance of about 73 ohms at its centre. That is a reasonable match to 50 ohm coax, with a standing wave ratio of roughly 1.5 to 1 at resonance. When you bend the two legs downwards, the impedance at the feed point falls. At an included angle of around 120 degrees between the legs the impedance is close to 50 ohms, and with a narrower angle it falls lower still. That is a happy accident, because a modest drop in impedance brings the antenna closer to the match you want, and in practice a well-built inverted V often gives a very low standing wave ratio at the centre of a band without any tuner.
Real installations are less tidy than the textbook. The height above ground, nearby trees and buildings, wire insulation and the sag of the legs all change the impedance, so measure rather than assume. A cheap analyser makes this simple, and our article on the NanoVNA antenna analyser walks through the measurement.
Height, Angle and Pattern
Two choices shape the antenna: how high the apex is and how wide the angle between the legs is. They interact, because for a given length of wire a higher apex with a wider angle keeps the ends higher above the ground.
- Apex height. The higher the apex, the more of the signal goes out at low angles and the less is absorbed by the ground. As with any horizontal antenna, height in wavelengths matters. Our article on antenna height and takeoff angle explains why, and why a lower antenna favours shorter distances.
- Included angle. A wider angle makes the antenna behave more like a flat dipole, with a clean horizontal pattern and a feed impedance a little above 50 ohms. A narrower angle brings the legs closer together, lowers the impedance, narrows the bandwidth and reduces the radiation, because the currents in the two legs partly cancel. Most builders aim for an included angle between about 90 and 120 degrees, and avoid going much narrower.
- End height. The tips of a dipole carry high voltage, so the ends should be well out of reach. Keep them above head height with a comfortable margin, and keep people and animals away from them while you transmit.
The pattern of an inverted V is a little more rounded than that of a flat dipole. The sloping legs add a vertical component to the radiation, which fills in the nulls off the ends of the wire. For general DX work and for casual contacts in all directions, that is an advantage, and it is one reason the inverted V is a popular antenna for portable and field operation.
Working Out the Sizes
The usual starting length for a half-wave dipole in feet is 468 divided by the frequency in megahertz, or about 142.5 divided by the frequency in metres. For 7.1 MHz on 40 metres that gives about 65.9 feet, or about 20.1 metres of wire in total, so each leg is about 10 metres. Because the antenna droops, the nearby ground and the wire insulation shift the resonance, so treat this as a starting point and plan to trim. Our article on velocity factor explains why practical wire lengths differ from the free-space figures.
To see how angle changes the geometry, take two 10-metre legs and an apex at 12 metres. The span between the ends is twice the leg length times the sine of half the included angle, and the end height is the apex height minus the leg length times the cosine of half the angle.
| Included angle | Span between ends | End height above ground | What it means |
|---|---|---|---|
| 120 degrees | about 17.3 m | about 7.0 m | Wide, high ends; impedance near 50 ohms; closest to a flat dipole |
| 100 degrees | about 15.3 m | about 5.6 m | A good compromise when the ground area is limited |
| 90 degrees | about 14.1 m | about 4.9 m | Compact; lower impedance and reduced bandwidth; ends are lower |
Rounded figures for a pair of 10 metre legs on an apex at 12 metres. A narrower angle needs less ground but brings the ends nearer to the ground and to people. If your mast is shorter than 12 metres, the end heights drop by the same amount, and at some point you must widen the angle or accept shorter legs and a different band.
Supports, Wire and Insulators
A single support makes the construction easy, but it must be strong enough for the wire and the feedline. The apex carries the weight of the centre insulator, the feedline and a choke if you fit one, and the tension from the legs pulls it sideways.
- The mast. A wooden pole, a fibreglass mast or a tall tree works well. If you use a metal mast, keep the antenna’s centre clear of it where possible, because metal close to the feed point detunes the antenna and distorts the pattern. A non-conductive section at the top is a good solution.
- The wire. Use stranded copper or copper-coated steel wire of a gauge strong enough to survive wind and the weight of ice in your climate. Insulated wire is longer-lived and quieter in rain static, but it resonates lower than bare wire, so cut it a little shorter.
- The centre insulator. Use a purpose-made centre insulator with a socket for the coax, and relieve the strain on the cable. Support the feedline so that its weight does not pull on the solder joints.
- The end insulators and rope. Use proper end insulators and ultraviolet-resistant rope. Tie the rope off to a stake or post that cannot be mistaken for a trip hazard, and mark the rope in daylight so people can see it. Consider a spring or an elastic section to take up movement in the wind.
Keep the legs symmetrical. If one leg is shorter or lower than the other, the antenna becomes unbalanced, and current flows on the outside of the coax, which can cause RF problems in the shack and a distorted pattern.
Feedline, Balun and Choke
A dipole is a balanced antenna, and coax is unbalanced. At the junction, some of the signal can flow back along the outside of the shield, which spoils the pattern and brings RF into the shack. A simple current choke or a one-to-one balun at the feed point reduces that current, and it is a cheap improvement for almost any dipole. Our guide to baluns and ununs explains how to choose one. Let the coax leave the antenna at right angles for a short distance, if you can, and avoid running it parallel and close to one of the legs.
Beyond a Single Band
A single inverted V works on its design band and, with a tuner, can be used on a few other bands, though its efficiency and pattern change. Two simple ways to cover more bands are common.
- A fan of inverted Vs. Several wires of different lengths share one feed point and one mast. Each wire is resonant on its own band, and the others have little effect. It takes more rope and more care to keep the wires apart, but it is simple to build and tune.
- Trap or loaded versions. Traps in each leg make one wire behave as different lengths on different bands. See our guide to the trap dipole antenna for how that works and what it costs in efficiency.
On the lower bands, a low inverted V also makes a useful regional antenna. When the apex is low and the angle is wide, much of the radiation goes steeply upward and returns close to the station, which is the principle behind near vertical incidence skywave propagation. Our article on NVIS propagation explains how to use that for regional contacts.
Putting It Up Safely
Raising a wire antenna looks easy, and that is when people get hurt. Before you throw a line over a branch or lift a mast, look up and check for power lines in every direction, including those that might swing into reach if the mast fell. Keep the whole structure, including the guy ropes, at least the full height of the mast away from any overhead wire, and never use a metal pole or a wet rope anywhere near one. Raise the mast with a helper, in calm weather, and do not climb a tree you have not tested. Plan the route of the feedline so that it does not cross a path at head height, and fit a lightning arrestor or a way to disconnect the coax during storms. Our guide to station grounding and lightning protection covers the permanent arrangements. A few minutes of preparation at this stage costs nothing and removes the biggest risk in the whole project.
Common Mistakes
- Cutting to the formula and walking away. The real resonance depends on height, droop and surroundings. Measure and trim.
- Making the angle too narrow. The antenna loses bandwidth and efficiency, and the ends come close to the ground.
- Letting the ends hang at head height. The tips carry high voltage. Keep them out of reach.
- Ignoring the choke. RF on the coax causes interference and pattern distortion, and a simple choke is cheap.
- Hanging the antenna on a metal mast without clearance. Metal close to the feed point detunes the antenna.
- Forgetting a strain relief. A heavy cable can break the connection at the centre insulator.
The Bottom Line
The inverted V antenna is a half-wave dipole that needs only one high support, and its feed impedance is conveniently close to 50 ohms. Keep the apex as high as you can, keep the included angle between about 90 and 120 degrees, keep the ends well out of reach, add a choke at the feed point and trim the wire against a measurement. With those few habits, a single wire hung from a single tree can give years of reliable HF operating.
Frequently Asked Questions
Is an inverted V better than a flat dipole?
It is easier to put up, because it needs only one high support, and its impedance is closer to 50 ohms. A flat dipole at the same height has slightly more gain broadside to the wire. In most gardens the inverted V is the more practical choice.
What angle should the legs make?
Most builders aim for an included angle between about 90 and 120 degrees. A wider angle behaves more like a flat dipole, and a narrower one lowers impedance, narrows bandwidth and reduces radiation.
Do I need a tuner with an inverted V?
Not on its design band if it is well built, because the impedance is close to 50 ohms and the antenna can give a low standing wave ratio across much of the band. On other bands, a tuner or a different antenna is needed.
How high should the ends be?
High enough that nobody can touch them while you transmit, because the tips carry high voltage. Keep them well above head height and keep people and animals away from them when you operate.
Can I use a metal mast?
You can, but metal close to the feed point detunes the antenna and distorts its pattern. Keep the centre clear of the mast where possible, or use a non-conductive section at the top.
Can an inverted V work on more than one band?
Yes. A fan of several wires from one feed point gives separate resonances on different bands, and trap versions use one wire for several bands. A single wire can also work on other bands with a tuner, though with changes in pattern and efficiency.