Antenna Height and Takeoff Angle Calculator

Antenna length decides what frequency an antenna is resonant on. Antenna height decides where the energy goes, and it is the factor that most often disappoints someone who cut the wire perfectly and still cannot work anyone far away. The number that matters is not height in feet, it is height in wavelengths, which is why the same 35 foot support is generous on 20 metres and inadequate on 40.

Why height sets the angle

A horizontal antenna over ground does not radiate into free space. Part of its energy goes upward directly and part goes downward, reflects off the ground and comes back up. Those two waves add together, and whether they reinforce or cancel at a given elevation angle depends on the extra path length the reflected wave travelled, which depends on height.

At a quarter wavelength above ground the reflected wave arrives back in phase with the direct wave straight up and out of phase at low angles, so the pattern is a single lobe pointing at the sky. At half a wavelength the reinforcement moves down to about 30 degrees. At a full wavelength it moves to about 15 degrees, with a second lobe higher up. Ground quality changes the depth of the nulls but barely moves the angles.

This is why the answer to "how high should my antenna be" is always in wavelengths and never in feet, and why an antenna that is a DX antenna on 20 metres is a regional antenna on 40 metres from the same support.

Height by band, in feet

Read the last column honestly, because it explains most of the antenna decisions people make. On 20 metres a half wavelength is 35 feet, which a tall tree or a modest mast reaches. On 40 metres it is 69 feet, which most domestic lots do not have. On 80 metres it is 138 feet, which effectively nobody has.

That single fact drives two conclusions. First, 20 metres is the band most people work the world on from a normal lot, because it is the lowest band where a realistic support produces a DX antenna. Second, on 40 and 80 metres the low-angle answer is usually a vertical rather than a horizontal wire, because a vertical's pattern does not depend on height at all.

The low dipole is not a failure

An 80 metre dipole at 35 feet is one tenth of a wavelength up and radiates almost entirely upward. It is a poor DX antenna and an excellent near-vertical incidence skywave antenna, and those are two different jobs rather than a good and a bad version of one job.

Near-vertical incidence skywave, usually shortened to NVIS, sends signals almost straight up so they refract back down over a circle a few hundred miles across with no skip zone. That is precisely what a state emergency net, a regional traffic net or a group of friends within a couple of hundred miles wants, and a high DX antenna is measurably worse at it because its energy goes over their heads.

Inverted V and sloping antennas

Almost nobody builds a flat top, because that needs two supports of equal height in the right places. The inverted V, with one central support and the ends coming down to fence posts, behaves as though it were mounted at roughly two thirds of its apex height. A V with its apex at 40 feet is therefore closer to a flat top at 27 feet, which matters when you are counting wavelengths.

Keep the included angle at the apex above about 90 degrees. Below that the two legs begin to cancel each other, the feedpoint impedance falls, and efficiency goes with it. A V that droops to 60 degrees is a compromise antenna pretending to be a dipole.

A sloper, one end high and one end anchored low, favours the downhill direction and picks up a vertical polarisation component that lowers the takeoff angle usefully. It is the most common shape on a lot with exactly one tall tree, and it works better than its reputation.

Verticals ignore all of this

A ground-mounted quarter-wave vertical radiates at a low angle from the moment it goes up, because its pattern is determined by the ground system rather than by height above ground. On 40 and 80 metres, where a half-wavelength support is out of reach, that is the entire argument for a vertical and it is a strong one.

The tradeoff is efficiency and noise. A vertical needs a radial field or it warms the soil, and it hears vertically polarised man-made noise from every direction. Operators with both frequently transmit on the vertical and listen on the dipole. The numbers are on the vertical calculator and vertical versus dipole for HF.

VHF and UHF: height is the whole story

Above 50 MHz, takeoff angle stops being the question and obstruction clearance becomes it. Signals travel a little past the visual horizon because of atmospheric refraction, and the usual approximation is that radio horizon in miles is about 1.42 times the square root of height in feet.

Thirty feet gives roughly 7.8 miles, sixty feet roughly 11 miles, and a repeater on a 1000 foot hill sees roughly 45 miles. That last figure is why a 5 watt handheld reaches fifty miles through a repeater and half a mile simplex: the repeater has the height, and the height is doing all the work. Run the numbers for your own case on the VHF range calculator.

Getting an antenna higher, safely

• A tree is the cheapest tall support there is. A Forester arborist throw line kit$21.99 puts a line over a branch fifty feet up from ground level, which is both safer and far more accurate than a ladder. Use QNR 3/16 in polyester antenna rope, 500 ft$49.73 in UV-stable polyester and leave a weight or a spring on the end so the tree can move in wind without breaking the wire.
• A push-up mast handles VHF verticals and light wire supports. An Easy Up 20 ft telescoping push-up mast$185.00 reaches 20 feet and must be guyed at full extension with something like a Threlaco 3-way galvanized guy wire kit, 50 ft$46.99.
• A roof tripod gets a vertical above the roofline. A Skywalker heavy duty roof tripod with mast$60.99 or a Skywalker J-pipe antenna mount, 38 in$49.99 on a fascia. Flashing and sealing the fixings properly is the entire job.
• A tower is an engineered structure. Permit, designed base, correct guying, and for most people a professional installer. Covered in masts, towers and rotators.

The height that is worth more than an amplifier

Raising a 40 metre dipole from 30 feet to 60 feet moves the main lobe from roughly 60 degrees to roughly 30 degrees. On a DX path that is worth several S units, because the antenna was previously sending its energy where nothing useful was happening. Going from 100 watts to 600 watts, by comparison, is 7.8 decibels, a bit over one S unit, and costs two thousand dollars and a new RF exposure evaluation.

This is the single most reliable trade in amateur radio: rope and height beat watts, almost always, and by a margin that surprises people who have only ever adjusted the power knob. The comparison with real numbers is on QRP versus 100 watts.