Quarter-Wave Vertical Antenna Calculator

A quarter-wave vertical is half a dipole standing on end, with the missing half supplied by a ground plane. That single sentence explains everything about it: why the radiator is exactly the length of one dipole leg, why the feedpoint impedance is half a dipole's, and why the radial field is not an accessory but the other half of the antenna.

Why 234, and why it is the same number as a dipole leg

A half-wave dipole is 468 divided by frequency in feet, and each of its two legs is 234. A quarter-wave vertical is one of those legs working against a ground plane, and the ground plane supplies an electrical mirror image of the second leg. The antenna is therefore a dipole in behaviour and half a dipole in hardware, which is the whole reason it exists: on 40 metres it needs 33 feet of vertical support instead of 66 feet of horizontal span.

The same end-effect correction applies. A quarter wavelength in free space is 246 divided by frequency, and real conductors resonate as though about five percent longer than they are, which gives 234. Thick tubing is worse than thin wire in this respect, so a commercial aluminium vertical typically ends up two to four percent shorter than the formula, and every one of them ships with a tuning chart for that reason.

The radial field is the antenna, not an accessory

This is where verticals earn their reputation for being poor performers, and the reputation is entirely deserved when the ground system is skipped. Current flowing out of the radiator has to return, and it returns through whatever conducts. If that is soil, the return current dissipates as heat in the ground and never radiates. Soil resistivity is not a small effect: a vertical over average ground with a token radial field can throw away half its power or more.

There are two entirely different strategies, and they follow different rules.

Elevated radials: few, resonant, and very effective

Raise the feedpoint at least eight feet, run four radials cut to the same 234 divided by frequency length, and slope them down at about 45 degrees. Four is genuinely enough, and the fifth and sixth add almost nothing measurable. Because they are elevated and resonant, they form the counterpoise directly rather than shielding a lossy ground, so the soil beneath barely matters.

Elevated radials also raise the feedpoint impedance towards 50 ohms as the droop angle increases, which is a convenient side effect: sloping them at 45 degrees typically lands close enough to 50 ohms that no matching network is needed at all. The downside is mechanical, since four sloping wires at head height need anchoring and become a hazard in a garden people walk through.

Ground radials: many, short, and about quantity

On-ground radials work as a shield rather than a counterpoise. They intercept the return currents before those currents enter the soil, so the useful measure is how much of the ground surface near the antenna is covered by wire, not whether any individual wire is resonant.

Note the trap in the SWR reading. Ground loss appears at the feedpoint as resistance, and it adds to the roughly 36 ohms of radiation resistance. A vertical with a terrible radial field can therefore present a beautiful 50 ohm match while radiating half its power into the lawn. A good SWR on a vertical is not evidence of a good antenna. The radial planner works through the wire budget for each option.

What you need to build one

• The radiator. A Gabil GRA-7350T telescopic HF whip$139.99 telescopic whip covers 3.5 to 50 MHz in a coil-tapped package that packs to 16 inches, which is the portable answer. For a permanent installation, aluminium tubing or a wire supported by a fibreglass mast is cheaper and better.
• Radial wire. Buy a spool. A Paladin 14 AWG stranded copper wire, 500 ft$74.50 gives you 500 feet, which is 32 eighth-wave radials on 20 metres with plenty spare. For a pre-cut portable set, a Super Antenna MR4010 radial set$29.99 saves the measuring.
• A feedpoint and a choke. Coax braid connects to the radials, centre to the radiator. Without a common-mode choke the coax shield becomes an unintended radial and the pattern goes with it. Twelve turns on an FT-240-31 ferrite toroid core$11.67 at the feedpoint fixes it.
• Coax. MOOKEERF RG-213 coax, 100 ft with UHF male$115.99 for a permanent HF run, or JEFA Tech 240-series flex coax, 50 ft with PL-259$49.99 for anything under about 50 feet.
• A support. An Easy Up 20 ft telescoping push-up mast$185.00 handles a telescoping whip or a wire vertical up to 20 feet. Guy it, and read the power line warning above before you pick the spot.
• Something to measure with. A NanoVNA H4 vector network analyzer$89.90 shows where the dip actually is, which matters more on a vertical than a dipole because the ground system moves it.

Ready-made verticals, and what they trade away

A commercial multiband vertical solves the mechanical problem and usually costs efficiency somewhere. The Comet CHA-250B broadband vertical$489.88 covers 3.5 to 57 MHz with no tuner and no radials, which is genuinely convenient and is achieved with a broadband matching network that absorbs power on the low bands. It is honest about that, and it is the right antenna for someone whose constraint is a homeowner association rather than decibels.

A trap vertical such as the Hustler 4-BTV trap verticalnot on amazon is resonant on each band it covers rather than broadbanded, which makes it more efficient and means it does need radials. The DX Commander Classic multiband verticalnot on amazon takes a third approach, hanging separate resonant wires for each band from one telescoping fibreglass pole, and it is sold direct by its maker rather than through the usual retail channel. Full field on HF verticals compared.

Where a vertical wins and where it loses

It wins on takeoff angle from a small lot. A vertical over a decent ground system radiates most strongly at 15 to 25 degrees above the horizon, which is the angle that reaches another continent. A horizontal dipole only does that at half a wavelength above ground, which is 35 feet on 20 metres and 69 feet on 40 metres. On 40 and 80 metres the vertical is often the only realistic low-angle antenna on a normal suburban lot.

It loses on receive noise. A vertical is omnidirectional and picks up man-made noise arriving vertically polarised from every direction, which describes almost all domestic electrical interference. Operators with both antennas routinely transmit on the vertical and listen on the dipole, and the difference on a noisy suburban lot can be two or three S units.

It loses on simplicity. A dipole is two wires and an insulator. A vertical done properly is a radiator, a ground system, a feedpoint assembly, a choke and a support, and the ground system is the part people skip. The full comparison with real numbers is on vertical versus dipole for HF.