Verticals and Radials

The vertical is the antenna for people who are short on trees, short on space or chasing distant stations. It takes up little room, it radiates in every direction around it, and it sends its signal out at the low angles that carry it a long way.

Some hams swear by verticals and others swear at them. Usually the difference is what lies underneath.

Half an Antenna

Stand a half-wave dipole on end and remove the bottom half: that is a quarter-wave vertical. The missing half has not gone away; the ground, or a set of wires called radials, now has to play its part. Over a perfect ground, the feed point of a quarter-wave vertical is around 36 ohms, roughly half of a dipole’s. Our Antenna Length Calculator gives a starting length for the radiator.

Real ground is far from perfect. RF current flowing back to the base of the antenna has to travel through soil, and soil is a poor conductor that turns some of that current into heat. Antenna efficiency is the radiation resistance divided by the total resistance, so every ohm of ground loss is power that warms the dirt instead of reaching the other station. How much you lose depends mostly on how well your soil conducts.

A ground rod does not fix this. It is important for lightning and electrical safety, but it touches only a small patch of earth. Radials spread out across the surface and give the return current a low-loss road home.

Radials on the Ground

For a vertical mounted at ground level, the radials lie on the surface or just under it. Because they are so closely coupled to the soil, their exact length does not matter; there is no such thing as a resonant radial lying in the grass. What matters is covering the ground around the antenna with wire.

• More radials are better, and if space forces them to be short, use more of them.
• Each new radial helps a little less than the one before, so the first ones you lay make the biggest difference.
• Just a handful of radials on the ground leaves a lot of loss in place.
• Inexpensive insulated wire held down with garden staples does the job and soon settles into the lawn.

The classic reference is a 1937 broadcast study in the Proceedings of the IRE, which recommended about 120 buried radials. You do not need that many, but it shows the direction to work in.

Elevated Radials

Raise the feed point off the ground, on a short mast, a roof or a deck, and the picture changes. Now the radials are tuned elements that need to be close to a quarter wave long for the band. Careful amateur measurements have shown that four elevated quarter-wave radials can perform about as well as a large radial field on the ground, with far less wire. Two radials can also work, though the pattern may be a little lopsided.

Elevated radials are fussier, though. With so few of them, the current needs to split evenly, so keep them symmetrical and at a similar height, and put a common-mode choke on the coax at the feed point. Sloping the radials downward raises the feed point impedance, which you can use to get a better match to 50 ohms.

The tips of tuned radials carry high RF voltage. Keep them above head height or out of reach, and never where someone could walk into one.

Low Angles, DX and Noise

A vertical with a good ground system puts much of its signal out at low angles toward the horizon. On HF, a low takeoff angle means a long first hop, which is exactly what you want for DX. A vertical near salt water does even better, because seawater increases radiation at low angles.

The flip side is that a vertical does little at high angles, so it is a poor choice for contacts within a few hundred miles on 80 and 40 meters. For those, a low horizontal wire is the right tool; see NVIS: Working the Next 300 Miles.

Verticals also tend to hear more local noise than horizontal antennas, especially from power lines and household electronics. Put the vertical as far from the house as practical. On the low bands, many vertical users add a separate antenna just for listening; see Receive-Only Antennas.

Multiband Verticals and Traps

Many commercial verticals cover several bands with traps. A trap is a coil and capacitor in parallel, tuned to one band. At that frequency it acts like an insulator and cuts off the tubing above it, so the antenna looks like a quarter wave on each band in turn. Traps add a little loss and narrow the bandwidth, but they make one radiator do the work of several.

Shortened verticals use loading coils or a capacity hat instead of full height. A loading coil near the middle of the radiator is more efficient than one at the base, and top loading also improves efficiency. Any shortened antenna has less SWR bandwidth than a full-size one. Whatever the design, it still needs radials, and on the ground they should spread out far enough to suit the lowest band you use.

The 5/8-Wave on VHF

On 2 meters and 70 centimeters, a vertical 5/8 of a wavelength tall has more gain toward the horizon than a quarter wave, which is why it is a favorite for mobile and base stations. It needs a small matching coil or network at the base to present 50 ohms. Going longer does not keep helping: past 5/8 wave, more of the signal heads upward. In How Do Long Dual Band Verticals Develop Gain?, Bill Rinker, W6OAV, explains why, and how longer commercial antennas get around it.

Putting One Up

1. Pick a spot clear of the house and well away from power lines. If the vertical fell, it must not be able to reach a line.
2. Contact 811 a few business days before you drive a ground rod or bury anything, so buried utilities get marked.
3. Lay as many ground radials as you can, or raise the feed point and use two to four tuned radials.
4. A ground-mounted vertical puts the radiator within reach. Evaluate RF exposure as Part 97.13 requires, and fence it or keep people back if needed.
5. Ground the base and the coax for lightning protection; see Grounding and Lightning Protection.

Then keep adding radials. A vertical that disappoints on day one often improves with every weekend of laying wire.