Installing a radial system is a must for any quarter-wave vertical antenna system. Without one, a vertical antenna is only half complete. The radials are the second half of the antenna, just like there are two elements to the common dipole. Radials contribute to the radiation efficiency of the entire vertical antenna system. The worse your ground conductivity, the more important your radial system becomes.
The big question is, How many radial wires are needed and what length should they be? A typical answer from most Hams is, “The more the better and make them as long as you can.” In 1937, the FCC set the “standard” for AM broadcast antenna radials at 120, each ½λ long at the working frequency. The traditional belief is the more total wire installed, the better the performance, especially with poor ground conditions.
For some reason, it became the norm for amateurs to use ¼λ radials at the antenna’s lowest frequency. But when buried or placed directly on the ground, radials couple into the earth so they don’t actually need to be resonant at all—and often their length is not overly critical.
From the Ground Up
Antenna manufacturers vary widely on their ground radial recommendations. For example, Hustler suggests you install two insulated wire 1/4λ radials for each band used on its BTV series vertical antennas. The same number is suggested for roof mounting. Butternut recommends a minimum of 30-60 radials at 65 feet each. Hy-Gain suggests numbers based on a chart similar to Table 1 below. It’s obvious there isn’t a consensus among vertical antenna makers.
If you need a number to start, twenty 32-foot radials will give you a workable system with most vertical antennas. You might consider 65-foot radials if you use the low HF bands. As we increase to 32 radials, improvement continues. As we move toward 64 radials, diminishing returns kick in and the improvement gets progressively smaller.
But if you dig into radial research over the last few decades, you’ll find there’s a tradeoff between the number of radials and their optimum length. John Stanley, K4ERO, wrote an article in QST a number of years ago to answer that question. Summarizing data contained in “Radio Broadcast Ground Systems,” Stanley compiled the following table, which appears in The ARRL Antenna Book, 22nd Edition.
Table 1: Six Possible Configurations for Ground Radials (Source: The ARRL Antenna Book)
The figures reflect results over average soil conditions–6-8 mS/m
Length of Radials 0.4λ 0.25λ 0.2λ 0.15λ 0.125λ 0.1λ
Number of Radials 120 90 60 36 24 16
Impedance 35Ω 37Ω 40Ω 43Ω 46Ω 52Ω
Low-Angle Loss 0 0.5dB 1dB 1.5dB 2dB 3dB
This table provides us with information about the “how many and how long” relationship, though the findings may not be what you expected. For example, the fewer radials you put down, the shorter they can be—notice the 16 radials on the chart at 0.1λ. That’s not suggesting a few short radials will work as well as many longer ones (they won’t). But it does mean that if you have a limited amount of wire for making radials, you can maximize your results by finding a balance between length and number of wires.
What if you already had sixteen 1/4λ long radials on the ground? Interpolating between the numbers in the table, you could expect to improve your low-angle loss by about 1.25dB if you decided to substitute 30 radials, each 0.125λ long. In this case, you are using the same total length of wire in less space.
As for placement, should radials be buried in shallow ground or placed on top of the ground? It really doesn’t make a difference, and the easiest solution is to lay the radials on top of the ground. Cut the lawn very short (about 1 inch), push the radials down on top of the grass, holding them in place with landscaping staples, then allow the grass to grow over them. Mow a bit higher than usual to avoid damaging the wire. After a month or two, the wires will disappear into the turf.
Elevated Radials
When mounting a vertical antenna on a pole, roof or tower, elevated radials are the solution. Elevated radials need to be made resonant at 1/4λ. They also need to be high enough (eight or more feet) to prevent people from clotheslining themselves on the wires or coming in contact with high voltage when you’re transmitting. Four resonant 1/4λ radials spaced at 90 degrees apart will provide a low-loss ground plane for a monopole vertical antenna, nearly equaling the performance from a 1/4 wave monopole antenna at ground level with 120 buried radials.
For multi-band antennas like the Butternut HF or Hustler’s BTV series, add four resonant 1/4λ radials for each band. You can share one radial for 40/15M, since 15M is nearly a 3rd harmonic of 40M. That would mean 12 radial wires, each with a tie-off point. If installation space is limited, you can cut that number in half by running two radials per band, 180 degrees apart. Worst case would be one radial per band evenly spread around the radiator. It works, but you limit the omnidirectional pattern, producing a slight null behind the radial.
In the Butternut vertical antenna manuals, they have a slick solution for a minimal number of radials on a four-band vertical. As shown in Figure 1, a piece of 300Ω twinlead is cut to length and notched to be resonant on a combination of bands. Four of these would take the place of the 12 individual wires mentioned earlier.
Figure 1. Multiband Twinlead Elevated Radial
When it comes to radials, here are some points to remember:
- There is little difference in performance between buried radials and radials laid directly on the ground.
- The efficiency of the antenna system relies less on soil conductivity as the number of radials increases—something we can control.
- Ground radials do not need to be resonant.
- The shorter your antenna, the more you need radials.
- Elevated radials should be electrically 1/4λ long.
- Four elevated radials can perform as well as an extensive ground radial system, but a greater number is likely to work even better.
- For a given length of wire, choose more short radials over fewer long ones. There is a relationship between the two and an optimum number-to-length ratio.
- Bare or insulated copper radials are the best choice, with insulated wire being more durable. Using thick wire doesn’t change ground loss enough to justify the higher cost.
- If you have limited space for ground radials, put down as many as you can wherever they’ll fit.
