Wednesday, July 28, 2010

Laying Down Radials

The key to effective ground-mounted vertical antenna is radials. While conventional AM broadcast practice is to use 120 1/2 wave radials, you don't need nearly that many to have an effective antenna. My shunt-fed 15m tall tower works just fine with 30 radials. None of these radials approaches 1/2 wave on 160m. Most are slightly less than 1/4 wave on 80m.

Even if they aren't very long, and don't come close to a hundred, it's a lot of wire to lay down. My vertical has about 2,000 feet of wire down today, and I'm looking to add about 1,000 more before the fall. It helps to have a system for laying them down.

The technique is very simple. The wire is laid on the ground, and the grass grows over it. After a few months, you will be hard-pressed to find the wire, or pull it up, even if you wanted to.

Start off by mowing the grass. You want this cut to be as short as you can make it without hurting the grass. Shorter grass will also make it a little easier to lay the wire.

Copper wire is expensive today, but is most compatible with the soil in most areas of the country. You don't need a heavy gauge. I started with some scrap 12 gauge cotton-covered wire I had. Insulation doesn't matter, and the wire should last longer if insulated. I would recommend 12 or 14 gauge THHN insulated solid house wiring, since it can be bought inexpensively at most home improvement stores. I would not suggest using anything smaller than 16 gauge, as the wire has got to take some abuse being on the ground. Hunt around at hamfests and offer to buy any spool of several hundred feet of wire for a buck or two.

At the base of the antenna, there must be some way to connect the radials. A few companies make expensive jigs for this purpose. I used a piece of 4 gauge copper wire, clamped to the tower legs. I put strips of stainless steel between the wire and the tower legs, to avoid dissimilar metals corrosion of the zinc plating. I then solder the radials to the wire ring using a 240 watt soldering gun. A little work with fine sandpaper makes this job go faster, as the copper wire has a thick layer of oxidation from being outside.

Once you've connected the radial to the base of the vertical, simply lay it out along the ground. As the ground is never perfectly flat, don't try to stretch the wire. Remember, we want to be able to mow over the wire. Stretching it will make it pop up in the low places along the ground.

To keep the wire close to the ground, so the grass will grow over it, I use a series of short clips. These are 3-3 1/2" pieces of 14-12 gauge wire bent into a U. You just fit them over the wire and press into the ground. You don't want to make them much more than 3 1/2", because they will bend when you try to push them in. You can generally do this by hand, but after putting in a few dozen clips, it will tend to hurt your thumb. I try to grab the clips with a pair of lineman's pliers and push them into the ground.

I generally walk along the wire heading away from the antenna, pushing the radial down with my foot and then adding a clip. If the soil is really dry, you may have trouble inserting the clips. In that case, you can wait for a good rain, or gently water the path of the radial. You'll need about one clip every 3-6 feet, so that's a few hundred clips if you are laying down 1,000 feet of wire. I built a little jig to make the bending process easier and more consistent.

For the next several days, it is good to inspect the radials to make sure kids or critters haven't pulled a wire up. I suggest not mowing for a few weeks. When you do, use the highest setting of the mower until the wire is clearly captured by the grass.

Radials definitely improve the performance of a vertical. Put down as much wire as you can, and you'll be pleased with the results.


  1. This is exactly what I'll be doing this weekend! Hoping to eventually feed my feedline & dipole as a T on 160.
    John AE5X

  2. Kids who REALLY want to pull them up WILL pull them up. Believe me.