Antenna Physics in the Field

The image antenna: the half of every vertical you cannot see

Short answer: every vertical antenna works together with a mirror image of itself in the ground. A Greyline is built to work with that partner, and the 4' Image Lift makes a good arrangement a little better. The textbooks call it the image. Almost nobody on the air talks about it, and it is a satisfying thing to understand. Here is the idea, where it comes from, and what to do with it.

What the masters say

Open John Kraus's Antennas to the chapter on antennas above ground and you find the method of images: a vertical conductor over a conducting plane behaves exactly as if a second, mirror-image conductor stood below the surface, carrying current in the same direction. The ground plane can be replaced by that image and the field above ground does not change. It is the reason a quarter-wave vertical over ground looks like a half-wave dipole — the other half is the image, and the image does real work.

Robert Zavrel, W7SX, carries the same picture into Antenna Physics: An Introduction, and adds the practical note: real soil is a less perfect mirror than the textbook plane, and how well it reflects varies with moisture and frequency. The image is still your partner; it is just a partner whose quality depends on the ground you were given.

The ARRL Antenna Book puts numbers on it in its chapters on ground effects and vertical antennas: the ground within a few wavelengths helps shape the low-angle pattern that DX lives in, and the relationship between the antenna and that ground is one of the levers a builder can pull.

Three authorities, one picture: a vertical and its image are a team, and the ground is the referee.

What that means for a Greyline

A Greyline is a vertical dipole: both halves of the radiator are above ground, fed at the insulator about four feet up. The antenna carries its own return current, which is why it needs no radial field. Radials are the traditional way to help the ground be a better mirror; a vertical dipole brings its own second half and lets the image be a bonus rather than a requirement. That is the design, and it is why owners on sand, rock, clay, rooftops and patios all report the same antenna.

Height above ground is simply one more lever. On 40, 80 and 160 meters the antenna's near field reaches down to the surface, and raising the feedpoint a few feet gives the low bands a little more room to work. It is a refinement, the kind a contester reaches for after everything else is right.

The 4' Image Lift

That is the idea behind installing the 4' Extension below the feedpoint instead of above it. The lower half of the dipole grows from four feet to eight, the feedpoint and everything above it rise four feet, and the upper radiator stays exactly as long as you built it. Nothing about the top of the antenna changes, so the bands it is strongest on do not change. You simply give the whole antenna a little more sky.

The upper radiator is the length we protect on 28-footers and down: up to about five-eighths of a wave on the highest band you want to keep, because beyond that the pattern starts to split and that band gets less useful. Putting the four feet underneath keeps every band you have. A 33 up with 8 down is 41 feet of antenna from a 33-foot footprint.

Why nobody talks about it

Because the radial-fed vertical turned the image into a chore: bury wire, and forget the mirror exists. A vertical dipole turns it back into physics you can see and use. That is worth a QSO.

Sources on the Bookshelf: Kraus, Antennas (method of images, antennas above ground); Zavrel, Antenna Physics: An Introduction; the ARRL Antenna Book (ground effects; vertical antennas). Related: how a vertical dipole works · how height pays.

Ham Radio is fun again! Pass it on... 73, Jon KL2A