Understanding Shortwave Radio Listening
and Antenna Design and Construction

Copyright © 1999 by Harold Melton KV5R. All Rights Reserved.

Understanding Shortwave Antennas: Page 9

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Continued from: Page 8

Directivity

As previously stated, a ½ wave antenna radiates off of the sides, perpendicular to the wire. Longwire antennas radiate toward the far end of the wire. Let's look at what happens to the directivity pattern of our 94-foot wire.

As we dial up the frequency, the pattern of the antenna changes. The following diagrams show relative signal strength, looking down from the top. As the frequency goes up, the two lobes split into 4, then get stronger toward the far end of the wire. The patterns are like doughnuts circling the wire - thus, the patterns extend upward as well as long the ground.

In each case, the wire is fed from the left end. Increasing the frequency has the same effect on the pattern as lengthening the wire. The two side lobes squash and divide into four (at 1 wave), then the directivity shifts toward the far end of the wire. They then come together as one long lobe. This is the primary negative design characteristic of longwire antennas.

You should keep these patterns in mind when stringing up a single-wire antenna, so that, at your favorite frequencies, a lobe is pointing toward the right part of the world. Other (slightly more complex) antenna designs avoid this problem of changing directivity by using several antenna elements of different lengths, as we shall see on the following page.

It's important to note that the height of the antenna above ground also affects the pattern. Lower antennas have higher radiation angles - thus, more energy is wasted into the sky. It would be nice to be able to get our antenna ½ wave above ground at the lowest operating frequency. This would mean that our 100-foot long antenna should be 100 feet high, but alas, this is hardly practical -- unless you have some old pine or redweed trees on your property. The general rule of antenna height is: higher = better. Surprisingly, however, even the top wire of a fence will do quite well.

You can bend the antenna wire around corners, but it's better if you do not. Try to keep most of the wire in a straight line.

Continued on: Page 10