Copyright © 1999-2011 by Harold Melton KV5R. All Rights Reserved.
Understanding Shortwave Antennas: Page 10
Outdoor Multi-Wire Shortwave Antennas
The easy way to overcome undesirable directivity shift is to design a multi-band antenna. The simplest multi-band antenna for shortwave listening is the multi-wire. This may be a Zepp (end-fed) or a dipole (center-fed).
The theory is simple: if a ½ wave antenna is best, then we need several of them, at different lengths, tuned to the various shortwave bands. Since the ½ wave element is most efficient, the one whose length is nearest our current frequency will predominate, while the others are relatively inactive. The top wire is the longest, and we use it to suspend the lower, shorter ones.
Note that ½ wave antennas are also resonant at 1½ waves. This is called “third-harmonic” operation. Knowing this, international radio treaty makers long ago placed the shortwave broadcast bands at convenient locations. The result is that we can use 4 wires to pick up 8 shortwave bands with excellent efficiency. Below are the calculations for this antenna (frequencies shown are approximate band centers for the meter bands shown):
|1||3.25 MHz (90 meter)||09.75 MHz (31 meter)||468 ÷ 3.25 = 144′ 0"|
|2||3.95 MHz (75 meter)||11.85 MHz (25 meter)||468 ÷ 3.95 = 118′ 6"|
|3||5.10 MHz (60 meter)||15.30 MHz (19 meter)||468 ÷ 5.10 = 91′ 9"|
|4||5.90 MHz (49 meter)||17.70 MHz (16 meter)||468 ÷ 5.90 = 79′ 3"|
The wires are spread 3-4 inches, held in place with simple Plexiglass spacers. Just cut a few pairs of the acrylic about 2 by 12 inches and run a few small bolts through them, pinching the wires between. Obviously, you stretch the whole mess out on the ground, assemble it, then pull it up with your rope and pulley.
The wires all join at the peak of the house and connect to the center wire of 50-ohm coax (RG-58). The shield of the coax connects to a wire which runs down to your ground rod. Solder and tape all connections to keep water out. Don’t forget the lightning arrestor.
If you have a big tree about 170 feet away, this antenna will give fabulous results.
The next design is a center-fed multiwire dipole (below). The big advantage is, since the array is supported at the center, you can use lighter (cheaper) materials, since each span is only 72 feet long. Also, using two tall trees puts more of the antenna higher off of the ground.
The only way you that may further improve on this design is to raise it higher. If you have thousands of dollars laying around, you can string it across three 100-foot towers - and probably get a write-up in a national magazine.
Ok - enough dreaming. Let’s get realistic here. We have no trees, and dozens of property association rules. We need a good shortwave antenna in the attic.