Copyright © 1999-2016 by Harold Melton KV5R. All Rights Reserved.
Understanding Shortwave Antennas: Page 8
Now that we know our antennas should be resonant, let’s see how to make them resonant.
First, are we transmitting or receiving? Transmitting antennas are much more critical than receiving antennas, although a receiving antenna can transmit if it is resonant (or otherwise matched) and if it can handle the transmitter’s output power. A transmitting antenna can also receive, and will do so particularly well on the frequency for which it was designed.
If you are running a CB or HAM station, you already know something about transmitting antennas. Amateur operators, in particular, must pass exam questions on antenna design and theory.
If we are operating a shortwave radio, we want it to quit fading out in the middle of our favorite shows.
Now that you are ready to get brilliant, learn rule #1 of antenna design: All antenna designs are a mixture of compromises. Just like boat hulls and airplane wings.
What we need to do, therefore, is identify our particular need and design an antenna which is optimized to fulfill that particular characteristic - whether it be directionality, gain, or bandwidth, or just all-around good performance.
I have been an antenna experimenter for over 35 years. My goal has always been to build cheap, simple antennas that work well. It’s all in the numbers. I once had a $25 multiband shortwave antenna in the attic that performed as well as any $150 manufactured SW antenna. I had a discone scanner antenna made of brazing rods and a PL-259 that I built for about $10. I once built a 10-element 2-meter yagi from scrap TV antenna parts. Some of my other antennas are shown in articles on this site.
You can string up a wire just about anywhere and get a good signal on shortwave. Fifty feet of very fine wire, strung along the ceiling on thumbtacks, will give you much more signal than the telescopic whip that comes with portable radios. Telescopic antennas are way too short for shortwave frequencies!
There is one fundamental rule for receiving antennas: It must be at least ¼-wave long at the lowest frequency you plan to use. Thus, if your lowest regular listening is on 3315 kHz, your wire should be 70 feet long - minimum. Obviously, the 5-foot whip antenna on your shortwave is just a bit too short. For much better performance, it shoud be ½ wave long on your lowerst frequency (a Zepp). Multiwire fan dipoles are best of all.
Let’s design a decent longwire antenna for general shortwave listening. We will want to listen down to 2500 kHz (2.5 MHz), then analyze its performance.
The formula for determining the ½ wave length of wire is: 468 ÷ f (MHz) = feet.
In our example, 2.5 MHz is our lowest frequency, therefore: 468 ÷ 2.5 = 187.2 feet of wire. That’s a lot more than most city lots can handle!
Okay, let’s say we’ll design it for 5 MHz, and be willing to accept slightly reduced performance down to 2.5: 468 ÷ 5 = 93.6 feet of wire. We can handle that.
Longwires are usually strung up something like this:
This arrangement keeps constant tension on the wire while allowing the tree to sway without breaking the wire.
Electrical suppliers carry 500-foot rolls of #14 stranded THHN wire (about $50). Electricians frequently have scraps and partial rolls. Farm supply stores carry #17 aluminum fence wire (about $15 for ¼-mile roll). Any wire will do — but some will last longer than others. Stranded, insulated wire (#14 THHN) works well.
Classic Longwire Installation
This page details the installation of a relatively safe, good-performing, long-lasting shortwave antenna. The #1 rule is: Where you scrimp on quality is where it will eventually break!
Yes, you can simply hang a wire out the window. But experience shows that a properly installed antenna that is mechanically and electrically sound, and a properly grounded radio, will consistently yield better performance and reliability. It really is worth the extra effort to use good materials, solder, and weather-proofing.
Wires should be insulated, stranded #18 - #14. A jack may be soldered on to plug into your radio. If the radio has no external antenna jack, solder an alligator clip to the antenna wire and clip it to the telescopic whip.
Which Direction Is Best?
Usually, directly away from overhead power lines.
If the lines run across the back of your property, go up the back of the house, over the roof, to a tree in the front yard. If the lines run across the front of your property, go up the front (or side) of the house, over the roof, to a tree in the back yard. You can also run a wire along eaves and/or the top rail of a wooden privacy fence.
Run the longwire as far from, and as perpendicular to, the power lines as possible. This will help reduce noise. If you have buried power lines, run your antenna any way you like. In the USA, pointing your longwire northeast will help bring in European stations in the daytime, on the higher shortwave bands.