Understanding Shortwave Radio Listening
and Antenna Design and Construction
Copyright © 1999-2012 by Harold Melton KV5R. All Rights Reserved.
Understanding Shortwave Antennas: Page 1
Who needs this article? Anyone who is a new shortwave radio listener. Shortwave listening is a bit more complicated than AM or FM radio listening, and satisfactory results depend upon designing and installing a good antenna.
Why? Shortwave radio signals travel great distances. They bend around the earth by reflecting off the ionosphere.
What’s that? The ionosphere is a finicky mirror! In the daytime, it reflects higher frequencies, and absorbs lower ones. At night, it reflects lower frequencies and passes higher ones. Around sunrise and sunset, the middle frequencies seem to travel best.
Sooooo... Is that daytime at my house, or at the far-away radio station? Good question! Answer: Somewhere in between. It means that higher frequency, daytime bands will bounce into the US from the west in the evening, because it’s still sunny over the Pacific. Likewise, stations from the east will bounce into the US in the early morning hours, increasing in strength through midday, then fading out as the sun goes down over the Atlantic.
This is too complicated! Not at all. You will soon learn to locate your favorite shortwave stations on the right frequency, depending on what time it is. Shortwave broadcasters may use several frequencies, rotating through them every day. When it’s time to change, they make an announcement.
What if I’m not listening when they make the announcement? Part of being an avid shortwave listener is collecting radio schedules. These are found on the Internet. You can also just spin the dial and listen, and keep notes of what is where, when, and make your own custom listening schedule.
Ok, so what else do I need to know to get started? Your little portable shortwave radio has a useless antenna. Oh, it’s fine for FM — but shortwave antennas need to be at least fifty feet long to be useful!
Oh no! I can’t put up some big, ugly antenna! All it takes is a very fine wire. It can run out the window to a tree, or stapled along under the eaves, or even in the attic. Another part of being a shortwave listener is designing and deploying the most clever antenna possible, balancing visibility, reliability, and performance.
Ok! I think I can do that! Sure! Anybody can do it. Millions of people all over the world use shortwave as their primary or only source of information.
So, just what is an antenna? Technically speaking, an antenna is an impedance matching transformer. It matches the low impedance of a transmitter or receiver to the extremely high impedance of the surrounding space. It converts power alternating in a wire into power alternating in free space (or air), and vice-versa.
Do I need to know about impedance matching? No, but if you want the best possible performance from your antenna, you should design it correctly. You need to know how to get enough signal to your radio to increase your listening options, and reduce annoying signal fading. A poor antenna converts very little signal, but a good antenna converts a lot more. Antennas also pick up noise — man-made and natural — and you want your antenna to receive more signal than noise. Also, you will want an antenna that is physically strong, so it will stay up through many years of storms. In most areas, it must also be well hidden.
Why does the antenna need to be so long? Because shortwaves are long (they are called “short” waves because longwaves are thousands of feet long!) If you only want to listen in the daytime, 50 feet of wire is fine. However, if you also want to listen to the lower, night-time frequencies, you’ll need 100 feet or more.
Why? The length of the antenna needs to agree with the length of the longest radio waves that you want to receive. Shortwave broadcasters use frequencies that are from about 50 to about 400 feet long, and an effective antenna needs to be at least one-fourth of that length — and one-half is much better.
I live in a Property Owners’ Association… Many people in the US now live in restricted neighborhoods. These usually have rules (which you agreed to by signing the contract) that prohibit outdoor antennas. They don’t want you messing up the neighborhood by exercising your constitutional rights, so they require you to enter into a contract that restricts your rights. I call it, “Communism by Contract,” for that is exactly what it is. It is also discrimination, because shortwave radio is just another information source, like TV and the Internet.
What to do? I’ll show you how to put up simple, effective shortwave antennas that are almost invisible. If your local communists can’t see it, they can’t object to it, since the rationale for their anti-antenna ruling is one of visual appearance. If they do object, you still have options. You can fight the ruling and get an exemption. You can fight on the grounds of a federal FCC ruling called PRB-1, which covers small satellite dishes, but may, in the spirit of the law, add weight to your argument. Or, you can simply hide your antenna better, such as running it under the eaves or in the attic.
Is this going to be complicated? Not at all. There are standard formulae for various antenna designs, and standard (common-sense) mechanical practices.
Why are there different designs? Again, it depends. You may want your antenna to transmit or receive equally well in all directions, or in one direction only. You may want it permanent, temporary, or hidden.
Oh! You mean, like a CB ground plane versus a TV antenna! Correct; and there are serveral other designs and parameters which we will consider herein.
Like what? Like whether the antenna design has a wide or narrow bandwidth.
Bandwidth?! You just read on!