Understanding Shortwave Radio Listening
and Antenna Design and Construction

— KV5R is disabled. Please help. —

Understanding Shortwave Antennas

Rev. 2.1, Sept. 2003. Added basic shortwave radio listening information.

Converted to web page April 2, 2003.

First published June 1999

Introduction

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!

SAFETY FIRST!

POWER LINE SAFETY

Every year, people are killed because they allow an antenna or support to contact overhead high-voltage power lines. Remember: Never, NEVER run any antenna wire, feedline, support line, or guy wire OVER OR UNDER power lines !!! The antenna, support, or guy can fall into the power line -- OR the power line can fall into them. A wet antenna support rope will conduct high-voltage. NEVER assume it is safe to run a rope or string over or under power lines.

NEVER raise a pole, mast, or tower is such a way that it could fall into a power line !!!

Get help. Tie off safety lines (dry nylon rope) perpendicular to and away from power lines. If your mast is 40 feet high, erect it 45 feet or more from power lines. NO EXCEPTIONS!

To reduce power-line induced noise in your shortwave radio, shortwave longwire antennas should be run perpendicular to, and away from, utility power lines.

LIGHTNING SAFETY

First of all, long antennas do not "attract" lightning. They merely act as long inductors that get a voltage spike from the nearby lightning arc. Nevertheless...

Every year, people are killed because they ignore simple lightning protection measures. The basic lightning arrestor is simply a grounded spark-gap device. You can buy them, or make one from an old spark plug.

All large antennas -- even horizontal wires run low to the ground -- increase the risk of electrocution and/or property damage from atmospheric lightning. The simple rule is: the longer the wire, the more voltage will be induced upon it by a nearby lightning strike. This voltage will leap off the end of the wire in a fat blue spark, and you will hear it pop. Any time you hear thunder, disconnect the antenna wire from the radio. Put the end in a glass or jar and lay it on the floor near the wall. If you have a radio ground wire, clip the antenna wire to it instead.

You don't need a direct strike to have lightning damage. Any strike within a mile will produce an electromagnetic pulse that will induce thousands of volts on your antenna wire, probably damaging your radio. If you get a direct hit, it will probably destroy your house, with or without a lightning arrestor.

NOTE: A lightening arrestor only drains away small pulses -- it will NOT protect you or your house from a direct strike. ALWAYS disconnect the antenna from the radio when storms are approaching! Do not leave an unattended radio connected to an outdoor antenna. Tell the whole family!

LEGALEZE

Antenna erection in the vicinity of power lines can be a FATAL activity! Your safety is your responsibility -- not mine! THE AUTHOR ASSUMES NO RESPONSIBILITY FOR YOUR USE OR MISUSE OF ANY INFORMATION HEREIN.

The author cannot guarantee nor warranty that the plans and information herein are perfect in every detail, nor that your use of them will satisfy your needs. The outcome is entirely your responsibility. They are intended simply as a guide for you to use in designing and building something to suit your own needs. Every antenna installation is different.

Radio Shack is Registered Trademark of Tandy Corporation. My mention of Radio Shack products in these plans should not be considered an endorsement, just that they are readily available and of reasonable quality. I am under no agreement with Tandy Corporation.

FALLS AND OTHER HOME HAZARDS

Every year people are killed or injured by falling off houses, out of trees, off ladders, etc.

DO NOT climb trees! Shoot a line over with a slingshot or fishing pole. Then, use the fishing line to pull over a stronger line (#18 nylon Mason's line). Then use that to pull up ¼-inch nylon (or better, UV-protected dacron). Then pull up a pulley, with more ¼-inch nylon looped through it. You CAN use trees without leaving the ground!

MAKE SURE ladders are strong and stable. Sink the legs of the ladder into the ground before climbing. Have a helper hold the ladder. Read the warning labels. Use common sense.

NOTE: It is not legal (under FCC regulations) to modify antennas on certain products like cordless phones and FRS radios. Check your owner's manual.

SHORTWAVE RADIO SPECTRUM USERS

The shortwave spectrum runs from the top of the AM broadcast band at 1.7 MHz, to 30 MHz. Within that range various "bands" of frequencies have been allocated (by international treaties) for various purposes. There are several main user types scattered throughout shortwave. They are:

• Broadcasters
• Amateurs
• Utility Stations
• Marine (ships)
• International Aircraft
• Government and military

Broadcasting is one-way radio, aimed at the general public. Broadcasters run high-powered transmitters (usually 100,000 watts or more) and are trying to reach an international audience. They are usually run by governments ("official" propaganda), or commercial shortwave providers that sell broadcast time to various individuals and organizations who have something to say. Most are religious or news programs. Some are "alternative" shows, focusing on unfiltered news, anti-establishment sentiment, doomsayers, and even outright hucksters. Some are fascinating, so beware of getting sucked into bogus "health" products, financial "opportunities," or extreme views. With shortwave, you're your own news editor. If you have led a sheltered life (TV brainwashed), you are in for a few shocks. You'll hear both good truth, and things too good or too bad to be true.

Amateurs ("Hams") are private, licensed radio operators who conduct two-way communications with other Amateurs, worldwide, for hobby, and sometimes emergency assistance, purposes. Amateurs may transmit in several radio modes, using power up to 1500 watts. Amateurs are not CBers, and vice-versa. They must master many radio-related topics, and pass exams, to receive their licenses. When you get tired of just listening, you may want to get your Amateur license and start talking to people around the world. However, Amateurs may not broadcast -- that is, make one-way transmissions to a general audience. If you want to broadcast, you can buy time from a shortwave station.

Utility stations may be one-way (like weather reports to ships and airplanes), or two-way (like radiogram messages to/from ships and airplanes. Some utility stations send coded messages to armies, or guerillas, or dope runners (assuming they're not the same). Many utility stations use radio teletype and FAX instead of voice. Some shortwave listeners make seeking out utility stations the main focus of the hobby. An interesting utility you will want to use is WWV, the national time standard -- on 5.0, 10.0, and 15.0 MHz. If you change your clocks twice a year, use WWV to get them exactly right. It's fun to keep your watch set to one second accuracy!

During major emergencies (like hurricanes), shortwave traffic will greatly increase on certain frequencies. Many Amateurs maintain emergency power sources and provide vital communications when public utilities are down.

Ships at sea, international airline flights, and governments also use parts of the shortwave radio spectrum.

SHORTWAVE BANDS

The shortwave spectrum is divided into bands. You need a basic familiarity with this to use a shortwave radio effectively. The shortwave spectrum is about 25 times larger than the AM broadcast band (.55 - 1.7), so you need to know what is where, and when.

The bands are called "Meter" bands and are identified by their approximate wavelength in meters.

• M = Meter
• Ham = Amateur
• BC = Broadcast
• P = Popular, somewhat congested band with many stations

Notice that 40 meter ham and 41 meter broadcast overlap. This is a big problem we are working to fix in the next international radio conference.

As you can see, the broadcast bands starting at 5.9, 7.1, 9.4, 11.65, 13.6, 15.1, 17.55 are the main place where most all your shortwave broadcast listening will be concentrated. Note, however, that some stations fall slightly outside of these ranges, so make sure to tune above and below them. Also, band edges, and broadcasters' frequencies, change from time to time. So, it pays to keep fresh frequency schedules. Search the internet for "shortwave shedule" and you'll find plenty. Locate one that you like, and that has fresh data. Shortwave listening magazines, and their web sites, are also good sources.

Most Amateur communications uses single-sideband. You need an SSB-equipped radio, or one with a "BFO" control, to receive them.

BASIC RADIO THEORY

Let's start here. Electricity, when flowing continuously in one direction is called direct current (DC). DC obeys the same rules as water in a pipe - flow is a function of pressure and resistance. More pressure and less resistance equals more flow. Congratulations - you just learned Ohm's Law.

Electricity, unlike water, can change its direction of flow very rapidly. Electricity that flows back and forth is called alternating current (AC). Utility power changes directions 120 times per second, AM-band radio about 2 million, FM-band radio about 200 million, and a microwave oven about 4 billion. Two changes, one forward and then one backward, are called one cycle or one Hertz. The number of cycles per second is called the frequency. The distance one cycle travels in one second is called the wavelength of that frequency. Since the speed is fixed (the speed of light), higher frequencies have shorter wavelengths, and lower frequencies have longer wavelengths.

Electricity travels down a wire at near the speed of light. At high frequencies, it becomes practical to shove extra electrons into one end of a wire then jerk them back out again long before the effect reaches the other end. The result of this is that, at such high frequencies, we don't need a closed loop to have a useful circuit - we can set up an electron oscillation in an open-ended conductor, much like a vibrating coil spring.

If the length of the conductor is 1/2, 3/2, or other odd-multiple half-wavelengths long, the conductor is said to be resonant. The length of the wire is in agreement with the length of the wave, at this frequency of oscillation.

When electricity flows, it creates a magnetic field. If the direction of the electricity alternates, the polarity of the magnetic field does, too. It also creates an electrostatic field. A radio wave is simply energy which is alternating between a magnetic field and an electrostatic field. This effect can go on indefinately, while there is a source of energy to sustain it. The energy at a given point diminishes with distance, because the size of the field is growing (and diffusing) as it goes outward.

When a conductor is resonant, it will convert virtually all of the electrical power into radio waves, and thus, is an efficient antenna. (Some power is lost to resistance, but never mind.)

There is a reciprocal law about antennas - any antenna which transmits well can also receive well. When receiving, a radio wave passes across the antenna, inducing an alternating electrical current in it. Variations in the frequency and/or amplititude are called "modulation." A radio receiver then makes the modulation intelligible (hopefully).

Not all antennas are resonant. A long piece of wire, for example, will pick up a little of just about everything, and induce enough current to drive a receiver. Transmitting antennas, however, are almost always designed to be resonant on the particular frequency of the transmitter.

In antenna books, the antenna itself is frequently used as the x-axis of the graph. This antenna is called a half-wave dipole - it has two poles, and is one-half wave long at its design frequency. The sine graphs for voltage and current are shown sitting atop the antenna.

DEFINITIONS

Cycle or Hertz - two changes of the polarity of energy, i.e., stop, forward, stop, reverse, stop.

Kilocycle or kiloHertz (kHz) - One kilocycle = one thousand cycles per second.

Megacycle or MegaHertz (MHz) - One megacycle = one million cycles per second.

Frequency - how frequent something repetitive happens. The frequency of alternating energy is usually expressed in cycles per second or Hertz.

Resonance - Input of alternating or pulsating energy timed to coincide with something's natural tendency to oscillate. Like pushing a child in a swing - you push at the right time, don't you? The swing and the child are a resonant pendulum, and you push forward just as the swing starts forward again. The resonance of a guitar string is determined by its length, weight, and tension. The resonance of a quartz crystal is useful as a stable time-base for electronic watches, computers, and radios. A particular length of wire or tubing is resonant at a particular frequency. Longer wires resonate at lower frequencies. Shorter, higher.

Wavelength - the distance energy travels, at the speed of light, during one complete cycle. Electricity in wire travels a tiny bit slower than light, therefore, a wavelength in wire is not exactly the same as a wavelength in free space (or air).

Bandwidth - how narrow or wide a range of frequencies a particular antenna design can cover efficiently. TV and scanner antennas have a wide bandwidth (hundreds of MHz). A CB antenna has a narrow bandwidth (0.5 MHz). Bandwidth is much more critical when transmitting than when receiving.

TYPICAL ANTENNA DESIGNS

• Longwire - any antenna which uses a single end-fed element which is several wavelengths long. A longwire is directional toward its far end: More wavelengths long = more directional.
• Monopole - like the single telescopic element used on portable radios. If the monopole is operated at one-half wavelength, it becomes a Zepp.
• Dipole - an antenna having two poles. Dipoles are usually ½ wave long, and have a narrow bandwidth. Dipoles radiate best off of the sides - perpendicular to the wire - assuming it is operating at the frequency for which it was designed. Dipoes are resonant at ¼ and ¾ wavelength. Dipoles are bi-directional. "Rabbit-ears" are simply "V" dipoles.
• Zepp - an antenna that is ½ wave long and fed at one end. It radiates like a dipole. If you shorten it or lower the frequency, it becomes a nonresonant monopole. If you lengthen it or raise the frequency, it becomes a nonresonant longwire. Zepps are bi-directional.
• Ground plane - an antenna which uses an artificial ground to act as an electrical mirror, making the antenna's radiating element "think" it's a vertical dipole. They are omnidirectional.
• Yagi - an antenna which uses a dipole with parasitic elements to make it directional.
• Log Periodic Dipole Array (LPDA) - like a TV antenna. The LPDA design is directional, and has an extremely wide bandwidth, because of all its different element lengths.
• Discone - an antenna that is a disc and a cone. They are omnidirectional and have an extremely wide bandwidth, so are very useful as scanner antennas.
• Feedline - the cable that carries signals between an antenna and a transmitter or receiver. This is usually coax or twinlead. The impedance of the feedline should match that of the radio and antenna, or the feedline will itself become part of the antenna, messing up your design plans.

SIMPLE ANTENNAS

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 low-power FM transmitter, or 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 25 years. My goal has always been to build cheap, simple antennas that work well. It's all in the numbers. I had a $25 multiband shortwave antenna in the attic which would outperform any commercially-made $150 antenna. I had a discone scanner antenna made of stainless steel welding rods which I built for about $3. I had a VHF yagi which would go five miles on ¼ watt, made of scrap TV antenna parts. The only antenna I have ever had to purchase is a satellite dish, because I can't build parabolic reflectors. I am currently running a 265-foot dipole fed with ladder-line and an antenna tuner. My next will be a 130-foot square horizontal loop. Having 5½ acres of trees helps!

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 ridiculous telescopic whip that comes with portable radios. Telescopic antennas are extremely 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 dipoles are best of all. Avoid all "trap" antennas.

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! No room!

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 $35). Electricians frequently have scraps and partial rolls. Farm supply stores carry #17 aluminum fence wire (about $12 for ¼-mile roll). Wal-Mart carries 100-foot rolls of telephone house wiring (about $10) (solder all 4 strands together at both ends). Any wire will do -- but some will last longer than others. Stranded, insulated wire (#14 green THHN) is best.

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 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 -- if you are a frequent, serious shortwave listener.

Wires should be insulated, stranded #18 - #14. A jack may be soldered on to plug into your radio. If the radio has no Ext. Ant. 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.

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.

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):

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.

INDOOR MULTIWIRE SHORTWAVE ANTENNAS

Indoor, or attic, antennas have several advantages: (1) no rain, no corrosion, no wind damage, (2) little possibility of lightning strike, (3) no problems with property association communists, and (4) no need for trees or other external supports.

It's possible to make a multiwire dipole out of 4 or 5 conductor TV antanna rotator cable. This type is a flat ribbon wire. Since it isn't very strong, it must be supported at frequent intervals. This means we can string it through our attic, diagonally, hanging it on the undersides of the rafters with bent nails.

Here comes the compromise: We will have to forego ½ wave operation on the lower bands. Below are the calculations for this antenna, which uses 5-conductor rotator ribbon wire (frequencies shown are approximate band centers for the meter bands shown):

Since this is a dipole (center-fed), we need to buy ((79.25 + 30.5) ÷ 2) + 2 = about 55' 2" of ribbon (leaves an inch on each end for connection). We then measure, mark, cut, and peel it all apart to make the two halves of the dipole (notice that the center wire is cut twice - 7 feet is discarded) (cut each conductor by pushing a small screwdriver through it):

If you can't get in the attic, you can lay it on top of the roof. Bring the coax out through a ridge vent, if possible. Waterproof the connections. NOTE: If you don't want to build it, Radio Shack sells this type antenna for about $35 (+ coax).

SIMPLE INDOOR SHORTWAVE ANTENNAS

Last but not least, let us examine simple wire antennas which may be designed and used indoors, for little or no cost. Such are need by renters and apartment dwellers, and are always better than the way-too-short telescopic whip antenna.

The Indoor Longwire

The simplest antenna which will drastically increase the amount of signal power (compared to the telescopic whip) is merely a much longer monopole -- i.e., 40 to 80 feet of fine wire, thumbtacked to the ceiling and connected to the radio.

• Go to Radio Shack and a get a little roll of fine enameled magnet wire, and if your radio has an antenna jack, get the male plug to fit it (usually a 1/8th-inch "earphone" plug). If not, get a small "alligator" clip. Stop by Wal-Mart and get some thumb tacks.
• Pick a layout through your house that will allow you to string up the longest wire along the ceiling, in a fairly straight line. For example, start at the far corner of the living room, go down the hall, to the far corner of the bedroom. String up the wire on the ceiling using thumb tacks, dipping down under doorways where necessary. Bring it down the wall at one end. Attach the plug for the radio. Scrape the enamel coating off of the wire, then solder it to the center pin of the plug. If your radio has no antenna jack, simply solder a small alligator clip to the end of the wire and clip it to the telescopic antenna.

You will be amazed when comparing a signal on the telescopic whip and the fine ceiling wire. When connecting the wire, the signal meter will jump way up. You will be able to pick up weaker signals, and will have less problems with fading. It will also pick up a lot more static and noise from electrical devices in your house...

How much better is the full-sized, outdoor, multiwire dipole antenna? On the strong signals, no better. But on the weakest ones, considerably better. If you regularly seek out weak signals, try to get a multi-wire up outdoors -- and considering purchasing a nice antenna tuner and an active audio filter. Mainly, a large outdoor antenna will have a much better singal-to-noise ratio. It will pull in more radio signal than electrical interference.

Indoor Electrical Noise

Almost all indoor antennas (including those expensive "active" ones) will pull in lots of electrical noise, requiring you to go around and turn off TVs, VCRs, computers, dimmers, florescent lights, and ceiling fans -- before your favorite shortwave radio program comes on. Even worse, the electrical interference may come from street lights or even neighbors -- sources you cannot control. This is why you should put up as big an antenna as possible, to get the induced signal power over the noise level.

NUTTY ANTENNAS

These are all the crazy "try it and see if it works" non-designed antennas. They usually work better than the telescopic radio antenna, but worse than the indoor longwire.

• House wiring: You can run a short piece of wire from the telescopic whip to the ground screw on the nearest electrical outlet, thus using the ground wires of the house as an antenna. The problem is that they go in all different directions and tend to cancel one another. But it's the thing to try if you can't even thumbtack a wire on the ceiling.
• Aluminum foil: KBOHAE reports that, "Aluminum foil can be used to make some very effective indoor antennas. Especially when these antennas must be physically short (less than 1/4 wavelength). I have tried various antennas for shortwave listening over the years. The most effective indoor antenna that I have found is made from 2 or 3 strips of aluminum foil attached to the back of a world map. This antenna has outperformed any inside wire antenna that I have tried. It has also outperformed some outside wire antennas." Thanks. This hadn't occured to the author before, but a slab of foil on the end of a wire should electrically lengthen it, like the capacitance hats on shortened verticals.
• Hidden wires: If the ceiling wire is too bold for your landlord, you can run a long wire under the baseboard -- just push it under (between the carpet and baseboard) with the back end of a butter knife. Works pretty good in upstairs rooms, but not worth the effort on the ground floor. The best hidden indoor wires are attic dipoles, stapled under the rafters, as shown on the previous page. They do require, however, access to the attic space, as well as a way to get the coax down to the radio. This is generally not for renters.
• Foil tape: If you are about to repaint, you can run a roll of foil tape along baseboards, then paint it. This works upstairs, and is the near-perfect hidden antenna.
• Gutters: Some building materials may be used as antennas, such as rain gutters. If it is metal and much longer than wide, it will work to some degree. Note however, that gutter joints tend to corrode and generate noise, particularly if you are near a broadcast transmitter.
• Thrown or draped wire: Buy a 100-foot roll of cheap indoor telephone wire ($8) and experiment with it. Run it out the window and throw it over the roof. Tie the far end to a tree. Twist all four wires together at both ends so it'll act like one wire, not four.
• Compressed antennas: A 100-foot antenna may be erected in about 40 feet of space by using two metal "Slinky" toys. Run a nylon rope through the Slinkys. String it up in the available space. Stretch the Slinkys out as far as possible. Connect the coax to the Slinkys in center-fed dipole fashion. This a decent attic antenna for lower frequencies than space would otherwise allow.

ADD-ON GADGETS

Like every other hobby, shortwave listening attracts sundry gadgets. Some of these are quite useful, some are not, and some are for specialty purposes only.

Active Antennas, Preamps, Prescalers

These connect between the antenna and the radio.

With an "active" antenna, the idea is to take the usual 5-foot telescopic whip and add a very sensitive amplifier -- thus adding another input stage to the radio, which is, presumably, more sensitive than the radio itself. The ads promise that they will work as well as a 60-foot longwire. Baloney! Modern portable shortwave radios, like Sony, Grundig, and Sangean, are as sensitive as any active antenna. Therefore, adding an active antenna will not improve anything. In other words, do active antenna makers have access to more sensitive transistors than the radio manufacturers?

However, if you do buy an active antenna, or preamplifier, make sure it has a "helical" filter, and relatively impressive "intermodulation suppression" specifications. This simply means that it will feed a relatively clean signal to the radio, not a jumbled bunch of amplified garbage from nearby stations.

A preamp is simply a way to make your existing antenna "active." Same as above. May be indicated for very old radios with poor sensitivity.

A prescaler is a sort of antenna tuning device. You tune your radio to the desired frequency, then you tune the prescaler to peak the signal. These are good devices for improving the selectivity and signal-to-noise ratio, and are particularly helpful of cheap radios. Most prescalers also include a preamp, but some are passive.

My recommendations: Ignore active antennas. Put up some wire. Ignore preamps. Get a modern radio which has all the preamp built-in that you'll ever need. Get a passive prescaler or random-wire tuner if needed. MFJ makes a nice collection of such things. Look for their ads in radio magazines, and order a catalog. www.mfjenterprises.com.

Audio Filters

These process the audio (speaker) output in some desireable way. You must plug them in between the radio's speaker (earphone) jack, and an external speaker (or headphones) which you have to purchase separately.

Audio filters allow you to "narrow" the bandwidth of the audio -- like turning a "Tone" control way down, but with more sophisticated control. It is a nice add-on, if you listen to CW (morse code), or if you regularly get interference from stations on nearby frequencies (like 5 kHz away) from your favorite stations. You can filter out the "hetrodyne" squeal -- but they will not filter out electrical noise. Be not deceived!

If you get an audio filter, make sure it covers what you want to do, like shortwave broadcast clean-up, or single-sideband, or CW (morse code) copy. Each of these requires different types of filtering. Some units have it all in one box, and of course, cost more. Ask an Amateur (Ham) radio operator -- we Hams read the radio magazines and keep up with these things.

Computer Interfaces, Radioteletype and Fax Demodulators

These connect to the speaker output of the radio, and usually to the serial input of a computer. They are used to receive and decode CW, radioteletype (many varieties), and weather fax transmissions.

My recommendations: The cheap ones ($100) are garbage, so don't waste your money. The better ones which cost several thousand dollars work very well. You need to have a real need-to-know (like surveillance work) to justify the expense. Even then, most of the signals are encrypted, and it's illegal to intercept them.

2003 Update: Computer Sound-card Software

Lots of “digital” modes can now be demodulated (and transmitted, if you have the license) using only a computer with a sound card and the appropriate software. See my web article Getting Started in Digital Modes for info and many links.

What to Avoid

• Don't mess with "active" antennas, unless you are trying to get shortwave in the car or from motel rooms.
• Don't mess with multiband, analog dial-type radios. You know the ones that have 21 shortwave bands, AM, FM, TV sound, etc. Get a good digitally-tuned, continuous-coverage (150-30000 kHz) shortwave, in the $150 to $400 range. Get one with "BFO" for sideband and CW listening. The Sangean 818 is the best radio made for less than $200 and I highly recommend it. C-Crane Co. is the best source.
• Don't mess with automotive radio shortwave converters or other such rinky-dink shortwave receiver kits. They all have a very limited frequency range, typically 1 or 2 megahertz -- and the useful shortwave band is is about 20 megahertz wide! Gimmicks to get you money…
• Don't waste money on "wall plug antennas" that promose to use your whole house wiring as a great antenna. They work no better than simply connecting to the ground screw in the center of the outlet plate.
• Don't blow $150 on fancy multiband trap-dipole antennas. Traps waste power. A $30 multiwire ribbon dipole works better. Don't waste $75 on a simple inverted Vee dipole which you can build yourself for $20 or less. Don't blow $135 on a name-brand discone antenna when you can build one for a few bucks.

ABOVE SHORTWAVE: THE DISCONE VHF SCANNER ANTENNA

This relatively small, strange-looking antenna may be used outdoors or in the attic, and is suitable for VHF and UHF transmitting and receiving. It has all these "way cool" features:

• Extremely broadband - over four octaves (ex.: 100-1600 MHz)
• Vertically polarized, omnidirectional
• 50 ohm coax feed with no matching or tuning
• Will handle 1500 watts or more
• Very low radiation angle puts power on the horizon

This antenna is excellent for scanners, FM-band micro-broadcasters, VHF/UHF business, law enforcement, GMRS, FRS, TV (7 and above), and VHF/UHF ham bands. It will do all of the above without ever taking it down to retune it. Just build it right (strong and weatherproof), put it up on a well-guyed mast, then use it for everything above shortwave. 

General Design Notes

• The aptly-named discone antenna is a disk and a cone. The cone (or skirt) is an equilateral triangle in cross section, where dimension "L" is equal to one-quarter free-space wavelength (0.25), which is as follows:
• 2952 ÷ ƒ(MHz) = L (inches)
• where ƒ is the lower cutoff frequency in megahertz.
• The diameter of the disc is 0.67L to 0.7L, and should be spaced about ½-inch above the point of the cone.
• The disc must be supported by an insulator block. The center of the coax connects to the disc, while the shield connect to the cone. The skirt of the cone is supported by four small aluminum tubes, flattened and bent at both ends. Old TV antenna elements work well as braces.
• Radio Shack sells a discone which uses eight elements spaced around the circle. They claim 25-1300 MHz coverage, which is ridiculous. Still, it is a fine, stainless steel antenna for $59.95, which will cover all VHF-UHF needs, including ham-band and FM-band transmitting.

These plans use dimensions which will set the lower cutoff frequency below 88 MHz, so that it may be used for FM micro-broadcasting. For cutoff to include:

Lowest Band: Cutoff: "L=":

• FM broadcast, 87 MHz, 34"
• Aircraft, 108 MHz, 27½"
• VHF-Hi, 138 MHz, 21½"

Eliminating the lower bands simply makes a smaller antenna, which is easier and cheaper to build and support. The antenna has a considerable surface area and wind load, therefore should be placed upon a well-guyed mast. The guys may attach immediately below the skirt of the antenna, without any need for insulators.

Ideally, the discone should be made of copper or aircraft aluminum ("Alclad") sheet metal, but a heavy screen (¼-inch galvanized hardware cloth) will work as well, with lower wind loading and less cost.

MATERIALS

• 4' x 9' of ¼" mesh hot-dipped galvanized hardware cloth
• One ten-foot section of 18-gage 1¼" TV mast
• One SO-239 chassis-mount type coax connector
• One PVC pipe cap (sized to fit snugly over the top of your mast)
• Six feet of old TV antenna element, or ½"x½" aluminum anglestock (to brace the bottom of the cone)
• Two 2" worm-gear clamps (radiator hose clamps)
• Sundry self-drilling sheetmetal screws, small machine screws (8-32) with double-flats, locks, and nuts.
• A small brazing torch and light-gage brazing rod would be helpful, but not essential.
• You might want to finish the edge of the disc and the lower edge of the cone with #9 steel tie wire, to stiffen it.

CONSTRUCTION

• Heavy leather gloves are required! Unless you like pain and blood.
• Cut out a half circle with a 36-inch radius for the cone. Leave a 1-inch tab, as shown, along the straight edge. This will be overlapped and sewn together with wire, to form the seam of the cone.
• Cut out a full circle with a 25-inch diameter for the disc.
• Roll the half-circle into a cone. Overlap the 1-inch lip and sew or braze it together. This overlapping is essential to ensure the cone has a nice round shape, and does not tend to "point" at the seam. Be careful not to dent the cone.
• Sew or braze #9 tie wire around the base of the cone, and around the edge of the disc, if desired, for stiffening.

Prepare the PVC cap as follows:

• Cut out a disc of some ¼-inch plastic, like Plexigless; the diameter to fit inside the PVC cap.
• Drill a hole exactly in center of cap for the wire.
• Solder a few inches of copper wire to the center pin of the SO-239. This wire will extend through the cap and connect to the 25-inch screen disc.
• Drill 5 holes in the plastic disc and mount the SO-239.
• Bring a few inches of wire from under one of the corner screws. This will exit through a small hole in the cap and attach to the skirt, to ground the SO-239 to the cone (skirt).
• Drill 4 holes in a circle around the top of the cap and mount the 25-inch screen disc to the cap, using 8-32 machine screws, flatwashers and nuts.
• Bring the wire through the hole in the center of the cap and push the plastic disc and SO239 assembly about half-way into the cap. Make sure the SO-239 mounting screws to not hit the screen disc mounting screws! If in doubt, put a couple of layers of duct tape over the screw ends. Glue the disc in place with silicone caulk. Seal the hole where the center wire exits the top of the cap. Solder the center wire to the disc.
• Form the cone and attach it to the PVC cap with machine screws and washers. Solder the ground wire to the cone.
• Run the coax feedline through the mast. Grease the PL-259 and attach it to the SO-239 in the cap. Mount the assembly on the mast.
• Finally, add 4 braces from the mast to the lower edge of the cone.

Before erecting the discone, test it with a ohmmeter. The center-to-shield of the coax should not be shorted. The shield to cone (skirt) should show short (0 ohms) and center-to-disc should of course show short.

SUPPORT PLANNING AND ERECTION SAFETY

It's a pretty good idea to plan and graph any proposed erection which could fall into power lines -- as opposed to killing yourself like several people per year do. This is easy to do using graph paper or a computer drawing program with a grid.

In the following example, a 40-foot aluminum tower will be anchored to a tilting base plate, bolted to a concrete pad, then attached to the house gable at 13 feet, then guyed near the top (39 feet) with one set of three 3/16" braided steel cables with turnbuckles. Up to 20 feet of 1½" heavy-wall steel mast pipe (commercial fence-post stock) will stand atop the tower, giving a height of 60 feet. The upper pipe mast will be guyed with three regular 7-strand galvanized guy wires.

Positioning the power line at the proper height and distance, then drawing an arc, will show where the tower must be placed, and the maximum height at that location, that will clear the lines.

To further eliminate the possibility of a line strike, the tower may be guyed with an additional safety cable extending away from the power lines. This will ensure that the top pole would topple before the tower, thus reducing the arc of fall to 40 feet. This failure mode assumes tornadic wind conditions. If it would take tornadic winds to topple the tower, the same would topple the power lines and poles! Thus, well-anchored steel cables, rated at over 2500 pounds working tensile, will totally prevent a power line strike.

Most electrocutions occur during tower erection. When pulling up an assembled tower or pole, at the very least a safety rope should be tied to the top, extending perpendicular, away from the power lines, tied securely to a tree trunk. If the tower falls during erection, the safety line will swing it down parallel to the power lines. Never try to man-handle a tower into position without safety lines !! One extra helper is ten times better!

No antenna, pole, mast, or tower should ever be raised alone, nor by unsupervised children.

Well, that’s all for now, folks! I hope this article has been helpful. If you wish to continue learning antenna theory and construction, obtain a few good books on the subject. I recommend you start with The ARRL Antenna Book.

73, de KV5R