understanding shortwave radio and antennas by kv5r

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Understanding Shortwave Radio Listeningand Antenna Design and Construction

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

Understanding Shortwave AntennasRev. 2.1, Sept. 2003. Added basic shortwave radio listening information.

Converted to web page April 2, 2003.

First published June 1999

IntroductionWho needs this article? Anyone who is a new shortwave radio listener. Shortwavelistening is a bit more complicated than AM or FM radio listening, and satisfactoryresults depend upon designing and installing a good antenna.

Why? Shortwave radio signals usually travel great distances. They bend around theearth by reflecting off the ionosphere.

Why do I need to know that? The ionosphere is a finicky mirror! In the daytime, itreflects higher frequencies, and absorbs lower ones. At night, it reflects lowerfrequencies and absorbs higher ones. Around sunrise and sunset, the middlefrequencies seem to travel best.

Sooooo... Is that daytime at my house, or at the far-away radio station? Goodquestion! Answer: Somewhere in between. It means that higher frequency, daytimebands will bounce into the US from the west (Australia, Japan) in the evening,because it's still sunny in the Pacific. Likewise, stations from the east (Europe, etc.)will bounce into the US in the early morning hours, increasing in strength throughmidday, 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 favoriteshortwave stations on the right frequency, depending on what time it is. Shortwavestations (24-hour transmitters) will use two, three, or even four different frequencies,rotating through them every day. When it's time to change, they make anannouncement.

What if I'm not listening when they make the announcement? Part of being ashortwave listener means 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, andmake your own custom listening schedule.

Ok, so what else do I need to know to get started? Your little portable shortwaveradio has a useless antenna. Oh, it's fine for FM -- but shortwave antennas need to beat least fifty feet long to be useful!

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run out the window to a tree, or stapled along under the eaves, or even in the attic.

Ok! I think I can do that! Sure! Anybody can do it. Millions of people all over the world use shortwave astheir primary or only source of information.

So, just what is an antenna? Technically speaking, an antenna is an impedance matching transformer. Itmatches the low impedance of a transmitter or receiver to the extremely high impedance of thesurrounding space. It converts power alternating in a wire into power alternating in free space (or air), andvice-versa. A poor antenna converts very little signal, but a good antenna will receive and convert a greatdeal more signal.

Do I really need to know that? No, but you do need to know how to get enough signal to your radio togreatly increase your listening options, and reduce annoying signal fading. You also need to understandthat antennas also pick up noise -- man-made and natural -- and you want your antenna to receive moresignal than noise.

Ok, so what is impedance? Impedance is like resistance, but more complex. Impedance includes capacitiveand inductive reactance.

Oh? So what's that? It doesn't matter. What's important here is that you realize that, when an antenna isproperly designed, there is an impedance match, and that usually means that the antenna is operating atmaximum efficiency in converting an electrical signal to radio waves (transmitting), and vice-versa(receiving).

I didn't realize it mattered that much. It depends - if you want the best possible performance from yourantenna, you must design it correctly. Also, you will want an antenna that is physically strong, and safe, soit will stay up through many years of storms.

Why does a shortwave antenna need to be so long? Because shortwaves are long (they are called "short"waves because longwaves are miles 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 toreceive. Shortwave broadcasters use frequencies that are from about 50 to about 400 feet long, and aneffective antenna needs to be at least one-fourth of that length -- and one-half is much better.

I didn't realize it mattered that much. It depends - if you want the best performance from your antenna,you must design it correctly. Also, you will want an antenna that is physically strong, and safe, so it willstay up through many years of storms. In most areas, it must also be well hidden.

I live in a Property Owners' Association. What can I do? Most people in the US now live in communisticproperty owner's associations. These usually have rules (which you agreed to by signing the contract) thatprohibit visible outdoor antennas. They don't want you messing up the neighborhood by exercising yourconstitutional 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 radiois just another information source, like TV and the internet.

They won't let me put up any outdoor antennas. I will show you how to put up simple, effective shortwaveantennas which are almost invisible. If your local communists can't see it, they cannot object to it, sincethe rationale for their anti-antenna ruling is one of visual appearance. If they do object, you still haveoptions. You can fight the ruling and get an exemption. You can fight on the grounds of a federal FCC rulingcalled PRB-1, which covers small satellite dishes, but may, in the spirit of the law, add weight to yourargument. 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, andstandard (common-sense) mechanical practices.

Why are there different designs? Again, it depends. You may want your antenna to transmit or receiveequally 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 designsand parameters which we will consider herein.

Like what? Like whether the antenna design has a wide or narrow bandwidth.

Bandwidth?! You just read on!


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SAFETY FIRST!

POWER LINE SAFETYEvery year, people are killed because they allow an antenna or support to contact overhead high-voltagepower lines. Remember: Never, NEVER run any antenna wire, feedline, support line, or guy wire OVER ORUNDER power lines !!! The antenna, support, or guy can fall into the power line -- OR the power line canfall into them. A wet antenna support rope will conduct high-voltage. NEVER assume it is safe to run a ropeor 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 is40 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 runperpendicular to, and away from, utility power lines.

LIGHTNING SAFETYEvery year, people are killed because they ignore simple lightning protection measures. The basic lightningarrestor 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 electrocutionand/or property damage from atmospheric lightning. The simple rule is: the longer the wire, the morevoltage will be induced upon it by a nearby lightning strike. This voltage will leap off the end of the wire ina fat blue spark, and you will hear it pop. Any time you hear thunder, disconnect the antenna wire from theradio. 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 anelectromagnetic pulse that will induce thousands of volts on your antenna wire, probably damaging yourradio. 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 adirect strike. ALWAYS disconnect the antenna from the radio when storms are approaching! Do not leavean unattended radio connected to an outdoor antenna. Tell the whole family!

LEGALEZEAntenna 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 INFORMATIONHEREIN.

The author cannot guarantee nor warranty that the plans and information herein are perfect in everydetail, nor that your use of them will satisfy your needs. The outcome is entirely your responsibility. Theyare 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 theseplans should not be considered an endorsement, just that they are readily available and of reasonablequality. I am under no agreement with Tandy Corporation.

FALLS AND OTHER HOME HAZARDSEvery 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 pullover 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 treeswithout 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 phonesand FRS radios. Check your owner's manual.


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Freq Band When1.8-2.0 160 M Ham Late Night2.3-2.4 120 M Bc Late Night3.2-3.4 90 M Bc All Night3.5-4.0 80/75 M Ham All night4.75-5.07 60 M Bc All Night5.9-6.2 49 M Bc, P All Night7.0-7.3 40 M Ham Eve, Night, Morn7.1-7.4 41 M Bc (foreign) Eve, Night, Morn9.4-9.9 31 M Bc, P Morn, Day Eve10.1-10.15 30 M Ham Morn, Day, Eve11.65-12.05 25 M Bc, P All day13.6-13.8 21 M Bc, P All day

SHORTWAVE RADIO SPECTRUM USERSThe shortwave spectrum runs from the top of the AM broadcast band at 1.7 MHz, to 30 MHz. Within thatrange various "bands" of frequencies have been allocated (by international treaties) for various purposes.There are three 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 bygovernments ("official" propaganda), or commercial shortwave contractors, who sell broadcast time tovarious 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, andeven outright hucksters. Most are fascinating, so beware of getting sucked into snake-oil (the biggest liarsare peddling bogus health products). With shortwave, you are your own news editor. If you have led asheltered life (TV brainwashed), you are in for a few big shocks. You will hear both good truth, and thingstoo good, or bad, to be true.

Amateurs ("Hams") are private, licensed radio operators who conduct two-way communications with otherAmateurs, worldwide, for hobby, and sometimes emergency assistance, purposes. Amateurs may transmitin several radio modes, using power up to 1500 watts. Amateurs are not CBers, and vice-versa. They mustmaster many radio-related topics, and pass exams, to receive their licenses. When you get tired of justlistening, 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 tobroadcast, 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 radiogrammessages 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 insteadof voice. Some shortwave listeners make seeking out utility stations the main focus of the hobby. Aninteresting utility you will want to use is WWV, the national time standard -- on 5.0, 10.0, and 15.0 MHz. Ifyou change your clocks twice a year, use WWV to get them exactly right. It's fun to keep your watch set toone 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 utilitiesare down.

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

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

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

M = Meter

Ham = Amateur

BC = Broadcast


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14.0-14.35 20 M Ham All day15.1-15.6 19 M Bc, P All day17.55-17.9 16 M Bc, P All day21.0-21.45 15 M Ham All day21.45-21.85 13 M Bc Not used much25.67-26.1 11 M Bc Not used much26.965-27.405 11 M CB Lots of noise28.0-29.7 10 M Ham All day, sometimes

Wavelength (feet) = 984 ÷ frequency (in megahertz).

Example: Middle of FM-band, 98 megahertz,

984 ÷ 98 = 10.04 feet.

Example: Bottom of AM-band, 0.55 megahertz,

984 ÷ 0.55 = 1,789 feet.

No, radio waves don't really go "squiggling"

through the air like a wave - we just draw them

that way on x-y (amplititude-time) graphs.

P = Popular, somewhat congested band withmany stations

Notice that 40 meter ham and 41 meter broadcastoverlap. This is a big problem we are working to fix inthe 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 placewhere most all your shortwave broadcast listening willbe concentrated. Note, however, that some stations fall slightly outside of these ranges, so make sure totune 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 findplenty. Locate one that you like, and that has fresh data. Shortwave listening magazines, and their websites, 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 THEORYLet's start here. Electricity,when flowing continuouslyin one direction is calleddirect current (DC). DCobeys the same rules aswater in a pipe - flow is afunction of pressure andresistance. More pressureand less resistance equalsmore flow. Congratulations- you just learned Ohm'sLaw.

Electricity, unlike water, canchange its direction of flowvery rapidly. Electricity thatflows back and forth iscalled alternating current(AC). Utility power changesdirections 120 times persecond, AM-band radioabout 2 million, FM-bandradio about 200 million, anda microwave oven about 4billion. Two changes, oneforward and then onebackward, are called onecycle or one Hertz. Thenumber of cycles persecond is called thefrequency. The distance onecycle travels in one secondis called the wavelength ofthat frequency. Since thespeed is fixed (the speed oflight), higher frequencieshave shorter wavelengths,and lower frequencies havelonger wavelengths.


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Section of wire, with electron concentration

alternately compressed and expanded.

They are "jiggling" back and forth,one-half wavelength distance.

This wire is 4 waves long at this frequency.

A half-cycle later, they have jiggled the other way.

When they moved, they emitted a radio wave.

Electricity travels down awire at near the speed oflight. At high frequencies, itbecomes practical to shoveextra electrons into one endof a wire then jerk themback out again long beforethe effect reaches the otherend. The result of this isthat, at such highfrequencies, we don't needa closed loop to have auseful circuit - we can setup an electron oscillation inan open-ended conductor,much like a vibrating coilspring.

If the length of theconductor is ½, 1, 3, orodd-multiple wavelengthslong, the conductor is saidto be resonant. The lengthof the wire is in agreementwith the length of the wave,at this frequency ofoscillation.

When electricity flows, itcreates a magnetic field. If the direction of the electricity alternates, the polarity of the magnetic fielddoes, too. It also creates an electrostatic field. A radio wave is simply energy which is alternating betweena magnetic field and an electrostatic field. This effect can go on indefinately, while there is a source ofenergy to sustain it. The energy at a given point diminishes with distance, because the size of the field isgrowing (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. Whenreceiving, a radio wave passes across the antenna, inducing an alternating electrical current in it. A radioreceiver then makes the signals intelligible.

Not all antennas are resonant. A long piece of wire, for example, will pick up a little of just abouteverything, and induce enough current to drive a receiver. Transmitting antennas, however, are almostalways 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 ahalf-wave dipole - it has two poles, and is one-half wave long at its design frequency. The sine graphs forvoltage and current are shown sitting atop the antenna.

DEFINITIONSCycle 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 persecond.

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

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


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Resonance - Input of alternating orpulsating energy timed to coincide withsomething's natural tendency tooscillate. 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 youpush forward just as the swing starts forward again. The resonance of aguitar string is determined by its length, weight, and tension. Theresonance of a quartz crystal is useful as a stable time-base forelectronic watches, computers, and radios. A particular length of wireor tubing is resonant at a particular frequency.Longer wires resonate atlower frequencies. Shorter, higher.

Wavelength - the distance energy travels, at the speed of light, duringone complete cycle. Electricity in wire travels a tiny bit slower than light, therefore, a wavelength in wire isnot 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 narrowbandwidth (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 isseveral 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. Ifthe 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 forwhich 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 radiateslike a dipole. If you shorten it or lower the frequency, it becomes anonresonant monopole. If you lengthen it or raise the frequency, itbecomes a nonresonant longwire. Zepps are bi-directional.

Ground plane - an antenna which uses an artificial ground to act as anelectrical mirror, making the antenna's radiating element "think" it's avertical dipole. They are omnidirectional.

Yagi - an antenna which uses a dipole with parasitic elements to make itdirectional.

Log Periodic Dipole Array (LPDA) - like a TV antenna. The LPDA designis directional, and has an extremely wide bandwidth, because of all itsdifferent element lengths.

Discone - an antenna that is a disc and a cone. They are omnidirectionaland have an extremely wide bandwidth, so are very useful as scannerantennas.

Feedline - the cable that carries signals between an antenna and atransmitter or receiver. This is usually coax or twinlead. The impedanceof the feedline should match that of the radio and antenna, or thefeedline will itself become part of the antenna, messing up your design plans.


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SIMPLE ANTENNASNow 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 receivingantennas, although a receiving antenna can transmit if it is resonant (or otherwise matched) and if it canhandle the transmitter's output power. A transmitting antenna can also receive, and will do so particularlywell 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 abouttransmitting antennas. Amateur operators, in particular, must pass exam questions on antenna design andtheory.

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 mixtureof 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 tofulfill that particular characteristic - whether it be directionality, gain, or bandwidth, or just all-around goodperformance.

I have been an antenna experimenter for over 25 years. My goal has always been to build cheap, simpleantennas that work well. It's all in the numbers. I had a $25 multiband shortwave antenna in the atticwhich would outperform any commercially-made $150 antenna. I had a discone scanner antenna made ofstainless steel welding rods which I built for about $3. I had a VHF yagi which would go five miles on ¼watt, 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 andan 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 finewire, strung along the ceiling on thumbtacks, will give you much more signal than the ridiculous telescopicwhip that comes with portable radios. Telescopic antennas are extremely too short for shortwavefrequencies!

There is one fundamental rule for receiving antennas: It must be at least ¼-wave long at the lowestfrequency you plan to use. Thus, if your lowest regular listening is on 3315 kHz, your wire should be 70feet long - minimum. Obviously, the 5-foot whip antenna on your shortwave is just a bit too short. Formuch better performance, it shoud be ½ wave long on your lowerst frequency (a Zepp). Multiwire dipolesare 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 2500kHz (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


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wire.

Electrical suppliers carry 500-foot rolls of #14 stranded THHN wire (about $35). Electricians frequentlyhave scraps and partial rolls. Farm supply stores carry #17 aluminum fence wire (about $12 for ¼-mileroll). Wal-Mart carries 100-foot rolls of telephone house wiring (about $10) (solder all 4 strands together atboth ends). Any wire will do -- but some will last longer than others. Stranded, insulated wire is best.

CLASSIC LONGWIRE INSTALLATIONThis 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 antennathat is mechanically and electrically sound, and a properly grounded radio, will consistently yield betterperformance and reliability. It really is worth the extra effort -- if you are a frequent, serious shortwavelistener.

Wires should be insulated, stranded #18 - #14. A jack may be soldered on to plug into your radio. If theradio 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 inthe 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.

Run the longwire as far from, and as perpendicular to, the power lines as possible. This will help reducenoise. If you have buried power lines, run your antenna any way you like. In the USA, pointing yourlongwire northeast will help bring in European stations in the daytime, on the higher shortwave bands.


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

As we dial up the frequency, the pattern of the antennachanges. The following diagrams show relative signal strength,


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looking down from the top. As the frequency goes up, the twolobes split into 4, then get stronger toward the far end of thewire. 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 thefrequency has the same effect on the pattern as lengtheningthe wire. The two side lobes squash and divide into four (at 1wave), then the directivity shifts toward the far end of the wire.They then come together as one long lobe. This is the primarynegative design characteristic of longwire antennas.

You should keep these patterns in mind when stringing up a single-wire antenna, sothat, at your favorite frequencies, a lobe is pointing toward the right part of theworld. 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 thefollowing page.

It's important to notethat the height of theantenna above groundalso affects the pattern.Lower antennas havehigher radiation angles -thus, more energy iswasted into the sky. Itwould be nice to be ableto get our antenna ½ wave above ground at the lowest operatingfrequency. This would mean that our 100-foot long antennashould be 100 feet high, but alas, this is hardly practical -- unlessyou 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 willdo quite well.

You can bend the antennawire around corners, butit's better if you do not.Try to keep most of thewire in a straight line.

OUTDOOR MULTI-WIRE SHORTWAVE ANTENNASThe easy way to overcome undesirable directivity shift is to design a multi-band antenna. The simplestmulti-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 isnearest our current frequency will predominate, while the others are relatively inactive. The top wire is thelongest, and we use it to suspend the lower, shorter ones.


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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 atconvenient locations. The result is that we can use 4 wires to pick up 8 shortwave bands with excellentefficiency. Below are the calculations for this antenna (frequencies shown are approximate band centers forthe meter bands shown):

Wire Fundamental 3rd Harmonic Length1 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 theacrylic 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 andpulley.

The wires all join at the peak of the house and connect to the center wire of 50-ohm coax (RG-58). Theshield of the coax connects to a wire which runs down to your ground rod. Solder and tape all connectionsto 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 supportedat the center, you can use lighter (cheaper) materials, since each span is only 72 feet long. Also, using twotall 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 ofdollars laying around, you can string it across three 100-foot towers - and probably get a write-up in anational 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 ANTENNASIndoor, or attic, antennas have several advantages: (1) no rain, no corrosion, no wind damage, (2) littlepossibility of lightning strike, (3) no problems with property association communists, and (4) no need fortrees 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 flatribbon wire. Since it isn't very strong, it must be supported at frequent intervals. This means we can stringit 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 thecalculations for this antenna, which uses 5-conductor rotator ribbon wire (frequencies shown areapproximate band centers for the meter bands shown):

Wire Fundamental 3rd Harmonic Overall Length Per Leg


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1 (top) 05.90 MHz (49 meter) 17.70 MHz (16 meter) 468 ÷ 05.90 = 79' 3" 39' 6"2 07.20 MHz (41 meter) 21.60 MHz (13 meter) 468 ÷ 07.20 = 65' 0" 32' 6"3 09.75 MHz (31 meter) 29.25 MHz (10 meter) 468 ÷ 09.75 = 48' 0" 24' 0"4 11.85 MHz (25 meter) 468 ÷ 11.85 = 39' 6" 19' 9"5 15.30 MHz (19 meter) 468 ÷ 15.30 = 30' 6" 15' 3"

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 thetwo halves of the dipole (notice that the center wire is cut twice - 7 feet is discarded) (cut each conductorby 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, ifpossible. Waterproof the connections. NOTE: If you don't want to build it, Radio Shack sells this typeantenna for about $35 (+ coax).

SIMPLE INDOOR SHORTWAVE ANTENNASLast but not least, let us examine simple wire antennas which may be designed and used indoors, for littleor no cost. Such are need by renters and apartment dwellers, and are always better than theway-too-short telescopic whip antenna.


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The Indoor LongwireThe simplest antenna which will drastically increase the amount of signal power (compared to thetelescopic whip) is merely a much longer monopole -- i.e., 40 to 80 feet of fine wire, thumbtacked to theceiling 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 anantenna 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, ina fairly straight line. For example, start at the far corner of the living room, go down the hall, to thefar corner of the bedroom. String up the wire on the ceiling using thumb tacks, dipping down underdoorways where necessary. Bring it down the wall at one end. Attach the plug for the radio. Scrapethe enamel coating off of the wire, then solder it to the center pin of the plug. If your radio has noantenna jack, simply solder a small alligator clip to the end of the wire and clip it to the telescopicantenna.

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

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

Indoor Electrical NoiseAlmost 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 maycome from street lights or even neighbors -- sources you cannot control. This is why you should put up asbig an antenna as possible, to get the induced signal power over the noise level.

NUTTY ANTENNASThese are all the crazy "try it and see if it works" non-designed antennas. They usually work better thanthe 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 thenearest electrical outlet, thus using the ground wires of the house as an antenna. The problem isthat they go in all different directions and tend to cancel one another. But it's the thing to try if youcan't even thumbtack a wire on the ceiling.

Aluminum foil: KBOHAE reports that, "Aluminum foil can be used to make some very effective indoorantennas. Especially when these antennas must be physically short (less than 1/4 wavelength). Ihave tried various antennas for shortwave listening over the years. The most effective indoorantenna that I have found is made from 2 or 3 strips of aluminum foil attached to the back of aworld map. This antenna has outperformed any inside wire antenna that I have tried. It has alsooutperformed some outside wire antennas." Thanks. This hadn't occured to the author before, but aslab of foil on the end of a wire should electrically lengthen it, like the capacitance hats on shortenedverticals.

Hidden wires: If the ceiling wire is too bold for your landlord, you can run a long wire under thebaseboard -- just push it under (between the carpet and baseboard) with the back end of a butterknife. Works pretty good in upstairs rooms, but not worth the effort on the ground floor. The besthidden 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 theradio. 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.


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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 andmuch longer than wide, it will work to some degree. Note however, that gutter joints tend to corrodeand 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 withit. Run it out the window and throw it over the roof. Tie the far end to a tree. Twist all four wirestogether 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 twometal "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 dipolefashion. This a decent attic antenna for lower frequencies than space would otherwise allow.

ADD-ON GADGETSLike every other hobby, shortwave listening attracts sundry gadgets. Some of these are quite useful, someare not, and some are for specialty purposes only.

Active Antennas, Preamps, PrescalersThese 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 sensitiveamplifier -- thus adding another input stage to the radio, which is, presumably, more sensitive than theradio itself. The ads promise that they will work as well as a 60-foot longwire. Baloney! Modern portableshortwave 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 accessto 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 relativelyimpressive "intermodulation suppression" specifications. This simply means that it will feed a relativelyclean 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 forvery old radios with poor sensitivity.

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

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

Audio FiltersThese process the audio (speaker) output in some desireable way. You must plug them in between theradio's speaker (earphone) jack, and an external speaker (or headphones) which you have to purchaseseparately.

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 youregularly get interference from stations on nearby frequencies (like 5 kHz away) from your favoritestations. You can filter out the "hetrodyne" squeal -- but they will not filter out electrical noise. Be notdeceived!

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

Computer Interfaces, Radioteletype and Fax Demodulators


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These connect to the speaker output of the radio, and usually to the serial input of a computer. They areused 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 oneswhich cost several thousand dollars work very well. You need to have a real need-to-know (like surveillancework) to justify the expense. Even then, most of the signals are encrypted, and it's illegal to interceptthem.

2003 Update: Computer Sound-card SoftwareLots of “digital” modes can now be demodulated (and transmitted, if you have the license) using only acomputer with a sound card and the appropriate software. See my web article Getting Started in DigitalModes 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 motelrooms.

Don't mess with multiband, analog dial-type radios. You know the ones that have 21 shortwavebands, 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. TheSangean 818 is the best radio made for less than $200 and I highly recommend it. C-Crane Co. isthe best source.

Don't mess with automotive radio shortwave converters or other such rinky-dink shortwave receiverkits. They all have a very limited frequency range, typically 1 or 2 megahertz -- and the usefulshortwave 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 greatantenna. They work no better than simply connecting to the ground screw in the center of the outletplate.

Don't blow $150 on fancy multiband trap-dipole antennas. Traps waste power. A $30 multiwireribbon dipole works better. Don't waste $75 on a simple inverted Vee dipole which you can buildyourself for $20 or less. Don't blow $135 on a name-brand discone antenna when Radio Shack sellsthe same one for $60.

ABOVE SHORTWAVE: THE DISCONE VHF SCANNER ANTENNAThis relatively small, strange-looking antenna may be usedoutdoors or in the attic, and is suitable for VHF and UHFtransmitting 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 doall of the above without ever taking it down to retune it.Just build it right (strong and weatherproof), put it up on awell-guyed mast, then use it for everything aboveshortwave.

General Design Notes

The aptly-named discone antenna is a disk and a cone. The cone (or skirt) is an equilateral triangle


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in cross section, where dimension "L" is equal to one-quarter free-space wavelength (0.25), which isas 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 thecone.

The disc must be supported by an insulator block. The center of the coax connects to the disc, whilethe 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 25-1300MHz coverage, which is ridiculous. Still, it is a fine, stainless steel antenna for $59.95, which willcover 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 usedfor FM micro-broadcasting. For cutoff to include:

Lowest Band: Cutoff: "L=":

FM broadcast 87 MHz 34"

Aircraft108 MHz 27½"

VHF-Hi 138 MHz 21½"

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

Ideally, the discone should be made of copper or aircraft aluminum ("Alclad") sheet metal, but a heavyscreen (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 thecone)

Two 2" worm-gear clamps (radiator hose clamps)

Sundry self-drilling sheetmetal screws, small machine screws (8-32) with double-flats, locks, andnuts.

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 thestraight 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 overlappingis essential to ensure the cone has a nice round shape, and does not tend to "point" at the seam. Becareful 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,


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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 thecap 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 inthe 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 SO239assembly about half-way into the cap. Make sure the SO-239 mounting screws to not hit the screendisc mounting screws! If in doubt, put a couple of layers of duct tape over the screw ends. Glue thedisc in place with silicone caulk. Seal the hole where the center wire exits the top of the cap. Solderthe center wire to the disc.

Form the cone and attach it to the PVC cap with machine screws and washers. Solder the groundwire 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 beshorted. The shield to cone (skirt) should show short (0 ohms) and center-to-disc should of course showshort.


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SUPPORT PLANNING AND ERECTION SAFETYIt's a pretty good idea to plan and graph any proposed erection which could fall into power lines -- asopposed to killing yourself like several people per year do. This is easy to do using graph paper or acomputer drawing program with a grid.

In the following example, a 40-foot aluminum tower will be anchored to a tilting base plate, bolted to aconcrete pad, then attached to the house gable at 13 feet, then guyed near the top (39 feet) with one setof 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 mastwill 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 thetower 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 cableextending away from the power lines. This will ensure that the top pole would topple before the tower, thusreducing the arc of fall to 40 feet. This failure mode assumes tornadic wind conditions. If it would taketornadic winds to topple the tower, the same would topple the power lines and poles! Thus, well-anchoredsteel 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 veryleast a safety rope should be tied to the top, extending perpendicular, away from the power lines, tiedsecurely to a tree trunk. If the tower falls during erection, the safety line will swing it down parallel to thepower lines. Never try to man-handle a tower into position without safety lines !! One extra helper is tentimes better!


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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 antennatheory and construction, obtain a few good books on the subject. I recommend you start with The ARRLAntenna Book.

73, de KV5R

Please see my new articles on KV5R.COM, too!

Copyright (c) 2002, 2007 by Harold Melton, KV5R. All Rights Reserved.May not be copied without express written permission. Feel free to link to this page.


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understanding shortwave radio and antennas by kv5r

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