Amateur Frequency Bands

Figure 9-1 shows the frequency spectrum indicating the frequency allocation to the various well known services, and indicating the common amateur frequency bands. The lowest amateur frequency is the 160-meter band. This band, just above the AM broadcast band, is subject to special frequency and power limitations which vary according to geographical location. The limitations have been established to prevent interference with LORAN (long-range navigation) stations. Table 9-1 indicates the major amateur bands by the common wave­length in meters. You will hear hams say that they are working at 15 meters or 10 meters, or 2 meters. This means they are transmitting and receiving on a frequency within the respective band.

Types Of Shortwave Antennas

  Many different types of antennas are used by hams depending on the band of frequencies in which they are operating. No matter what amateur band is considered, the basic antenna theory discussed in the previous chapters still applies. For example, the free space radiation pattern and other characteristics of a half-wave 3 GHz dipole is the same as a half-wave dipole at 3 MHz. Only the size is different. The 3 megahertz dipole is 165 feet long, whereas the 3 gigahertz dipole is about 2 inches long!

Electrical Length vs Physical Length

Because it gives the half-wave length in feet, a somewhat easier equation for the half-wavelength of an antennas is:

                492              

Half-wave Length (in feet) =                      Frequency (in Megahertz)

However, the half-wave length that one actually uses for the physical length is about 5% shorter than that calculated with the above equation. The reason for this is that the electrical length of the antenna is different than its physical length. The antenna wire is usually fastened to insulators by making several twists or wraps at the ends. This produces what is known as “end effect.” End effect, and the fact that energy travels along the antenna wire at slightly less than its free space velocity, makes the effective electrical length about 5% longer than the physical length.

Long Wire Antennas

Probably the most common type of antenna used by early radio amateurs was a long wire antenna. This type of antenna is a horizontal wire antenna which is at least one wavelength long, sometimes much longer. It may or may not have a terminating resistor, as shown in Figure 9-2. When no terminating resistor is used, it is bidirectional, with the maximum response off both ends. If the terminating resistor is used, maximum response is off the terminated end.

Zepp (or Zeppelin)

A type of end-fed antenna shown in Figure 9-3 is called a Zepp antenna. They get their name because they were first used on the German lighter-than-air dirigibles called Zeppelins. This antenna is one-half wavelength at the lowest frequency.

The impedance at the end is much higher (e.g. 1000 ohms or more) than the -impedance at the center of a resonant dipole (75 ohms). Usually widely spaced

2-wire transmission line is used to couple to the antenna, and a matching box must be used between the transmitter and the transmission line.

Shortwave Listening

For the person interested in antennas for shortwave listening, a long wire antenna that is portable (Radio Shack catalog no. 278-1 374) and can be used for quick, temporary setups is a coiled unit that clips over a telescoping antenna and can be extended to 23 feet like a measuring tape.

Vertical Antennas

Another very popular amateur antenna is a vertical antenna. In Chapter 1 we showed the omnidirectional pattern of a vertical quarter-wavelength antenna. Generally, the radiation pattern of the vertical antenna is not deflected upwards by the ground nearly as much as the pattern of the horizontal antenna. Figure 9-4 shows how increasing the length of the vertical antenna affects the radiation pattern, Increasing the frequency while keeping length constant produces exactly the same result.

Loops

A loop consists of one or more turns of wire wrapped around a supporting structure that is shaped into a rectangle or a circle. Figure 9-5 shows examples of loop antennas. A loop is quite directive and relatively compact, which makes it attractive for portable equipment. Its small size makes it easy to rotate for either maximum sensitivity or minimum interference. It is usually much smaller than the wavelength at which it is used. Its characteristics can be modified by changing the antenna Q (quality factor). This is usually done with a fixed or adjustable magnetic (ferrite) core. The radiation resistance of small loops is so low that it is nearly impossible to match a transmitter to them, so that loops are used primarily for receiving.

Beam Antennas

An antenna is called a beam antenna when its radiation/reception pattern is a very narrow beam to make it a very directional antenna. We’ve seen many examples of Yagi antennas which are beam antennas. Hams use beam antennas at all frequencies. You may have noticed very large Yagi type arrays mounted on a tower in the backyard of an amateur radio operator. In many cases, the antennas can be rotated. Beam antennas are about the only type shortwave antennas besides loops that can be rotated easily to permit efficient transmission and reception in any direction.

Quad-Cubical Antennas

An antenna that combines directivity and loop antenna characteristics is the quad-beam antenna shown in Figure 9-6. It is one of the more effective long range shortwave antennas. It consists of two or more square loops of wire supported by spreaders that were often made from bamboo, although fiberglass is more common today. The length of each side of the square is equal to a quarter wavelength of the transmitted signal. Loops may be either driven or parasitic. If parasitic elements are used, they have the same dimensions as the driven element. By tuning with shorted tuning stubs, the parasitic loop may be made to function as either a reflector or director, giving the antenna its directivity characteristics. In Figure 9-6, the parasitic element is a reflector.

Collinear Arrays

A collinear array consists of two or more elements mounted in-line along the same axis, with the current in each element in the same direction (in phase) as all other elements. The radiation pattern is bidirectional, with maximum radiation at right angles to the line of the elements. Although large collinear arrays may contain from 48 elements to as many as 128 elements, amateur arrays rarely contain more than 4 elements.

Other Antennas

There are other less frequently used types, such as the V and rhombic antennas, and broadside and endfire arrays. Information on these types, as well as more detailed descriptions of the antennas described above can be found in The ARRL Antenna Handbook, published by the American Radio Relay League, New­ington, Connecticut, an association for radio amateurs.

Antenna Placement

Most of the predictions of antenna performance are based on free-space behav­ior. Objects surrounding an antenna after it is mounted may affect its performance significantly. The erection of new buildings, the growth of trees, adjacent wires or towers are common environmental changes that occur to change an antenna’s performance.

As we have stated many times throughout this book, one of the best ways to isolate an antenna is to mount it high above other obstructions. This may be very difficult for the very large and heavy antennas used for the lower frequency amateur bands. If the antenna is not mounted high enough its radiation/recep­tion pattern will be affected. For example, Figure 9-7 shows vertical radiation patterns of a horizontal antenna at various heights above ground. Generally the lower the antenna, the more the radiated (or received) wave is directed upwards. Consequently, the antenna position adversely affects long range communica­tion. Also, the impedance of an antennas is dramatically reduced for heights much less than one quarter wavelength.

SHORTWAVE ANTENNA PROJECTS

Now that we know the type of antenna used for various applications, let’s look at a number of antenna project that you can build for your particular frequen­cy application.

We have chosen antennas that will be particularly useful for people that lis­ten on shortwave, listen to scanner radios, are active ham operators, or that have business and mobile radio applications. Diagrams with dimensions, parts lists and construction details are included in order to make it easy for you to build useful antennas.

The only equations we will use will be those needed to calculate antenna element lengths, which are derived from the basic equation for wavelength given in Chapter 9 and repeated here.

x, wavelength in meters =                 300 (meter/sec)

Frequency (In megahertz)

Since a meter equals 3.28 feet,

x, wavelength in feet =                      984 (feet/sec)    

Frequency (In megahertz)

Thus, a half-wavelength in feet,         =          492                 

                                                   2      Frequency (in Mhz)

In thin conductors there are self-inductance effects that make the physical length 5% shorter than the theoretical or “Free Space” length. Therefore, we use the following equation for one-half wavelength antennas,

Dipole length in feet, x =               468             

2              Frequency (in Mhz)

And for a one-quarter wavelength antenna,

Element length in feet, x =            234             

                                  4       Frequency (in Mhz)  

SHORTWAVE LISTENING

Project No.1 — Long Wire Antennas

The oldest antenna used by Shortwave Listeners (SWLers) has been the long wire. We talked about long wire antennas in Chapter 9 and showed the radia­tion pattern of a long wire. Antennas have “reciprocality”. This means that the receive pattern is identical to the transmit pattern. However, when receiving shortwave signals, it is not as important to match the feed line. Just run a wire 20 to 150 feet to some nearby support.

Trees work well, other buildings, or even a pole put up for this purpose. It is good to get the antenna as high off ground as practical, but keep your long wire antenna away from electrical wires! Not only for safety, but also because noise can be coupled from power lines.

On the lower frequencies, a long wire will favor signals off the sides. As you go up in frequency it will tend to favor signals off the ends. If you have enough room, run long wire antennas in different directions to favor different parts of the world. However, if you do, you should use a video switch like the one in Project No. 4 to isolate each antenna. Figure 11-1 shows a typical installation.

Parts List for Long Wire Antenna

        QtY.         Description                                  Catalog No.

         2              Insulators — Stand-off screw mount  15-853

   (choose type)             — Stand-off masonry            15-820

                                   — Stand-off strap mount       15-823

                                   —Mini-egg                          278-1335

As required Antenna wire — 70’ roll 14-gauge

stranded copper                         278-1329

         1              Ground wire — 40’ 8-gauge aluminum 15-035

         1              Ground rod —8’ copper-plated          15-529

Project No. 2— Vertical Wires

Another very simple antenna for listeners to shortwave radio is the vertical antenna. We showed the radiation pattern of a vertical antenna in Figure 9-4.

A vertical antenna is very simple to make, just run 8 to 15 feet of wire straight up from your radio. Your vertical will be omni-directional and pick up signals equally well from all directions. Your vertical will also pick up less man­made noise. That’s because most man-made noise is horizontally polarized.

Any kind of wire will do, speaker wire, power cord wire, magnet wire and the like, as long as it is stiff enough to stand alone. Or you can secure the top end of it to hold it as long as you do not short it out. Just strip the insulation off the bottom end and attach to your radio’s antenna connection. Figure 11-2 shows the simple installation.

Each wire in the fan is a dipole antenna cut for a shortwave band. Every SWL’er has their favorite bands. Listed are the dipole lengths for the common shortwave broadcasting bands. The lengths are calculated for the top frequency in the band.

              SW Band (Meters)                    Length (In feet)

                       49M                                        75ft

                       41M                                        63ft

                       31M                                        47ft

                       25M                                        38ft

                       21M                                        34ft

                       19M                                        30ft

                       16M                                        24ft

                       13M                                        21ft

Your author has used as many as 5 dipoles on one antenna. It is necessary to keep the ends of the individual dipoles at least a foot apart to prevent interac­tion between dipoles. A typical installation is shown in Figure 11-3.

Choose the end insulator that will work best for your installation. Use the nylon rope to tie the other end of the insulator to a tree or pole. Each dipole will have a different length. Make sure that you solder all wires together at the “Dogbone” insulator. Poor solder joints will produce unreliable and noisy con­nections. You may have to use an adapter to match the RG-59U coax connec­tor to your receiver.

Parts List for Fan Dipole Antenna

    Qty.             Description                                  Catalog No.

 As required     Antenna wire — 70’ roll 14-gauge

                        stranded copper                         278-1329

     1                “Dogbone” insulator                     278-1336

 2 per dipole     End insulators — Screw mount     15-853

 (choose type)          — Masonry mount               15-820

                               — Strap mount                    15-823

 As required     RG-59U coax                              278-1315

     1                Cable tie                                     278-1622

                                                                         or 278-1652

 As required     Light nylon rope or heavy duty fishing line

                       RF adapter connector                  To match receiver

Project No.4 — The Complete SWL Station

Here is an installation for the avid listener to shortwave radio. With various skip angles and reflections from other metal objects around your station, it is hard to predict which antenna will work best. And the “best” antenna will change with the time of day and the station you are listening to. In this installation, there are several antennas and a video switch as shown in Figure 11-4a. By having sever­al antennas, you can switch around until the signal is clearest. The high isola­tion and low loss of a video switch is ideal in this application.

Your long wire and vertical antennas can be easily converted to “F” con­nectors for use with the video switch. Just cut in half a 75-ohm coax patch cable with F connectors on each end, available at Radio Shack. Solder the cen­ter conductor of the coax to your antenna wire, as shown in Figure 11-4b. This makes a compatible connector to the video switch. The other half of the cable can be used between the switch and the receiver. However, you may have to use an adapter at the receiver depending on the receiver input connections. If you use the fan dipole antenna, you may need an adapter connector to match the RG-59U connector of the antenna to the video switch. Radio Shack has many adapters available. Choose the video switch with the number of inputs equal to the number of antennas you want to use. The parts list only includes the additional parts required for the switching. The parts for each antenna are listed previously with the antenna.

Parts List for SWL Station Switching                                   

        Oty.                   Description                               Catalog No.

        1               Video switch —2 inputs                       15-1247

   (choose type)                     —3 inputs                       15-1248

                                             —2 inputs, high-isolation  15-1249

   As required Ant. patch cable —4 ft.                             15-1533

         RF adapter connector                               To match receiver

The section, “shortwave antennas” was written  by Alvis J. Evans and the section, “shortwave antenna projects” was written by Kent E. Britain. These selected sections are only two from a book called ANTENNAS Forth Edition and is available at Radio Shack (part #62-1083B). The information here is a small part of what is covered within the book mentioned. The information contained here is provided free and for educational purposes only.