Shortwave Antennas shortwavetech.8k This chapter is about antennas used by amateur radio operators. They are classified under the title of shortwave antennas. Such antennas are designed using ... The Radio Amateur We’ve all heard the term “ham” radio operator. Or we have seen an unusual series of numbers and letters and the words “amateur radio operator” on automobile license plates. Both names—ham and amateur—identify a person whose hobby is experimenting with radio. Amateur radio operators have been around since the birth of radio communications nearly a century ago. In the early years, the amateurs were assigned all of the unused frequency spectrum, which at that time was anything above 1500 kilohertz (1.5 MHz). Recall that wavelength can be calculated from the following equation: 300,000,000 (meters/sec) Wavelength (meters)= Frequency (cycles/sec) If we substitute 1.5 MHz for the frequency as shown, then 300,000,000 Wavelength= 1,500,000 = 200 meters The wavelength is 200 meters. As a result all the frequencies above 1.5 MHz were called (and still are today) “short waves” because their wavelength was shorter than 200 meters. The early amateurs using “short waves” achieved unheard of long distance communications. Commercial communications, seeing this success, began using “short waves,” and started crowding out the amateurs. To settle the issue, an international conference allocated the frequencies to be used for various activities, and reserved specific bands of frequencies for use by amateurs. 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. Besides the bands in Table 9-1, there are some 12 bands in UHF, SHF and EHF from 900 MHz to 300 GHz. Recently, conferences in Switzerland have resulted in further reallocation of certain portions of the frequency spectrum. If you desire more information, refer to FCC Rules and Regulations, Volumes II and VI, which are available with updated corrections in many libraries, or can be obtained from the Superintendent of Government Documents, U.S. Government Printing Office, Washington, DC. 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
Posted on: Tue, 05 Aug 2014 23:22:41 +0000
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