Focus On Early Secret

 Basic Radiowave and Antenna Parameters Terms and definitions commonly encountered in the study of antennas and radio wave communication can be found in Institute of Electrical and Electron- ics Engineers (IEEE) [19, 20] and European Telecommunications Standards Institute (ETSI) [21] standards, as well as in FCC rules [3] and ITU-R reports and recommendations [22]. Here, we will present some of the parameters and definitions that are especially relevant and specific to the problems in this textbook. Antenna factor (AF) in a receiving antenna is the ratio of electric field strength to the voltage across the terminating impedance connected to the antenna. By common usage in the telecommunications industry [23], the antenna factor is sometimes stated under conditions where the field incident on the antenna is not uniform along the antenna. Antenna factor is therefore, by common usage, a parameter that is measurement site dependent. Antenna illumination efficiency, sometimes called the...

 The Immediate and Short Distance Sky-Wave books can be had just for the asking. Write to: Commanding General USAESEIA ATTN: ASC-E-TP Fort Huachuca, Arizona 85613-5300 or call AUTOVON 879-7685. Every division or higher level unit should have a book for their area. There are 35 volumes covering all areas of the world. For our use, we will use column 5 from the ISD book and column 7 from the GW book. 11. Let's do a couple of sky-wave frequency selection problems. a. Situation 1. Let us say that we have a radio net with only two stations which are 50 miles apart. The radio set used is the AN/GRC-142. We are using radioteletype during a period of low sunspot activity. We need to select two frequencies for 24-hour communications. We will use a doublet antenna erected less than a quarter wavelength above the ground for our highest frequency so that our radiation pattern is straight up. Keep in mind that we might have to vary the antenna height from 40 feet down to 10 feet (any lower than...

 FREQUENCY SELECTION. 1. Prior to selecting frequencies for a radio circuit, thought must be given as to what type of antenna will be used. Often, during displacement or during an alert, at least two types of antennas will be used -- a whip while moving; a doublet or suitable compromise antenna while mobile at a halt. Antenna selection will determine the choice of frequency, not the other way around. Looking at the various Immediate Sky-Wave Distance (ISD) charts, page 125 to page 141 (we will use column 5 on the charts) for various antennas, we see that the most reliable antenna is a doublet with reliability dropping until we reach the poorest antenna -- the whip. We also see on the ISD charts that for the same distance, the frequency increases with each type of antenna, with the whip having the highest frequency. What conclusions can we draw from these comparisons? First, we must have two frequencies for sky wave use -- a day and a night frequency. Also, when forced to use a whip...

PRACTICAL HALF-WAVE ANTENNAS 1. Introduction. a. We have discussed how to calculate a half-wave. Now, let's discuss the patterns half-wave antennas make. We have shown in figures 34 and 35, the radiation pattern of an antenna in free space. Since our antennas must be erected over earth, the patterns created are different. b. The ground has the greatest effect on the medium and high frequency antennas which are mounted fairly close to it in terms of wavelength. 2. Ground Effects. a. If a horizontal antenna is erected some distance above ground, its radiation pattern is as shown in figure 37. Notice that some of the energy travels directly to the distant station. Notice also that part of the energy strikes the ground directly in front of the antenna. As we have learned earlier, phase reversal takes place and may cancel out the direct wave if the ground-reflected wave and the direct wave arrive at the same time and are out of phase. If they arrive in phase, the ground reflected wave a...

 The transmission line transfers the RF power from the transmitter to the antenna. There are two general types of transmission lines: resonant (tuned) and nonresonant (untuned). (a) Resonant feeder line is the same length as the antenna. It is rarely used in tactical applications. (b) A nonresonant transmission line is one that has an SWR of less than 1.5. In order to achieve this, the impedance of the antenna and the transmission line must match. An antenna tuning unit is used in some applications to match the transmitter to the line and antenna. (1) Single-wire feed. A single wire can be used as a nonresonant feed line. Because the impedance of a single-wire feed is 500 to 600 ohms, a point on the half-wave antenna must be selected that will match the impedance of the line. The antenna impedance varies from 2500 at the end to about 73 ohms in the center. A point 14 percent from the center of the antenna will provide the 500 to 600 ohms required (A of figure 31). To reduce radiati...

 Half-Wave and Quarter-Wave Antennas 1. Basic Theory. a. The antenna is part of the electrical circuit of the center;">transmitter and receiver. As mentioned earlier, radio waves travel in free space at 300,000,000 meters per second. Our antenna is not in free space but erected over and near terrain features which affect antenna length. For that reason, the physical length of the antenna is shorter than the electrical length. b. There are several factors which cause the antenna to be physically shorter. As the diameter of the antenna wire increases, the velocity or speed of the radio waves is slowed, decreasing antenna length. c. Another factor that affects antenna length is the feed line that connects the transmitter to the antenna. The insulators also affect antenna length. This is called end effect and is compensated by making the antenna 5 percent shorter. Thus to find antenna length you use the formula N = number of half waves L = .95(492/F) or said another way L = N-.05(...

 SEO for WordPress Posts WordPress posts have a few more SEO options than WordPress pages. We still have the Yoast WordPress SEO plugin options that we saw in the previous section on 'WordPress Pages', and they are used in exactly the same way. If you need to noindex, follow a post (as we will do later when we look at setting up category pages), then you do it using the 'Advanced' tab of that plugin's options. Also, you will notice that if you go to Posts –> All Posts, in the WordPress Dashboard, there are a few extra columns tacked on to the end of the table: I would suggest you ignore these columns, as they are part of the keyword optimisation features in the plugin. Remember I advised you earlier not to use those features as they are likely to hurt, rather than help, your site's rankings).  Post Categories One of the benefits of using posts for publishing your main, visitor-orientated content is that you can group them into categories. These act as organis...