This is a useful band for moderate distance transmissions because the coverage usually extends further than VHF and UHF discussed next module , but is not as affected by ionospheric disturbances as HF. The reception distance over ocean is considerable greater than over land, making it particularly useful for Naval use.
Like all the lower frequencies, transmission in this band are usually ground waves, which limits the transmission distance, but makes them less susceptible to ionospheric disturbances than HF. HF - The Navy makes extensive use of this band for communications.
It is also used for long range "over-the-horizon" radar. Due to the skywave transmission mode, HF radiation can travel great distances, sometimes to the other side of the earth. Due to its versatility and large coverage area, this is a very crowded band and the military can only use a few frequency regions scattered throughout this band.
The most efficient transmissions require fairly large antennas, therefore it is most useful when at least one of the stations is on shore. The antenna size limits its use on aircraft. It cannot be used for satellite communications since it is reflected by the ionosphere. Many of the former uses of HF by the Navy are now being taken over by satellite communication systems.
However, we expect that the Navy will continue to use HF for quite some time in the future. The primary drawback to HF use is that it is highly susceptible to changes in the ionosphere and therefore several frequencies must be available for use. The rest of this module will examine how the ionosphere affects HF transmission and how to predict these effects. Today's Navy uses a large number of radio frequencies for communications, targeting, search and rescue, navigation and other uses.
The radio spectrum is becoming a crowded place. It is necessary for the Navy to use a wide variety of frequencies to accomplish its mission. In this module we will examine the lower radio frequencies. The environmental factors affecting these frequencies primarily deal with what is going in the upper reaches of the atmosphere, a region called the ionosphere. The characteristics of the ionosphere and how it affects electromagnetic radiation will be discussed later.
All antennas are passive devices. An ideal isotropic antenna which is only theoretical would radiate the signal out in all directions with no gain 0 dBm. In reality, antennas reduce the signal strength in some directions and increase the signal strength in others, providing gain. Omnidirectional antennas radiate out perpendicular to the direction of the antenna in donut or flattened torus pattern, as shown in Figure 1. Examples of omnidirectional antennas include dipole and monopole antennas.
A dipole antenna consists of two metal conductors in line with each other. Monopole antennas have a single conductive line and are mounted over a ground plane.
The ground plane plays a critical role in the quality of the transmission. For lower frequencies a larger ground plane is necessary; in these cases, the earth is often used. Examples of monopole antennas include whip antennas and mast radiators, such as the ones sometimes used in AM broadcast towers. By redirecting some of the energy of the signal, the antenna can provide gain to the overall signal strength; a dipole antenna could gain between 1 and 5 dBm.
More directional antennas such as Yagi antennas can provide even greater gains, on the order of 6 dBm to 15 dBm, by providing a very narrow transmission beam. Yagi antennas consist of multiple elements used to focus the transmission beam and produce larger gain. Figure 3 shows a radiation pattern from a MHz Yagi antenna with 13 dBi of gain.
Directional antennas not only provide better gain; they also help reduce the amount of interference received at the antenna by producing an overall signal loss from directions where the antenna does not point. If there is a known interferer in proximity, placing the antenna such that there is a loss from that direction can help alleviate interference.
Due to the specific directional nature of the Yagi and other directional antennas, they are limited to applications where the antenna can be pointed at the destination, such as in point-to-point networks.
Additionally, too much gain on an antenna can cause it to violate local regulatory restrictions for radiated output power. Refer to the user manual on the transceiver or with a local regulatory body for emissions rules. Often, to place an antenna in the best location for transmission, a cable will be required to connect the transceiver to the antenna. Thus, not only are high-frequency common-mode signals not rejected; they are distorted, producing offsets.
For some applications, where RF interference is a strong possibility, the AD difference amplifier has wideband common-mode rejection and is designed for line-receiver applications; it may be a useful substitute for an instrumentation amplifier. Sensors are often connected to their signal-conditioning electronics by long cables. Radio engineers have a term for such long pieces of wire; they call them antennas. The long feeders from sensors to their electronics will behave in the same way and will serve as antennas, even if we do not wish them to do so.
It does not matter if the sensor case is grounded-at high frequencies the reactances of the case and feeders will allow the system to behave as an antenna, and any high- frequency signals E -field, M -field, or E-M -field which it encounters will appear across any impedances. The most likely place for them to end up is at the amplifier input. Precision low-frequency amplifiers can rarely cope with large HF signals, and the result is error — commonly a varying offset error.
An easy free lunch can always be obtained by persuading an innocent to bet on his or her circuit being free of such problems. Using a ham radio HT on the two-metre MHz band, one watt at a distance of one meter for one second will win you your free lunch almost every time. But a less-dramatic test can be equally convincing.
Disconnect the sensor and its leads. Short-circuit the amplifier input terminals to each other and to the amplifier circuit common probably ground with the shortest possible links and measure the amplifier output; observe its stability over a few minutes. Now remove the short-circuit, replace the sensor leads and place them in their normal operating environment. Disable the excitation and short-circuit the signal leads at the sensor end.
Again measure the amplifier output, and its variation with time. Weep quietly. It is often possible to see what is happening by using a high-frequency oscilloscope or a spectrum analyzer, which is more sensitive but less easy to interpret to measure the HF noise, both normal mode and common-mode, at the amplifier input; but normal mode measurements must be treated with some suspicion, because the oscilloscope itself — and its power- and probe leads — may themselves introduce signals and invalidate the measurement.
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