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Antennas for RFID Transponders
Published in Albert Lozano-Nieto, RFID Design Fundamentals and Applications, 2017
Given these two regions, it seems obvious that antennas for RFID transponders should be designed to operate in the far-field region. However, the boundaries between far-field and near-field regions depend on the relationship between the physical dimensions of the antenna and the wavelength of the propagating signal. In particular, the dimensions of the antenna should be comparable to the wavelength of the signal in order to achieve optimal performance. The wavelength of signals operating in the LF region is around 2.4 km (1.5 miles), while the wavelength for signals operating in the HF region is 24 m (78 feet). Therefore, at these frequencies it would not be practical to build antennas with dimensions similar to the wavelength of their signals. Any antennas that can be built in a practical manner for RFID transponders operating in the LF or HF ranges will be electrically small and therefore highly inefficient. In contrast, RFID signals operating in the UHF region have a wavelength of approximately 30 cm. Although this dimension is still very large for building practical antennas, the following sections will explore how they can be modified to obtain antennas for UHF transponders that will operate in the far-field region.
Propagation
Published in Geoff Lewis, Communications Technology Handbook, 2013
Solar cycles. Although the production of flares and eruptions from the sun’s surface occurs in a random manner in the short term, they repeat fairly predictably over an 11-year cycle. Cycle 22 started in September 1986 and the activity peaked during 1990. Such solar activity chiefly affects propagation in the HF bands. A close relationship has been noted between abnormally quiet days (AQD) (days when solar activity is minimal) and the magnitude of the following peak. During such periods, the MUF can rise intermittently as high as 50 MHz, and the 21 and 26 MHz bands can sustain reliable communications for considerable periods. This correlation, which holds good as far back as 1885, the earliest year for which data is available, allows future peaks to be predicted for several days ahead.
Aeronautical Communications Channel Characteristics and Modeling From Legacy toward Future Satellite Systems
Published in Athanasios G. Kanatas, Athanasios D. Panagopoulos, Radio Wave Propagation and Channel Modeling for Earth–Space Systems, 2017
Ana Vazquez Alejos, Manuel Garcia Sanchez, Edgar Lemos Cid
The HF frequencies currently used by the aeronautical mobile services for distress, safety, and other communications, including allotted operational frequencies, suffer from harmful interference and are often subject to difficult propagation conditions (ERC, 2001; WRC, 2003). Unauthorized operations using aeronautical frequencies in the HF bands are continuing to increase and are already a serious risk to HF distress, safety, and other communications. Enforcing compliance with these regulatory provisions is becoming increasingly difficult with the availability of low-cost HF SSB transceivers (ERC, 2001; WRC, 2003).
QAM and PSK modulation performance analysis over narrow band HF channel
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2022
By definition, High Frequency HF band is a set of individual broadcasting bands with a frequency range of 3 to 30 MHz (2009). Recently, HF communications attracted the attention of the researchers due to their importance in many applications such as long-distance broadcasting, military and maritime communications, and aeronautical systems. The increase demand of developing HF systems is due to its inherent features: 1) low cost; there is no need to use expensive equipment. 2) More robust to jamming and hacking. 3) The ability to provide long-distance communications at relatively low-transmitted power (Xu, Yang, and Wang 2005). On the other hand, many challenges need to be solved before using HF communications. HF communication suffers from the instability of the ionosphere layer which is very sensitive to changes in temperature. This instability affects the quality of the link (Diakhaté et al. 2016) and leads to a decrease in the available and usable channels. In addition, by the existence of the large number of users, traffic and interference between signals will also increase. For this reason, HF systems usually operate on 3 kHz channels to overcome the abovementioned issues (Lamy-Bergot et al. 2015; Bernier et al. 2013). Note that 3 kHz channels are the minimum width channel to work with single tone signals in HF systems. In addition, fading resulting from the multipath of the traveling wave (more than 5 Hz) is one of the issues to be respected to ensure a good connection between the transmitter and the receiver (Department of Defense 2011; Measurement 2006). Furthermore, the size of the antenna is another problem of the wideband HF communication systems. In this case a big antenna (30x30m-sloping VEE antenna) is needed to ensure that all the band has been used, however, this tends to reduce the mobility property of the transceiver. In addition, the work studied the ability to use some advanced modulation schemes in HF communication systems in order to minimize the tremendous energy used in such systems especially in military applications. In addition, the environmental effects were studied and analyzed to facilitate the selection of appropriate scheme and power level based on the data rate needed.