U.S. Department of Commerce / NTIA poster shows frequency allocations across the radio spectrum. AM and FM radio, TV broadcast, as well as WiFi, Cellular, maritime navigation, and even space communication frequencies may be identified on the chart.
and efficiency. Directivity is the measure of how the energy of the radiation is directed in space, while efficiency is the measure of how much of the feed power was converted into useful electromagnetic radiation. Obviously the most efficient frequency of a dipole antenna is when the excitation frequency matches the half-wavelength size of the arms. The starting point for the RF circuit is the signal generator. In a laboratory, an RF signal generator produces the signal to test the proper functionality of RF circuits to ensure proper field functionality. RF generators produce a well-defined signal frequency, amplitude, and phase. This signal is then fed into a device under test, and the resulting amplitude and phase changes are measured. The changes are captured in a matrix called the “S” parameter matrix. In real-world applications, such as a satellite comms system (see sidebar), the role of the generator is assumed by the transmit/
receive radio that combines multiple functions including RF signal generation, modulation, encoding and transmission of the signal to and from the antenna. In practice, signals are transmitted between LRU’s and antennas on purpose-built coaxial cables, the construction of which consists of: • Center Conductor • Dielectric • Outer Conductor • Jacket (if required for environmental protection) The term coaxial refers to the inner conductor and the outer shield sharing a geometric axis. The electromagnetic field carrying the signal exists only in the space between these two conductors. Flexible coaxial cables may be selected with a stranded center conductor which aids flexibility but increases loss in like-for-like designs. Conformable or Semi- Rigid cables typically have a solid center conductor which is friendlier to loss-budgets but more difficult to route. The outer conductor on flexible cables can be a conductive braid screen, tape, or combination. Better screening, such as a double-screen or screen-plus- tape, offer lower loss, but again can affect flexibility. Special “conformable” cables have a tin-dipped outer braid (sometimes with a tape serving as an auxiliary shield underneath). Semi-Rigid cable types have solid outer conductor (commonly copper or aluminium). See sidebar for more cable details.
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Radiation pattern of vertical half-wave dipole; vertical section shown in linear scale (top) and in decibels (bottom)
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QwikConnect • January 2023
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