Antennas. Yi Huang

Antennas - Yi Huang


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2–4 GHz S 0.15–0.075 m Short wave, mobile radio 4–8 GHz C 7.5–3.75 cm Compromise between S and X, radar 8–12 GHz X 3.75–2.5 cm Radar, satellite 12–18 GHz Ku 2.5–1.7 cm Satellite and radar 18–27 GHz K 1.7–1.1 cm Satellite and radar 27–40 GHz Ka 11–7.5 mm Communications and radar 40–75 GHz V 7.5–4.0 mm Communications and radar 75–110 GHz W 4.0–2.7 mm Communications and radar Frequencies of some popular wireless systems 535–1605 kHz AM radio broadcast band 3–30 MHz Short‐wave radio broadcast band 13.56 MHz NFC 88–108 MHz FM radio broadcast band 175–240 MHz DAB radio broadcast band 470–890 MHz UHF TV (14‐83) 698–960 MHz Cellular mobile radio (2/4G) 1710–2690 MHz Cellular mobile radio (2/3/4G) 3.3–3.8 GHz Cellular mobile radio (5G) 1.227 GHz GPS L2 band 1.575 GHz GPS L1 band 2.45 GHz Microwave, Bluetooth, Wi‐Fi 3.1–10.6 GHz UWB band 5.180–5.825 Wi‐Fi bands

      Although the whole spectrum is infinite, the useful spectrum is limited and some frequency bands, such as the UHF, are already very congested. Normally significant license fees have to be paid to use the spectrum, although there are some license‐free bands: the most well‐known ones are the industrial, science, and medical (ISM) bands. The 433 MHz and 2.45 GHz bands are just two examples. Cable operators do not need to pay the spectrum license fees, but they have to pay other fees for things such as digging out the roads to bury the cables.

      The wave velocity v is linked to the frequency f and wavelength λ by this simple equation:

      (1.14)equation

Schematic illustration of frequency vs wavelength

      Logarithmic scales are widely used in RF (radio frequency) engineering and antennas community since the signals we are dealing with change significantly (over 1000 times in many cases) in terms of the magnitude. The signal power is normally expressed in dB (decibel), which is defined as

      (1.15)equation

      Thus, 100 W is 20 dBW, or just expressed as 20 dB in most cases; 1 W is 0 dB or 30 dBm; and 0.5 W is −3 dB or 27 dBm. Based on this definition, we can also express other parameters in dB. For example, since the power is linked to voltage V by P = V2/R (so PV2), the voltage can be converted to dBV by

      (1.16)equation

      Thus, 300 kVolts is 70 dBV, and 0.5 V is −6 dBV (not −3 dBV) or 54 dBmV.

      1.4.1 Electric Field

      The electric field (in V/m) is defined as the force (in Newtons) per unit charge (in Coulombs). From this definition and Coulomb's law, the electric field E created by a single point charge Q at a distance r is

      where

       F is the electric force given by Coulomb's law ;

        is a unit vector along r direction which is also the direction of the electric field E.

       ε is the electric permittivity of the material. Its SI unit is Farads/m. In free space, it is a constant:(1.18)

      The product of permittivity and the electric field is called the electric flex density, D, which is a measure of how much electric flux passes through


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