1.2: RF and Microwave Engineering

An RF signal is a signal that is coherently generated, radiated by a convey antenna, propagated through air or space, collected by a welcome antenna, and then amplified and data extracted. An RF circuit operates at the same frequency as the RF signal that is transmitted or received. That is, the frequency at which a circuit operates does not define that it is an RF circuit. The RF spectrum is function of the electromagnetic ( EM ) spectrum exploited by humans for communications. A broad classification of the EM spectrum is shown in table 1.1.1. today radios operate on from \ ( 3\text { Hz } \ ) to \ ( 300\text { GHz } \ ), although the upper end will increase as technology progresses .
Microwaves refers to the frequencies where the size of a circuit or structure is comparable to or greater than the wavelength of the EM signal. The division is arbitrary but if a racing circuit structure is greater than \ ( \frac { 1 } { 20 } \ ) of a wavelength, then most engineers would regard the circumference as being a microwave racing circuit. For immediately the microwave frequency crop is broadly taken as \ ( 300\text { MHz } \ ) to \ ( 300\text { GHz } \ ). At these frequencies distributed effects, sometimes called transmission line effects, must be considered .
One of the key characteristics distinguishing RF signals from infrared and visible light up is that an RF bespeak can be generated with coherent phase, and information can be transmitted in both amplitude and phase variations of the RF signal. such signals can be easily generated up to \ ( 220\text { GHz } \ ). The necessary hardware becomes increasingly more expensive as frequency increases. The amphetamine limit of radio frequencies is about \ ( 300\text { GHz } \ ) today, but the limit is extending lento above this as technology progresses .
The RF bands are listed in Table \ ( \PageIndex { 1 } \ ) along with generation modes and representative applications. The generation of RF signals in free space follows one or more paths from a transmitter to a receiver at any frequency, with differences being in the size of the antennas needed to transmit and receive signals. The size of the necessary antenna is related to wavelength, with the distinctive dimensions ranging from a quarter of a wavelength to a few wavelengths if a reflecting telescope is used to focus the EM waves. On earth, and dependent on frequency, RF signals propagate through walls, diffract around objects, refract when the insulator ceaseless of the culture medium changes, and reflect from buildings and walls. The extent is dependent on frequency.

Above crunch the generation at RF is affected by atmospheric loss, by charge layers induced by solar radiation sickness in the amphetamine standard atmosphere, and by density variations in the air caused by heating angstrom well as the cutting of air with acme above the ground. The ionosphere is the uppermost separate of the standard atmosphere, at \ ( 60\ ) to \ ( 90\text { kilometer } \ ), and is ionized by solar radiation, producing a brooding surface, called the D layer, to radio signals improving to \ ( 3\text { GHz } \ ). The D layer weakens at night and most radio signals can then pass through this weakened layer. The E layer extends from \ ( 90\ ) to \ ( 120\text { kilometer } \ ) and is ionized by X-rays and extreme ultraviolet radiation, and the ionize regions, which reflect RF signals, form ionized cloud that last entirely a few hours. The F layer of the ionosphere extends from \ ( 200\ ) to \ ( 500\text { kilometer } \ ) and ionization in this level is due to extreme ultraviolet radiotherapy. refraction results from this charged layer preferably than contemplation, as the charges are widely separated. At night the F layer results in what is called the skywave, which is the deflection of radio waves around the earth. At low frequencies a radio wave penetrates the earth ’ south open and the wave can become trapped at the interface between two regions of unlike permittivity, the earth region and the atmosphere region. This radio wave is called the surface wave or ground wave .
note
When a radio sign near \ ( 60\text { GHz } \ ) passes through air an oxygen atom of two constipate oxygen atoms vibrates and EM energy is transferred to the mechanical energy of vibration and frankincense heat .
Propagating RF signals in air are absorbed by molecules in the atmosphere primarily by molecular resonances such as the bending and extend of bonds. This crouch and stretching converts department of energy in the EM wave into vibrational energy of the molecules. The transmittance of radio receiver signals versus frequency in dry air at an elevation of \ ( 4.2\text { kilometer } \ ) is shown in Figure \ ( \PageIndex { 1 } \ ). The first base molecular resonance encountered in dry air as frequency increases is the oxygen resonance, centered at \ ( 60\text { GHz } \ ), but below that the assimilation in dry air is very modest. attenuation increases with higher water system vapor pressure ( with a rapport at \ ( 22\text { GHz } \ ) ) and in rain. Within \ ( 2\text { GHz } \ ) of \ ( 60\text { GHz } \ ) a sign will not travel far, and this can be used to provide set communication over a few meters as a local data link. Regions of low attenuation ( i.e. high gear transmittance ), are called windows and there are numerous low loss windows .

Band Frequency wavelength Propagation mode/applications
TLF Tremendously low frequency \(< 3\text{ Hz} > 100,000\text{ km}\) Penetration of liquids and solids/Submarine communication
ELF Extremely low frequency \(3-30\text{ Hz}, 100,000-10,000\text{ km}\) Penetration of liquids and solids/Submarine communication
SLF Super low frequency \(30-300\text{ Hz}, 10,000-1,000\text{ km}\) Penetration of liquids and solids/Submarine communication
ULF Ultra low frequency \(300-3,000\text{ Hz}, 1,000-100\text{ km}\) Penetration of liquids and solids/Submarine communication; communication within mines
VLF Very low frequency \(3-30\text{ kHz}, 100-10\text{ km}\) Guided wave trapped between the earth and the ionosphere/Navigation, geophysics
LF Low frequency \(30-300\text{ kHz}, 10-1\text{ km}\) Guided wave between the earth and the ionosphere’s D layer; surface waves, building penetration/Navigation, AM broadcast, amateur radio, time signals, RFID
MF Medium frequency \(300-3,000\text{ kHz}, 1000-100\text{ m}\) Surface wave, building penetration; day time: guided wave between the earth and the ionosphere’s D layer; night time: sky wave/AM broadcast
HF High frequency \(3-30\text{ MHz}, 100-10\text{ m}\) Sky wave, building penetration/shortwave broadcast, over-the-horizon radar, RFID, amateur radio, marine and mobile telephony
VHF Very high frequency \(30-300\text{ MHz}, 10-1\text{ m}\) Line of sight, building penetration; up to \(80\text{ MHz}\), skywave during periods of high sunspot activity/FM and TV broadcast, weather radio, line-of-sight aircraft communications
UHF Ultra high frequency \(300-3000\text{ MHz}, 10-1\text{ cm}\) Line of sight, building penetration; sometimes tropospheric ducting/1G–4G cellular communications, RFID, microwave ovens, radio astronomy, satellite-based navigation
SHF Super high frequency \(3-30\text{ GHz}, 10-1\text{ cm}\) Line of sight/5G cellular communications, Radio astronomy, point-to-point communications, wireless local area networks, radar
EHF Extremely high frequency \(30-300\text{ GHz}, 10-1\text{ mm}\) Line of sight/5G cellular communications, Astronomy, remote sensing, point-to-point and satellite communications
THF Terahertz or tremendously high frequency \(300-3,000\text{ GHz}, 1,000-100\:\mu\text{m}\) Line of sight /Spectroscopy, imaging

board \ ( \PageIndex { 1 } \ ) : radio frequency bands, basal propagation mechanisms, and selected applications.

RF signals diffract and indeed can bend around structures and penetrate into valleys. The ability to diffract reduces with increasing frequency. however, as frequency increases the size of antenna decreases and the capacity to carry data increases. A very well compromise for mobile communications is at UHF, \ ( 300\text { MHz } \ ) to \ ( 4\text { GHz } \ ), where antennas are of convenient size and there is a good ability to diffract around objects and even penetrate walls. This choice can be seen with 1G–4G cellular communication systems operating in several bands from \ ( 450\text { MHz } \ ) to \ ( 3.6\text { GHz } \ ) where antennas do not dominate the size of the handset, and the ability to receive calls within buildings and without telephone line of view to the base station is well known .
RF bands have been promote divided for particular applications. The
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figure \ ( \PageIndex { 1 } \ ) : atmospheric transmission at Mauna Kea, with a stature of \ ( 4.2\text { kilometer } \ ), on the Island of Hawaii where the atmospheric blackmail is \ ( 60\ % \ ) of that at ocean level and the vent is dry with a precipitable water vapor level of \ ( 0.001\text { millimeter } \ ). After [ 5 ] .
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name \ ( \PageIndex { 2 } \ ) : rectangular waveguide with inner dimensions of \ ( a\ ) and \ ( b\ ). normally \ ( a ≈ b\ ). The EM waves are confined within the four metallic walls and propagate in the \ ( ±z\ ) commission. little current flows in the waveguide walls and so resistive losses are small. Compared to coaxial lines orthogonal waveguides have very humble loss .
frequency bands for radar are shown in board 1.3.1. The L, S, and C bands are referred to as having octave bandwidths, as the upper frequency of a isthmus is twice the lower frequency. The other bands are half-octave bands, as the upper berth frequency limit is approximately 50 % higher than the lower frequency limit. The lapp letter ring designations are used by other standards. The most important alternative band designation is for the waveguide bands. These bands refer to the useful range of operation of a rectangular waveguide, which is a orthogonal tube that confines a propagate signal within four conducting walls ( see Figure \ ( \PageIndex { 2 } \ ) ). The waveguide bands are shown in table 1.3.2 with the ceremonious letter designation of bands and standardized waveguide dimensions. Compared to coaxial lines orthogonal waveguides have very abject loss.\ ( ^ { 1 } \ )

Footnotes

[ 1 ] A semirigid coaxial line with an out conductor diameter of \ ( 3.5\text { millimeter } \ ) has a loss at \ ( 10\text { GHz } \ ) of \ ( 0.5\text { db/m } \ ) while an X-band waveguide has a personnel casualty of \ ( 0.1\text { dB/m } \ ). At \ ( 100\text { GHz } \ ) a \ ( 1\text { millimeter } \ ) -diameter coaxial line has a loss of \ ( 12.5\text { dB/m } \ ) compared to \ ( 2.5\text { dB/m } \ ) loss for a W-band waveguide .