UNIT I TWO PORT RF NETWORKS-CIRCUIT REPRESENTATION
INTRODUCTION:
Microwaves are electromagnetic waves with wavelengths ranging from 1 mm to 1 m, or
frequencies between 300 MHz and 300 GHz.
Apparatus and techniques may be described qualitatively as "microwave" when the wavelengths of
signals are roughly the same as the dimensions of the equipment, so that lumped-element circuit
theory is inaccurate. As a consequence, practical microwave technique tends to move away from
the discrete resistors, capacitors, and inductors used with lower frequency radio waves. Instead,
distributed circuit elements and transmission-line theory are more useful methods for design,
analysis. Open-wire and coaxial transmission lines give way to waveguides, and lumped-element
tuned circuits are replaced by cavity resonators or resonant lines. Effects of reflection, polarization,
scattering, diffraction, and atmospheric absorption usually associated with visible light are of
practical significance in the study of microwave propagation. The same equations of
electromagnetic theory apply at all frequencies.
While the name may suggest a micrometer wavelength, it is better understood as indicating
wavelengths very much smaller than those used in radio broadcasting. The boundaries between far
infrared light, terahertz radiation, microwaves, and ultra-high-frequency radio waves are fairly
arbitrary and are used
variously between different fields of study. The term microwave generally refers to "alternating
current signals with frequencies between 300 MHz (3×108 Hz) and 300 GHz (3×1011 Hz)."[1]
Both IEC standard 60050 and IEEE standard 100 define "microwave" frequencies starting at 1
GHz (30 cm wavelength).
Electromagnetic waves longer (lower frequency) than microwaves are called "radio waves".
Electromagnetic radiation with shorter wavelengths may be called "millimeter waves", terahertz
radiation or even T-rays. Definitions differ for millimeter wave band, which the IEEE defines as
110 GHz to 300 GHz.
Discovery
The existence of electromagnetic waves, of which microwaves are part of the frequency spectrum,
was predicted by James Clerk Maxwell in 1864 from his equations. In 1888, Heinrich Hertz was
the first to demonstrate the existence of electromagnetic waves by building an apparatus that
produced and detected microwaves in the UHF region. The design necessarily used horse-andbuggy materials, including a horse trough, a wrought iron point spark, Leyden jars, and a length of
zinc gutter whose parabolic cross- section worked as a reflection antenna. In 1894 J. C. Bose
publicly demonstrated radio control of a bell using millimetre wavelengths, and conducted research
into the propagation of microwaves.
Plot of the zenith atmospheric transmission on the summit of Mauna Kea throughout the entire
gigahertz range of the electromagnetic spectrum at a precipitable water vapor level of 0.001 mm.
(simulated)
Frequency range
The microwave range includes ultra-high frequency (UHF) (0.3–3 GHz), super high frequency
(SHF) (3–30 GHz), and extremely high frequency (EHF) (30–300 GHz) signals.
Above 300 GHz, the absorption of electromagnetic radiation by Earth's atmosphere is so great that
it is effectively opaque, until the atmosphere becomes transparent again in the so-called infrared
and optical window frequency ranges.
Microwave Sources
Vacuum tube based devices operate on the ballistic motion of electrons in a vacuum under the
influence of controlling electric or magnetic fields, and include the magnetron, klystron, travelling
wave tube (TWT), and gyrotron. These devices work in the density modulated mode, rather than
the current modulated mode. This means that they work on the basis of clumps of electrons flying
ballistically through them, rather than using a continuous stream.
A maser is a device similar to a laser, except that it works at microwave frequencies.
Solid-state sources include the field-effect transistor, at least at lower frequencies, tunnel diodes
and Gunn diodes
Uses
Communication

Before the advent of fiber optic transmission, most long distance telephone calls were carried via
microwave point-to-point links through sites like the AT&T Long Lines. Starting in the early
1950's, frequency division multiplex was used to send up to 5,400 telephone channels on each
microwave radio channel, with as many as ten radio channels combined into one antenna for the
hop to the next site, up to 70 km away.

Wireless LAN protocols, such as Bluetooth and the IEEE 802.11 specifications, also use
microwaves in the 2.4 GHz ISM band, although 802.11a uses ISM band and U-NII frequencies in
the 5 GHz range. Licensed long-range (up to about 25 km) Wireless Internet Access services can
be found in many countries (but not the USA) in the 3.5–4.0 GHz range.

Metropolitan Area Networks: MAN protocols, such as WiMAX (Worldwide Interoperability for
Microwave Access) based in the IEEE 802.16 specification. The IEEE 802.16 specification was
designed to operate between 2 to 11 GHz. The commercial implementations are in the 2.3GHz,
2.5 GHz, 3.5 GHz and 5.8 GHz ranges.

Wide Area Mobile Broadband Wireless Access: MBWA protocols based on standards
specifications such as IEEE 802.20 or ATIS/ANSI HC-SDMA (e.g. iBurst) are designed to operate
between 1.6 and 2.3 GHz to give mobility and in-building penetration characteristics similar to
mobile phones but with vastly greater spectral efficiency.

Cable TV and Internet access on coaxial cable as well as broadcast television use some of the
lower microwave frequencies. Some mobile phone networks, like GSM, also use the lower
microwave frequencies.

Microwave radio is used in broadcasting and telecommunication transmissions because, due to
their short wavelength, highly directive antennas are smaller and therefore more practical than they
would be at longer wavelengths (lower frequencies). There is also more bandwidth in the
microwave spectrum than in the rest of the radio spectrum; the usable bandwidth below 300 MHz
is less than 300 MHz while many GHz can be used above 300 MHz. Typically, microwaves are
used in television news to transmit a signal from a remote location to a television
station from a specially equipped van.
Remote Sensing

Radar uses microwave radiation to detect the range, speed, and other characteristics of remote
objects. Development of radar was accelerated during World War II due to its great military utility.
Now radar is widely used for applications such as air traffic control, navigation of ships, and speed
limit enforcement.

A Gunn diode oscillator and waveguide are used as a motion detector for automatic door openers
(although these are being replaced by ultrasonic devices).

Most radio astronomy uses microwaves.

Microwave imaging; see Photoacoustic imaging in biomedicine
Navigation

Global Navigation Satellite Systems (GNSS) including the American Global Positioning System
(GPS) and the Russian ГЛОбальная НАвигационная Спутниковая Система (GLONASS)
broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz.
Power

A microwave oven passes (non-ionizing) microwave radiation (at a frequency near 2.45 GHz)
through food, causing dielectric heating by absorption of energy in the water, fats and sugar
contained in the food. Microwave ovens became common kitchen appliances in Western countries
in the late 1970s, following development of inexpensive cavity magnetrons.

Microwave heating is used in industrial processes for drying and curing products.

Many semiconductor processing techniques use microwaves to generate plasma for such purposes as
reactive ion etching and plasma-enhanced chemical vapor deposition (PECVD).

Microwaves can be used to transmit power over long distances, and post-World War II research was
done to examine possibilities. NASA worked in the 1970s and early 1980s to research the
possibilities of using Solar power satellite (SPS) systems with large solar arrays that would beam
power down to the Earth's surface via microwaves.

Less-than-lethal weaponry exists that uses millimeter waves to heat a thin layer of human skin to an
intolerable temperature so as to make the targeted person move away. A two-second burst of the 95
GHz focused beam heats the skin to a temperature of 130 F (54 C) at a depth of 1/64th of an inch
(0.4 mm). The United States Air Force and Marines are currently using this type of Active Denial
System.
Microwave frequency bands
The microwave spectrum is usually defined as electromagnetic energy ranging from approximately
1 GHz to 1000 GHz in frequency, but older usage includes lower frequencies. Most common
applications are within the 1 to 40 GHz range. Microwave frequency bands, as defined by the
Radio Society of Great Britain (RSGB), are shown in the table below:The term P band is
sometimes used for Ku Band. For other definitions see Letter Designations of Microwave Bands
Health effects
Microwaves contain insufficient energy to directly chemically change substances by ionization,
and so are an example of nonionizing radiation. The word "radiation" refers to the fact that energy
can radiate, and not to the different nature and effects of different kinds of energy. Specifically, the
term in this context is not to be confused with radioactivity.
A great number of studies have been undertaken in the last two decades, most concluding they are
safe. It is understood that microwave radiation at a level that causes heating of living tissue is
hazardous (due to the possibility of overheating and burns) and most countries have standards
limiting exposure, such as the Federal Communications Commission RF safety regulations.
Synthetic reviews of literature indicate the predominance of their safety of use.
History and research
Perhaps the first, documented, formal use of the term microwave occurred in 1931:
"When trials with wavelengths as low as 18 cm were made known, there was undisguised surprise
that the problem of the micro-wave had been solved so soon." Telegraph & Telephone Journal
XVII. 179/1
Perhaps the first use of the word microwave in an astronomical context occurred in 1946 in an
article "Microwave Radiation from the Sun and Moon" by Robert Dicke and Robert Beringer.
Scattering parameters
"Scattering" is an idea taken from billiards, or pool. One takes a cue ball and fires it up the table at
a collection of other balls. After the impact, the energy and momentum in the cue ball is divided
between all the balls involved in the impact. The cue ball "scatters" the stationary target balls and
in turn is deflected or "scattered" by them.
In a microwave circuit, the equivalent to the energy and momentum of the cue ball is the amplitude
and phase of the incoming wave on a transmission line. (A rather loose analogy, this). This
incoming wave is "scattered" by the circuit and its energy is partitioned between all the possible
outgoing waves on all the other transmission lines connected to the circuit. The scattering
parameters are fixed properties of the (linear) circuit which describe how the energy couples
between each pair of ports or transmission lines connected to the circuit.
Formally, s-parameters can be defined for any collection of linear electronic components, whether
or not the wave view of the power flow in the circuit is necessary. They are algebraically related to
the impedance parameters (z-parameters), also to the admittance parameters (y-parameters) and to
a notional characteristic impedance of the transmission lines.
A visual demonstration of the meaning of scattering may be given by throwing a piece of chalk at a
blackboard....
Definitions.
An n-port microwave network has n arms into which power can be fed and from which power can
be taken. In general, power can get from any arm (as input) to any other arm (as output). There are
thus n incoming waves and n outgoing waves. We also observe that power can be reflected by a
port, so the input power to a single port can partition between all the ports of the network to form
outgoing waves.
Associated with each port is the notion of a "reference plane" at which the wave amplitude and
phase is defined. Usually the reference plane associated with a certain port is at the same place with
respect to incoming and outgoing waves.
The n incoming wave complex amplitudes are usually designated by the n complex quantities an,
and the n outgoing wave complex quantities are designated by the n complex quantities bn. The
incoming wave quantities are assembled into an n-vector A and the outgoing wave quantities into
an n-vector B. The outgoing waves are expressed in terms of the incoming waves by the matrix
equation B = SA where S is an n by n square matrix of complex numbers called the "scattering
matrix". It completely determines the behaviour of the network. In general, the elements of this
matrix, which are termed "s-parameters", are all frequency-dependent.
For example, the matrix equations for a 2-port are
b1 = s11 a1 + s12 a2 b2 = s21 a1 + s22 a2
And the matrix equations for a 3-port are b1 = s11 a1 + s12 a2 + s13 a3
b2 = s21 a1 + s22 a2 + s23 a3
b3 = s31 a1 + s32 a2 + s33 a3
The wave amplitudes an and bn are obtained from the port current and voltages by the relations a =
(V + ZoI)/(2 sqrt(2Zo)) and b = (V - ZoI)/(2 sqrt(2Zo)). Here, a refers to an if V is Vn and I In for
the nth port. Note the sqrt(2) reduces the peak value to an rms value, and the sqrt(Zo) makes the
amplitude normalised with respect to power, so that the incoming power = aa* and the outgoing
power is bb*.
A one-port scattering parameter s is merely the reflection coefficient gamma, and as we have seen
we can relate gamma to the load impedance zL = ZL/Zo by the formula gamma = (zL-1)/(zL+1).
Similarly, given an n by n "Z-matrix" for an n-port network, we obtain the S matrix from the
formula S
= (Z-I)(Z+I)^-1, by post-multiplying the matrix (Z-I) by the inverse of the matrix (Z+I). Here, I is
the n by n unit matrix. The matrix of z parameters (which has n squared elements) is the inverse of
the matrix of y parameters.
2-
port scattering matrix.
In the case of a microwave network having two ports only, an input and an output, the s-matrix has
four s-parameters, designated
s11
s12
s21
s22
These four complex quantites actually contain eight separate numbers; the real and imaginary
parts, or the modulus and the phase angle, of each of the four complex scattering parameters.
Let us consider the physical meaning of these s-parameters. If the output port 2 is terminated, that
is, the transmission line is connected to a matched load impedance giving rise to no reflections,
then there is no input wave on port 2. The input wave on port 1 (a1) gives rise to a reflected wave
at port 1 (s11a1) and a transmitted wave at port 2 which is absorbed in the termination on 2. The
transmitted wave size is (s21a1).
If the network has no loss and no gain, the output power must equal the input power and so in this
case
|s11|^2 + |s21|^2 must equal unity.
We see therefore that the sizes of S11 and S21 determine how the input power splits between the
possible output paths.
NOTE s21 relates power OUT of 2 to power IN to 1, not vice versa as it is easy to think at first
sight.
Clearly, if our 2-port microwave network represents a good amplifier, we need s11 rather small
and s21 quite big, let us say 10 for a 20dB amplifier.
In general, the s-parameters tell us how much power "comes back" or "comes out" when we "throw
power at" a network. They also contain phase shift information.
Reciprocity
Reciprocity has to do with the symmetry of the s-matrix. A reciprocal s-matrix has symmetry
about the leading diagonal. Many networks are reciprocal. In the case of a 2-port network, that
means that s21 = s12 and interchanging the input and output ports does not change the transmission
properties. A transmission line section is an example of a reciprocal 2-port. A dual directional
coupler is an example of a reciprocal 4-port. In general for a reciprocal n-port sij = sji.
Amplifiers are non-reciprocal; they have to be, otherwise they would be unstable. Ferrite devices
are deliberately non-reciprocal; they are used to construct isolators, phase shifters, circulators, and
power combiners.
Examples of scattering matrices.
One-port S-matrix
This is a matrix consisting of a single element, the scattering parameter or reflection coefficient.
You may think of it as a 1 by 1 matrix; one row and one column.
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A matched transmission line, s11 = 0
A short circuit, at the short, s11 = 1 angle -180 degrees
The input to a 0.2 lambda line feeding a short cicuit, s11 = 1 angle -324 degrees
Your turn. A normalised load 2+j1?
Your turn again. The input to a transmission line of length 20.35 lambda connected to a normalised
load 2+j1?
Two port S-matrices
These are 2 by 2 matrices having the following s parameters; s11 s12 s21 s22.
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A 0.1 lambda length of transmission line
s11 = 0
s12 = 1 angle -36 degrees
s21 = 1 angle -36
s22 = 0
A 10dB amplifier, matched on input and ouput ports
s11 = 0
s12 = small
s21 = 3.16 angle -theta
s22 = 0
An isolator having 1dB forward loss, 21 dB backward loss, matched on ports 1 and 2
s11 = 0
s12 = 0.0891 some angle
s21 = 0.891 some angle
s22 = 0
Your turn. A 9 cm length of waveguide of cross sectional dimensions 2.5 cms by 1.8 cms at a
frequency of 8GHz
Your turn again. The waveguide above has a capacitative iris placed mid way along it. Assume that
30% of the power incident on the iris gets through it, and 70% is reflected. Hint. represent the
normalised load admittance of the iris as y = 1 + js and calculate s for 70% power reflection. That
will give you the phase shift on reflection as well.
Three port S-matrices
These are 3 by 3 matrices having the following s parameters
 Your turn. Write down the s matrix for a perfect circulator with -70 degrees of phase shifts
between successive ports.
 Your turn again. A coaxial cable is connected in a Y arrangement with each arm 12.3
wavelengths long. At the junction, the cable arms are all connected in parallel. Write down its S
matrix and comment on this method of splitting power from a TV down lead to serve two
television sets.
Stability.
If a 1-port network has reflection gain, its s-parameter has size or modulus greater than unity. More
power is reflected than is incident. The power usually comes from a dc power supply; Gunn diodes
can be used as amplifiers in combination with circulators which separate the incoming and
outgoing waves. Suppose the reflection gain from our 1-port is s11, having modulus bigger than
unity. If the 1-port is connected to a transmission line with a load impedance having reflection
coefficient g1, then oscillations may well occur if g1s11 is bigger than unity. The round trip gain
must be unity or greater at an integer number of (2 pi) radians phase shift along the path. This is
called the "Barkhausen criterion" for oscillations. Clearly if we have a Gunn source matched to a
matched transmission line, no oscillations will occur because g1 will be zero.
If an amplifier has either s11 or s22 greater than unity then it is quite likely to oscillate or go
unstable for some values of source or load impedance. If an amplifier (large s21) has s12 which is
not negligibly small, and if the output and input are mismatched, round trip gain may be greater
than unity giving rise
to oscillation. If the input line has a generator mismatch with reflection coefficient
g1, and the load impedance on port 2 is mismatched with reflection coefficient g2,
potential instability happens if g1g2s12s21 is greater than unity.