The Virtual Microwave laboratory is a MHRD funded project under NMEICT mission that provides simulated microwave instrumentation for students at remote locations as a part of Sakshat Virtual Laboratory. This site provides a platform for real experience through simulation experiments to a distant student from Microwave Physics group, Dayalbagh Educational Institute, Agra. The virtual microwave laboratory offers the following support to its remote students:

• Step by step instructions
• Simulations and multimedia experiences
• Better use of software
• Awareness building on handling of mistakes
• Building a sense of logical thinking and independence.
• Virtual microwave lab curricula are designed to meet the need of the class.
• Repeating virtual experiments student can practice or develop conceptual understanding.
• Giving room to be creative without being restricted by resource or safety concerns.
• Providing an opportunity to the student for self analysis and reflection in the laboratory.
• Building good understanding of the subject.

Introduction to Microwaves

What are Microwaves

Microwaves are generally considered to be a specific part of wide radio frequency spectrum. The frequency window from 300 MHz to 30 GHz is typically considered to be microwaves. Above 30 GHz, the term millimeter wave is used. Our High School physics suggests that we are considering wavelengths ranging from 30 cm down to 10 mm.

Well known application area where microwaves are widely in use are satellite communication, radar navigation, radiometric remote sensing of environment, non-destructive testing. The tremendous growth in mobile and wireless communication in past decade has particularly caused a respective increase in the need for microwave and millimeter wave components. The topics of microwave in general and its application to wireless technology in specific are through of having a certain air of mystery to them. It is generally considered to that to completely understand the phenomenon of high frequency circuits it is necessary to have a vast mathematical background. That is not the case. Microwaves can be learnt by anyone who wants to learn the subject and the only prerequisite is the desire to learn.

Ministry of Human Resource and development, Government of India has taken a noble initiative under National Mission on Education Through Information and Communication Technology (NMEICT) to present students with lucid learning materials on specialized subject areas to make learning fun.

In pursuit of this, Microwave Physics Laboratory has developed Virtual Simulation Microwave laboratory explaining the basics of high frequency circuit design in an interesting and methodical manner, which will facilitate the learner to understand, explore, innovate and simulate new designs for microwave components.

Definition of Microwaves

The first step in learning about microwaves is being able to define the word microwave in very easily understood terms, that is: "A radiowave operating in the frequency range of 500 MHz to 20 GHz that requires printed circuit components be used instead of conventional lumped components."

This definition shows that microwaves need to be treated differently from low-frequency circuits. First of all, the terms megahertz (MHz) and gigahertz (GHz) indicate frequency in cycles per second (hertz). The term mega (designated as 106) means that the signal is traveling at a certain number of million times per second. The term giga (designated as 109) means the signal is traveling at a certain number of billion times per second. Thus, you can see that the frequencies we are working with are very high.

To understand how high the frequencies are, let us look at some common designations for frequency ranges. The first is very high frequency (VHF). This frequency range is 30 to 300 MHz. This range does not fit the range we described earlier for microwave application, but it is one that is familiar to many people because television is in this frequency range. Another common designation to many people is ultra-high frequency (UHF), which is also a range in which some television channels are located. This frequency range is 0.3 to 3 GHz. You can see that the upper end of this range is well within the frequency range that we designated for microwaves (500 MHz to 20 GHz). Another range that takes in most of the frequency range for microwaves is super-high frequency (SHF), which is 3 to 30 GHz.

Skin Effect

The lumped circuits referred to in the definition are the carbon resistors, mica capacitors, and small inductors you see in your AM-FM radio or television set. The reason those components cannot be used is a phenomenon called skin effect, which is the concept that high frequency energy travels only on the outside skin of a conductor and does not penetrate into it any great distance. The concept of skin effect can best be understood by the following example. If you tie a string to a ball and then twirl the ball around your head at a slow speed, you will see that the ball just sort of lumbers around and stays fairly close to your head as you spin it around. If you spin it faster and faster, it begins to stretch out and be straight out away from your head and body. The force that causes that to happen is centrifugal force.

Now, let us relate the speed of the ball to frequency (slow speed is low frequency, high speed is high frequency). As the frequency gets higher, a centrifugal force also is present. The force is inductance that is set up in the transmission line simply because a current is flowing in that transmission line. This force, which we refer to as a microwave centrifugal force, keeps the energy from penetrating the surface of the transmission line and makes it fol- low a path along the skin of the line rather than down into the entire cross-sectional area, as in low-frequency circuits. Thus, we have a skin effect which determines the properties of microwave signals.

Corresponding to the idea of skin effect is a term called skin depth. This is how far the microwave energy actually penetrates a conductor. This depth is dependent on the material being used and on the frequency at which you are operating. For example, the skin depth in copper at 10 GHz is 0.000025 inch; for aluminum at 10 GHz, it is 0.000031 inch; for silver, it is 0.000023 inch; and for gold, it is 0.000019 inch. Thus, it can be seen that the energy truly does travel along the top edge of the metal. This can be emphasized even more when you consider that for a microwave circuit board with copper on it, the thickness of the copper is 0.0014 inch.

Since the high-frequency signals and transmission lines do not allow energy to penetrate very far into a conductor, it makes no sense to have round (radial) wire leads on components for microwave applications. The energy would travel only on the skin of the lead and be very inefficient. That is why you see ribbon leads or no leads with solder termination points on most microwave components. It also is why you do not see many physical compo- nents on a microwave circuit board. They are there, but they are distributed over a large, thin area and result in the same values as a lumped device that would be used at lower frequencies; hence, the term distributed element components. Those components are what prompt many people to look at a microwave circuit and ask, “Where are all the parts?” With these facts in mind, we can see that microwaves are high-frequency waves that require special circuit-fabrication techniques.
With a definition set forth, it now is time to get into the terminology of microwaves and wireless technology, that is, the jargon and the buzz words used by those in the microwave field.

Decibels

The first term we will look at is decibel (dB). A decibel, which is a relative term with no units, is a ratio of two powers (or voltages). The decibel value can be positive (gain) or negative (loss). If an output power of a device (or system) is measured, an input power is measured, the ratio of the two taken, and the log of the ratio is multiplied by 10, you have a decibel value for that particular gain or loss. (When using voltages, the multiplication factor is 20.) The term decibel tells you only how much a device increases or decreases a power or voltage level. It does not tell you what that power or voltage level actually is. That is valuable in determining a system’s overall gain or loss. For example, if we had a filter with a 2-dB loss, an amplifier with a 20-dB gain, an attenuator with a 6-dB loss, and another amplifier with a 12-dB gain, the overall setup (or system) would have a +24-dB gain (Figure 1.1). The value is found simply by adding the positive decibels (+32), then the negative deci- bels (-8), and taking the difference (+24).

Whereas decibel is a relative term, decibels referred to milliwatts (dBm) is an absolute number, that is, decibels referred to milliwatts are specific powers (milliwatts, watts, and so forth). To determine decibels referred to milliwatts you need only one power. If you have a power of 10 mW (0.010W), for example, you would take that power, divide it by 1 mW, take the log of the result, and multiply it by 10 (+10 dBm, in this case). As can be seen, the value of +10 dBm tells you that a definite 10 mW of power are available from a source or are being read at a specific point. That differs greatly from +10 dB, which only means that there is a gain of 10 dB (gain of 10).  So whenever you require absolute power readings, use decibels referred to milliwatts.

Characteristic Impedance

A third term you should be familiar with is characteristic impedance. When you think of impedance, think of something in the way. A running back in football is impeded by a group of 300-pound defensive linemen; an accident on the freeway impedes the flow of traffic; and alcohol impedes one’s driving skills. All these examples show some parameter in the way of normal operations. Characteristic impedance is an impedance (in ohms) that determines the flow of high-frequency energy in a system or through a transmission line. The characteristic impedance most often used in high-frequency applications is 50Ω. This value is a dynamic impedance in that it is not an ohmic value measured with an ohmmeter but rather an alternating-current (ac) impedance, which depends on the characteristics of the transmission line or component being used. You would not, for example, place an ohmmeter between the center conductor and the outer shield of a coaxial cable and measure anything but an open circuit.

It was mentioned earlier that the characteristic impedance most often used in high-frequency applications is 50Ω. The question that comes about is: Where does this figure 50-Ω come from?

the maximum power handling capability of a particular transmission line or system is 30Ω, while the lowest attenuation for a transmission line or system is 77Ω. The ideal characteristic impedance, therefore, is a compromise between these two values, or 50Ω. Thus, you can see that this is not an arbitrary number giving high power handling capability with low attenuation.

Another point to be brought out for this parameter is that the value of characteristic impedance is the same at the input of a transmission line or device as it is 30 cm away, 1m away, or 1 km away. It is a constant that can be relied on to produce predictable results in your system.

Voltage Standing Wave Ratio (VSWR)

The term voltage standing wave ratio (VSWR) is used to characterize many areas of microwaves. It is a number between 1.0 and infinity. The best value you can get for the VSWR is 1:1 (notice that it is expressed as a ratio), which is termed a matched condition. (A matched condition is one in which systems have the same impedance, so no signals are reflected back to the source of energy.) To understand the concept of a standing wave, consider a rope tied to a post. If you hold the rope in your hand and flip your wrist up and down, you see a wave going down the rope to the post. If the post and the rope were matched to each other, the wave going down the rope would be completely absorbed into the post and you would not see it again. In reality, however, the post and the rope are not matched to each other and the wave comes right back to your hand. If you could move the rope at a high enough rate, you would have one wave going down the rope and one coming back at the same time. That would result in the waves adding at some points and subtracting at others. There would be a wave on the line that was “standing still,” which is where the term standing wave comes about.

The amplitude of a standing wave depends on how well the output is matched to the input. In high-frequency microwave applications, the standing wave ratio depends on the value of the impedance at the output of a trans- mission line compared to the characteristic impedance of the transmission line. It also can be shown that the standing wave ratio is a comparison of the impedance at the input of a device compared to the impedance at the out- put of the device that is driving it. A perfect match is indicated by no standing waves. A drastic mismatch like an open circuit or a short circuit shows a large amplitude standing wave on the transmission line or device. That would indicate a very large mismatch between devices or between the transmis- sion line and the load that was at its output. Remember that the larger the mismatch, the larger the VSWR on the transmission line or at the input or output of a device.

Return Loss

A term that goes along with standing wave ratio is return loss. The return loss (in decibels) indicates the level of power being reflected from a device due to a mismatch. If we have a perfect match between a transmission line and a load at its output, very little, if any, power is reflected, and the difference between the input level and the reflected power is a large number of decibels. If there is a short circuit or an open circuit at the output of the trans- mission line, basically all the power is reflected back, and there is very little difference in decibels between the two. Thus, the return loss for a matched, or near-matched, condition is a large negative number of decibels; the value for a large mismatch is basically 0 dB. It is important to point out that the return loss is a negative number, because it is a loss. Sometimes it is difficult to understand that we have a much better match in a circuit when we have a higher value of return loss. Usually you do not want more loss in your circuits, but in this case, it is a good situation.

Reflection Coefficient

Another term used to describe a matched or mismatched condition in microwaves is reflection coefficient. The reflection coefficient is the percentage of power reflected from a mismatch at the end of a transmission line or at the input or output of a circuit. If there is a perfectly matched condition, the reflection coefficient is 0 (0%); if there is an open circuit or a short circuit at the end of a transmission line, the reflection coefficient is 1 (100%). Any mismatch condition between those two extremes is between 0 and 1. The designation for the reflection coefficient is either ? or G, depending on the text you are using. This text uses ? to designate reflection coefficient. So, if we want to have a good match for a system or a transmission line, we want to have a low reflection coefficient. If a high reflection coefficient appears, it is an indication of a large mismatch somewhere and, consequently, a high VSWR at that point.

Wavelength

Another term that comes into play with both microwaves and wireless applications is wavelength. A wavelength is the length of one cycle of a signal, as illustrated in Figure below:

Wavelength is designated by the symbol λ. As can be seen in Figure 1.4, one wavelength is the distance between two points that have a repeat value. If, for example, we measure 0.1V at one point on the wave, one wavelength will be where the wave has 0.1V again. Values used throughout high-frequency applications are λ/2 (half-wavelength) and λ/4 (quarter-wavelength). Those terms are discussed in more detail later, but for now we can say that a signal repeats itself every half-wavelength and is exactly the opposite every quarter-wavelength. A 0V signal will be zero volts every half-wavelength and maximum voltage every quarter-wavelength. The most important point to remember about wavelengths is that you should always look for points that have the same value to determine how long a wavelength is. That does not necessarily need to be where the signal is at zero, although it helps to get a good reference at those points.

Frequency

A term concerning wavelength is frequency. This simply means how many times the electromagnetic wave repeats itself in 1 second. As an example, at the low end of the microwave spectrum, we have a frequency of 1.0 GHz. This says that the wave repeats itself 1,000 million times in 1 second (1 billion times per second). We have previously defined such terms concerning frequency as gigahertz and megahertz, so we have now completed the definition and characterization of one of the more fundamental terms used in microwaves and wireless technology: frequency.

Short Circuits

A term that usually means you have a problem is short circuit. For high-frequency work, however, a short circuit is an intentional condition, an actual short circuit that has 0O if measured with a meter. A short circuit comes in handy to establish an accurate reference point along transmission lines. Care must be taken in the use of a short circuit for any application; it still is a short to dc and will short your current to ground. Remember that a short circuitis a short at 0Hz(dc), at 1kHz, at 10MHz, at 20GHz, and soon. It is always a short, so remember to correct for it.