System Design: Tips & Tricks
This page provides tips & tricks for system design by experienced Telex RadioCom technicians.
- Introduction to Wireless Intercoms
- Wireless intercoms have a long and important history as part of the communication professional’s repertoire. They have gone through many changes and technological improvements over the years to bring us to where we are today. The purpose of this chapter is to allow you to become familiar with the history, general workings, and special considerations of wireless intercoms. This includes their advantages and disadvantages so that in the following articles we may explore the wild, sometimes weird, but almost never boring, world of wireless intercom systems design.
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- Special Considerations
- Wireless communications are here to stay. They have become an integral part of the total professional communications package. There are, however, many factors associated with wireless that need to be understood and addressed that do not come into play with hardwired communications systems. In this section, we look at the special considerations that must be considered when deciding whether or not to implement a wireless system. The first area of study is the RF spectrum and how it can be used to implement a wireless intercom system. Traditionally, wireless intercoms have been a function of broadcast television productions, and as such have used, at least in part, a spectrum that falls under FCC Code 47 CFR, Part 74 in addition to itinerant frequencies. The spectrum most commonly used falls into two areas: VHF systems from approximately 154 MHz to 216 MHz, and UHF systems from 460 MHz to 608 MHz and 614 - 806 MHz. As mentioned in an earlier section, large chunks of this spectrum have either been reallocated, or will soon be reallocated. The FCC has found that auctioning spectrum is a good way for the commission to move from an expense center to a profit center, and they are pursuing it with a passion.
Wireless intercoms, like any other wireless system, require at least one transmitter to function. Under FCC rules, all transmitters must be licensed before operation (there are some very low power transmitters that can operate under Part 15 and do not need to be licensed, but that doesn’t apply to any modern RF intercom systems). There are different forms to obtain various types of licenses depending on what area of the spectrum your system will operate in, who will be operating the system, and what the system will be used for. The law is very clear in that no one is permitted to operate a transmitter typically used for wireless intercom systems without first obtaining an FCC license.
Wireless equipment often operates in areas of the RF spectrum that are designated for TV channels, but are unused in a given area. In all cases low power transmitters used by wireless intercoms and wireless mics must operate on a secondary, non-interfering basis. This means that wireless users must not cause harmful interference to television or other receivers, and must accept all interference sources. In keeping with this, the FCC rules state that VHF systems must not be operated within 50 miles of a television transmitter occupying a similar spectrum. The rules further state that UHF systems must not be operated within 75 miles of a television transmitter occupying a similar spectrum. See Figure 1, for a depiction of what a television station’s assigned spectrum looks like.
Refer to Figure 2 for the standard frequency allocations of television transmitters.
Having touched briefly on the FCC rules, I must inform you the vast majority of users, not only of wireless intercoms, but of wireless mics and IFBs as well, do not obtain licenses. In fact, historically, many users of UHF wireless gear, outside of television broadcasters or people working with broadcast entities, could not even qualify to get an appropriate license. Right, wrong or indifferent, this has been the case. Telex Communications, Inc. strongly recommends that every wireless system be licensed and operated in strict accordance to FCC rules. Your local wireless dealer can help you understand the requirements and regulations that apply to you.
There has been progress though. The FCC has, as of late, worked with users, other than broadcast, to facilitate a win-win licensing scheme that may, in the future, help to ensure all systems are licensed and operated according to FCC rules. In any case, each user must in all good conscience, research his ability to license and operate wireless equipment, and govern equipment purchase and implementation accordingly.
In addition to the FCC rules, wireless users must also consider how best to avoid harmful interference to ensure uninterrupted and intelligible communications. One of the best ways to go about selecting the area of spectrum you will use is to do a frequency survey. By using a spectrum analyzer or other specialized receiver, it is possible to look at potential interference sources and avoid them. Picking an area of spectrum that is free from external interference sources will go a long way in helping you select frequencies that offer trouble free operation.
In addition to a clear spectrum, you must also consider the intermodulation affects of the specific frequencies you pick. An in depth study of this topic is beyond the scope of this book, but we will touch on the subject to give you a general overview. Intermodulation (IM) or intermod as it is often called, happens when two or more frequencies mix in a nonlinear device and produce a number of related different frequencies known as intermodulation products. We look at intermodulation in more detail in the next chapter, but suffice to say, choosing a manufacturer or dealer that is qualified to pick intermodulation free frequencies is necessary.
Now let’s look at cost. Wireless intercom systems cost substantially more at initial purchase than do hardwired communications systems. For a comparable number of users, wireless systems cost between two to ten times as much as hard-wired partyline systems, depending on system type and configuration. Because of this increased cost factor, it is important to determine which members of the production team must be wireless and which can be tethered by a wire. Of course, everyone wants to be wireless and has a great reason why they need a wireless beltpack, but in the end you must make the budget meet the overall production needs.
The added cost and special consideration factors that wireless communications systems have to be concerned with are far outweighed by the increased flexibility and functionality they offer. It is important to choose a wireless system with all of the facts and considerations in front of you so you can have years of trouble free operation to come.
Figure 1
Figure 2
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- Introduction to the Design of Wireless Intercom Systems
- The design, and subsequent operation, of a wireless intercom system is, like any wireless network, highly dependent on numerous factors. Some of these factors you will have control over, but many you will not. The key to successful wireless system design, whether it be intercoms, talent audio, or roving camera, is to gather the information related to all of the variables before you get started and then match the system components and architecture to your specific requirements. There is no such thing as a one size fits all wireless system. In this chapter, we explore some Radio Frequency (RF) theory that allows you to have a better understanding of how RF works. We will also look at many of the key components of a wireless communications system and how they go together to create the desired effect.
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- Back-to-Basics
- In this section, we discuss the theory of how RF signals act and how they are affected by various conditions. There is some math discussed here, but only enough to convey the principles at hand. The idea is to give you a good working knowledge of RF principles, not make you an expert in Bessel functions. Old RF pros can probably skip this section, although a refresher of this material is usually appropriate.
First, let’s answer the question, “What is RF?” Contrary to popular belief, the frequency of a signal does not determine whether it is an RF signal or not. The defining factor for RF signals is the medium through which they propagate. All energy that travels in waves propagates through some medium, which allows the wave to move from one location to another. In the case of sound, the medium is typically air, water, or some other physical mass. RF signals on the other hand, regardless of frequency, always propagate or move through the electromagnetic spectrum. Where as sound needs some physical mass to move, RF signals do not. The electromagnetic spectrum exists everywhere (as far as we know), and enables RF signals to move through the vast vacuum of space where sound waves could never go.
A brief look at the properties of the electromagnetic spectrum can tell us a lot about the RF signals that move through it. As you can see, the name electro-magnetic is really a combination of two words, electron (or electronic) and magnet (or magnetic). The reason for this is that waves that propagate in the electromagnetic spectrum have two separate and distinct components, an electrical and a magnetic. As you can see in Figure 1, these two components exist at right angles to each other, as well as, to the direction of propagation. The electrical component, or field as it is called, is represented by the letter E and the magnetic field by the letter H. (No, I don’t know why they use H, but they do!)
RF signals at different frequencies have different propagation characteristics and are affected by external forces in different ways. The reason for this is the ratio of the magnitudes of the electrical and magnetic components of an RF wave varies dramatically as frequency changes. The magnetic component of an RF wave is much greater than the electrical component at very low frequencies. As the frequency increases, the electrical component increases and the magnetic component decreases, until, at very high frequencies, the electrical component is much greater than the magnetic.
This is not just “gee whiz” information. The different makeup of RF waves at different frequencies is what allows us to use the signals for different and sometimes unusual applications. For instance, at super low frequencies, such as 5 Hertz, where the magnetic component is extremely dominant, the US Navy has been able to propagate RF signals through the Earth’s core to communicate with submarines on the other side of the world. Try that at 13 GHz! In a more pertinent example, at much higher frequencies the highly reflective nature of the mostly electrical component wave can cause self-interference, known as multipath. Multipath can cause an RF signal to be unusable at a very short distance from the transmitter if not properly handled. We will discuss multipath in more detail later in this chapter.
Now that we know what RF is, we can discuss what it does, how we can use it and how it is affected by outside forces. In its most basic form, an RF system puts information on an RF signal, sends it to a remote location and retrieves the information in exactly the same form as it originally existed. Let’s look at this most basic system and define some terms so we can talk about this process more easily. Refer to Figure 2.
In Figure 2, the transmitter is a device that has an input for information, audio, data, or some other form of intelligence called a source signal, that needs to get from here to there. The transmitter then takes that information and puts it onto an RF signal. The RF signal is called a carrier because it, in effect, carries the source signal as it propagates. The process of actually putting the source signal onto the carrier is called modulating the carrier, which normally is referred to simply as modulation. The carrier that has had the source signal applied is then broadcast into the air (actually the electromagnetic spectrum) via an antenna. The antenna is a transducer that allows the carrier to be efficiently broadcast or received.
Once the signal is broadcast into the air, it propagates out away from the transmit antenna and eventually reaches the receive antenna. The area between the transmit antenna and the receive antenna is called the propagation path, or just path. At the receive antenna, the signal, which is now much weaker, is collected and enters into the receiver. The receiver’s job is to find the one unique carrier from the transmitter and strip off the source signal so it exactly matches the original information. This process is called demodulation.
Now, let’s look at the RF wave as it moves along the propagation path. We know that RF propagates or moves from one point to another, and that propagation can be affected by the frequency of the wave. Now we’ll find out how RF waves normally act in typical environments. You can think of an RF signal that radiates out into open space from a specific point, such as a transmit antenna, like the waves generated by throwing a pebble into a pond as shown in Figure 3. The energy carried by the wave moves away from the original point in all directions equally and each vector that can be drawn from the center point represents RF energy traveling away from the point of origin in a straight line as shown in Figure 3.
In addition, the RF wave continually gets weaker as it moves away from the transmit antenna. The rate at which the wave becomes weaker can be calculated via the inverse square law 1/D2 where D = distance traveled by the wave. This is a very important concept because it shows why a wave that travels twice as far as another wave of equal magnitude is not half as strong. Take the following example:
Two transmitters TXA and TXB both emit signals that are exactly the same at 1 Watt of power. The signal from TXA travels 10 units. Power at that point can be calculated by 1/102 x 1W or 0.01 x 1W. That means there is 0.01W of the TXA signal left after it has traveled 10 units. Now let’s say that the TXB signal travels twice the distance of TXA or 20 units. Power at that point can be calculated by 1/202 x 1W or 0.0025 x 1W. That means there is 0.0025W of the TXB signal left after it has traveled 20 units. As you can see, the signal that traveled twice as far was not ½ the power, but ¼ the power of the first signal.
Because of the inverse square law, the effective radiated power (ERP) of a given transmitter must increase by a factor of four times to achieve twice the operating range. This information is important in determining the necessary power for a wireless system for a given range. It is always important to use the minimum power necessary to accomplish the task, so that excess power does not affect other systems and cause undue harm.
The theoretical range of an RF system is important to know, but it is the functional range you must be more concerned with. The functional range of a system takes into account a certain cushion factor called fade margin that will ensure the signal coming from the transmitter to the receiver will not only be detectable, but will also be usable. This is less of a concern in communications systems as you can tolerate less fade margin than in an on-air wireless microphone system, because a small momentary dropout will not critically affect communications as it would program audio.
RF waves travel away from the source in a straight line until that path is interrupted or disturbed by some outside influence. Figure 4 shows an RF wave being reflected and thus changing the path of some of the RF energy. If you remember earlier when we mentioned multipath, this reflected energy is the cause of that phenomenon. Before we can discuss multipath in more detail, though, we must learn about another aspect of the RF wave and how it changes when it encounters a reflective surface.
Polarization is the term that describes the orientation of an RF wave. Remember back to when we discussed the two components that make up an RF wave, the electrical and the magnetic. We said that the E field was the electrical component and that the H field was the magnetic component. The polarization of an RF signal is determined by the orientation of the E field. If the E field is perpendicular to the plane of the Earth, the wave is said to be vertically polarized. If the E field is parallel to the plane of the Earth, the wave is said to be horizontally polarized. See Figure 5.
Transmit and receive antennas of the same system must be oriented in the same direction (plane) to have a proper transfer of the carrier. In theory, if a transmit antenna is oriented vertically, thus producing a vertically polarized carrier, and the corresponding receive antenna is oriented horizontally, the receive antenna will not be able to see the vertically polarized wave at all. In practice, there will always be some polarization shift in the path and the receiver will see a very small signal if it is close enough to the transmitter. To avoid this problem, antennas in a given RF system should always have similar orientation.
There are other forms of polarization, such as circular polarization, which can be used to help counteract the effect of multipath, but for now we will use horizontal and vertical polarization for our discussion. It is important to note here the difference between polarization and phase, as the two terms are often confused. Phase refers to the relationship of the sinusoidal energy of two or more waves, not to the orientation of the electrical component. See Figure 6. Two identical waves that are in phase, and are combined, add to make a larger wave. Two identical waves that are out of phase by exactly 180°, and are combined, cancel each other out. See Figure 7. Waves that are not exactly identical in either frequency, amplitude, or phase will have a composite sum that may increase the overall amplitude at some points, and either reduce or eliminate the overall amplitude at others. See Figure 8. It is critical to have a good understanding of these two principles as we start to discuss multipath.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
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- Interference
- As mentioned earlier, multipath can be described as a form of self-interference caused when a reflected RF carrier arrives at the receive antenna along with an RF carrier that has taken a direct path. See Figure 1. The reason multipath is so detrimental to the successful operation of an RF system has to do with the nature of the relationship of the reflected signal to the direct path signal.
The direct path carrier takes the most direct, and consequently, the shortest path from transmitter to receiver. The reflected carrier, on the other hand, takes a longer path, from the transmitter to the reflective surface, and from the reflective surface to the receiver. The waves leaving the transmitter antenna are all in phase, but because the direct carrier and the reflected carrier travel different distances, thus taking slightly different lengths of time, the two carriers are out of phase, and of different amplitudes (remember the inverse square law), when they reach the receive antenna. The two carriers are combined at the receive antenna and, being out of phase, they cancel each other out so that little or nothing can be detected by the receiver. This causes a momentary interruption in the RF wave, which is called a dropout. Dropouts are manifested in audio RF systems by a loud click or pop surrounded by noise. Proper system design and careful antenna placement can go a long way to reducing the effects of multipath on a wireless communications system. We discuss how to avoid multipath later in this chapter.
The next concept that you must be familiar with to move forward in the design of your wireless intercom system is receiver desensitization or desensing. As mentioned earlier, desensing happens when a transmitter is in close proximity to a receiver, even if that transmitter is not on or near the receiver’s operating frequency. Receiver desensitization happens because receivers must maintain critical voltage and current levels throughout the front-end stages, and a strong (i.e. close by) transmitter can cause these levels to vary greatly. As these levels are changed over a wide range, the receiver performance will be greatly degraded. The greater the physical distance between transmitter and receiver, the less the receiver will be affected. Likewise, the greater the frequency separation between the two, the less the receiver performance will be affected.
Selecting frequencies that are “clean,” or free from the effects of intermodulation products, is essential to good wireless communications. Intermodulation is often one of the prevalent sources of system interference. We touch on just the basics of intermod here so you can get a sense of what it is and how it works. As stated in an earlier chapter, intermodulation, or IM as it is often called, happens when two or more frequencies mix in a non-linear device and produce a number of related different frequencies known as intermodulation products. These IM products can cause severe, harmful interference to a wireless intercom system if they fall on or near any of the operating frequencies of that system.
For intermodulation interference to take place, at least two transmitters must be broadcasting at the same time on frequencies that have a definite, calculable relationship with the affected receiver. In many cases of IM interference the receiver can detect and demodulate the IM product almost as cleanly as if one of the interfering transmitters was on the operating frequency of the receiver. Turning off either one of the two (or more) transmitters will cause the IM interference to cease.
Because there is a fixed and calculable relationship between frequencies, intermodulation products can be calculated and avoided. Here is an example of some of the more common IM products that can be calculated:
2A – B = C
2(651.500 MHz) – 650.000 MHz = 653.000 MHz
A – B + C = D
184.000 MHz – 190.600 MHz + 188.200 MHz = 186.400 MHz
3A – 2B = C
3(518.200 MHz) – 2(520.500 MHz) = 513.600 MHz
There are, of course, many other combinations that can cause harmful interference. These examples give you a good idea of how the calculations work, but for comprehensive frequency selection, an advanced computer program must be used.
It is important to note that intermod products are not created in the air; they are the result of the mixing of signals in non-linear devices such as amplifiers or other usually active elements. The most common place for this mixing to take place is in the active receiver RF circuitry. Once RF signals get past the receiver front end and get to the first RF amplifier and beyond, mixing of those signals can and will take place. If the intermod products that are generated fall on or near the operating frequency of the receiver, harmful interference will be heard.
Good quality receivers have front ends that are passive, linear devices that limit the range of frequencies that will enter the rest of the receiver circuitry. Making sure you pick wireless intercoms with well-designed front ends is critical to proper operation in hostile RF environments.
The next most common place for IM products to be generated is in the final amplifier of a transmitter. Because the transmit antenna can and does also act as a receive antenna, strong RF signals from nearby transmitters can make their way into the non-linear, active, final amplifier and produce intermod products. These products can then be broadcast out with the intended signal and cause harmful interference. It is important to note that IM products do not have to end up exactly at a receive frequency. Sometimes, they can be of sufficient power at relatively close frequencies to create a desensing situation.
Reducing the effect that intermodulation can have on your wireless intercom system comes down to a few important principles. First, and foremost, you must pick frequencies that are intermod free with each other and with surrounding transmitters. Second, you should pick wireless intercom systems that have well designed receivers and transmitters with appropriate passive filtering. Third, you must manage the positioning of antennas and beltpacks within the system to optimize operational potential.
Figure 1
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- Transmitters & Receivers
- To be able to select the appropriate wireless communications equipment you need to understand the basic operations of transmitters and receivers, and which aspects are important to proper operation. In this section, we cover generic functional block diagrams of transmitters and receivers, and point out the most critical aspects of each. While design variations are great between manufacturers, the block diagrams that follow represent the most basic designs.
Let’s start with the transmitter (see Figure 1). The primary job of the transmitter is to take in a source signal, modulate it onto an RF carrier, and then deliver it to the transmit antenna for broadcast into the electromagnetic spectrum.
First, an audio signal is brought in and any necessary audio amplification is done via the Mic/Line Input section. Next, the signal is sent through a Compressor circuit to ensure the levels of the input signal are held within acceptable limits. The signal is then mixed with a reference frequency in the Modulator. This reference frequency can be the main carrier frequency, or (as in most cases) it is a base frequency that results in a composite signal.
Note There are many different types of Modulators, as well as, many different types of modulation. A detailed discussion of their detailed workings is beyond the scope of this book.
The signal is then sent to the Amplifier/Multiplier. If the signal is already on the desired transmit frequency, it is only further amplified. If, however, the signal is only a composite signal, then it is frequency multiplied to reach the desired operating frequency. The signal is then sent to a Final Amplifier where it reaches its maximum power level. Usually this is slightly more than the actual output power as measured at the output connector. The reason for this is to make up for the losses induced by the Output Filter and Impedance Matching circuit(s).
The Output Filter and Impedance Matching circuits are generally passive and therefore, do not provide any means of amplification. As such, they can only reduce the output signal levels. The Output Filter is a very narrow bandpass filter that removes any unwanted harmonics from the signal. The Impedance Matcher provides the necessary interface between the transmitter and the Antenna/Transmission Line to ensure maximum power transfer. If the Antenna/Transmission Line is not properly matched, significant loss can occur. In some situations, it is possible for this to cause damage to either the transmitter, transmission line, and/or antenna.
Now let’s look at the receiver and it’s primary functional aspects (see Figure 2). The receiver in a wireless system is the exact compliment of the transmitter, but is usually much more sophisticated and complex in design. Its job is to receive the signal from the receive antenna and extract the source signal so that it matches the original exactly. In practice, there will always be some modification or distortion of the source signal in the course of transmission, but good quality wireless systems minimize this to a level that is indistinguishable.
As in the transmitter, the antenna will be covered in the next section. The receiver starts with the front-end filter. The front-end filter is extremely important to successful operation in high RF level environments. The front-end filter is the first line of defense. Its job is to limit the number of potential interfering frequencies that could affect the receiver. It is usually a passive, linear section and it must be impedance matched to the antenna for proper signal transfer. Linearity is the most important factor in a front end, even more so than how tight or narrow the section is. A high degree of linearity will ensure that no intermodulation products are generated in the front end before extraneous RF signals are filtered out. Having a front-end that is relatively tight and that is extremely linear is critical if the system is to work properly under worst-case RF scenarios.
The next section of the receiver is the first RF amplifier. The first RF amp’s job is to take the extremely low level RF signal coming through from the front end and bring it up to a usable level. The incoming RF signal at the first RF amp can vary dramatically from less than 0.5 ìV to almost the value of the transmitter output. The key for the first RF amp is that it should be able to handle very small, as well as, relatively large incoming signals within it’s linear region of operation. See Figure 3. To maintain a good linear region,
RF amps normally require a high current drain, which can negatively affect battery life. A compromise between linearity and effective battery life must be managed carefully.
The next receiver section we look at is the first local oscillator (LO). The job of the first LO is to provide a reference signal that is a fixed distance from the operating frequency of the system. It is very important the first LO be stable over a wide range of temperatures. In fixed crystal systems, one or more crystals cut to a specific relationship of the operating frequency are used to generate this highly accurate reference signal. A different crystal is necessary for each operating frequency. In synthesized units, a single reference crystal is used in a phase-lock-loop to provide the signal for any operating frequency needed by the receiver.
The First LO feeds the reference signal to the Mixer where the incoming RF carrier is mixed, or beat with the reference signal, to produce the First Intermediate Frequency (IF). The frequency of the First IF is the difference in frequency between the incoming RF carrier and the First LO reference signal. Unfortunately, what comes out of the Mixer is not a just the First IF, it is the algebraic sum and difference of the two signals being mixed plus numerous other harmonic junk. To get to the point where you have a clean First IF consisting of just the desired frequency, the signal is passed through to the First IF Filter. The First IF Filter is extremely important to proper receiver operation. It is a passive, very narrow (often 50 to 250 KHz), and precise filter that eliminates the vast majority of unwanted signals so the true First IF can be processed correctly. It is very important that the First IF Filter be sharp, as well as, very linear. Any non-linearity in the filter will cause unwanted distortion of the demodulated source signal.
Next, the signal is sent to the second Mixer where a second IF frequency is produced in the same way the First IF was obtained. The Second LO is the same frequency for any RF carrier frequency the receiver is capable of because the first LO takes care of the frequency differences and produces an always-constant First IF frequency for the Second IF to handle. Again, the Second IF signal as it leaves the Second Mixer is full of harmonic junk and needs to be filtered by the Second IF Filter. The Second IF filter eliminates unwanted harmonic energy and prepares the signal to be demodulated.
The next phase of the receiver is the Demodulator. There are several types of demodulators used by wireless manufacturers today and it would be beyond the scope of this book to discuss them all in detail. Suffice to say; through a type specific process the Demodulator extracts the source signal from the Second IF carrier. The quality of the Demodulator circuit is critical to good audio quality. Any type of signal distortion or modification that takes place in the demodulation process will cause the final signal to be a less than perfect reproduction of the original source signal.
Figure 1
Figure 2
Figure 3
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- Antenna & Cable Considerations
- Antennas and cables (transmission lines) are one of the least thought about aspects of a wireless system among RF novices. Good quality antennas and cables, however, are some of the most important aspects to establishing and maintaining a quality RF link. In addition, because antennas and cables are more easily changed and in general are less expensive than other system components, they can be a “quick fix” for many RF problems found in common wireless communications systems. In this section, we cover some of the more common types of antennas and the operating characteristics of each. In addition, we take a look at coaxial cable and what is required when selecting cable for your system.
To adequately look at and evaluate the strengths and weaknesses of some common antenna types, we first must have a very general knowledge of antenna theory. Antenna theory is a course of study unto itself and we will not even scratch the surface in this brief section, but it should be enough to understand some basic principles. To start, let’s ask the question, “What is an antenna?” To answer that question we must look at what an antenna does. In a transmitter, the antenna takes electrical energy and allows it to be propagated out into the electromagnetic spectrum. In a receiver, the antenna “gathers” the RF signal and converts it back into electrical voltages and currents. In either case, the antenna acts as a transducer to change the form of the RF energy.
All real world antennas have a pattern or specific shape with which the RF energy is released or captured. There is no such thing as an antenna that sends energy out equally in all directions. The primary reason for this is that you have to get the signal to the antenna via a transmission line and that line must be connected to the antenna some how. That connection will always cause a disruption or altering of RF propagation in some direction. In theory though, it is nice to talk about a perfect antenna. This perfect antenna radiates equally in all directions and is called an isotropic radiator. See Figure 1.
We can look at how all other antennas emit RF energy as a comparison to our perfect antenna. The isotropic radiator is said to have zero antenna gain. Antenna gain is an often-misunderstood term, so we will cover it here. Let’s start by saying that a passive antenna is not an amplifier and cannot increase to total RF energy being emitted or received. Having said that, an antenna can and does focus the RF energy in a specific direction or directions. This focusing of energy causes greater RF energy levels in those directions and weaker energy levels in the remaining areas as shown in Figure 1.
We can think about this by looking at a water balloon. If we had a water balloon that was a perfect sphere, it would accurately represent the pattern of an isotropic radiator located in the balloon’s center. All of the RF energy is equally dispersed in all directions. If you squeezed the balloon’s center with your hands, a corresponding bulge would appear on either end. The balloon is not any larger or smaller than it was, it has only changed shape. This is how a real world antenna works. When energy is focused in one direction, it must always be at the expense of energy going in another direction.
The most basic form of real world antenna is the dipole. The dipole has 2.15 dBi of antenna gain over an isotropic radiator. That means there is 2.15 dB more signal in the direction that the energy is focused than there would be if the antenna were an isotropic radiator. Antenna gain is specified in one of two ways: dBi or dBd. It is very important to know which specification is being used when comparing antennas. dBi, as stated above, is referenced to the uniform radiation of an isotropic radiator. dBd, on the other hand, is referenced to a dipole. Most antenna manufacturers like the dBi spec because the number is bigger, but since there is no real world antenna that represents the 0 dB mark, many engineers prefer dBd. In reality, either specification is fine as long as you are comparing apples to apples. In the remainder of this book, all antenna gain references will be in dBd, referenced to a dipole, unless otherwise specified.
Important: If you need to convert from dBi to dBd, simply subtract 2.15 dB from the dBi number. If you need to convert from dBd to dBi, simply add 2.15 dB to the dBd number. There are two basic groups of antennas, omni directional and directional antennas. Omni directional antennas are generally low gain antennas used in the center of operational areas. Because the RF energy in omni directional antennas is in 360° and not in one specific direction, the antenna gain must always be low. The isotropic radiator and dipole antennas are both examples of omni directional antennas. Normally, omni directional antennas will be found with antenna gain less that 5 dBd. Gain in omni directional antennas is achieved by flattening the vertical angle of the pattern as shown in the dipole example in Figure 1.
For proper propagation to take place, the length of an omni directional in critical. The theoretical minimum length for an omni directional antenna is ½ the wavelength of the RF carrier to be served. In many cases, this ½ wavelength is too long to be practical so a ¼ wave antenna is used instead. It is extremely important to note that for a ¼ wave antenna to work properly, it must have a corresponding ground plane that is equal to or greater than the length of the antenna itself. It is for this reason that a ¼ wave antenna that works just fine when it is attached directly to the back of a wireless receiver has very poor coverage when operated at the end of a length of coaxial cable. The cable does not provide the necessary ground plane for proper ¼ operation as the receiver does. This is a very common mistake made by RF novices who are trying to improve RF performance and end up killing it instead!
Directional antennas, on the other hand, seek to focus the area of coverage to something less than 360° to form a flashlight like coverage pattern. Directional antennas are normally used on the edge of a coverage area. They can have very high antenna gain factors in excess of 20 dBd. Normally though, in conventional wireless communications systems, size and cost limit directional antenna gain to less than 12 dBd.
Directional antennas have the advantage of not only focusing the RF energy in a given direction, but also attenuating energy from undesired areas. This is very important for receive antennas in areas with high levels of RF. If positioned properly, a directional receive antenna can increase the desired RF energy while attenuating unwanted, potentially interfering RF energy from other areas. See Figure 2.
There are two very commonly used directional antennas in wireless communications systems today, Yagi and Log Periodic antennas. We will not cover the technical differences of these antennas here, but we will discuss the functional differences. Just as in omni directional antennas, directional antennas must be tuned or “cut” to a specific frequency range. This is fine when there is only one RF frequency, but if you are using a range of frequencies through a single antenna, it is important to ensure that all of the RF signals will be in the effective range of that antenna. The primary difference of Yagi and Log periodic antennas is the range of frequencies they can handle. Yagi antennas normally handle a relatively narrow range of RF frequencies, while Log Periodic antennas can achieve much larger effective frequency ranges.
On the surface, it would appear the wide frequency range of the Log Periodic antenna would make it the obvious choice, especially when you consider that Log Periodics are generally also much smaller than Yagis. This however, is not always the case. Consider the application where there are strong off frequency interference sources (virtually all high RF level applications!). In these situations, the off frequency rejection of a Yagi antenna can greatly improve system performance and decrease harmful interference. In general, it is a good idea to choose an antenna that is just wide enough to handle the desired operating frequencies.
One more note on directional antennas. Because FCC rules concerning transmit power (Effective Radiated Power or ERP) take into account the antenna gain of the transmit antenna, high gain transmit antennas may not be used on transmitters in most wireless communications applications. The good news is that high gain antennas on the receive side of an RF system are also very effective for increasing system range and are commonly used.
We reiterate one more important antenna concept. As stated in an earlier section, antenna polarization is critical to proper system operation. Transmit and receive antennas of the same system must be oriented in the same direction to have a proper transfer of the carrier. In theory, if a transmit antenna is oriented vertically, thus producing a vertically polarized carrier, and the corresponding receive antenna is oriented horizontally, the receive antenna will not be able to see the vertically polarized wave at all. In practice, there will always be some polarization shift in the path and the receiver will see a very small signal if it is close enough to the transmitter, but system range will be greatly reduced. To avoid this problem, antennas in a given RF system should always have similar orientation.
Now, lets take a brief look at the role coaxial cable (transmission line, feedline) plays in the big picture. See Figure 5. Unless an antenna is attached directly to the receiver or transmitter in an RF system, coaxial cable is the usual means used to span the gap. The importance of choosing the right coaxial cable cannot be over-stressed. When choosing cable to use in your RF system three main factors must be considered:
1 The cable must be properly impedance matched (correct characteristic impedance). Most wireless systems today are 50-ohm impedance systems. That means the final amplifier and filters in the transmitter, the front end of the receiver and both transmit and receive antennas, are designed to work using 50 ohms as the nominal impedance. It is extremely important to choose coaxial cable that is also 50 ohms. Coaxial cable that is used in video applications is normally 75 ohms, not 50 ohms. Don’t ever use video cable in RF transmit applications. An explanation of why this is bad is beyond the scope of this book, but trust me on this one, it is a bad thing, don’t do it.
2 Consider the loss per foot of coaxial cable at your system’s operating frequency. In VHF systems, it is usually easy to select cable with acceptable loss for runs of 100 feet or more. In UHF applications however, it gets a little tougher. See the coax loss chart below in Figure 4. In general, it is a good idea to never have more loss in the transmission cable than you have antenna gain in the system. This is a good rule of thumb that will keep you out of trouble most of the time.
3 Consider how the system is used. Is this a fixed installation, or a mobile one? If the system is being moved frequently, you want to use coaxial cable that has a stranded center conductor. Just like other types of wire, coaxial cable with a stranded center conductor will tolerate being flexed repeatedly without degradation in performance. However, this doesn’t mean you can tie a knot in the cable, or crimp it in a door and expect it to work perfectly.
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
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- Installation
- Having all the right gear and all the proper frequencies selected is a good first step to having a top notch, highly effective, wireless communications system. However, having the right stuff is not enough, it has to be installed properly or it is all for not. In this section, we take the time to cover the most common dos and don’ts of installing a wireless system that actually works!
We’ll start with the general conceptual strategy for selecting a location for the RF equipment to live. Unlike hardwired communications systems, that can be tucked away almost anywhere, wireless systems must have prime real estate locations due to the extremely limited length of the coaxial cables that connect the transmitter and receiver to their respective antennas. As discussed in the previous section, the length of the antenna cables in a wireless system should rarely exceed 100 feet, and in some cases, they should be kept much shorter due to frequency and cable loss. Because of this, selecting the location of transmitter and receiver equipment is absolutely critical to system performance.
First of all, it is necessary to determine all of areas where coverage is absolutely necessary. These are the ‘no compromise’ areas, and your system must be designed and installed to consistently meet or exceed these minimum operational requirements. Anything you can get after these areas is gravy. Select a location for the wireless base station that is centrally located in the “must work” area whenever possible. Obstacles like buildings, cars, trees outside, walls, cameras, lighting, and equipment racks inside all act as factors to limit range. If they are in the direct line of site between the base station antennas and the wireless beltpacks, it is important to locate the base antennas as high as possible. Getting a few extra feet up will often make a large difference in overall system performance.
When installing the wireless base station it is important to avoid locating it near computer or other microprocessor controlled equipment. All computer type equipment radiates RF energy that can cause harmful interference in even the best wireless equipment. Likewise, your RF equipment may interfere with the operation of the computer equipment. Try not to have your wireless base station in the same rack as lighting controllers, audio processors or other highly RF radiant electronic equipment. Whenever possible, locate the wireless base station in its own enclosure to avoid harmful interference to or from other gear.
The specific location of the antennas is also extremely important. Antennas should never be placed in close proximity to large metal surfaces that are parallel to the active element of the antenna. A large metal surface can cause numerous problems in most wireless systems. Try to locate antennas in the middle of a room (when inside) as far away from reflective surfaces as possible. When using omni directional antennas inside, try hanging them upside down with the ground plane on top, near the ceiling. This will provide the most effective radiation pattern. It is also a very good idea to get as much space between the transmit antenna and the receive antenna as is practically possible. As was covered in an earlier section, this will avoid the phenomenon known as desensing and increase system range. It is important to note that having the receive and/or transmit antennas as high up as practical is often more beneficial than increasing the output power of the transmitter(s).
As covered in the previous section, it is critical to make sure that antennas are polarized the same on both the transmit and receive end of the wireless system. In wireless communications systems, the deciding factor lies with the beltpacks. Since the antennas on the beltpack will normally be vertically polarized it is important to ensure that the base station antennas are also vertically placed. Never mount an antenna using part of the working or active elements. Most antennas that are designed to be mounted come with specific mounting hardware. Trying to rig some other “innovative” mounting solution will almost always result in reduced system range and performance. Also, don’t paint or otherwise cover you antennas. Some paint has a metallic component and will greatly affect system performance in a negative way.
In some extreme applications, it may be desirable to have multiple antenna locations for an individual wireless base station. This technique can greatly increase the effective range of the system if the everything is done just right. If not, it will most assuredly degrade overall system performance. The first thing to remember is that splitting the transmit antenna is never permissible! This is tantamount to setting up a frequency interference source right next to your operational area of coverage.
Splitting receive antennas can sometimes be a good idea. The key thing to remember here is the line impedance must be properly maintained. This means you should not use a standard “T” connector to perform the split. The most common device used for this function is a Mini Circuits splitter. This device maintains the 50-ohm impedance on all legs. It is important to remember there is no such thing as a free lunch. Splitting antennas comes at a price. When you add a second antenna the signal from each antenna is reduced by at least 3dB. This loss then needs to be figured into the total loss/gain calculation for proper system performance. Multiple antenna configurations can be very challenging. When faced with the need to do so, it may be time to consult with an RF professional for help.
As covered in the previous section, make sure you have selected a coaxial cable that is of the correct impedance and has a loss per foot low enough to support the length necessary without having more loss than you have antenna gain. Be sure to note with omni directional antennas, it is sometimes acceptable to have a dB or two more cable loss than you have antenna gain. When running RF coaxial cable, make sure the cable is not bent sharply as to crimp the cable. The magic of coaxial transmission lines is a direct result of the relationship between the center conductor, the dielectric and the shield. If a cable is pinched in a door, or bent sharply around a corner, the characteristics of the cable can be changed dramatically and have a significant negative affect on system performance.
Electromagnetic fields generated by other radios, AC power, arc welders or… well you get the idea, can also have a negative affect on your wireless communications system. Avoid placing antennas near any device that has a strong electromagnetic field associated with it. Also, do not route antenna cables in the same runs as high voltage AC lines. Whenever possible, try to keep antenna cables by themselves. The thing to remember is the RF signal at the receive antenna of a typical wireless intercom system can be less than 0.5ìV, that’s 0.0000005 of a volt! It does not take much to disrupt such a small signal and anything we can do in the installation process to prevent that disruption is time well spent.
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