Tips for police cruiser installations Pre-planning, care in running wire and cables, a master ground and proper antenna placement are keys to a successful installation. Good quality control is also an important element in preventing unnecessary callbacks.

Major urban, with an increasingly large number of suburban and rural, public safety departments are equipping their patrol fleets with modern, powerful cruisers. If these vehicles come to the installer fresh from the dealer or supplier, a tremendous amount of work is in store for the technical staff. In addition to a light bar, siren horn(s), radios, antennas, wig-wag (headlight drivers) and corner strobe lights, the installer will also add many new police tools. These tools can range from speed radar to video camera and recorder system to Doppler direction-finding and tracking systems to computers giving officers a direct link to databases. As always, pre-planning is a key element to a successful installation. Let’s begin with a new Ford Crown Victoria and look at what is required to turn it into a modern police tool.

The outside light bar, pusher bumper, corner strobe units and other structural items may best be installed by the garage or mechanic who will service the vehicle in the future, rather than your radio shop. The light bar wiring and controls can easily be installed by the radio shop in order to keep complete records–more on this later. Outside siren horns should be mounted at the same time. With that done, measure the space in and under the dash available for equipment. Keep in mind that you must allow an interior “safe zone” for air bag deployment. Ensure installed equipment does not interfere. If in doubt, check with the dealer or expert service personnel familiar with the size of inflated air bags. Don’t forget to allow space behind or near radio mounts for wiring bundles under and through the dash and firewall. With the smaller size of new vehicles and the large amount of police equipment normally found today, you will almost always be forced to use a console mount to hold the multiple control heads, radios and light controls. Remaining controls for the VCR system are mounted in the overhead, safely out of the way.

Once you have measured and loosely fit the console, check the operator’s preference for the stacking of equipment and control heads. Customer involvement in this early stage can eliminate potential problems and help to build a good working relationship. Now you have a good grip on where the wiring will have to run. As an aside, most installers find they must use the trunk area to mount R/T modules, siren amps, auxiliary equipment or other units. Given the amount of gear carried in the trunk by most safety officers, care must be used in placement of communication and signaling equipment. A plate rigidly mounted on the side of the trunk will help, or consider using the underside of the rear deck. The technicians here at the Municipality of Anchorage, AK, radio shop have indicated to me that the underside of the deck works well for them and provides good protection for the equipment. Given the extreme cold temperatures found here, however, radios are mounted inside. In a warmer climate, like the desert Southwest, consider problems related to heat-loading in trunk-mounted equipment.

Set the mount or console aside and pull the wire and cable. Use care to protect the wires or cabling run under the floor. Feeding antenna cable of the ‘A’ post (windshield support post) and to the antenna is easy–a smooth fish tape will help avoid damage to the headliner. Carpet can be removed temporarily, if necessary. Before you drill through the firewall, double check both sides! I have embarrassed myself by drilling into odd things behind the firewall. Some vehicles have grommeted holes that may be suitable if enlarged. Always protect the wires with a grommet or other chafe-reducer.

A master ground will go a long way toward stopping interference before it starts. Running a tie wire between the battery, body, frame and the equipment console, all tied to a common point, is best. Use the correct gauge of wire, based on equipment current draw, and avoid splices. Place the ground point where you can easily check and service it. A master power (positive battery) buss may help in the installation. A master switch or fuse near the battery is essential for safety. You should always consider a low-voltage cutout or magnetic relay to prevent battery drainage while the vehicle is off or unattended. Finally, make notes or a line drawing of the installation wiring; nobody lives forever, and sometimes you do go on vacation, so give your coworkers a break and document your work.

Now that the wiring is in, permanently mount the console and radios. Once all equipment is mounted, check that each system will power up as you connect it. This will simplify any troubleshooting, should any be necessary to bring the system on line. With that done, you can then finish the wiring by using tie wraps, looming or clips to secure everything. Marking or tagging the individual wires in the harness, indicating the unit supplied, will save time in the future, should problems arise. At this point, a copy of the wiring harness drawing or diagram and wire list/marking should be placed in the vehicle service file. This should include the type and size of fusing, and modifications such as ‘wig-wag’ headlights, switching for light bar and so on. Now back to the outside.

Antennas and related cable require some thought. Antennas should be placed away from light bars, strobes or other obstructions that could distort or block the transmitted signal. Additionally, interactions between systems need to be considered. New video recorders found in public safety vehicles often use low-power “wireless” or radio mics. The recorder/receiver unit is mounted inside the trunk, in a sturdy, lockable “box.” Radar units and in-vehicle repeaters are other examples of potential systems that could cause conflicts. For those installations where undercover work is anticipated, replacement automobile-style antennas or mag-mount DDRR-type antennas can be considered. If you mount antennas under the vehicle body, say a mag-mount DDRR, take care to attach the cable to avoid snags or chafe, which could cause damage.

Take the time to check the quality of your work when finished. Test each system and the total of installed systems before turning the vehicle over to the user. Always take the time to go over all the installed systems in a short “training” session with the operator. Even experienced public safety personnel may have questions that are best answered at the beginning rather than as a trouble callback. You could combine the final checkout with a training session to save time and effort.

Pre-planning, care in running wires and cables, a master ground and proper antenna placement are some of the keys to a successful installation. Proper documentation, operator training and systems testing are keys to prevention of unnecessary downtime and callbacks. Integration of new technology–video, radar or direction-finding systems, can be simplified with this system-based approach. Best of luck on your next install.

Acknowledgments: To Gene Sole, manager; Edward Wise, David Dias and Jess McDonnell, technicians at the Municipality of Anchorage, AK, radio shop; and Officer Stillwell of the traffic unit. Thanks to all of these professionals for taking the time to answer questions and for photography.

Some land mobile RF data modems just seem to work better than others. Often, a modem will work well when the vehicle is stationary, but behave poorly when the vehicle is in motion. Sometimes, the exact opposite is true. Sometimes a mobile data network consisting of 9,600bps or 19,200bps modems will have an inbound channel throughput of less than several kilobits per second (kbps). This article describes several key mobile RF data modem performance measurements. It then describes how well-engineered data modems achieve good performance in the land mobile environment.

The first article in this series (November 1995) showed that RF signal fading is a natural phenomenon that causes burst errors in land mobile data transmissions. Because fading limits the performance of land mobile RF data modems, understanding its behavior is important. In contrast to land mobile data systems, the performance limitation for modems intended for satellite use is a low signal-to-noise ratio (SNR). Frequently, poor land mobile RF data modem performance can be traced to the use of modems that are simply not designed for vehicular use.

System performance measures There are many measures and specifications used to describe land mobile RF data modems, but two sets of curves are most important. The first curve specifies the decoding performance of the modem as a function of received signal level (RSL) and vehicular speed. The second curve specifies data throughput performance when a large number of modems are using the same inbound (mobile-to-base) frequency.

Decode performance curves Figure 1 at the left shows a set of decode performance curves. The horizontal axis represents the average RSL measured at the receiver’s antenna input. The vertical axis represents the percentage of messages successfully decoded. Because many modems allow for varying message lengths, these tests should be performed using a nominal length data packet. The static curve on the right shows decode performance in a non-fading environment. This curve can be generated using an RF service monitor, and it provides a useful baseline for the modem. The curves on the left show the decode performance in a fading environment. These three curves represent different vehicle speeds. Mobile fading simulators, available from various manufacturers, are used for faded performance measurement.

Decode performance curves are normally used for system coverage calculations. Customer requirements provide the required message-decode probability. The minimum required mean signal level is then determined from the faded curves on the left. Next, this level is entered into the system link budget and converted to an estimated coverage area using computer coverage analysis. As a practical matter, the data from the static curve often are entered into the link budget, and an extended coverage area is determined. The extended coverage area shows the increased service area that is often available when vehicles are stationary.

Throughput curves Figure 2 on page 34 shows a set of calculated inbound data throughput curves. These curves show the amount of inbound data successfully decoded by the base station receiver and passed to the host computer. The curves shown are typical for a system using digital sense multiple access (DSMA) contention control. (See sidebar, page 38.) Throughput curves are commonly found in system documentation, but frequently are misunderstood. An example of the usefulness of the throughput curves is shown in Figure 3A at the right. The user’s peak throughput demand is compared to the maximum throughput of the system. Figure 3B on page 40 shows the success probability of a signal transmission as a function of the operating point. Figure 3C on page 40 shows decode success probability as a function of the number of retries.

Four design techniques for vehicular modems Error control refers to the process of adding redundant bits to data packets so that transmission errors either are detected or are corrected by the receiving modem. An important system design consideration is the balance of error detection and correction. Consider a system with only the ability to detect and reject corrupted packets. The data throughput of such a system would be quite low because the RF fading process nearly always guarantees errors. Next, consider a system that would attempt to correct every packet received. When badly corrupted packets would be received, this system would attempt correction and would pass corrupted messages to the user. A properly designed system optimizes the balance for maximum data throughput.

A simple example of error correction would be to send messages three times and to vote at the receiver, as shown in Figure 4A on page 42. The code shown in Figure 4A is conceptually simple, but two weaknesses limit its use. First the 200% redundancy significantly affects throughput. The second weakness is that two strategically placed errors could cause an undetected error.

A practical and frequently used code is the (63,45,9) Reed-Solomon code shown in Figure 4B on page 42. It is used in several commercially available systems. This code adds 108 bits of redundancy to a 270-bit data payload (40% redundancy) and will correct any nine bit errors. In addition, if the errors arrive in bursts, which is the typical case for a mobile channel, the code will correct for even more errors.

Data interleaving is explained best with an example. Consider a 9,600bps system using a 900MHz RF carrier. For this system, fades of greater than 15dB below the mean RSL cause the modem to take errors. At a vehicular speed of 45mph, the expected rate of fades below 15dB is 24.1Hz, and the typical duration of the portion of the fade below 15dB is 1.2msec or 12 bits.1 If the data were sent in continuous, sequential blocks of 60 bits as shown in Figure 5A on page 44, the amount of redundancy required for successful transmission in this environment would be 66%. If the interleaver shown in Figure 5B on page 44 were used, the required amount of redundancy drops to 15.4%.

Interleaving takes advantage of the fact that in a fading environment, the average error rate is low. The interleaver de-correlates the burst errors so that the receiver’s error decoder is presented with the average error rate instead of the higher burst error rate. The result of using the interleaver is that less redundancy is required for a given performance level.

Well-designed modems use a combination of error correction, error detection and interleaving. Note that the curves on the left side of Figure 1 are close together or nearly independent of the vehicle speed. This is the result of an error control strategy properly designed for vehicular use. If the faded curves are spread out horizontally, i.e., if the decoding performance varies with vehicular speed, the error control strategy is incorrect. This commonly happens when satellite or telephone line modems are used for vehicular applications.

Data compression is a technique that allows the modem to take advantage of the inherent redundancy of most data messages or graphic images. The basic idea is simple. Frequently transmitted characters should be converted to extremely minimal channel uses. Infrequently used characters should be represented using longer channel uses. A good, though not optimal, example is Morse code. This code represents the most commonly used letter in English, the letter “e,” by a single dot. Conversely, the least frequently used letter, “z,” is represented by the longer channel use of two dashes and two dots.

To understand the achievable performance of data compression techniques, it is useful to understand two basic distinctions. First, data compression techniques are either lossless or lossy. Second, they are either adaptive or non-adaptive. These properties are independent of one another, as shown in Figure 6 on page 46.

A lossless data compression scheme compresses source data so that the decompressed data match the source exactly. When text or transactional messages are compressed, lossless methods generally are required. Lossy data compression techniques do not pass all of the source information. Instead, they transmit only the data that are most important for the application. Lossy data compression is extremely useful for image transmission because the compression performance is high, and the losses are nearly undetectable.

Non-adaptive techniques compress data according to a fixed rule. This works well if the data compression rule matches the statistics of the data being transmitted. For example, the Morse code matches English text. In a hypothetical language where “z” would be the most common letter and “e” the least common, the Morse code would yield a much lower throughput. Straightforward techniques can be used to generate a unique, non-adaptive compression code (a Huffman code) for almost any system requirement.

Adaptive techniques continually analyze the incoming data and modify the compression rule as required. The performance of these techniques is somewhat independent of the statistics of the data because the compressor adapts itself to them. For example, if an officer were to type an accident report with frequent uses of the word “vehicle,” the compression algorithm would detect this and would represent “vehicle” with a minimal channel use. The most common adaptive data compression is the Lempel-Ziv algorithm, which frequently is used for computer file compression utilities.

Both adaptive and non-adaptive techniques are useful in mobile data systems. Status or short text messages are best compressed using non-adaptive data compression; longer transmissions are best compressed adaptively. Conceptually, data compression strategies can be mapped as points on Figure 6.

Acquisition performance is a modem characteristic that directly affects system throughput. Acquisition time is the amount of time required for a receiving modem to determine that an incoming signal is present. In DSMA systems (see sidebar), the two largest constituents of the contention window are the mobile attack time and the base modem acquisition time. The acquisition curve in Figure 7 on page 48 shows the mean and standard deviation of acquisition time as a function of the receive signal level. This curve shows a good acquisition performance because the modem acquires reliably at quite low signal levels. A result of poor acquisition performance is that, although messages may be passed at low signal levels, the increase in the contention window significantly degrades data throughput.

Will it work? Designing a modem to work in an RF-fading environment is an essential part of proper system design, but in itself does not ensure a successful system deployment. A modem should be flexible enough to satisfy varying system requirements. For example, if coverage over a large, sparsely populated geographic area is required, 600bps may be the correct rate. If the required coverage area is a small, densely populated area, then 19,200bps may work well. If the coverage area is large, and the number of frequencies is limited, the modem must be able to operate in a multisite, multifrequency system. The modem should also provide features that reduce installation and maintenance costs as well as features that maximize system reliability. Finally, developers of software applications should find the modem easy to use.

Conclusion Understanding mobile data equipment can be an arduous task. Research articles continually describe exciting new techniques for wireless communications, and manufacturers sift through these research results, developing those which are practical and useful. Users and potential users of this equipment are presented with tremendous amounts of information.

The intention of this article and Part 1 in the November 1995 issue has been to explain RF data modem performance in terms of some basic fundamentals. The first fundamental is the RF fading process, common to all wireless data systems, which was described in Part 1. Next, the basic performance measures, decode performance (sensitivity) and data throughput were described. Finally, four design techniques were described. Some or all of these techniques are used by most state-of-the-art RF data modems.

Reference 1. Reudink, D.O., “Properties of Mobile Radio Propagation Above 400MHz,” IEEE Transactions on Vehicular Technology, November 1974.

RF modem series Part 1–“How Mobile Radio Fading Affects Data Transmission,” November 1995. Part 2–“How Mobile Radio Fading Affects RF Data Modems,” August 1996. Back issues for the past two years are available for purchase from Intertec Publishing customer service. Call customer service at 800-441-0294 or 913-341-1300. Article photocopies are unavailable from the publisher, but they can be ordered from UMI Information Store, 800-248-0360 or 415-433-5500 ext. 282; fax 415-433-0100.

As is typical of many paging entrepreneurs, Jeff Grazi serves as president, chief engineer and technician for his small company, Communications Unlimited, Denver. To maintain a competitive edge, he depends on system reliability and customer service. His greatest fear is a system-wide failure that goes unannounced until customers call to complain. By that time, a failed control link or paging transmitter may have been off the air for hours, and customers may have begun to shop elsewhere for paging service. One solution for small paging system operators has been to monitor their channels continuously. For 10 years, Grazi used receivers in his office, home and car to monitor his three channels. This approach constantly consumes an operator’s time and attention, instead of only requiring attention when a problem occurs.

An automatic and silent method of monitoring the network would be an improvement over active monitoring. For example, sending test messages over the network at regular intervals for decoding by an automatic monitor connected with an alarm would make it possible to detect a network failure soon after its occurrence.

My company devised an eight-output, remote-controlled switch that decodes alphanumeric and numeric POCSAG messages. Each output is individually controlled, and each can be programmed with a time delay ranging from one second to 12 days. An on-board serial port allows the switch to feed received messages into a personal computer, a printer or a moving-message electronic sign.

The remote-controlled switch is an industrial pager that receives the paging signal over the air and decodes the specific capcodes for which it is programmed. The eight remote control outputs operate from the paging message itself. The messages consist of a six-character password followed by a two-digit function code. (See Figure 1 below left.) Because the unit operates in either the alphanumeric or numeric mode, alphanumeric characters, both upper and lower case, can be used as the system password. To turn on output No. 1, the six-character password is sent, followed by the command “11” (output No. 1, on). To turn output No. 1 off, the command is 10 (output No. 1, off). A command of 91 or 90 turns all of the outputs on or off. In addition, each output time delay can be programmed independently so that output No. 1 can be set for five minutes, output No. 2 for 15 minutes, and so on.

For Grazi’s purposes, he sends a page at regular 10-minute intervals. This page in turn resets a 12-minute delay on output No. 1 so it only times out when the specific page is not received by the remote-controlled switch after 12 minutes. By connecting the switch to unused inputs on his alarm system, Grazi is notified within a few minutes of a system failure. (See Figure 2 on page 62.)

The next step was to devise a way to cause the system to send the same paging message to the three remote-controlled switches on each of Grazi’s three channels at the desired intervals, automatically. One solution was to use an external autodialer with a timer that causes it to dial a paging number to drop a message every 10 minutes. This single paging number was chained together to allow pages to be sent on all three channels simultaneously. This method allows each in-bound direct inward dialing (DID) trunk to be tested, as well as the system itself. However, it requires a dedicated telephone line and the associated monthly expense.

By studying the programming manual for his Unipage paging terminal, Grazi found that he could generate the page automatically from the TNPP (Telocator Network Paging Protocol) card in the terminal chassis. This method eliminates the need for outboard equipment and the phone line, and it operates forever or until the programming is changed.

The first test site for the remote-controlled switch used at Communications Unlimited was at Grazi’s home, where it was connected to one of the unused zones on a burglar alarm. The use of only one output was required for monitoring the paging system. The other outputs were used to control the furnace, hot tub and electric blanket! With the time-delayed outputs set for a few seconds, Grazi could use his cellular phone to adjust his home electronic thermostat in five-degree increments while driving home.

The home alarm monitoring company was informed that an alarm on “zone No. 8” required that Grazi be notified immediately of a system failure. It worked, and the first time after installation that the paging system failed and the remote-control page was not received within the 12-minute window, Grazi was alerted by the alarm company of the system failure. That part was satisfactory. In addition, though, Grazi was met by Denver police officers as he backed his vehicle out of his driveway, on his way to check the paging system. Another call to the alarm company with another explanation of “zone No. 8” alarms prevented further armed police responses to a paging system failure.

Grazi since has installed other remote-controlled switches in other cities to act as his sentry against system failures. Because the 10-minute page is a general broadcast, all of the remote-control receivers can be reset simultaneously with the same page.

Grazi plans to install the remote-controlled switch at hospitals served by his system so hospital employees who depend on paging for emergency calls can be notified immediately of a system failure. During the interim, they can use the public-address paging system as a backup until the radiopaging system once again is operational.

The sounds of paging signals no longer intrude on Grazi’s personal time. His secretary reports that, with his newfound freedom, Grazi can spend less time in the office. Grazi himself reports a new lease on life, including being able to sleep a full night without the background noise of his scanners and the ability to pursue his half-dozen other businesses while the remote-controlled switch keeps watch over his paging system.

Before deciding to install transmitting equipment for a new frequency at a heavily congested radio communications site, an electromagnetic compatibility (EMC) study should be conducted. Hardly any study is more complex. It takes into consideration broadband transmitter noise, receiver desensitization and intermodulation interference (“intermod”).

Once the proposed facility is shown to be compatible with respect to those three criteria, one more analysis, an intermodulation level maintenance calculation, might need to be performed. The evaluation is important at sites where mobile (including portable) units might operate nearby. It is less important at most mountaintop sites where users are not likely to come close. The analysis predicts the likelihood of intermod affecting the receiving frequency of the mobile units. Most site-related intermod studies only consider intermod that may affect the base station receivers at the site.

The analysis is simpler when the base station transmitters to be studied feed the same antenna through a combiner. It is more complex when the transmitters use separate antennas, perhaps on different towers. For example, you will have to use your own judgment whether to include in the analysis a transmitter on the nearest frequency if it is situated at another nearby tower when another transmitter separated from the frequency under study by only a few more kilohertz is part of the same combining group with the proposed new transmitter. For transmitter intermodulation to be strong enough to cause a problem, the power levels involved must be strong and the transmitters must be close to one another, or both.

The path one signal must travel to enter another transmitter’s output circuit and mix with the other transmitter’s signal to produce intermod is complex and subject to much attenuation. Part of that attenuation is the path loss between the two transmitters’ antennas.

Figure 1 on page 50 shows attenuation vs. distance for horizontal and vertical separation between antennas for different frequency bands. Table 1 at the left lists attenuation figures for vertically spaced antennas. Antenna gain affects attenuation, too, when the antennas involved are on the same horizontal plane.

Another component of the attenuation lies within internal transmitter filters and external filter cavities. Refer to manufacturer’s specifications and duplex curves for these attenuation figures that apply to one transmitter at another transmitter’s frequency.

Next, derive the conversion factor (CF) from the formula:

CF = 57log(1.5 + [DELTA]F) – (9.9) – (9log[DELTA]F)

where

[DELTA]F = the difference in megahertz between each two transmitters’ frequencies.

The possible levels of intermod that might be produced on the mobile unit receive frequency can be calculated using the path loss figure, the antenna gain figure (if applicable), the transmitter cavity and circuit filter attenuation figure and the conversion factor.

Here are examples of possible configurations to be evaluated and how to maintain an acceptable level of intermod in each case.

Example No. 1 — Transmitters on the same antenna. The two transmitters are transmitter A (157.8MHz) and transmitter B (160.6MHz). Transmitter A has 18dBW (63W) output and a bandpass filter with a 1.5dB insertion loss. Transmitter B’s feedline loss is 3dB, its bandpass filter has a 1.5dB insertion loss and its antenna gain is 7dB.

First, calculate the conversion factor: CF = 57log(1.5 + [DELTA]F) – (9.9) – (9log[DELTA]F) = 57log[1.5 + (160.6MHz – 157.8MHz)] – 9.9 – 9log(160.6MHz – 157.8MHz) = 57log(1.5 + 2.8) – 9.9 – 9log2.8 = 22.2dB

The transmitter intermod level (TIL) then is calculated as follows: TIL = PTX – (IL[sub TX] + 2FL[sub M] + CF + LL) + AG[sub TX]

where P[sub TX] = transmitter A power output IL[sub TX] = filter insertion loss at transmitter A FL[sub M] = filter loss of the mixer (transmitter B) at transmitter A’s frequency. Refer to manufacturer’s specifications to evaluate attenuation at desired frequency offset. CF = conversion factor LL = transmitter B’s feedline loss AG[sub TX] = transmitter B’s antenna gain

It is important to double the actual band-pass filter attenuation figure for this calculation, because the frequency response curve is symmetrical. The incoming signal of transmitter A must pass through transmitter B’s filters, mix, and then pass again through the same filters at the intermod product frequency; thus, the outgoing intermod is subject to the same attenuation as the incoming signal from transmitter A. For other types of filters such as band-rejects, the only attenuation is at the tuned frequency. (See Figure 3 on page 56.)

Replace the variables in the TIL equation with values from the example, and calculate TIL as follows: TIL = 18dBW – (1.5dB + 10dB + 22.2dB + 3dB) + 7dB = -11.7dBW.

Figure 2 illustrates the signal path.

If signals from the transmitters mix, an intermod product with an output level of about 211.7dBW would result, and a mobile unit on 163.4MHz passing near the site would be affected by this signal. The intermod frequency, 163.4MHz, is equal to two times transmitter B’s frequency minus transmitter A’s frequency, or 2B – A = 163.4MHz.

The acceptable levels of intermod products vary according to frequency band. At VHF, the acceptable level is -80dBW, and at UHF, -70dBW.

In this particular situation, an additional 68.3dB of attenuation is required to achieve the -80dBW acceptable intermodulation level to protect mobile and portable units near the transmitter.

How to achieve the required attenuation will be explained after the next (and more complex) situation.

Example No. 2 — Transmitters on same tower but different antennas. The proposed transmitter A is on 154.20MHz with an output power of 16dBW, a line loss of 5.2dB, a bandpass filter insertion loss of 1.5dB and an omnidirectional antenna with 9.5dB gain 30 meters above ground.

The closest transmitter, transmitter B, is on 160.80MHz with a combiner with 5dB attenuation at transmitter A’s frequency, a line loss of 3dB and an antenna with 7dB gain at 26 meters above ground. In this case, a new variable is involved: the attenuation between the two antennas. Figure 1 reveals an attenuation of about 39dB for a spacing of 4 meters. Because the antennas are polarized vertically and spaced vertically, gain is not taken into account.

As in the first example, calculate the conversion factor as follows: CF = 57 log(1.5 + DF) – (9.9) – (9logDF) = 57 log(1.5 + 6.6) – 9.9 – 9log(6.6) = 34.5dB

where

[DELTA]F =160.8MHz – 154.2MHz = 6.6MHz

The transmitter intermod level then is calculated as follows:

TIL = P[sub TX] – (IL[sub TX] + LL + AI + 2LL[sub M] + 2FL[sub M] + CF) + AG[sub M] = 16.0dBW – (1.5dB + 5.2dB + 39dB + 6dB + 10dB + 34.5dB) +7.0dB = -73.2dBW

where:

P[sub TX] = transmitter A power output IL[sub TX] = filter insertion loss at transmitter A LL = transmitter A’s feedline loss AI = isolation between antennas LL[sub M] = line loss at mixer (transmitter B) FL[sub M] = filter loss (attenuation) of transmitter B at transmitter A’s frequency. Refer to manufacturer’s specifications to evaluate attenuation at desired frequency offset. CF = conversion factor AG[sub M] = antenna gain of mixer (transmitter B)

Figure 4 above illustrates the signal path.

An intermod signal at 167.4MHz will affect a nearby mobile when the transmitter outputs mix. Because the system works at VHF, an additional 6.8dB of attenuation is required to reach the acceptable -80.0dBW level.

Use judgment when only a small amount of attenuation remains to be achieved to meet the minimum requirement. Reality and theory often differ, and the equipment may perform better than the manufacturer’s specifications.

The next, and last, example is the most complex. The principle is the same as in the previous cases, except the attenuation between antennas can be calculated as free-space loss. The equation for free-space loss in decibels is 32.3 + 20 log(frequency) + 20 log(distance in miles), but we will use the equation for free-space loss for distances measured in kilometers. Other new variables to consider are the antenna gain figures for transmitter A and transmitter B.

Example No. 3 — Transmitters on different antennas and towers. The proposed transmitter A is on 158.0MHz with an output power of 21.0dBW, a line loss of 4.3dB, a combiner insertion loss of 3.2dB and an omnidirectional antenna with 8.0dB gain 34.5 meters above ground.

The closest transmitter, transmitter B, is on 161.2MHz with a combiner with 12.5dB attenuation at transmitter A’s frequency, a line loss of 5.2dB and an antenna with 9.5dB gain 37 meters above ground. The distance between the antennas is 30 meters (0.030km).

First, evaluate the loss between antennas. Because the antennas are separated horizontally, path loss and antenna gain must be taken into account, which was not the case with co-located, vertically separated antennas. The formula for path loss (PL) in decibels is:

PL = 28.2 + 20 log(frequency) + 20 log(distance in km) = 28.2 + 20 log(158.0MHz) + 20 log(0.030km) = 41.71dB

The conversion factor is: CF = 57 log(1.5 + [DELTA]F) – (9.9) – (9 log[DELTA]F) = 57 log(1.5 + 3.2) – 9.9 – 9 log(3.2) = 23.86dB

The transmitter intermod level is: TIL = P[sub TX] – (IL[sub T] + LL) + AG – PL + 2AG[sub M] – ( 2LL[sub M] + 2FL[sub M] + CF) = 21.0dBW – (3.2dB + 4.3dB) + 8dB – 41.71dB + 19dB – (10.4dB + 25dB + 23.86dB) = 60.47dBW

where:

P[sub TX] = output power of transmitter A IL[sub TX] = filter insertion loss at transmitter A LL = line loss of transmitter A AG = antenna gain of transmitter A PL = isolation between antennas (path loss) AG[sub M] = the antenna gain of mixer (transmitter B) LL[sub M] = line loss at mixer (transmitter B) FL[sub M] = the filter loss (attenuation) of the mixer (transmitter B) at transmitter A’s frequency. Refer to manufacturer’s specifications to evaluate attenuation at desired frequency offset. CF = conversion factor

An additional 19.53dB of attenuation is required to reach the minimum -80dBW signal level for VHF intermod.

Figure 5 shows the path.

In this case, the antenna gain and line loss of the mixing transmitter is involved twice, because the incoming and intermod signals follow the same path. This is why they are multiplied by 2.

For obvious reasons, our examples used the same transmitter as being the closest in frequency and distance. This is rarely the case, but it is possible. Remember to conduct the same calculations by switching the mixing transmitter with the proposed transmitter so that both intermod possibilities are verified.

When additional attenuation is required, many devices may be used. The point is to increase the attenuation of the signal trying to reach our transmitter. To do so, band-pass filter cavities may be added to those already in use. They have the advantage of attenuating all other strong signals that may cause intermodulation. Another possibility is to use notch or band-reject filters, but they only offer protection against one specific signal and, unlike a band-pass, they do not offer any attenuation at the intermod product’s frequency. When minimum insertion loss is important, though, one may prefer the notch filter.

For many, the ultimate device is an isolator or circulator, which acts as though it were a diode on the transmitter output. The insertion loss is low, and isolation is quite high, typically 40dB. Manufacturers offer them in dual packages, so isolation is doubled. They have the advantage of offering attenuation that does not vary with frequency spacing.

Reference SYSPARC User’s Manual, a computer program for antenna sites sold by the Department of Industry Canada.