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Page 2 of 2
TOWER SHADOWING
By Ira Wiesenfeld, P.E.
PART 2: REAL WORLD SITUATIONS AND OTHER FACTORS THAT AFFECT SYSTEM PERFORMANCE
In Part 1 of this series, we defined what Tower Shadowing was and some of the basics of antenna systems. In this part, we will look at some of the other factors that can cause radio systems to not have the range that was predicted or was previously providing to the users of the system.
MANUFACTURER DIAGRAMS
Most of the major antenna manufacturers have diagrams that show how the side mounting of an antenna to a tower distorts the omni-directional pattern of the antenna. These are usually drawn as polar graph diagrams based upon an omni-directional antenna. If you have an antenna that has an offset pattern, then you have to combine the antenna offset pattern with the distortion pattern to see the actual pattern that would be present for that antenna with a side mounting.
Some manufacturers do not acknowledge shadowing, but it does exist and does need to be part of the system design.
COMPUTER MODELING
The effects of a tower on an omni-directional antenna will be influenced by the size of the tower legs, the size of the tower face, how far the antenna is from the tower, where the antenna is in relation to the legs vs. the face of the tower, what frequency the antenna is operating on, how much coaxial transmission line is on the tower at that level, and what is the free space radiation pattern of that same antenna.
The formula to do this computing is quite complex, has some constants that were determined by empirical measurements, and will take quite some time to set up on a computer.
One of the top engineers in the world on antenna patterns has developed a computer software program that allows the engineer to input into the data input fields the pertinent antenna and tower parameters, and the output will be a graph showing the detail pattern and numeric attenuation for a given antenna on a given tower. In fact, most of the antenna manufacturing companies have used his software to show the offset patterns caused by towers.
The name of the engineer is Arthur K. Peters, and he sells this program called Obspath for under $1,000.00. If you tried to duplicate his work, you will spend many weeks of programming time to match his program. I have not collaborated with Mr. Peters, so he does not endorse this mention of his name or is in any way connected to him. I have known of Mr. Peters since my early days in radio, but our paths have never crossed.
EMPIRICAL MEASUREMENTS
Radio system performance can be measured with the test equipment available and the measured performance should match the predicted performance if everything is working properly. This section will detail some of the tests and some of the anomalies that will be seen in the course of normal radio system observation.
Before making any field measurements, you should verify the transmitter power output, frequency, modulation levels, and do a Frequency Domain Reflectometery sweep of the antenna and coaxial system. (See MRT Magazine, April 2003 for an explanation of this technology if you are not familiar with it.)
You can then make a starting point for checking a system’s range by drawing on a map the radials from the tower or building every 30 degrees or every 45 degrees, and you have these radials exactly 1 mile in length. You will find that on many systems, the signal strength will not match the predicted value, but you can move your test set antenna by just a few inches from the set point on the map and the signal strength will move by as much a 15 dB. There are actually two different phenomenons occurring to cause this variation in signal strength.
The first item that can cause a variation in signal strength is called RAYLEIGH FADING. This is where the signal strength will follow a pattern of peaks and valleys as you follow a radial path to or from the tower. The peaks will occur every 1/2 wavelength, and the valleys will occur 1/4 wavelength in distance past each peak. All radio systems have this characteristic. This is why you can move a few inches with your Walkie-talkie or mobile unit and go from a weak signal to a strong signal. There is nothing wrong with your system, as this is a normal part of every radio system.
The second item that causes radio signals to have peaks and valleys is found on antennas that are not top mounted by themselves on a tower. When an antenna is side mounted on a tower, or has other metallic objects where the signal can bounce off the object, there will be reflections. These reflections can arrive in-phase at the reception point, and the signal strength will be stronger than normal. When the signal arrives out of phase, the signal strength will be lower than normal. This will occur on every radio system where the antenna has metal objects near where the signal can bounce off of the object. The overall effect is called SPOKING.
If you think that you are being affected by tower shadowing, then draw your radials every 2 degrees in the area that you believe is being shadowed. You will immediately see if shadowing is a problem with the results of this empirical study.
OBSTRUCTIONS
A single tree can cause a signal to drop from 0 dB to as much as 20 dB (x 100 power). Likewise, a building can have an effect of 0 dB to more than 40 dB (x 10,000 power), depending upon the materials and metal content of a building. A hill or drop in elevation can have enough attenuation to possible kill the signal.
There are computer propagation programs on the market that can predict what the coverage will be, including the effect of the obstructions and ground elevations. When the radio manufacturers or radio dealers run their programs, they usually are very conservative in their predictions because they normally have a motto which is UNDER PROMISE and OVER DELIVER. This has two good effects for them. Firstly, they always deliver more in the system performance that they had the customer expecting. Secondly, the dealer or manufacturer can sell more equipment, because their studies indicated that the system required higher towers, more sites, or both. As the end customer, you want the actual coverage studies, as you probably have more spending requirements than you have a budget for if you are like most organizations. Even though the manufacture or radio dealer is willing to provide you propagation studies for free, this can sometimes cost you more in the long run. You might want an independent study if you are making a large investment or if you have a mission critical situation. Spending a few hundred dollars can save you thousands of dollars in the long run.
RF INTERFERENCE
If the range of a system is less than the expected predictions have in actuality, it is usually due to RF Interference. The source of the interference might be from your own system, somebody else’s system, or from a myriad of electromagnetic energy sources that are beyond the scope of this article to explain. In short, many times the range is deficient, but the hardware in you system is working properly.
If you suspect interference, there are tests that can be performed to confirm that this is the case. If the tests do confirm that RF Interference does exist, then there are quite a few options that can be followed to correct these problems. Please note that not every interference problem can be corrected without somebody having to move one of the radio systems or the redesign of one of the associated components.
MOBILE AND PORTABLE UNIT PERFORMANCE
In some cases, the problem may or may not be caused by the infrastructure radio system. If you have quite a few mobile units and portable units not meeting factory specifications, you will think that your system does not work correctly, but in reality, you have multiple mobile units and portable units not working making you think that it is the fixed equipment in trouble.
Mobile units need to have good antennas and installations. I had a Sheriff’s Department using “INMATE RADIO SERVICE COMPANY” (IRSC) which was the polite way of saying the prisoners who were convicted thieves and drug dealers were the cheap labor. The Sheriff was using these people to install the radios into the squad cars. The local radio shop that had formally been maintaining the old system had told the fleet service manger to be sure and use “ALL” of the silicon lubricant that came with each antenna and fill the antenna connection fitting with the silicon to keep the moisture out of the connection after he did not get the bid for the new system. I am sure that he was laughing all of the way home over this prank. The silicon is used on the part of the mount to keep moisture out of the vehicle, but is not supposed to have any contact with the antenna radiator that uses a pressure fit to make the RF connection to the antenna rod. The IRSC employees used every drop of the silicon into the mount, which made an insulator between the supplied NMO mount center conductor button and the antenna rod. The range of the new system was less than 1 mile for the sheriff’s squad cars, but over 30 miles for my car, my walkie-talkie, and the mobile units that were installed anywhere other than the sheriff’s dolly port.
To compound the problem, the radio dealer that did get the bid for the equipment had told IRSC that they did not need to solder the PL259 connectors that went into the back of the radios. There were shorts and opens in over half of the radios installed by IRSC and other shops that had listened to this dealer. You CANNOT take a shortcut on the installation and expect things to work as the engineer designed the system.
It took almost 6 months to correct all of the fleet installation problems with the mobile units. It is much easier to just put the fleet equipment in properly than to chase installation problems.
When you trade in a squad car or other emergency services vehicle, you should buy a new antenna and cable. The $30 to $50 that you save will eventually cost you more than that when you have problems later down the road. Hopefully, nobody gets injured because of the decision of a bookkeeper or accounting manager. The radio is truly the lifeline for public safety personnel, and used antennas on an installation should NEVER be allowed.
Just as mobile units have their problems, so can portable radios. The batteries should have sufficient energy to last though an entire shift. For police departments this might be 8 hours, or it could be 12 hours. For fire personnel, many of them operate 24 hours straight, and they might need more than one battery per shift. The batteries are only good for 500 recharge cycles, so most batteries should be replaced after two years of use. I have been at agencies that use their batteries for three to six years, yet they wonder why the radio system just does not work like it used to. Also, a portable radio worn on someone’s belt will have shadowing and antenna detuning. (See MRT Magazine November 2007 for further information on this problem.)
Just because a radio system is not performing like it is supposed to or once did, the problem may not be in the fixed infrastructure equipment. Always consider the mobile and portable radios as a part of the “system”.
CONCLUSION
Tower shadowing can and does occur on many radio systems. The effects can be predicted, and used to enhance the system performance if this factor is known and controlled. If your system is not performing as expected, this article has also explained some of the factors that could possibly be causing your system to not work as the engineer designed it. Whether you have commercial radio system, a public safety system, or even an amateur radio system, the effects of shadowing are real and can be overcome. If you do have tower shadowing, just make sure that it is in a direction where the shadow does not affect the system performance for the primary territory that you are trying to cover. We hope the above information has helped to clear up any confusions or misunderstandings of Radio System performance and or expectations.
Ira Wiesenfeld, Principal Engineer - Ira Wiesenfeld and AssociatesIra Wiesenfeld, P.E., is a consulting engineer who has been involved with commercial radio systems since 1966. He has spent time working in the broadcast, two-way, mobile telephone, paging, microwave, military, and public safety radio systems, and has consulted with most of the major manufacturers in the radio industry. Ira is the author of Wiring for Wireless Sites, available from Delmar Thompson / Prompt Publishing (www.electronictech.com).
Ira has a BSEE from Southern Methodist University in Dallas, Texas; a FCC General Radiotelephone Operator License; is a Senior Certified Radio Technician from the Electronics Technicians Association - International; and is a licensed Professional Engineer in the State of Texas. He holds an Advanced class Amateur radio license WA5GXP which he has held since 1963. He has been married to his wife Natalie for 36 years, and has lived in Dallas, Texas his entire life.
Ira can be reached at
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EFFECTS ON REAL SYSTEMS
In radio and electronics, the numbers are listed in Decibels, but not everyone understands these relationships. Below is a brief explanation of decibels.
Numbers can be expressed in two forms. The first is the number itself. The second is how much the number 10 must be raised by an exponent to get that number. The examples below will help you understand this:
EXAMPLE
100 = 102
1000 = 103
0.0001 = 10-4
2 = 100.301
3 = 100.4771
Logarithms are useful because extremely large and extremely small numbers are easier to express. The difference between the two numbers is not related to the numbers themselves. The advent of the scientific calculator has made the use of logarithms very easy. There are two types of logarithms, natural and common. The common logarithm is based on powers of 10 and is calculated using the key LOG and is different than the natural logarithm, which is calculated by using the LN key on the calculator. The LN key will not be used in these applications.
Gains of amplifiers, loss of power, audio levels, earthquakes and many other quantities are expressed in numbers based upon logarithms. Human senses such as light intensity, hearing and pain intensity are also measured by the body logarithmically. The common level expressed in decibels is based upon a mathematical formula based upon logarithms.
DECIBELS
Decibels are the unit of relative measure used throughout the radio world. The following formulas are used for calculating decibels:
POWER
DBPower = 10 x LOG (POUT / PIN)
DBVoltage = 20 x LOG (VOut / VIn)
If you always have the OUT divided by the IN level, the following will always hold true:
A positive number represents GAIN.
A negative number represents LOSS.
If the Input and Output are the same, the gain will be 0 dB.
EFFECTIVE RADIATED POWER
A radio transmitter system consists of the transmitter, antenna peripheral equipment, coaxial line loss, connector loss and antenna gain or loss. The EFFECTIVE RADIATED POWER (ERP) is the amount of power that would be the equivalent if a transmitter was placed on the tower with a unity gain antenna.
The transmitter power, less the losses, added with the gains, gives the ERP.
Power (ERP) = Power (Transmitter) – Power (Losses) + Power (Gains)
dBm
dBm is a unit of ABSOLUTE POWER, where dB is a relative number. The following chart will help show this:
0 dBm = 1 milliwatt
+ 30 dBm = 1 Watt
+ 60 dBm = 1000 Watts
- 30 dBm = 1 microwatt
dBm = 10 Log [(Pout) / 1mw]
When you are working in radio, the impedance is 50 Ohms.
When you are working in telephony, the impedance is 600 Ohms.
Either way, the power levels or voltage levels are the same.
POWER RATIOS AND DECIBELS
X2 = 3 DB
X4 = 6 DB
X10 = 10 DB
X100 = 20 DB
X1000 = 30 DB
X ½ = -3 DB
X 1/10 = -10 DB
VOLTAGE RATIOS AND DECIBELSX2 = 6 DB
X4 = 12 DB
X10 = 20 DB
X 100 = 40 DB
X ½ = -6 DB
X 1/10 = -20 DB
Now that we understand a little bit about Decibels, let us now differentiate between the common uses and misuses of this term.
When working with radio signals, the levels can be extremely large or extremely small, and as a result, the use of Decibels is necessary to express these levels. The following 4 sections will help in separating the uses of the term dB.
dB
When you use the term dB, you are only comparing one signal value to another signal value. When looking at a filter loss, amplifier gain, or antenna gain or loss, then the use of the term dB is the correct choice.
dBm
dBm is an absolute value. If you had a 100 watt transmitter, that would equate to a +50.0 dBm level. If you had a 2 microvolt signal, that would equate to a -100 dBm signal. If you have a 0.5 microvolt (uV) signal, that would be a -112 dBm. Most two-way radio receivers operate in the range of -113 dBm to -124 dBm, while most wireless LAN receivers operate with a sensitivity level of -70 dBm to -100 dBm. WiFi systems with extended range operate with a sensitivity of -130 dBm. GPS receivers have a sensitivity of -135 dBm. Most Communications Service Monitors have a sensitivity of -100 dBm.
dBd
dBd is used when expressing antenna gain based on the difference between the signal of a dipole antenna verses the signal gain from the given antenna. Antennas derive gain by redirecting the signal from undesirable or non-useful directions to more useful directions. This gain can be measured or calculated, and is expressed in dBd.
dBi
As a ruse to make uninformed engineers and technicians think that an antenna has more gain than it really has, some manufacturers started using the term dBi to represent the gain over an ISOTROPIC point in space, which exists in theory only. There is a 2.1 dB difference between a dBd and a dBi. An example would be a 13.0 dBd gain antenna would also have a 15.1 dBi gain. In reality, it is the same antenna.
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