A stacked deck
In Part 1 of this series, we defined “tower shadowing” and some of the basics of antenna systems. In this part, we look at some of the other factors that inhibit radio system performance.
Most of the major antenna manufacturers have diagrams that show how mounting an antenna to a tower side distorts the omnidirectional pattern of the antenna. These are usually drawn as polar graph diagrams. If a side-mounted antenna has an offset pattern, it must be combined with the distortion pattern to see the actual pattern that would be present.
The effects of a tower on an omnidirectional 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 versus the face of the tower; the frequency on which the antenna is operating; the amount of coaxial transmission line on the tower at that level; and the free space radiation pattern of the 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 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. If you tried to duplicate his work, you will spend many weeks trying to match his program.
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 and modulation levels, and also conduct a frequency-domain-reflectometery sweep of the antenna and coaxial system.
Then establish a starting point for checking a system’s range by drawing on a map the radials from the tower or building, every 30° or every 45°, to 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 be affected by as much as 15 dB. Two different phenomena cause this variation in signal strength.
One is called Rayleigh fading. This is where the signal strength will follow a pattern of peaks and valleys along the radial path to or from the tower. The peaks will occur every ½ wavelength, and the valleys will occur ¼ 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; again, this is a normal part of every radio system.
The other factor that causes radio signals to have peaks and valleys occurs when antennas 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. When these reflections arrive in-phase at the reception point, the signal strength will be stronger than normal. When the signal arrives out of phase, the signal strength will be weaker than normal. This will occur on every radio system where the antenna has metal objects near where the signal can bounce off the object. The effect is called spoking.
If you think that you are being affected by tower shadowing, draw your radials every 2° in the area that you believe is being shadowed. You will immediately see whether shadowing is a problem.
A single tree can cause a signal to drop by as much as 20 dB (x 100 power). Likewise, a building can diminish signal strength by more than 40 dB (x 10,000 power), depending upon its materials and metal content. In some cases, a hill or drop in elevation can cause enough attenuation to kill the signal.
Computer propagation programs 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.” As the end customer, you want the actual coverage studies. And though the manufacturer 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.
If the range of a system is less than predicted, it usually is due to RF interference. The source of the interference might be from your own system, somebody else’s system, or from myriad electromagnetic energy sources that are beyond the scope of this article. Often the range is deficient but the hardware in your 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 redesign one of the associated components.
In some cases, the problem may not be the fixed infrastructure but rather mobile and portable units that are not working properly within factory specifications. Mobile units need to have good antennas and installations. One sheriff’s department learned this the hard way when it used inmates — the ultimate in cheap labor — to install radios into its squad cars. The local radio shop that had been maintaining the previous system — and which lost the bid for the new system — had told the fleet service manger to use all of the silicon lubricant that came with each antenna and to fill the antenna connection fitting with the silicon to keep the moisture out of the connection. (I am sure that he was laughing all of the way home over this prank.)
The silicon is used on 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 inmates applied every drop of the silicon to the mount, which created an insulator between the supplied non-magnetic-option 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 more than 30 miles for the mobile units that were installed anywhere other than the sheriff’s dolly port.
Making matters worse, the radio dealer that did get the bid for the equipment had told the inmates that they did not need to solder the PL259 connectors that went into the back of the radios. There were shorts and opens in more than half of the radios installed by the inmates 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 at the same time. The problems you will encounter down the road will cost you more than the $30 to $50 you saved by going with used antennas and cables. 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 never should 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 seen agencies use their batteries for three to six years, yet they wonder why their radio system just does not work like it used to. Also, a portable radio worn on someone’s belt will experience shadowing and antenna detuning. (See Why isn’t my walkie working?, November 2007, for further information on this problem.)
Again, just because a radio system is not performing like it is supposed to or as it 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.
Ira Wiesenfeld, P.E., is a consulting engineer who has been involved with commercial radio systems since 1966. He can be reached at firstname.lastname@example.org.