In summation we can see that the effect of the newly constructed tower changed by varying the height of the tower, and the distance from the AM transmitter.  The closer the tower is to the AM station, the more likely the tower will have to be detuned.  Also, the closer the tower is to a 90-degree tower, the more likely the tower will have to be detuned.  Remember also that these factors do work collectively to determine whether the tower requires detuning or not. One point should be mentioned though.  Even though the tower does not require detuning, the FCC still requires coordination with the AM station if it is within the AM Coordination distance of 1.0 kilometers for a non-directional AM broadcast station and 3.0 kilometers for a directional broadcast station.  Arriving at some conclusion not to do anything  because a tower is short and 2.90 kilometers from the AM station would not be a good move.  But, having a basic understanding of why some towers require detuning and others do not is a good thing.  An important thing to know is that not all towers have to be detuned!  In fact, only a small percentage of the towers do require detuning.  The detuning apparatus is expensive and must be maintained once installed, so you only want to detune a tower that really requires it. The last of the major factors was the bearing from the radio station that the tower is constructed.  I think that you will also find this very interesting.

90 - degree Tower

70-degree tower

50-degree tower

Towers primarily effect AM Broadcast towers by: 1.  The height, as it approaches 90 electrical degrees. 2.  The distance from the AM Station - the farther away,   the lower the effect. 3.  The amount of illumination of the tower, which is determined by the power, proximity and bearing  of the radio station. Any one factor or  combination of factors may cause a tower to require detuning.

Summary

Think of the analogy of the moon reflecting off a lake.  If the moon is full, you will have maximum reflection.  If the moon is only one-quarter or less, or if it is cloudy, you will have less reflection.  The same happens with the tower.  The more the illumination from the tower, the more the signal will reflect back out and cause problems for the AM station. The factor of illumination is the real reason the distance from the AM station affected the percentage of re-radiation the way that it did.  By moving the tower further away from the AM station, the illumination was less, thus the re-radiation was less.  So, a low power AM broadcast station will normally cause less problems than a high power one will. In the case of the height of the tower, the shorter the tower the less efficient it was, therefore the illumination was less.

Both patterns are for the same station, with the same newly constructed tower with the same height and distance from the station.  The only apparent difference is where the tower was constructed in reference to the bearing of the station. What has happened is that the pattern on the right has far less signal than the pattern on the left.  By having less signal hitting the tower, less signal will be re-radiated.  A term  often use in the industry when describing the effect on a tower is "illumination."  The tower has to be illuminated by a certain amount of signal before it can re-radiate the signal back out.  Remember that we said the problem was that a re-radiated signal would go back out and add to or subtract from the original, thus distorting the stations antenna pattern.  (non-directional also)  However,  if there is very little signal in the first place, then very little signal can be re-radiated, thus, very little effect on the stations antenna pattern.  This phenomenon will occur if the tower is constructed in a pattern minimum, but it can also occur if the station is a very low power station.  All the previous patterns we analyzed were for a one kilowatt station.  The percentage of re-radiation of the minimum directional antenna field strength will be different for other power outputs. If the station is a 50,000 watt station (50 kilowatts), the tower will certainly have more effect on the AM station than it would for a 250 watt station.

The pattern on the left is for the 90-degree tower, built in the stations main lobe of approximately 180-degrees.  The pattern on the right is built in a pattern minimum, called a null.  Most likely the tower would not require detuning, unless it was built close to a stations monitoring point.

3.05% of the minimum directional antenna field strength.

22.11% of the minimum directional antenna field strength.

14.07% of the minimum directional antenna field strength.

Once again, same pattern.  We will now move out to a distance of 3.0 kilometers, which by the way is what FCC 22.371 states is the end of the coordination distance for directional broadcast stations.  (Check my discussion in FCC Rules) Now the re-radiation of a percentage of the minimum directional antenna field strength is only 14.07%.  Yes, the tower still should be detuned, but for a 70-degree tower, it percentage would decrease to only 1.79%, which means it probably would not have to be detuned.

22.11% of the minimum directional antenna field strength.

2.0 kilometers 90-degree tower

When we double the distance to 2.0 kilometers away from the AM broadcast transmitter site, the re-radiation as a percentage of the minimum directional antenna field strength was cut in half.  That percentage is now only 22.11%. The tower would still require detuning.  If we had the 70-degree tower, the re-radiation as a percentage of the minimum directional antenna field strength would be approximately 3.58%.  In this case, the tower probably would not need to be detuned. The same would happen to the 50-degree tower.  The re-radiation would be cut in half.  Note that the two major factors work collectively, and that not just one factor will determine whether the tower needs to be detuned or not.

In summary, it can be seen that the height of the constructed tower can have a  significant affect on a AM broadcast station.   Keep in mind that as a constructed tower approaches 90-degrees or higher, the impact increases dramatically.  However, item two of our major factor list was the distance from the AM station that a tower is constructed.  In this case we will again evaluate three patterns with a distance of 1.0-kilometers, 2.0 kilometers, and 3.0 kilometers.  We already evaluated the above patterns at 1.0-kilometers.  I will keep the same pattern, with the same frequency and the same two-tower directional antenna pattern.  I will also evaluate it at the worse case scenario of a 90-degree tower, where the re-re-radiation as a percentage of the minimum directional antenna field strength was 42.22%

90-degree tower

42.22% of the minimum directional antenna field strength.

Remembering from AM Broadcasting that a 90-degree tower is a very efficient radiator and a common height for AM broadcast stations, now lets increase the height an additional 10.8 meters to that of a 90-degree tower.  This is approximately 49.4 meters (162-feet) at 1270 kilohertz.  (Degree Calculator) The re-radiation as a percentage of the minimum directional antenna field strength is now 42.22-percent, which would be a definite detune.  The reason for the increase amount of re-radiation is that now the newly constructed tower is a very efficient at 1270 kilohertz, and since it is an efficient radiator, it is also an efficient re-radiator.  A really observant person would also catch on to the fact that a 162-foot tower is a good height for a cellular / PCS tower, thus, the problem! Now, if you didn't want to have the expense of detuning this tower, simply lower the height, or buy the radio station!

The next tower is a 70-degree tower.  Same criteria:  Tower is constructed 1.0 kilometers from the station, same 1270 kilohertz, 2-tower directional antenna.  The only thing that changed was the height of the constructed tower.  A 70- degree tower at 1270 kilohertz would be approximately 38.3 meters.  (Distance Calculator)  The re-radiation as a percentage of the minimum directional antenna field strength has jumped to 7.16%.  Normally, it would now be recommended to detune this tower.  Remember, the only thing that changed was the increase of 10.8 meters (approximately 35-feet) of tower height.  The reason for the change is that the constructed tower is becoming more efficient at 1270 kilohertz.

50-degree tower  1270 kilohertz  2-tower Directional Antenna (DA)

The first tower evaluated is a 50-degree.  At 1270 kilohertz, this would be a height of 27.5 meters.  Again, this is a new tower at 1.0 kilometers from the AM transmitter site.  This tower would not normally required detuning, even though it is only 1.0 kilometers from the AM station, and is in the main lobe of the of the stations pattern.

To see how the pattern can be effected by the height of the tower, consider the same pattern once again.  This is a  two tower directional antenna with a power output of 1000 watts (1 kilowatts), operating on 1270 kilohertz.  We will evaluate the construction of a tower 1.0 kilometers from the transmitter site of the radio station at a bearing of 180-degrees.  We will look at this tower with three different heights; 50-electrical degrees, 70-degrees, and 90-degrees.  Structures are evaluated and a percentage of effect is calculated.  The term used is "percentage of the minimum directional antenna field strength".  For reference purposes, normally anything over a 5-percent effect would require that the tower be detuned.

Starting with the first factor, the height of the newly constructed tower.  In AM Broadcasting, under the heading of tower height, it was discussed that a tower above 90-electrical degrees is a very efficient radiator.  Transmitting antennas and receiving antennas are essentially the same; therefore, if a 90-degree tower is an efficient radiator at a certain AM frequency, it will also be efficient receiving antenna.  A major problem occurs when this new tower, acting as an efficient receive antenna, re-radiates (or re-transmits) the signal from the radio station.  This signal will now add or subtract from the signal coming from the AM transmitting site (just like the directional antenna systems works for the AM station).  This  distorts the pattern of the AM broadcast station which can  cause the station to be out of tolerance with its antenna system.  This can also cause interference to other broadcast stations that the AM station is suppose to protect.  It is important to realize that it is the height of the tower, or any electrical conductor, such as a ground wire or transmission line when it approaches 90-electrical degrees at the AM frequency, that effects the antenna pattern.  It is not the addition of antennas on the tower, or the frequency of the cellular or PCS service.  It does not even matter if there are no antennas on the tower.  The interference is cause by the height of the structure.  It is not like transmitter intermodulation that is caused by two or more RF transmitters.

The station is able to determine if its pattern is working as designed by checking its parameters at the transmitter site and by reading the monitor points.  If all are within tolerance, then the pattern is working as designed. The problem with all of this is that if another tower is constructed near this directional antenna system, it can distort this directional antenna pattern.  In the case of a non-directional station, adding another tower in its vicinity can in effect make the station operate as a directional station. There are three major factors of a new tower that can effect the operation of either a non-directional or a directional broadcast station.  They are: 1.  The height of the newly constructed tower. 2.  The distance from the AM station of the new tower. 3.  The bearing from the AM station to the new tower.

For the pattern on the left, the  nulls are located at approximately 45 and 290-degrees. The main lobe is located between 150 and 180 degrees, with a smaller main lobe at 350-degrees. To maintain this pattern and insure protection to other broadcasters, the FCC would require the station to monitor certain parameters (ratio of current between towers and the phase between the towers) at the transmitter site.  In addition. monitor points would most likely be established in the direction of the nulls.  When the station finishes the construction of its transmitting site, and files with the FCC its request for the station license, points will be established by the engineer with a maximum field strength at that point.  When the FCC issues the station license, those points will be listed on the license along with directions to the location of the points and the maximum allowable field strength at those points.  The points are called monitoring points.

0.77% of minimum directional antenna field strength

7.16% of minimum directional antenna field strength

                                                                                      



































































































                                                                                   

The purpose of this tutorial is to provide some insight as to why cell towers, whether guyed, self-supporting, monopoles, and even wooden poles, can adversely effect the pattern of an AM broadcast station.  The problem became acute when the number of cellular towers began to increase in the early 1980's.  AM broadcast stations found their antenna patterns being effected by these towers to the point that the FCC mandated AM Coordination. As is discussed in AM Broadcasting, and as way of review, the radio station tower is the actual antenna,  unlike wireless  carriers who use the tower to mount their antennas on.  There are two basic types of AM broadcast station antenna systems.  The single tower station is termed a non-directional antenna.  A station that uses two or more towers is termed a directional antenna system.  Directional broadcast stations have used up to twelve towers; however, typically the number is under five.  A station operates in a directional mode to protect other broadcast stations operating on the same frequency or adjacent frequencies from interference.  Often a station can operate a higher output power by having a directional antenna system. The height of AM broadcast towers is determined by the frequency of the station, just as it is with any transmitting antenna.  One wavelength is equivalent  to a height of 555.6 meters (1822 feet) at a frequency of 540 kilohertz and 176.5 meters (578.8 feet) at a frequency of 1700 kilohertz, which covers the AM broadcast band.  This is based on the formula for the velocity of propagation, 300/f (MHz).  Take note that the height decreases with the higher AM frequencies.  Degree Calculator, located on this web site,  will calculate the electrical height of a tower in electrical degrees from the height of a tower given in meters.  Later it will be shown how this has a distinctive effect when towers are constructed in the vicinity of AM broadcast towers.  Also, AM broadcast towers are often described in terms of their electrical height rather than their physical height.  A one-wavelength tower would be 360 electrical degrees; a quarter-wavelength tower would be 90-degrees, while a 5/8 wavelength tower would be 225-degrees.  A quarter-wavelength tower is 90-degrees regardless of whether the frequency is 560 kilohertz or 1700 kilohertz.  Many of the licensed AM broadcast stations operate non-directional. with a single tower.   In theory, a non-directional station broadcasts with equal signal strength (which is measured as field strength, in mV/m) in all directions.  In reality, the ground conductivity, terrain and other factors may alter that, but for purposes of discussion, the non-directional (ND) station will be considered a circle, with equal signal strength in all directions. If another tower is placed in close proximity to the first tower, the non-directional antenna pattern will be changed.  By varying the amount of power in each tower, the phase relationships of the towers, and the spacing between the towers in electrical degrees, a directional antenna pattern (DA) can be formed.  By controlling these three factors, more signal can be placed in one direction than another.  The main purpose though for the directional antenna system is to protect other AM broadcast stations that operate on the same frequency or an adjacent frequency from interference by supplying less signal in given directions. If you consider that each tower is radiating a signal based on the power, phase and spacing, at a distance removed from the transmitter, the signals will add or subtract from each other depending on the phase of the arriving signals.  The system is designed so that a minimum signal (null) can be placed in directions to protect other stations.