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TUTORIAL
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.
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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. |
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.
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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. |
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3.05% of the minimum
directional antenna field strength. |
 
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22.11% of the minimum
directional antenna field strength. |

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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. |

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22.11% of the minimum
directional antenna field strength. |
2.0 kilometers
90-degree tower
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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. |
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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% |
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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! |

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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. |
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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.
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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.
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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.
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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.
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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. |
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0.77% of minimum
directional antenna field
strength |
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7.16% of minimum directional
antenna field strength |
 
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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.





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