1. CECOM LAR Tropo Operation Course
Theory of Operation
CECOM LAR Course
Logistics Assistance Division
Fort Monmouth, NJ
In this class we will cover some of the theory involved in Troposcatter Communications.
Theory of Operation

2. CECOM LAR Tropo Operation Course
Troposphere
Average of 10 km
Weather
Refractive/Reflective
The troposphere is the lowest, most dense layer of the atmosphere and extends from the Earth’s surface to an average of 10 km. This is where most of the weather takes place.
There are a number of theories explaining Tropospheric scatter communications ending with a small fraction of the transmitted radio energy being diverted towards a receiving station.
Tropospheric Scatter Communications can be puzzling and frustrating.
There are a number of terms and definitions you should understand to be an effective Tropo Communicator.
Theory of Operation

3. CECOM LAR Tropo Operation Course
Propagation
Line of Sight (LOS)
Troposphere Scatter
Diffraction
The dictionary defines propagation as; “transmission (esp. sound waves or electromagnetic radiation) through a medium”.
The medium is this case is the Troposphere.
3 propagation modes of operation for the AN/TRC-170.
LOS
Tropo
Diffraction
Theory of Operation

4. CECOM LAR Tropo Operation Course
Line of Sight
Two stations must have a direct ray path between the antennas.
Generally LOS is used no more than 35 miles (57 Km).
Substantially less transmit power is required for LOS propagation.
The most important factor in LOS communications is the First Fresnel Zone.
Theory of Operation

5. CECOM LAR Tropo Operation Course
First Fresnel Zone
The transmitted signal from the antenna is not a straight-line. It is actually closer to that of a cone.
The Fresnel Zones are an infinite number of areas that are broadcast out of the antenna. The zones vary in signal strength and phase.
We are only concerned with the first Fresnel Zone, which is depicted by the gray area in this figure.
This area should be 60% free of obstructions. In the diagram above, if the antennas were 30 miles apart the height of the First Fresnel Zone at 15 miles is 84 feet in diameter.
Ideally, 51 feet of this should be clear of obstructions. To complicate matters some obstructions can actually increase the signal several decibels (dB).
These factors must be considered during the planning stages. You should be aware of what the First Fresnel Zone is.
Theory of Operation

6. CECOM LAR Tropo Operation Course
Tropospheric Scatter
The most used propagation mode for this system.
This distorted figure depicts a typical Over-the-Horizon communication network.
Several terms are used when discussing Tropospheric Scatter Communications.
We will explain a few of these complex and often misunderstood terms.
Theory of Operation

7. CECOM LAR Tropo Operation Course
Common Volume
Due to the angle required for successful communications only a certain portion within the troposphere will cause the scattering effect needed between two stations.
The intersection of the antenna beams from both stations is referred to as the Common Volume.
The red lines represents the signal beam width.zones.
The blue line represents center pointing angle of the antenna.
The yellow area is called the Common Volume.
For best communications, common volume should exist near the mid-point between the two stations and be as large as possible.
Theory of Operation

8. CECOM LAR Tropo Operation Course
Scatter Angle
The angle of the two antenna beams interception is referred to as the Scatter angle.
The smaller this angle is, the more energy that will be scattered from the Common Volume into a receiving antenna.
When the angles are too great, little to no energy is diverted into the receiving station.
For this reason, the take-off-angles will typically be as close to 0 (zero degrees) as possible, which also creates a larger common volume.
Theory of Operation

9. CECOM LAR Tropo Operation Course
Multi-Path Spread
In this diagram, diamond shape represents the Common Volume.
The arrows represents the transmitted signal.
As the signal travels into the common volume it is refracted at different points in the Common volume. This results in the same signal being received at different times.
It causes distortion on the recovered signal. If the Multi-Path Delay Spread is too great, it will prevent recovery of the received signal. The receiving station will see a large Receive Signal Level (RSL) and a high Bit-Error-Rate (BER).
The AN/TRC-170 is designed to use the Multi-Path Spread to enhance communications. However if the Multi-Path Spread is too great it will adversely effect communications.
Theory of Operation

10. CECOM LAR Tropo Operation Course
Path Loss
The amount of loss present between the antenna at the transmit station and the antenna at the receive station
Expressed in dB
Directly affects the RSL
FACTORS
Weather
Propagation Mode
Station Distance
Antenna Angle
Antenna Size
Only a fraction of the transmitted energy (signal) arrives at the receiving station. A numeric value is given to the amount of signal that is lost through the troposphere.
This value is referred to as Path Loss and is expressed in decibels (dB). The distance between stations (Path Length) will also affect the Path Loss.
The effect of too much path loss is little to no Receive Signal Level (RSL).
The factors effecting the path loss are;
Weather
The type of propagation you are using.
The distance between stations.
The angle and size of the antenna.
The allowable path losses for the AN/TRC-170 TROPO system in the Tropo Mode are between a nominal 200 and 250 dB.
Theory of Operation

11. CECOM LAR Tropo Operation Course
Diffraction
Spreading radio waves over an obstacle and expanding into the region behind it.
4 types of diffraction propagation
The dictionary defines diffraction as: “the spreading of a wave motion, as light, as it passes an obstacle and expands into the region that is behind the obstacle and hence not directly exposed to the incoming waves”.
Many Tropo Communicators will realize this type of propagation is used or planned. It depends on your area of operation. Military units in the Southwestern United States use this routinely.
There are four conditions, which classify a Tropo Radio as diffraction.
Theory of Operation

12. CECOM LAR Tropo Operation Course
Single Horizon
1. Here we the classic Diffraction propagation. Both stations share a common horizon.
The transmit signals are directed at the top of the obstacle. The signal is then diffracted over to the receiving station.
A rule of thumb is the obstacle should be approximately 2/3 and 1/3 the distance between stations.
As an example; if the distant between stations was 90 miles. One station should be approximately 60 miles from the mountain; the other should be 30 miles.
Theory of Operation

13. CECOM LAR Tropo Operation Course
Reduced Fresnel Zone
2. Here we looking at a LOS propagation with less than 60% clearance of the first Fresnel Zone. Because of the inverse bending, a LOS path may at times become a diffraction path. It is possible to have Diffraction propagation and not realize it until close examination of the terrain between stations.
Theory of Operation

14. CECOM LAR Tropo Operation Course
Two Horizons
3. In figure 1-7 we see diffraction propagation with two horizons between which a LOS path exists. This is a more complex diffraction path to calculate and is seldom attempted.
Theory of Operation

15. Closely Spaced Two Horizons
CECOM LAR Tropo Operation Course
Closely Spaced Two Horizons
4. Here we see diffraction propagation with two horizons, which are closely spaced, and no LOS path exists. The path loss for this type of diffraction propagation cannot be calculated. Communication links of this type are unpredictable and test shots should be performed to validate usable locations.
Theory of Operation

16. CECOM LAR Tropo Operation Course
Fading
Rapid
Caused by the troposphere
Constantly changing
Affects all frequencies at different times
Slow
Caused by the troposphere, gradually
Seasonal Influences
Affects many frequencies at the same time
Fading: Because the Troposphere is a turbulent medium, communication links suffer a phenomenon termed as Fading. Fading is the reduction of signal strength at the receiving station. There are two types of Fading classification; Rapid Fading and Slow Fading. These are explained below;
RAPID FADING: The term Rapid Fading is known as Fast Fading or Scintillation Fading and usually comes from the Troposphere itself. The random variation in the Troposphere’s refraction qualities causes this fading phenomenon. It is constantly changing from moment-to-moment and from point-to-point. This is the same mechanism that cause stars to twinkle and distant objects to exhibit “heat shimmer” on hot days. The fast fading will be most pronounced when the atmosphere is well mixed with moisture and turbulence. A well-mixed atmosphere is the normal conditions for best communications. Rapid fading affects all frequencies at different instances in time.
SLOW FADING: This is caused primarily by changes in the variation of refractivity with altitude in the atmosphere that develop gradually over a period of time. The frequency and intensity of slow fading are both subject to seasonal influences. It has a tendency to occur simultaneously at different frequencies and on different paths between stations. Slow fading may cause prolonged loss of communications.
Theory of Operation

17. 4 Requirements for Tropo Com
CECOM LAR Tropo Operation Course
4 Requirements for Tropo Com
High Power (2 KW)
High gain Antenna (40.5 of 36.5 dBi)
Sensitive Receivers
Diversity Reception (Quad-V2, Dual-V3)
In order to establish and maintain a Troposhere Scatter Communications systems there are 4 things you must.
High Power Amplifier.
High Gain Antennas the 9.5 foot dish on the V2 provides a gain of 40.5 dBi. The 6 foot dish on the V3 provides us a gain of 36.5 dBi.
Very sensitive receivers are required for this system.
One of the two types of diversity reception are required. The V3 provides Dual diversity reception while the V2 can provide a Quad Diversity Reception.
Theory of Operation

18. CECOM LAR Tropo Operation Course
Diversity Reception
Dual Space
2nd Antenna/Receiver
100 wavelength separation
Used on V2 and V3
Dual Frequency
2nd Antenna/Transmitter
100 wavelength separation
Used on V2 only
Diversity Reception: The dictionary defines diversity as variety. By receiving more than one signal you have diversity reception. Diversity Reception is used to overcome the effects of Rapid Fading. The AN/TRC-170 has two types of diversity reception available; Dual Frequency Diversity and Dual Space Diversity. These are explained below:
DUAL SPACE DIVERSITY RECEPTION: This is accomplished by adding a second antenna and receiver to the station. The antennas are separated by at least 100 wavelengths (20.8 Feet). This separation causes the two received signals to fade independent of each other. The two fading signals are used to generate a single stable signal. This is the only form of diversity reception available to the AN/TRC-170(V)3.
DUAL FREQUENCY DIVERSITY RECEPTION: This is accomplished by adding a second transmitter and antenna to the station. The second transmitter is used on a separate frequency at least 100 MHz apart. This separation will ensure that Rapid Fading will not affect both frequencies at the same time. The signal recovery circuits can then use the two fading signals to develop one stable signal.
Theory of Operation

19. CECOM LAR Tropo Operation Course
Quad Diversity System
QUAD-DIVERSITY RECEPTION: This is accomplished by combining the above two diversity reception methods along with antenna polarization. Refer to figure 1-9 on page 1-8 for the following explanation. Transmitter 1 & 2, transmit using frequencies F1 & F2 (at least 100 MHz apart). Antenna 1 is transmitting a Horizontally Polarized signal while Antenna 2 is transmitting a Vertically Polarized Signal. These transmitted signals are sent to the receiving station. At the receiving station we have Receive Antenna 1 & 2. Both of these antennas have two feed horns, which are Horizontally, or Vertically polarized. These four receiving antenna feeds are connected to four separate receivers; RCVR 1, RCVR 2, RCVR 3 & RCVR 4. The signals at the receivers arrive via four different paths with two different frequencies. These four signals are then sent to a diversity combiner (Tropo Modem). This is the best defense against the effects of Slow Fading. This is the preferred operating mode of the AN/TRC-170(V)2.
Theory of Operation

20. Characteristics of the TRC-170s
CECOM LAR Tropo Operation Course
Characteristics of the TRC-170s
Refer to Page 1-9 in your Handbook
Recap the characteristic
Theory of Operation

21. CECOM LAR Tropo Operation Course
Service Range of the V2
This charts gives us a rough idea of the maximum range a Tropo System can be used. The area on the left (yellow) shows the range with the associated data rate that can be expected for a BER of 1 X As you can see the range decreases as the data rate increases.
The area on the right (green) shows the range with the associated data rate that can be expected for a BER of 1 X As you can see the range decreases as the data rate increases.
The rule of thumb planning range for the V2 Tropo system is 150 miles.
Theory of Operation

22. CECOM LAR Tropo Operation Course
Service Range of the V3
This charts gives us a rough idea of the maximum range a Tropo System can be used. The area on the left (yellow) shows the range with the associated data rate that can be expected for a BER of 1 X As you can see the range decreases as the data rate increases.
The area on the right (green) shows the range with the associated data rate that can be expected for a BER of 1 X As you can see the range decreases as the data rate increases.
The rule of thumb planning range for the V3 Tropo system is 100 miles.
Theory of Operation

23. CECOM LAR Tropo Operation Course
Review
Propagation Modes
Common Volume
Scatter Angle
Factors effecting Tropo Communications
Advantages of Tropo Communications
Characteristics of the AN/TRC-170(V)3
What are the 3 modes of propagation?
Explain Common Volume.
What is scatter angle? What happens if this too great?
What factors affect tropo?
What are the advantages of tropo?
Theory of Operation

24. CECOM LAR Tropo Operation Course
Summary
3 types of Propagation
4 Equipment requirements for Tropo Com
Quick Set-up and deployment
Large Data Rate Capability
Reliable Communications
Recap
Theory of Operation