Any of the Network Model plots below can be imported and plotted in mapping software such as Maptitude or Google Earth.
Line-of-sight plots with the fresnel zones. These plots can be used on their own or to determine if path blockage exists and possible repeater locations. The results can also be used in Field Strength Studies.
The predicted received carrier levels and fade margins can be provided with other factors. These other factors are often called Path Reliability. This is the anticipated Communication Outage due to rain, snow and atmospheric condition. The results are useful in the design of critical microwave systems such as studio to transmitter links (STL) and in similar applications requiring “continuous” communications.
• BROADCAST AND CAREY
The Broadcast and Carey Field Strength Methods are both based on the FCC Rules and Regulations. The two methods are essentially simplified statistical methods of estimating field strength and coverage. They are based only on the transmitter effective radiated power (ERP) and height above average terrain (HAAT).
These models are limited in their usefulness. The limitations are that only points from three (3) to sixteen (16) kilometers away from the transmitter are accounted for, the terrain is averaged, and neither obstruction information or shadowing is used. The main use of these models is in FCC licensing in rule parts 22 and 73.
The Bullington Field Strength Model is typically used from a very low height antenna to a very high one and over rough or hilly terrain. It is reportedly useful at frequencies as low as 30 MHz, however, our experience indicates that it is unreliable until the frequency exceeds 80 MHz. A further drawback is that paths over long smooth terrain, such as a path over 100 miles of water, yields unreliable data
It is an analytical model that considers both terrain and man-made obstructions. Terrain elevation, from our database, and any available obstruction files are used to compute knife-edge diffraction losses. The obstruction penetration into the first Fresnel zone of the path is computed along with the decibel loss of the corresponding penetration loss. The net received field at points along radials are a summation of effective radiated signal level, free space and the numerous calculated losses.
The results of this study, when properly used, are excellent and have been used for many years as a standard by the National Oceanic and Atmospheric Administration for the protection of the quiet zone at Table Mountain in Boulder, Colorado.
The Okumura model is based upon numerous measurements, on multiple frequencies, in Japan. It is useful in Urban and Suburban areas. The typical USA situation is far from that of Japan. The Suburban model is representative of a large domestic metropolitan residential area with multiple groups of "row" houses. Our model accounts for a reduced degree of urbanization that is representative of the domestic situation.
This model is extremely accurate when properly used. The terrain information is computed automatically by our database. The degree of urbanization, terrain slope, roughness, receiver location relative to nearby hills, valleys, general street orientation in the service area, localized obstructions etc., must be entered manually. This data can be collected from municipal maps, field surveys, aerial photography overlays, property deeds, FCC Tower Files and FAA Obstruction Files. This model is typically used for 800 MHz cellular and 1900 Mhz PCS design, but has a wide range of usefulness above 150 MHz.
Longley-Rice is a well-established field strength model. It uses our elevation database to compute terrain roughness and radio horizons. Our implementation of this model accounts for average climate conditions, soil conductivity etc.
The model is reported to be useful above 30 MHz. Our experience indicates that “accurate” results can be obtained above 80 MHz. It is used where the length and terrain is very flat.