The terrain around the station has a significant impact on the strength of the signal radiated from their antennas. A location in a valley isn't going to "get out" as well as the located on a ridge line above surrounding terrain. But beyond these "obvious" cases, most need help in understanding how the surrounding terrain impacts their station and how they may be able to mitigate these effects by choice and placement of antennas.
Modeling an antenna over real terrain gives you a visual picture of how terrain impacts performance. You can use a model to:
Determine optimum height for antennas on an existing tower
Compare different tower locations for performance
Compare different sites for performance
K0UO uses a software package called HFTA or HF Terrain Analysis.
HFTA models horizontal antennas at specified heights over terrain and plots the modeled antenna gain at different elevation angles.
The Fresnel region is the area where the radiation field pattern or shape is still being formed. It may or may not include induction field areas. Physically large arrays like K0UO's have a physically large Fresnel zone extending out a few wavelengths. The field impedance may or may not have already been established in the Fresnel zone.
The K0UO QTH is surrounded by a natural low-lands called wetlands on a creek bottom, which is highly alkaline and has a high salt content. The normal conductivity of the nearby farmland up to two miles away is very high, it is red soil, which is high in iron. What is Electrical Conductivity (EC)? It is the ability of a material to transmit (conduct) an electrical current and is commonly expressed in units of milliSiemens per meter (mS/m). The more acidic or basic something is, the more ions there are. The higher ions the better the electrical conductivity is. Therefore, the more acidic or basic in the soil, the higher the EC will be. To test, the Wenner "4-point or 4 pin Method" is used, which is by far the most used test method to measure the resistivity of soil for broadcasters and comm-sites. The basic premise of the soil resistivity test is that probes spaced at 5’ distance across the earth, will read 5’ in depth. The same is true if you space the probes 40’ across the earth, you get a weighted average soil resistance from 0’ down to 40’ in depth, and all points in between. This raw data is usually processed with computer software to determine the actual resistivity of the soil as a function of depth. The photo below shows 3 of the woodpoles holding the radiating antenna cables surrounded by water, when dryer the soil has a very high salt consistency and is red dirt (high in iron). Up to 6 Beverage receive antennas in the winter months each 1000 to 1500ft are used in this area and the winter wheat fields near by.
My Cat opening up land below the antenna cable, there is 6 to 7 acres under each antenna