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 is a great graphical tool but only models the antenna over at most four azimuth values at a time. A good program is intended to evaluate different antenna heights over terrain in all directions.
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.
Above, 4-point or 4 pin Method Test Unit that K0UO uses.
Acceptance testing in the Far Field is complete (12/2019) for the Curtain Array. Metrology is the scientific study of measurement just the ability to measure alone is insufficient; standardisation is crucial for measurements to be meaningful. The test verified on 40 & 20 meters the design maximum gain, the azimuth of the maximum gain, steering of both direction and azimuth, design sidelobes, and the back-to-front ratio. A calibrated, W&G EMR meter for both the E and H field is used, with the use of a portable tower in the far field at precise predetermined positions. This allowed the for the most suitable method of conducting the test measurements. The analysis and its feedback mechanisms are a major part of K0UO's projects.
All test at K0UO is done in the Far Fields or the Fraunhofer zone which is the area where changes in distance from the antenna no longer produce a noticeable change in pattern shape or field impedance.
Modern programs based on NEC2/ NEC4 try to model loss (absorption of RF) in the ground Electrical Conductivity under and around an antenna (using the Sommerfeld-Norton ground model). Older modelers are constrained to using an inadequate method of modeling loss and ground reflections at low angles. To form a skywave, all of the relevant interaction between the antenna and the ground under it happens within a distance of a 1 to 5 wavelengths radially out from the antenna. Some antennas are more sensitive to ground losses than others. Surrounding objects in the near field of the antenna like buildings, trees, tower, antennas and fences, also play a big role that models do not show. The real world patterns may not conform to theoretical model unless you have tested the Electrical Conductivity of the ground. Only after you actually measure, the RF radiation pattern of what you simulate, will you learn what the antenna is truely doing at your location.
My Cat opening up land below the antenna cable, there is 6 to 7 acres under each antenna
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 are used in the winter months each 1000 to 1500ft are used in this area and the winter wheat fields near by.
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.
A Ground Is Just a Ground--Unless It Is a Model of a Ground
By L. B. Cebik, W4RNL/SK
