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Rhombic Antenna testing in the Far Field

Updated: Apr 23

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 truly doing at your location.

Acceptance testing in the Far Field can be done with a large drone. A large drone that can handle the payload is what you need. The Drones that can be preprogrammed to fly at precise distances, and height (XYZ) can give you very accurate information on takeoff angles, and front to back gain.

Metrology is the scientific study of measurement just the ability to measure alone is insufficient; standardization 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 side-lobes, 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 or drone 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.

Capture area or Effective Aperture is determined by antenna gain and the wavelength, not by antenna physical size.

All the Rhombic and Vee Beam antennas are field tested to confirm the design values, which concluded that the amplitudes & phases of the currents in the radiators conform with the antenna model. The antennas are readjustment as needed for max gain and best F/B. The radiation pattern of an HF antenna is formed as a result of reflection by the ground, and it may also be modified by currents flowing in the support structure. Data regarding the gain, accuracy of beam shape, and slew angle, as well as sidelobe level and the amplitude of the radiation both in the minima and to the rear of the antenna, this was determined through real measurements. It is difficult to predict from the amplitudes and phases of the current flowing in the radiating elements. If significant discrepancies between design and actual performance are found, such measurements are advantageous as changes are made.

  • K0UO proof tests fall into three categories:

  • (1) Comprehensive evaluation of radiation patterns, impedance, and gain for the antenna on 40 & 20 meters. (80, 30, 17, 15, 10 & 6 meters were considered secondary, but also tested)

  • (2) The minimum practical tests provided proof of performance of the installed antenna on 40 meters @ night time "F layer", and Daytime with "D layer absorption".

  • (3) Compares forward gain at the desired azimuth and elevation angle to average gain over the entire hemisphere

  • E & H meters below are used for the field testing

  • Above: Wandel & Goltermann E&H Field power density meter with fiber optic cable to PC, the meter-probe is on the crank-up test tower and uses a fiber cable down to the PC for data collection.

  • Also can use a E&H Field power density meter mounted on a large commercial drone.

  • K0UO is using Ace HF Pro and IONSUM, which are computer programs using the output of the IONCAP prediction method to determine the most suitable frequency band and required antenna gain under specific averaged conditions. The acronym stands for IONCAP SUMmary. Propagation predictions form an essential tool in the management of a HF wanting to work DX Stations. The data from such predictions are used to specify the types and operating frequency ranges required to work the DX by allowing for changes in the antennas take off angle to achieve maximum signal to the desired direction. Which it is used to contact DX stations utilizing both long path or short path on F layer, or to contact North American stations on daytime with high D layer adsorption (160, 80, 60 & 40 meters). Some reflection can be obtained from the D region, but the strength of radio waves is reduced; this is the cause of the marked reduction in the range of radio transmissions in daytime on the lower bands.

  • K0UO uses real time ionosonde (vertical HF RADAR ionospheric height-finder) data and ACE-HF Network, by Long Wave Inc which allows K0UO to analyze the entire HF spectrum using a single fixed transmitter location and multiple receive locations. The 64 Bit application has enhanced features and uses Google Earth and Google Maps to provide detailed HF Area Coverage Maps. K0UO uses, and is part of real time GPS observables network to measure properties of the electron density such as the total electron content (TEC). The TEC is a measure of the total number of electrons that would be contained in a cylinder that extends up vertically above a given point on the earth all the way through the ionosphere K0UO uses, and was part of a real time GPS observables network to measure properties of the electron density such as the total electron content (TEC). The TEC is a measure of the total number of electrons that would be contained in a cylinder that extends up vertically above a given point on the earth all the way through the ionosphere. The ACE HF Pro, by Long Wave Inc allows K0UO to analyze the entire HF spectrum using a single fixed transmitter location and multiple receive locations. The station uses ionosonde (vertical HF RADAR ionospheric height-finder) data.

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. The main ground is a 5400 feet deep (an oil well casing). 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 wood pine poles 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 to1500ft are used in this area and the winter wheat fields near by.

Models vs. Prototypes: Why Field Adjustment Will Always be Necessary

L. B. Cebik, W4RNL



General Steve Walz

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