Model and then do Far Field Testing
Updated: Nov 22, 2021
Acceptance testing in the Far Field is complete (12/2019) for the Curtain Array. 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.
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.
All the 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 fell 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
Above: Wandel & Goltermann E&H Field power density meter with fiber optic cable to PC, the meter-probe is on the crankup test tower and uses a fiber cable down to the PC for data collection
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 D Layer (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.
The antennas have real gain
For the Best Modeling data on Rhombic antennas see, http://on5au.be/Cebic/longwire-rhombic.pdf