Model and then do Far Field Testing
- skylarkcolo
- Oct 29, 2023
- 5 min read
Updated: Feb 18
Acceptance testing in the Far Field for Arrays.
My test verified 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 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, towers, light poles, 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.
I have model all antennas using EZNEC and use HFTA (High Frequency Terrain Analysis) to evaluate the take of angle of the various antennas over real ground
All of my 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 side-lobe level, and the amplitude of the radiation, both in the minima and to the rear of the antenna, which must be 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. (160, 80, 30, 17, 15, 10 & 6 meters were considered secondary, but also tested) on the big Rhombics.
(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 100ft 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 with the daytime D Layer higher absorption (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.
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.
The antennas have real gain
Rhombic or Yagis
VK3MO Ian has a stacked rhombic antenna with 8 wavelengths on a leg giving a total of 1340m of radiating wire. The upper rhombic is at 40M and the lower rhombic is at 21M. The rhombic had a gain of 23dBi at a take off angle of 5 degrees on 20M and is directed at New York. The rhombic was modelled using EZNEC and it has 3dB more gain than the 5/5/5/5 yagis Both the yagis and the rhombic have a take off angle of 5 degrees which allows a comparison between the two antennas in the direction of New York. Ian sees see the 3 dB advantage which validates the accuracy of the NEC antenna modelling software.
It can be truthfully be said, that "a Rhombic antenna occupies more space per db of gain than any other antenna" The Rhombic is a very high-gain antenna however it requires a lot of acres, and the efficiency when terminated is only about 50%. An alternate impedance-termination system, which was only used for a few large broadcast stations where input powers were above 50 kw, is called the re-entrant line termination. Clyde Haehnlen SK, developed the specifications for the Voice of America antenna system at the Bethany, OH Relay Station. That re-entrant Rhombic was 90% efficient by re-phasing the power instead of heating up termination resistors, in this system, the Rhombic is terminated in a transmission line, which in turn is coupled back to the input through the proper voltage-matching and phasing networks. Thus, the energy in the dissipation line is fed back to the antenna, so that considerably less than 50 percent of the energy is wasted. The old VOA Bethany site in Ohio had efficiency up to over 90%. This feeds-backs the wasted RF energy "In-Phase", back into the feeder end of the antenna. For any variation from the stubs frequency, the stub must be returned.
I am now the only station using re-entrant line termination equipment, which is re-phasing the power instead of heating up termination resistors.
Clyde provided me with design information for re-phasing a few years ago before his passing.
Models vs. Prototypes: Why Field Adjustment Will Always be Necessary
L. B. Cebik, W4RNL/SK
For the Best Modeling data on Rhombic antennas see, https://www.antenna2.net/cebik/content/a10/wire/lw4.html
Comentários