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(Feb 16 1998)
Why all these details ??Since the method of measuring antenna performance by comparision of far field amplitudes and phases is not commonly used and since it leads to a considerable amount of data processing it is described in great detail here.All data and software needed is presented in full detail in such a way that the evaluation process can be repeated by anyone interested. It is possible to investigate how sensitive different parameters in the computer model are to changes. For example it is possible to move the center of rotation along the boom or perpendicular to see if the change that occurs can be compensated by changing other parameters. This site serves as a back up for software and general information about this method for later use at better sites than the one used at the Nordic VHF Meeting on Öland 1997.
Description of the dataThe model calculates the expected complex voltage that would be observed when the AUT is rotated at a fixed frequency. These model patterns are all normalised so the average power from all directions is 1 for each frequency.The experimental patterns are normalised in the same way, and the phase of all points is shifted by a fixed angle to make the phase zero in the forward direction. The amplitude and phase change with frequency not only because of the AUT - the fixed antenna also adds a contribution. The attenuation and phase shift caused by mismatch has a large influence on the change with frequency of phase and amplitude. It is not meaningful to build a model that includes the absolute phase and absolute amplitude without accurate measurements of feedpoint impedances, cable lengths and geometric data. The VSWR changes with frequency and NEC2 does not model it well for folded dipoles. The model produces 51 radiation patterns at 51 different frequencies for each antenna. For 17M2 an additional series of measurements was made at different heights, but with the elevation angle = 0 at all heights. In this way 17M2 was measured twice at 10 meters and 0 degrees. The height series was scaled in amplitude and phase to be similar to the angle series for the common geometry (10 metres and 0 degrees) because unfortunately the network analyser was recalibrated between these two series.
K1FO 51 frequencies in 32 directions
Running the STEP3.EXE program with the files supplied with it produces .DAT files containing amplitudes and phases for the measured data and for the model. These files also give separately the different waves that sum up to the model.
Global parametersSome parameters are common for all measurement series. These parameters are contained in the file GLOBAL.TXT which is one of the input files to step3.
----------------------------------------------------------- RFLAMPL144 0 1 0.705472 0.00005 RFLAMPL432 0 1 0.826093 0.00005 RFLDISTANCE 0 1 1000. RFLHEIGHT 0 1 0. RFLPHASE144 0 1 3.609131 0.00002 RFLPHASE432 0 1 5.024800 0.00002These are the remaining parameters for the first reflection, the one from the ground between the towers. The 1 in the third column tells that these parameters belong to the first reflection. The values are in the fourth column and RFLDISTANCE = 1000 means that FIXTOWER and TOWDISTANCE are to be used to get the geometry for a reflection from the ground between the two towers. RFLAMPL gives the amplitudes as a fraction of the amplitude of the direct wave and RFLPHASE gives a phase shift that is added to the phase shift due to the path length being longer than the direct wave path.
----------------------------------------------------------- RFLAMPL144 0 2 0.012967 0.00002 RFLAMPL432 0 2 0.014963 0.00002 RFLDISTANCE 0 2 -92.441969 0.0002 RFLHEIGHT 0 2 6.0 RFLPHASE144 0 2 0.721643 0.00002 RFLPHASE432 0 2 5.290989 0.00002These parameters describe reflections from a building about 100 meters behind the AUT. (92.44 m from parameter fitting)
----------------------------------------------------------- RFLAMPL144 0 3 0.008875 0.00002 RFLAMPL432 0 3 0.028954 0.00002 RFLDISTANCE 0 3 -78.945276 0.0002 RFLHEIGHT 0 3 0.5 RFLPHASE144 0 3 5.072266 0.00002 RFLPHASE432 0 3 4.428474 0.00002These parameters describe a stone wall and perhaps some parked cars in front of the building. It is clearly seen from the data that there are two different (or maybe more) reflections from behind. It is not possible with the limited number of angles around 180 degrees to determine the different heights, so the heights were fixed at reasonable values. Allowing the heights to find optimum values gives unrealistic positions for the reflection points but it does not change the sum of the two reflected waves significantly.
-----------------------------------------------------------The second column is 0 if a parameter should not be changed. If the second column is made = 1 for a parameter, the STEP3 program will try to find a better value for that parameter. The last column gives the step size used by the optimisation algorithm when calculating derivatives.
Antenna specific parametersFor each antenna there is a file ANTPAR.TXT in respective subdirectory. These files are used by all three evaluation steps. The file for 17M2 is used as an example:----------------------------------------------------------- PATHLEN 0 250.This parameter is manually set to a reasonable value as described in STEP1. 250 meters is the free space distance that is required to compensate for the total delay through cables and the air between the towers for the 17M2 antenna. |