Low-frequency RF Coupling To Unconventional (Fat Unbalanced) Dipoles Page: 26 of 44
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4 Experimental Validation
Because the dipole equations and computer simulations could be used for critical safety
analysis, a laboratory study was attempted to validate the accuracy and limitations of the
equations and modeling codes. Even on the faster rising edge, the lightning current changes
relatively slowly from an RF perspective, around a microsecond. Therefore, the lightning
wavelength is much longer than the dimensions of the dipoles. These low radio frequencies
allow the use of a 2-meter high transverse electromagnetic (TEM) cell to create known and
reasonably spatially-uniform electric fields to excite our dipoles [1.6 - Crawford]. The study was
performed in the frequency domain from 10 kHz to over 1 MHz using a commercial signal
generator that produces a few voltages. There are two advantages to performing frequency
domain studies rather than using lightning-like pulses. First, the sine waveform generates more
average power for a given measurement period, and thus will improve the signal-to-noise ratio.
Second, the frequency response of the antenna system provides important insights about the
coupling efficiency in the spectrum of the lightning current. While the antenna has a flat
response over the lightning current spectrum, adding the load will change the circuit frequency
response. It acts like a high-pass filter. This is easier to observe in the frequency domain than in
the time domain.At all frequencies, the dipole voltage will drop with
any loading, such as from a cable and digitizer. To
minimize the impact of the measurement
instrumentation on the antenna voltage, a high-
impedance probe was. A Tektronix P6247, a high-
impedance active differential, voltage probe with low
input capacitance (<1 pF) that significantly reduces the
loading was selected [4.1 - Tektronix]. (See Figure 4.1.)
A 12-inch dipole was fabricated on a thin circuiti
board, and the antenna arms were fabricated on
Figure 4.1. Baseline antenna used in
opposite sides of the thin board. Based on computer
validation study with differential high-
simulations, the higher than air dielectric constant
impedance voltage probe.
around the arms has little impact in the antenna
characteristics. Two small wires connecting the
antenna to the probe were kept as short as possible, a few millimeters. A light-colored vertical
foam board in the center of Figure 4.1 supports the probe. Near the dipole, the cable is routed
on an equipotential line.
The connection diagram for the equipment used in the antenna characterization is shown in
Figure 4.2. The output of the TEM cell was terminated in a 500 load to reduce reflections.- 26 -
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Ong, M. M.; Brown, C. G.; Perkins, M. P.; Speer, R. D. & Javedani, J. B. Low-frequency RF Coupling To Unconventional (Fat Unbalanced) Dipoles, report, December 7, 2010; Livermore, California. (https://digital.library.unt.edu/ark:/67531/metadc834261/m1/26/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.