Loop Antenna Characterization for ELF and SLF
Measurements
Caracterizaci
´
on de Antenas Lazo para Mediciones en ELF y SLF
G. I. Quintana
1
, R. Alonso
2
and W. G. Fano
3
Universidad de Buenos Aires, Electronic Department, Electromagnetic Radiation Laboratory
Paseo Colon 850. (1063) Buenos Aires. Argentina
1
gonza.quintana94@gmail.com
2
ninjaramiro@gmail.com
3
gustavo.fano@ieee.org
Abstract—Electromagnetic Fields are present in the Earth,
due both to natural and artificial emissions. The Electric
and Magnetic fields generated by natural events like volcanic
eruptions and earthquakes, are aspects of tectonomagnetism,
volcanomagnetism and tectonoelectricity, in the electromag-
netic spectrum from radio frequencies (RF) to submicrohertz
frequencies. This paper is dedicated to the characterization
of loop antennas for measuring electromagnetic precursors of
seismic movements and other ELF (3-30 Hz) and SLF (30-300
Hz) natural effects. The antenna factor, impedance, quality
factor Q and sensitivity of the antennas are measured. These
characteristics are of great importance in the choice of the loop
antenna to be used in the framework of the UBACyT research
proyect Study of Electromagnetic Disturbances Produced by
Seismic Movements.
Keywords: Electromagnetic Precursors, Magnetic Field,
Loop Antenna, Antenna Factor.
Resumen— Los campos electromagn
´
eticos que se encuentran
presentes en la Tierra, son debidos tanto a emisiones de
la naturaleza como producidos por el ser humano. Los
campos el
´
ectricos y magn
´
eticos que son generados por
eventos naturales como erupciones volc
´
anicas y terremotos,
son aspectos de tectomagnetismo, volcanomagnetismo y
tectoelectricidad, en el espectro electromagnetico desde
las frecuencias de radio (RF) hasta las frecuencias de los
submicrohertz. Este trabajo se dedica a la caracterizaci
´
on de
antenas lazo para medir los precursores electromagn
´
eticos
de movimientos s
´
ısmicos junto con otros efectos naturales
en ELF (3-30 Hz) y SLF (30-300 Hz). El factor de antena,
la impedancia, el factor de calidad Q y la sensibilidad de
las antenas son medidos. Estas caracter
´
ısticas son de vital
importancia para la elecci
´
on de la antena lazo a utilizar en el
marco del proyecto de investigaci
´
on de UBACyT Estudio de
Perturbaciones Electromagn
´
eticas Producidas por Movimientos
S
´
ısmicos.
Palabras clave: Precursores Electromagn
´
eticos, Campo
Magn
´
etico, Antena lazo, Factor de Antena.
I. INTRODUCTION
During the past few decades, a remarkable increase in the
quality and quantity of electromagnetic data recorded before
and during eruptions and earthquakes [1] are evidence that
seismic movements are preceded by anomalous electromag-
netic signals.
The Electromagnetic Precursors signals have been dis-
cussed in many publications [1], [2], [3] and [4]. The most
accepted theory is that electromagnetic waves are generated
as a consequence of microfractures in the rocks. The electric
charges of opposite sign, created on opposite sides of the
microfractures form electric dipoles separated by a distance
d. This separation is modulated by the mechanical vibrations
of its walls (originated by rupture of the atomic bonds),
giving rise to dipole oscillations and electromagnetic waves
[5]. This emissions are in the ELF (3-30 Hz) and TLF (<
3 Hz) range.
Another natural phenomena at ELF are the Schumann
resonances, which are electromagnetic resonances in the
cavity formed by the ionosphere and Earth’s surface (that
can be modeled as a conductor for low frequencies) excited
by lighting discharges. The first 3 peak frequencies, 7.8 Hz,
14 Hz and 20 Hz, had been measured at the location of Villa
Alpina, C
´
ordoba, Argentina [6]. This measurement had the
purpose to get evidence of electromagnetic precursors of
earthquakes in a quiet zone. This location was chosen due
to the very low artificial noise of the power lines radiation
[6].
The most important naturally occurring VLF signal is the
whistler. A whistler is created from a lightning stroke that
passes first to the ionosphere and then to the magnetosphere
above. These particles are then guided along the Earth’s
magnetic field, returning to ground to the opposite hemi-
sphere [2].
Fig. 1. Shielded loop antenna.
Revista elektron, Vol. 2, No. 2, pp. 95-100 (2018)
ISSN 2525-0159
95
Recibido: 15/06/18; Aceptado: 26/07/18
2r
1
Unshielding Loop Antenna
Receiver
Fig. 2. Unshielded loop antenna.
Due to the Electromagnetic Compatibility regulations
(EMC), measurements of Electric and Magnetic Fields are
of great importance. A very useful method for measuring
the Magnetic Field of electromagnetic waves is by means
of a loop antenna, which can be shielded or unshielded.
A shielded loop antenna can be seen in Figure 1 and an
unshielded loop in Figure 2. The radiation pattern of a loop
antenna, in which the size of the loop is much smaller than
the wavelength, is described in Figure 3. As all measure-
ments are made within the ELF and SLF ranges, Figure 3
represents the directional dependence of the received power
of the antennas used in this work.
The unshielded loop will receive the electromagnetic
radiation and the quasistatic induced noise. This noise will
typically come from fluorescent lamps, brush motors, corona
discharge from HV power lines, and the radiation from
the line scan of TV sets [7]. The shielding loop, on the
other hand, is practically insensitive to stray fields induced
noise, which is quite important as nearly all of the energy
content of long wave and medium wave interference locally
generated is quasistatic [7]. The gap observed in Figure 1 in
the shielding, prevents the currents around the loop due to
possible atmospheric effects and electrostatic discharges [8].
Magnetic loops have long been used by EMC personnel to
“sniff” out sources of emissions in circuits and equipment
[8].
In order to measure the ELF and SLF spectrum, two
circular loop antennas are constructed and compared. These
loops, together with a low noise amplifier and a computer
(using the free software GNU-Radio to compute the FFT),
allow to sense the normal component of the incident Mag-
netic Field and it’s frequency spectrum. By using an array of
three of this loops, the 3 components of
B can be measured.
This paper is dedicated to the characterization and com-
parison of these two antennas. As all the measurements have
been made in the laboratory, in the absence of those unde-
sired effects, unshielded loops are used. In the future, when
using these loops for capturing electromagnetic precursors
and other natural electromagnetic emissions (as Schumann
resonances), a suitable shield will be needed to avoid the
above mentioned problems.
0.5
1
0
30
60
90
120
150
180
210
240
270
300
330
Loop
Antenna
Fig. 3. Radiation patterns of a circular short loop antenna.
II. ANTENNA FACTOR
The Antenna Factor or Correction Factor of a loop an-
tenna relates the magnitude of the incident Magnetic Field
(H) and the induced voltage (open circuit voltage) at the
antenna terminals (V). It is defined thus:
K =
H
V
A
V m
(1)
The Maxwell-Faraday Law expression in differential form
is [9]:
×
E =
B
t
(2)
where:
B[T ]: is the magnetic flow density.
E[V/m]: is the electric field.
t[s]: is the time.
In order to obtain the open circuit induced voltage at the
terminals of the loop antenna, integration is performed at
both sides of eqn. (2) :
V
oc
=
φ
t
(3)
where: φ =
R
B ·
ds is the magnetic flux.
Consider a short single turn loop, excited by a harmonic
electromagnetic wave as a function of time where the
wavelength of the electromagnetic wave received is much
greater than the perimeter of this loop. The magnetic flux is
then:
φ = B
Z
ds = µ
0
HA (4)
The open circuit induced voltage of N turns of the loop
antenna is [9]:
V
oc
= N
0
H
t
(5)
This voltage can be written thus [10].
V
oc
= Nωµ
0
HcosθA (6)
where θ is the angle between z and H and ω = 2πf.
Revista elektron, Vol. 2, No. 2, pp. 95-100 (2018)
ISSN 2525-0159
96
http://elektron.fi.uba.ar