BX-245 • 27
© Box 73 Amateurfunkservice GmbH 2019
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Tips on measurement practice
Measuring complex impedances poses
many options. The measurement examples
listed below can therefore only hint at
these. Further explanations of the physical
and mathematical correlations can be
found in specialist literature. It is assumed
that a master calibration was performed
with or without the cable attached (de-
pending on the measurement setup). If the
test structure is changed later, e.g. by re-
placing the cable, calibration must be car-
ried out again. When making antenna
measurements note that the FA-VA5 is an
“active” analyzer which generates and
outputs RF to the antenna under test.
Therefore, the measurement time
should be kept as short as possible to
avoid the possibility of causing inter-
ference to other users. Tests on fre-
quencies outside the amateur radio
bands should be avoided.
Multi-Frequency Measurement of
Impedance and SWR of an Antenna
The antenna is connected to the Analyzer
directly at the base or via a cable previ-
ously included in the calibration by SOL
compensation. Using the multi-frequency
measurement, an overview of the SWR or
impedance curve can be generated. The
centre frequency and the frequency span
are set as required for the spectrum to be
measured. The marker mode can be used
to “approach” an SWR minimum.
In antennas with high Q-factors (e.g. mag-
netic antennas), more attention is needed
to obtain accurate measurements. The
maxima and minima are so narrow that
these may not be displayed depending on
the selected frequency span. The answer
is to carefully select the centre frequency
and to reduce the frequency span (sweep)
range accordingly to ensure meaningful
results are obtained.
Measuring impedance and SWR of
an antenna at one frequency
The antenna is connected directly to the
base or via a cable to the analyzer. You
will need to decide if the whole system of
antenna and cable or only the antenna
should be measured. If the latter applies,
the cable must be disconnected at the base
of the antenna (usually designed as a plug
connection).
The analyzer should be set to a single fre-
quency measurement of SWR or
impedance. Then the target frequency is
entered. The cable is now included in the
measurement: remove the feeder cable
from the base of the antenna. SOL Cali-
bration should now be done for this fre-
quency via the menu item SOL One fre-
quency for the current frequency by con-
necting the calibration elements to the end
of the antenna cable as previously de-
scribed in Calibration.
Next, reconnect the cable to the antenna
base and the actual SWR or impedance of
the antenna can now be measured and dis-
played. The way of tuning the antenna for
optimal SWR and / or resonance depends
on the antenna type. For example, a
monoband dipole with balun may be tuned
by shortening or lengthening the dipole
legs. If there is an impedance with nega-
tive reactance (capacitive), the antenna is
too short. On the other hand, if the Anten-
na is too long for the measured frequency,
a positive reactance (inductive) will be
displayed. The aim is to achieve a resis-
tance of 50 Ω and with an imaginary part
of the impedance of zero (resonance case).
Measuring Capacitance and
Inductance
For measurement of components at fre-
quencies up to 30 MHz, simple adapters
made of 2.54 mm pin headers and sockets
and matching SOL reference elements has
proven to be successful. For the measure-
ment, mode
LCR Meter
is used. The target
frequency must be set and then calibration
performed using menu item
SOL One fre-
quency
for the existing test setup. The
