18 • BX-240
© Box 73 Amateurfunkservice GmbH 2017
n
Tips on measurements in
practice
The application possibilities of measuring
a complex impedance are very diverse.
The measurement examples listed below
can therefore only be used to a certain ex-
tent. Further explanation of the physical
and mathematical contexts can be found
in the relevant technical literature.
It is assumed that a master calibration
(
SOL All Frequencies
) has been carried
out with or without connected cables (de-
pending on the measurement setup). If the
setup is later changed e. g. by changing the
cable, then the compensation must be car-
ried out again.
On Measurements in General
One must be aware when measuring on
antennas, that the FA -VA4 is an ‘active’
device that generates RF energy and de-
livers it to the antenna. The power at the
BNC socket is about 3dBm.
And is sufficient to cause interfer-
ence to other stations, therfore the
duration of all measurements should
be kept as short as possible and tests
on frequencies outside of the Ama-
teur Radio bands should be avoided.
Multi Frequency Measurement of
Impedance and SWR of an Antenna
The antenna is connected directly to the
base point or via a cable previously inte-
grated into the calibration by means of
SOL compensation and the analyzer. An
overview of the SWR or impedance pro-
file can be generated using the multi fre-
quency measurement. The centre frequen-
cy and the frequency range must be set to
the range to be measured. A minimum
SWR can be ‘targeted’ via the Marker
Mode.
For high quality antennas (e. g. magnetic),
increased attention is necessary when tak-
ing a measurement. Under certain circum-
stances, depending on the chosen frequen-
cy range the maxima and minima are so
narrow that they are not displayed. Here
the measurement frequency can only be
further limited and the span reduced ac-
cordingly.
Measurement of Antenna Impedance
and SWR for a single frequency
The antenna is connected directly to the
analyzer or via a cable. It is to be decided
whether the whole system of antenna and
cable or just the antenna is to be measured.
If the latter is connected then any cable
(usually plug in connectors) must be dis-
connected. The analyzer must be set to a
fre quency to measure SWR and or Im pe -
dance. The target frequency is then en-
tered. Via the menu item
SOL One Fre-
quency
(see paragraph
Calibration
), the
cable is again included for the actual fre-
quency under measurement by successive-
ly connecting the test elements (instead of
the antenna) as described above. Lastly the
cable is connected to the antenna and the
actual antenna SWR or impedance is dis-
played.
The method of tuning an antenna for opti-
mal SWR or resonance depends on the an-
tenna type. For a monoband dipole with
Balun this may be achieved by shortening
or lengthening the antenna elements. For
an antenna with negative reactance (ca-
pacitive) the elements are too short, in the
case of positive reactance (inductive) then
the elements are too long. The aim is to
achieve an effective resistance of 50 Ohms
with zero impedance (at resonance).
