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ROM-Elektronik GmbH
In order to be able to determine the radiant flux density exactly with Equa-
tions (3) and (5), you must measure not only the received power, but you
must also determine the frequency. For this, spectrum analyzers are gene-
rally used, which are very expensive because of their complicated struc-
ture.
If it is a question of the effect of electromagnetic waves on persons, howe-
ver, the radiant flux density generally does not have to be determined ex-
actly. Here it is of primary importance to know the order of magnitude of
the radiant flux density, in order to make a judgment of the potential dan-
ger. The RF analyzer HFA-3 has been developed for this purpose. It consists
of a very sensitive RF power measuring device (detector). Since the detec-
tor cannot determine the frequency of the radio frequency radiation, no
high-precision measurements are possible with it for the above explained
reasons.
Uncertainties of measurement
From the above-mentioned facts, it can be identifiable that, with the mea-
surement of the radio frequency, uncertainties of measurement (measu-
ring errors) must be expected.
Which order of magnitude of the measuring errors must be dealt with? If
we look at the best RF measuring devices which can be bought, they most-
ly have an uncertainty of measurement of ±3 dB. In the measuring of out-
put, 3 dB means a factor of 2! For the measured value, this means that the
true value or can be twice as large or only half the indicated value. Expres-
sed in percent, we therefore have an uncertainty of measurement of ±50%
with a very good measuring device!
In case of more economical devices, these measuring errors are often far
greater. However, what is the situation with the HFA-3? We must consider
the inadequacies of the antenna and the measuring device. This is becau-
se the combination of the two of them should supply the "correct" measu-
red value.
A high level of linearity is necessary for the measuring device (in order to
keep the measuring error low). High-frequency amplifiers generally have a
more or less linear frequency response. Without special measures, this un-
suitable characteristic can affect the overall linearity.
Only by means of large-scale development efforts and with close coope-
ration and support through the University of the German Armed Forces,
Munich, could a suitable radio frequency circuit finally be developed,
which does not indicate the disadvantages mentioned. The results of these
efforts are represented in the following graphics.
In order to achieve a corresponding sensitivity, antennae are required
which supply a sufficient outlet voltage in a large frequency range. Log-
arithmic-periodic antennae are very well suited for this. Therefore this an-
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ROM-Elektronik GmbH
Terminal
With this display, the outputs peak value and average value are displayed
both with regard to amount and with the respective dimension. The values
are displayed as they come over the interface ("endless loop" in the measu-
ring device). A clock cycle is not available.
As soon as the measurement is started (click on the icon Start Measurement,
or out of the menu) the read-out values are represented in the text field
Measured Values under each other, where the peak value and average
value alternate.
The last measured values in each case are represented for diagnostic pur-
poses in the separate text fields Peak Value and Average Value in a larger
font.
The filter (frequencies) in the selection list can be adju-
sted manually or automatically. The maximum values
for every filter range are evaluated and can be stored
during the stored long-term measurement.
Selection list: Filter
In this selection list are listed the available recording
duration for the long-term recording. When the adju-
sted time has expired, the measurement is ended au-
tomatically
Selection list: Recording duration
In this selection list are listed the two possible recording
widths. The recording width is the time, which is repre-
sented as maximal during the long-term recording. If
the recording duration exceeds the recording width,
the graphics are scrolled automatically. The recording
width is stored with that i.e. if a measurement is loaded
again, only so much of the measurement curve is re-
presented that fits into the recording width. The remai-
ning part of the measurement curve is made visible by
shift of the horizontal scroll bar ("scrolled").
Selection list: Recording width
