LAPLACE INSTRUMENTS RF200 User Manual - page 9
RF200/500 user manual
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antenna height
MHz
3-6
0-3
-3-0
-6--3
-9--6
-12--9
-15--12
-18--15
To overcome this effect, the standards call for the antenna to be mounted on a mast so
that it can be varied in height over a range of 1 to 4 metres. For each frequency there
will be a height at which the two signals are in phase and additive. This is the height
at which that frequency is measured.
Fig 5 shows the relationship between frequency, antenna height and signal
gain/attenuation. The white areas correspond to gain and the dark areas represent
attenuation.
If using an open field site on dry soil, the reflection will be small and the
gain/attenuation effect will be minimal. However, if any metal surface is in the
vicinity (filing cabinet for instance) this chart gives some idea of the consequences!
Note that the chart is correct for the conditions listed only. If for instance the
polarisation is changed to vertical the plot will completely change.
Fig 5 Ground plane effect
Conditions:
Product height:
0.8m
Antenna distance: 3m
Polarisation: Horizontal
Ground plane: Metal sheet (ideal)
4.0 Test Site Calibration
Any area or test cell used for far field radiation testing should be calibrated. Purpose
made cells such as a G-TEM cells are supplied ready with a calibration sheet which
defines the characteristic relationship between UUT emissions and cell output vs
frequency.