Impact of antenna sidelobes on MAMBO observations
Tests with different sidelobes for a 23cm antenna.
K. Dassas 12/04/02
The goal of these simulations is to determine the effect of sidelobes when looking at altitudes
close to the Martian surface in order to constrain the maximum extension of the sidelobes
acceptable for Mambo antenna.
Design
The main lobe of the antenna response function has been designed on the basis of the ODIN
antenna adapted to MAMBO (central frequency, antenna size).
In practice, the main lobe and the sidelobes have been modeled using a gaussian function.
Only the first sidelobes are simulated.
The figure below shows the reponse function of the 23cm antenna with sidelobes of –10db.
Figure 1 : 23cm antenna, sidelobes –10db
y1=1
-10dB=10log(y0/y1)
y0=0.1
q
Simulations
Simulations have been performed with MOLIERE.
The surface temperature is set 30K higher than the temperature of the very low atmosphere.
The Emissivity is set to 1.
To maximize the contrast between the atmosphere and the surface (and thus the contamination
by the sidelobes looking at the surface), calculations were performed at wavelength far from
any gasesous lines. Only the pressure induced CO2 continuum is taken into account.
Sensor altitude is of 400km.
Simulations have been performed with sidelobes of –10dB, -15dB, -20dB, -25dB, -30dB.
Results
Figure 2 : Difference between the signal measured by an antenna with sidelobes and the
signal obtained by a perfect antenna without sidelobes. Each curves correspond to a
different tangent altitude (se below)
In the studied case, the field of view (FOV) corresponding to the half power beam width for
the main lobe has a radius of about 4.2 km. The total field of view (first zero) radius is ~5 km
o The impact of the sidelobe is thus maximal around the tangent altitude 5 km (case
4 km and 6 km on figure 2) when the main lobe is only sensing the atmosphere, and
the side lobe is also sensing the ground
o Above or below this level, the contrast between the contamination by the sidelobe and
the main signal is reduced.
o The asymptotic behaviour for small sidelobes is not straightforward to understand.
The “contamination” by the sidelobes becomes negligible at -30dB when the main
lobe does not include any surface (cases 14, 12, 10 and even 6 km) whereas it can
remains significant (1.5 K) if the main lobe includes some surface (cases 4, 2, 0 km).
This is probably explained by the fact that the total solid angle of the sidelobes sensing
the surface is much higher in the later cases (TBC).
Conclusion
The conclusion is not straightforward, since there is no obvious threshold below which a
sidelobe contamination is acceptable. Nevertheless, 2 K seems to be a realistic choice. On this
basis. We can conclude from this graph that a design with –25dB sidelobes is acceptable (the
difference between –30dB and –25dB is not tremendous).
Comments an suggestions are welcome !