PIM Characterization of RF Shielded Fully
Anechoic Chamber
Puneet Kumar Mishra, Satish Kumar Bandalmudi, Singh Kamlesh Satyadev, Purushothaman S.,
Renuka R., Nagesh S.K., M.R. Thyagraj, G.V.C. Rajan, and K.V. Govinda
Electrical Design & Measurement Division, System Integration Group
ISRO Satellite Centre (ISAC), HAL Airport road, Bangalore-560017, India
puneet@isac.gov.in
Abstract—Passive Inter Modulation (PIM) affects the operation
of high power satellites and sensitive receivers. Since satellite
uses multiple channels in a transponder chain it is required to
conduct PIM test at satellite level to ensure that S/C system is
free from PIM. These tests are generally performed in RF
shielded fully anechoic chambers. Before performing test it is
very important to know that whether PIM is generated from
satellite transponder chain or from external environment. So, it
is required to characterize inherent PIM level of the facility. In
this paper, procedure for PIM characterization of RF shielded
anechoic chamber and measured results are discussed.
Keywords—Passive Inter Modulation, RF shielding, anechoic
chamber
I. INTRODUCTION
When, more than one frequency f1 and f2 are passed
through non linear passive component, PIM products
±mf1±nf2 (where m and n are integers) are generated. Such
PIM products which have power level above the threshold
sensitivity of the receivers appears as interference in the
communication channel of the spacecraft. So, PIM analysis
and measurements have to be carried out in advance to avoid
such kind of failures. PIM can occur in waveguide joints,
antennas and reflector surfaces.
The various causes for PIM product generation and their
minimizing techniques are reported by Weinstein [1].
Schennum et.al.[2] had discussed sources of PIM generation
in Intelsat VIIA spacecraft and methods used to suppress
them. PIM tests will be generally done in RF shielded
anechoic chambers specially designed to cater PIM
measurements. One such facility to carry out PIM
measurement is reported in [3] and has operating frequency
range from 240 MHz-20 GHz. The RF shielded chamber at
ISAC, where it was planned to perform spacecraft (S/C) PIM
testing along with other communication payload testing, was
evaluated for any possible facility generated PIM products.
Mitigation of PIM products (if any) is a pre-requisite prior to
launch of the spacecraft. It is therefore important to
characterize the test facility to gain a confidence that it is not
prone to PIM generation and thus avoids the ambiguity about
the PIM generation and its source if seen during S/C testing.
The frequency of interest is to perform the PIM
characterization of the chamber at lower operating range of
the frequencies. Also, if chamber is qualified for lower
operating range of frequencies, its improved performance at
higher operating range of frequencies is guaranteed. Rawlins
et. al. [4] had reported PIM characterization of the ESTEC
compact range at 1.5 GHz. No one else has reported in open
literature this kind of characterization in the frequency range
of 200MHz-300MHz.This paper is divided into four sections.
In section II, brief overview of the RF shielded anechoic
chamber is discussed. In section III, Measurement setup and
procedure are discussed. Measurement results are discussed
in section IV and lastly conclusion of the work is discussed.
II. RF SHIELDED ANECHOIC CHAMBER
RF Shielded fully Anechoic Chamber at ISAC-Bangalore
is a state-of-the-art facility of its kind. It is primarily built to
carry out System Level EMI/EMC Tests on Spacecraft over
the frequency range from 200 MHz to 40 GHz, in accordance
with the MIL-STD-461E [5]. The chamber has dimensions of
19.5 m × 14.6 m × 16.0 m. The electric field shielding
effectiveness of the chamber is better than 100 dB over the
entire frequency band.
III. MEASUREMENT SETUP AND PROCEDURE
The measurement set up is shown in the figure 1. The two
signal generators and Spectrum Analyzer (SA) phase locked
by 10 MHz reference signal were kept outside the anechoic
chamber. The two frequencies of interest are considered as
243.955 MHz, 254.15 MHz and resulting 9th order PIM
frequency is 294.93 MHz. Two antennas Tx1 and Tx2 were
used as transmitting antennas and one antenna Rx was used
as receiving element. The test setup involved connector joints
between test elements. Two separate transmit paths were
adopted to ensure PIM will not generate from such joints.
Each signal path consist of signal generator followed by High
Power Amplifier (HPA). It was ensured that O/P from the
HPA is around 100 W. Appropriate cooling mechanism has
been employed for cooling of HPA. The requirement was to
identify PIM level of the order of -180 dBc at carrier power
of 50 dBm, which means SA shall be able to measure the
signal of -130 dBm or in other words noise floor should be
better than -130 dBm. Following precautions were taken
before start of the measurement. All the doors of anechoic
chamber were properly closed (To avoid unwanted
interference from outside EM environment). All
ferromagnetic materials on nearby measurement area were
removed. All threaded joints between the elements were
connected properly. The crane in the anechoic chamber was
kept at extreme end of the chamber i.e. in home position.
between the antenna and crane hook is 5 m. The
corresponding SA plot is shown in figure 4. PIM product was
observed and the level was -177.41 dBc. The measurement
was repeated at different positions of the receive antenna
inside the quiet zone. The observed worst case PIM level is
of the order of -165 dBc.
Figure 3 : PIM result (SA trace) when crane is in postion1
Figure 1 : Test setup for PIM characterization of anechoic chamber
IV. MEASUREMENT RESULTS
The measurement was done in two phases. In the first
phase, to measure inherent PIM level of the chamber,
following procedure was followed: 1. Crane is positioned in
its home place. 2. Receive antenna was kept on the walk way
absorbers. 3. Signal f1 and f2 were transmitted from the
respective antennas after required amplification. Reflected
signal from the RF absorbers was monitored on the SA.
Figure 2 : PIM result (SA trace) when crane is in home postion
Figure 2 shows the monitored signal on SA. It is clear from
Figure 2 that there is no PIM product upto -135 dBm level.
The cable used from the receive antenna to SA has a loss of
1.2 dB at the measured frequency. The power of the
carrier ( ) reaching the antenna input is 49.04 dBm. The
Gain of Transmit and Receive antenna (G) is same. EIRP of
carrier (
) is 56.54 dBm.
=
+
+ −
!"#
Hence it can be concluded that there is no PIM product upto 182.84 dBc level. In the second case, PIM is generated
intentionally by keeping the crane at multiple positions. Due
to paucity of space, observations of only two position of
crane are discussed here. Position 1: Crane was positioned
vertically above the antennas and distance between the
antenna and crane hook is 2 m. PIM product was seen and is
shown in figure 3. Using the equation 1 the measured PIM
level was calculated as -168.89 dBc. Position 2: Crane was
positioned vertically above the antennas and distance
Figure 4 : PIM result (SA trace) when crane is in postion2
CONCLUSION
A measurement procedure and precautions are listed in the
paper to characterize a RF shielded anechoic chamber for
PIM measurement. The measurement was carried out at a
carrier EIRP of 56.54 dBm and upto -180dBc there was no
PIM observed when the crane is in its home position.
Similarly any metallic structures which are in the vicinity
could induce the PIM and can be picked up by S/C receivers.
This clearly indicates that Facility can be successfully used
for accurate PIM measurements (upto -180 dBc) generated by
any DUT in the frequency range from 200MHz to 40GHz.
ACKNOWLEDGEMENT
Authors are thankful to Dr. S.K. Shivakumar Director,
ISAC, for his encouragement and guidance towards this
work. Authors are also thankful to Shri Rajeev jyoti, GD,
ASG, Shri. M.B. Mahajan, and Shri. Alok Kumar,
ASG/SAC, Ahmedabad, for providing necessary hardware to
conduct this work.
REFERENCES
[1]
[2]
[3]
[4]
[5]
David Weinstein, “Passive Intermodulation Distortion in Connectors,
Cable and Cable Assemblies”.Amphenol Corporation, Jan 2008.
G.H. Schennum, G.Rosati,“ Minimizing Passive Inter Modulation
Product Generation in High Power Satellites,” Proc.IEEE, 2006.
www.ets-lindgren.com
A.D. Rawlins, J.S.Petit and S.D. Mitchell, “PIM Characterization of
the ESTEC Compact Test Range,” European Microwave Conference,
Amsterdam, 1998.
MIL STD 461 E, August,1993