Telecoms, networking & broadcasting
Technical
Radio frequency interference
measurements
by Gerhard Petrick
A group of South Africans have risen to the challenge of building a measurement system with unrivalled sensitivity and frequency range and
successfully operating and deploying this delicate system to remote and rugged areas in Southern Africa.
South Africa is one of four countries that submitted
proposals to host a new generation radio
telescope. Radio astronomers and researchers
from across the globe are collaborating in a
mega-science project that aims to establish a
gigantic radio telescope with a collecting area
of one square kilometre. The Square Kilometre
Array (SKA) telescope project aims to establish a
science instrument that will be a 100 times more
sensitive than any radio telescope in operation
to date. Groundbreaking scientific discoveries
are expected.
A key component of the bid to host the radio
telescope relates to an assessment of the
radio environment. Just as light and poor air
quality would interfere with and limit optical
astronomy, radio signals from broadcast- and
telecommunications transmitters limit the
ability of a radio telescope to detect very weak
signals from space. To secure the ability to do
groundbreaking science the SKA would have to
be sited in a very radio-quiet environment.
Bidders to host the SKA telescope were
consequently required to run 12 month radio
frequency interference (RFI) measurements at
their respective candidate sites. An independent
team of experts from the Netherlands would
visit the candidate sites in South Africa, China,
Australia and Argentina and perform an
independent measurement as verification of
the local results.
The South African SKA project office decided to
include measurements at test points around the
Parameter
Specification
Frequency range
70 MHz - 26.5 GHz
System temperature
< 300 K
System gain
60 dB
Operating temperature
-10 oC to 45 oC
Deployment
Mobile via 4 x 4 terrain
Continuous
38 hours (mode 1)
measurement duration
258,1 hours (mode 2)
32
Fig. 1: Block diagram of RF assembly.
Fig. 2: RFI measurement point established in December 2004.
core site as well as potential remote sites that will
be required for the telescope.
The prescribed measurements required an
instrument set-up with specifications well beyond
that of even highly specialised military gear. A
team of experts at the Hartebeesthoek Radio
Astronomy Observatory assembled the first
of three measurement systems to be used in
the South African measurement campaign in
October 2004.
The South African measurement system was
built around an Agilent spectrum analyser.
Two antennas from Rohde & Schwarz coupled
to a network of low noise radio frequency (RF)
amplifiers and switches were used to cover the
measurement frequency range.
A pneumatic mast was used to lift the RF assembly
to the prescribed height and an elevation- and
azimuth rotator to facilitate automated antenna
pointing. A switching control system was
designed and a PC used to provide the system
control and data-logging functions.
Whilst the Hartebeeshoek team prepared
the measurement systems a detailed study
August 2006 - EngineerIT
of the radio frequency spectrum utilisation
was commissioned. With the support of the
Independent Communications Authority of South
Africa (ICASA) and the respective spectrum users,
a comprehensive data-base of transmitters in
South Africa was compiled. The data-base with
more than 270 000 entries was geo-referenced
allowing the display of each transmitter and
its parameters on interactive maps within a
geographical information system (GIS).
Fig. 3: Screen-shot of sporadic emissions as recorded
during site establishment. (The trace shows sporadic
short-term emissions captured using the
max hold function).
The South African RFI measurement team was
the first in the field. Measurements commenced
at the candidate core site in the isolated Karoo
near the town of Carnarvon in the Northern
Cape.
Fig. 7: Spectrum analyser in EMC enclosure.
An air-conditioned office container provided
shelter to the measurement team and
equipment and power was provided from a
20 kVA diesel generator. A mobile pneumatic
mast installed on a trailer was positioned to the
south of the container and the RF assembly and
measurement antennas mounted thereon.
Fig. 4: Bonded and grounded rotating collar.
Fig. 5: Copper loop in Gazebo pole.
Whilst one was conscious of the demands, the
uniqueness of the system and the specifications,
operating a system with an amplification factor
of close to 1-million brought with it a number of
unforeseen challenges.
A few manual measurements highlighted a
large number of signals that were not listed on
the transmitter database for the core site. Whilst
the sensitivity of the system made it possible to
accurately detect transmitters from as far away
as Kimberley (414 km) the presence of signals
not listed on the database suggested that the
team was measuring effects caused by the
measurement system, human activity and other
equipment on site in addition to the real radio
environment. These signals were all below 1 GHz
and had not been observed during tests in the
cluttered and busy radio environment at the
Hartebeesthoek radio astronomy observatory.
It was essential that these signals were eliminated
as their presence suggested a poor radio
environment at the South African candidate
site. The signals could be categorised as either
sporadic or continual.
Fig. 6: Screen-shot of sporadic emissions as traced
back to the Gazebo frame. (The yellow trace shows
normal static conditions with the blue trace showing
sporadic short term emissions captured using the max
hold function) Note: The broadband noise due to
equipment self-RFI is evident in the yellow trace.
34
Fig. 8: UPS in EMC enclosure.
Sporadic emissions, although more difficult to
trace, were identified as most detrimental to
meaningful RFI measurements. These sporadic
emissions related mainly to short-term emissions
or static discharges. Viewed over time, these
emissions would effectively lift the noise floor
between 150 MHz to 450 MHz by more than
20 dB.
Continued observations confirmed that some
sporadic emissions could be linked to human
Fig. 9: System 3 in ISSA Houwteq EMC chamber.
activity on site. Static discharges from a casual
brush through the hair or over clothing caused
signals significantly stronger that the real
transmitters being measured.
Other sporadic emissions were linked to the
wind blowing. On-site investigations identified
the rotating collar on the pneumatic mast as
the origin of some of wind-related emissions.
The assembly which consists of a metal collar
on nylon bushes was located very close to the
receiving antennas and created significant
emissions with movement caused by the wind
vibrations. The problem was dealt with by
August 2006 - EngineerIT
Fig. 10: System 2 at the Karoo core site.
Fig. 12: Actual RFI measurement points visited (Orange Pentagons) and theoretical SKA sites (8 February 2006).
bonding the collar and mast sections together
and grounding these.
frame was removed from site and the wind
related emissions eliminated.
An extensive grounding system was installed but
sporadic emissions linked to the wind blowing
remained. These emissions were finally traced
back to a Gazebo frame that had been set-up
as vehicle shade close to the measurement
antennas.
A number of continual and equipment related
emissions were also identified.
With the
assistance of experts from the ICASA regional
office in Bloemfontein these static emissions
were tracked back to a vehicle alarm system,
PC switching noise, harmonics of oscillators, a
control chip oscillating and the UPS.
The modern Gazebo the frame components
and pipes are factory fitted with bungee cords
to simplify setting up. The sections of bungee
cord are connected with short copper loops.
The copper loops within the pipes effectively
constituted inductive and capacitive radio
frequency
components.
Measurements
confirmed that vibrations on the Gazebo frame
resulted in short impulse-like noise spikes from
150 MHz to well beyond 450 MHz. The Gazebo
Although significant care had been taken to
make the components in the measurement
system “radio quite” the sensitivity of the system
was such that these measures did not suffice.
Follow-up lab measurements highlighted
amongst others that the spectrum analyser
- although meeting onerous military electromagnetic compatibility (EMC) specifications
- was itself radiating signals detectable and
objectionable to the SKA
measurement system.
Fig. 11: Joint ICASA and SKA South Africa RFI measurement team
during training programme.
EngineerIT - August 2006
It became clear that
meaningful RFI measurement
would require significant
reduction of the unwanted
signals.
The mitigation
measures applied would
have to ensure absolute
reliability and EMC integrity
of the system even it if were
to travel over the rough
Karoo roads. In addition
the uncertainty associated
with signals generated from
human activity would have
to be addressed through
automation.
The system design was consequently re-visited
and transformed into an autonomous selfcontained mobile system. The entire mobile
system with generator and air-conditioning
unit could now be assembled, tested and
characterised in an EMC chamber before
deployment into the field.
The South African consultancy MESA Solutions
was engaged to assist the SKA team in mitigating
self-generated RFI. MESA, in collaboration with
SKA team members, conducted RFI mitigation
studies on the SKA mobile unit systems. Based on
the results of these studies, various remediation
measures were successfully implemented on
SKA’s three mobile units. These included:
The use of a removable laptop PC for setting up
and starting the measurement control system
PC via a network connection. This eliminated
the need for a permanently installed monitor,
keyboard or mouse, all of which produce
significant RFI.
Installation of the spectrum analyser, commercial
PC, UPS and control electronics in specially
designed EMC enclosures that contained the
RFI identified from each unit.
Sealing of any connection points of the controland RF cabling to ensure that no RF noise
escapes.
The three mobile unit systems were qualified
in terms of RFI performance in the anechoic
EMC chamber at the Institute for Satellite and
Software applications (ISSA) near Cape Town.
The 100 dB isolation of the chamber and
its large physical size allowed for the testing
of the systems fully assembled and with the
35
antennas at the 5 m measurement height for all
operational modes.
The RFI was evaluated for the prescribed
measurement state with the units’ air-conditioner
in various operating modes and with both mains
and diesel generator supplies.
A specialised group of ICASA employees working
in the ICASA regional offices volunteered to
participate in the project. These individuals are
all highly skilled RF measurement experts with
years of experience in identifying, tracking and
eliminating RF interference.
Very
low-level
RFI
signals
at
a
handful
of
frequencies
between
70 MHz and 26 GHz were characterised and
the optimal operational mode (air-conditioner
on and in cold mode) implemented in the
standard operating procedure.
A detailed standard operating procedures
(SOP) for joint ICASA and SKA South Africa RFI
measurements was compiled. These procedures
ensured that the scheduled measurements
would yield reliable, repeatable and operatorindependent results.
The fully mitigated and characterised System 2
was deployed back to the core site in July 2005.
The fully mitigated and characterised System 1
and System 3 were deployed for measurements
around the central area and remote stations in
August / September 2005.
At a time where ICASA receives very little positive
press coverage it is great to report that these
ICASA representatives did South Africa proud
in getting the measurement systems deployed
and returning reliable results in record time.
The South African RFI measurements campaign
was conducted in close co-operation between
the South African SKA Project Office and
representatives of ICASA.
36
In addition to the 12 month measurements at
the Karoo core site RFI measurements were
conduced at more than 35 points around the
core site and included planned remote SKA
sites in Botswana, Mozambique and Namibia.
The measurement results were consolidated
and a comprehensive RFI report submitted to
the international SKA project office in March
2006.
A follow-up article will outline the findings of the
study.
The international SKA project office is currently
evaluating the respective bid documents
received. A short-list is expected in September
2006. A final decision on the SKA host country is
expected in 2008.
For more information visit www.ska.org.za
Gerhard Petrick worked as RFI project manager
with the SA SKA project office. He is currently
head of technology at the National Electronic
Media Institute of South Africa and writes in his
personal capacity.
Contact Gerhard Petrick,
Nemisa,
Tel (011) 484-0583,
gerhard.petrick@gmail.co.za ☐
August 2006 - EngineerIT