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RF CABLESThe Overlooked Satellite Component
Andrew Kurzrok and Maria CaliaAmphenol Times Microwave Systems
SmallSat 2021, Communications SessionSSC21-X-06
HOW COMPLICATED CAN IT BE?AN RF CABLE IS A PASSIVE MICROWAVE COMPONENT
CARRIES ANALOG SIGNAL FROM A TO BPerformance is well-characterized by undergraduate-level math
FAILURE IS COSTLYThe interconnect is the bridge to the entire system behind it• Communications antenna• Payload• Signal transport and processing
BUT, RF CABLES GO SOMEWHERE REMOTE, BY DEFINITION• Between isolated boxes• Into an exposed environment• Through complicated routings
WHEN FAILURE IS NOT AN OPTION
RF cabling design decisions can mean the difference between mission success and failure.
Surveys have found: • 17% of cubesat failures due to comms sub-
systems (1)• Suspected instances of cable failures
leading to mission loss (2)
All too often, we find engineers have specified RF cables that either do not optimize for system performance or risk failing during the mission.
References(1) Swarthout, Michael. “The First One Hundred CubeSats: A Statistical Look.” Journal
of Small Satellites. Col. 2, No. 2, pp. 213-233. (2) Venturini, Catherine C. “Improving Mission Success of CubeSats.” The Aerospace
Corporation, Report TOR-2017-01689. June 12, 2017.
3 RULES TO ENSURE YOUR RF CABLES AND CONNECTORS SUCCEED
DO NO HARM
UNDERSTAND THE TRADEOFFS
SIMPLIFY
1
2
3
1- DO NO HARM
Outgassing
Whiskering
Multipaction
Vibration
Radiation
Temperature
1 2 3
4 5 6
Do not compromise! Watch out for companies that have limited technology.
RF cables failure mechanisms can be managed through judicious selection of materials and manufacturing techniques.
For the best performance, it’s critical to have access to a wide variety of:
• Materials• Cable Constructions• Connectors Designs• Assembly Techniques
1- DO NO HARM
Outgassing
1
Select plastics that meet ASTM E595!
Cables often have plastics that can pose an off-gassing risk• The target number for acceptable TML is less than 1% and
for CVCM is less than 0.1%
Even if your mission is not sensitive to condensation of outgassed volatiles, most rideshare operators require outgassing compliance
1- DO NO HARM
Multipaction
2
Multipaction: a resonance effect that occurs in vacuum when RF fields accelerate electrons causing them to impact with a surface, which depending on its energy, will release one or more electrons into the vacuum.
Impact: loss/distortion of the RF signal and damage to the RF components or subsystems
Each situation is unique! • Connector geometry, signal frequency, ambient
temperature, and power level all matter
Mitigation: design connectors for multipaction resistance• Reducing the number and nature of the cavities • Overlapped dielectrics • Fully insulated interfaces
1- DO NO HARM
Radiation
3
Radiation exposure: degradation of a coax’s plastics, resulting in loss of performance
Flexible cables: TID <100 MRadSiO2 semirigid: TID 100 Mrad+
Location matters:• Inside the bus/shielded?• Mission profile?
1- DO NO HARM
Whiskering
4
Choose your materials carefully to do no harm.
Coax cables can be soldered or plated with pure tin, poses a whiskering risk
Lead alloys are common to avoid risk and should be specified
Spaceflight is exempted from RoHS
KNOW YOUR ENVIRONMENT
The impact on your dielectric
The Impact on your metal-expansion and the changes in electrical performance
The impact on your connector
2-UNDERSTAND TRADEOFFS
Attenuation vs. Mechanical 1Electrical vs. Environmental
1. Materials2. Temperature Range3. Radiation 4. Vibration
2
Mission Specific Considerations3
2-UNDERSTAND TRADEOFFS
Attenuation vs. Mechanical 1
All else equal: larger diameter cable = lower attenuation / length, but heavier and less flexible
Tradeoffs:• Conductor conductivity• Dielectric constant• Cable diameter
0
10
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30
40
50
60
70
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100
0 5 10 15 20 25 30
Att
enu
atio
n (
dB
/10
0 f
t.)
Frequency (GHz)
Attenuation of Various RF Cable Materials and Constructions
Solid PTFE (RG-402), 0.141" OD LD PTFE (SPFLT-140), 0.139" OD TF4 (PT-140), 0.144" OD
Solid PTFE (RG-304), 0.280" OD LD PTFE (SPFLT-310), 0.310" OD TF4 (PT-318), 0.315" OD
2-UNDERSTAND TRADEOFFS
Electrical vs. Environmental2Considerations over:
Temperature• Attenuation• Phase→the PTFE “knee”
Radiation• Shielding• Non-plastic dielectrics such as SiO2• Mission length/reliability expectations
2-UNDERSTAND TRADEOFFS
Mission Specific Considerations3The tradeoffs described so far are not exhaustive. Depending on the mission, other critical considerations include:
• Thermal stability of the cable
• Magnetic moment performance
• Vibration
3- SIMPLIFY
Multiport Connectors
Marker Bands
Simplify assembly, installation, and test by taking advantage of:
Keying
Go/No Go connectors
3 – SIMPLIFY
RISK MANAGEMENT AND DEPENDABILITYBad things happen. Can your supplier adjust quickly?
GET THE PERFECT SOLUTION AS QUICKLY AS A COMMERCIAL OFF THE SHELF OPTION
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