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

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Att

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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?

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