PROBE STATIONS  •  TEST SERVICES  •  CABLE ASSEMBLIES

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D-COAX, INC. • P.O. Box 999

Newberg, Oregon 97132  USA

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Coaxial Cable

This coaxial cable could be improved at the design phase in a number of ways.

 

a.) Conductor losses are the starting point of a coaxial construction.

1.) Pick the very largest center conductor that still meets the other requirements.

2.) Center conductor conductivity should be the very highest. (Refer to table 7 for metal W / sq.cm.)

3.) Center conductor should have the flexibility for the application.(solids for high freq. and stranded for flex life)

4.) Center conductor compatibility with termination requirements. Some industry standard connectors have size limitations. The customer may tolerate a certain size center conductor and no more.

 

b.) Dielectric machine-ability, mould ability, extrudability, tape wrap process (whichever is applicable)

1.) Dielectric Cost.

2.) Process friendly (yield)

 

b) Pick the very largest center conductor that still meets the other requirements.

1.) Center conductor conductivity should be the very highest. (Refer to table 7 at bottom for metal W/ sq.cm.)

2.) Center conductor should have the flexibility for the application.(solids for high freq. and stranded for flex life)

3.) Center conductor compatibility with termination requirements. Some industry standard connectors have size limitations. The customer may tolerate a certain size center conductor and no more.

 

c) Pick a shield coverage at 100% or as close to it allowed by design constraints.

1.) For braid wire shields, the strands grouped together, then layed across the next group are measured in strands per inch (picks).  This diamond shape openings in this pattern will allow high frequencies to radiate, reflect, and dwell to cause performance limiting behavior. Typically these phenomena begin in the earliest stages at 50 MHz.

2.) Serve shields may be used to increase flexure. Sometimes serve shields allow openings when the cable is flexed and must have a second shield in the opposite lay direction to abate problems. Many (a lot) of low angel parallel shield wires layered one atop another have demonstrated the (Longitudinal to transmission) least area paths for the current. Further more, it allows for 100% coverage since all the multiple layers aid each other when the cable is bent.

3.) Foil shields have been used alone and with a drain wire piggyback to allow for termination to ground. These shields are not usually meant for high flex life applications. At frequencies above50 MHz, the overlapping of the shield may provide a bigger surface area for the current path and create more loss and reflections.

4.) Extruded copper tube (semi-rigid coax) has been the benchmark for low leakage, low loss, low reflection coaxial cable. The shield is highly polished (on the inside) longitudinally to the transmission path and the shield has no openings, overlaps, or drain wires.

 

The measurement range can be improved in resolution, stability, bandwidth, and non-aliasing.

a) The magnitude resolution for very small magnitudes may be enhanced by:

1) Increasing the quantity of measurement points. (Looks at more frequencies so that small problems in the cable will not be skipped or smoothed out)

2) Adding averaging. (will stabilize each point before finalizing the location on the screen)

3) Follow all connector cleaning and torque protocols. (will reduce reflection and repeatability errors)

 

b) The magnitude resolution for very large negative magnitudes may be enhanced by:

1) Add a low power amplifier on the input to the D.U.T. and sufficient attenuation (padding) on the output of the D.U.T. to protect the Analyzer from damage.(will provide enough needed signal through the D.U.T. to be measured by the Analyzer at extreme levels.)

2) a)  1) through 3).

 

c) Bandwidth resolution can be enhanced by maximizing the IF bandwidth, increasing averaging, increasing sweep time, and increasing the amount of points. Breaking up the full bandwidth of the Analyzer into smaller segmented calibrations will also provide high resolution.

 

d) Non-aliasing will be discussed to some length in the Time Domain section. (Eliminating as many Reflection-causing problems in the connectors and D.U.T. will be a major help.)