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PTFE INSULATED
RF COAXIAL CABLES PER
U.S.A. MIL-C-17
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PTFE (Teflon®) is known to be the best Dielectric for RF Coaxial and Triaxial Cables.  We offer these cables as per USA MIL-C-17 or IndianJSS 51100 ranging from subminiature 1.0 mm OD and standard miniature to high power sizes. There is also a choice of flexible or semi-rigid constructions, Characteristic Impedance (Zo) of 50, 75 or 95 ohms (or as specified), solid or semi-air-spaced PTFE, and Low-noise (anti-microphonic) cables.

Please click here for a listing of some common items, with construction and typical values of various parameters. 

PTFE Insulated RF Coaxial Cables

COAXIAL CABLE IS A TRANSMISSION LINE

The essential function of coaxial cables is to transmit high frequency energy and signals with low loss, with reflectionless matching and without Phase and Amplitude distortion.  For such transmission, Characteristic Impedance and Attenuation are important concepts.  Characteristic Impedance (Zo) is determined by the ratio of dielectric OD to conductor diameter, hence a large cable and a very small cable can have the same Zobut the smaller cable will have higher losses.  The problem in high energy transmission is the losses because of i) cable dielectric, ii) cable construction, iii) radiation, and iv) interference with adjoining circuits. 

Coaxial cable cross sectionThe coaxial cable construction has been invented to overcome these problems, in which signals and energy are contained within an enclosed space.  It consists of a core conductor, surrounded by pure PTFE dielectric (solid or foamed or air-spaced), covered with a metal shield, and finally encased in an overall jacket.  The shield provides the return path as well as the confinement for RF energy, and is generally grounded.  Transmission through coaxial cables is called an unbalanced transmission because the centre conductor and the shield are not reversible (as opposed to the balanced construction of atwisted pair). 
 

 

Twister pair cable
 a) Two-wire twisted-pair cable     b) Two-wire twisted-pair shielded cable

Shield can be SPC (silver plated copper) round wire single or double braid, round wire served (helically wrapped), flat foil served (helically wrapped with drain wires), foil and braid combination with drain wires, and other materials. 

Braid shield
Braid Shield

Round wire served shield
Round Wire Served Shield

Jackets provide outer cover to keep the cable clean, smooth and to prevent ingress of moisture into shield, as well as for isolation of shield.  Materials used are PTFE (our specialty), VFG (Fibre Glass braid covered with lacquer Varnish, over sintered PTFE tape moisture seal), PVC or PU.  High temperature melt extrudible jackets such as FEP (Fluorinated Ethylene Propylene) and PFA (Per Fluoro Alkoxy) are also likely to be available in future.

PTFE Jackets over Coaxial Cables are Difficult -- But They are our Specialty

In early versions of MIL-C-17, the miniature PTFE coaxial cables were specified with PTFE jacket.  However, PTFE jackets on coaxial cables are very difficult to process.  This is because when the jacket is cured at high temperature, the core dielectric also undergoes expansion and bites into the metal shield.  This tends to disturb the delicate ratio between the dielectric OD and the conductor diameter.  During the years, we have gained considerable insight into the complex behaviour of coaxial cables with PTFE jackets and can exercise control over the ultimate performance to a great extent.  Due to the difficulty with PTFE jackets, MIL specs moved on to melt-extrudible FEP (or other materials) jackets, and also added the requirement of sweep testing of coaxial cables (ascertaining the behaviour of cable at a large number of frequencies in the specified range).  Our listing of coaxial cables groups various items with similar size and properties together for FEP, PTFE, VFG (Varnished Fibre Glass) jackets etc. 

100% Testing of Coaxial Cables

Coaxial cables offered by us are 100% tested on a Time Domain Reflectometer (TDR) to control Characteristic Impedance within the permissible range (say, 50 +/- 2 ohms, 75 +/- 3 ohms etc.).  We also ascertain the Attenuation (loss) by insertion method on representative samples.  This test is done without the use of connectors and is a worst case situation.  With the proper connectors on both ends of the cable, the actual insertion loss will be even lower.  It is worth mentioning that for most common coaxial cables, the actual insertion loss of cables processed by us is 10 to 25% lower than the permissible maximum given in US MIL-C-17.  Please refer to the list of coaxial cables for a comparison of typical measured loss with that specified by MIL-C-17.

Test set up for RF Coaxial Cables
Coaxial cables Test set up including 
TDR, Power Meter, Signal Generator, Frequency Counter and Capacitance Meter
 

Typical 50 ohm wave form on TDR
Typical 50 ohm wave form on TDR

SUBMINIATURE RF COAXIAL AND TRIAXIAL CABLES

In addition to the range of standard RF coaxial and triaxial cables as per MIL-C-17, we also offer Miniature and sub-miniature coaxial cables which are useful for high density RF circuitry with soldered connections (without connectors) to help reduce mismatch, reflections, radiations, attenuation and RFI.

Centre conductors are single or multi-strand SP HSCA (Silver Plated High Strength Copper Alloy) or SPCCS (Silver Plated Copper Coated Steel); smallest size is 0.125 mm, multistrand 7/0.4 mm.  The dielectric is pure PTFE concentrically multi-layer wrapped and sintered.  The addition in radial thickness due to shield and jacket is about 0.125 mm for SP HSCA foil helix and 0.2 mm for round wire helical shield.

Typical cable diameters are : For 50 ohms 0.9 mm; for 75 ohms 1.5 mm; and for 95 ohms 2.5 mm.  The cables are fully tested on Time Domain Reflectometer (TDR) to within+ 10% (or better on demand).

Similar constructions have been suitably incorporated in the manufacture of low microphonic noise cables (less than 5 microvolts) for sensitive transducer interconnects and very low capacitance cables for instrumentation (e.g. 1.8 mm OD with 40 pF/m).

SPECIAL FEATURES  These cables are supple enough to be used in delicate systems (such as Echography), and yet far more reliable than corresponding expanded/unsintered PTFE tape wrapped cables in respect of crush-resistance and long-term stability in hostile surroundings.  Manufacturing experience of hundreds of kilometers over past several years and the established low prices will permit the designer to create new systems or upgrade technology for improved reliability and performance in existing systems.

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