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elspec Whitepaper

Facts about PTFE coaxial cables - Whitepaper #1

Coaxial cables with low losses and a broad temperature range usually use PTFE (Polytetrafluoroethylene) as the isolating material. At elspec we also use PTFE: for the last 32 years it has been the most frequently-ordered basic material besides copper, silver and steel. It is indispensable as requirements have become in-creasingly high, and precise, stable measurements are a necessity.

Therefore, it is of particular concern for the RF-specialist elspec to give an overview of the latest re-search about, and applications of PTFE. We wish to share insights about the properties and the limita-tions of this material, and summarize the various publications on the topic.


Coaxial Plug Connectors in High-Frequency Applications

Coaxial lines lend themselves well to the wire-bound transmission of high-frequency signals. The have no lower frequency limit and thus are able to carry DC, as well. Moreover, the propagation velocity is constant across the whole useable frequency range as long as the cable is kept in the transversal electro-magnetic (TEM) mode.

A disadvantage is the additional attenuation in the dielectric between outer and inner conductors. Compared to a WR-90 waveguide with 10.8 dB/100m at 10 GHz, a 1/2” Flexwell cable can merely offer 40.6 dB/100m. On the other hand, the waveguide is solid pipe and it is not easily bent which is no issue at all for a coax cable.


Degradation of electrical properties of coaxial cables

At Helmholtz-Zentrum Dresden-Rossendorf (HZDR) two types of coaxial cable have been exposed to ionizing radiation from October 2015 to October 2016. The first sample was a 8 meter piece of Teledyne Storm Microwave Phase Master® 160 and the second sample a 10 meter piece of Harbour Industries LL142.

They were installed on the back of a beam dump at the ELBE FEL (U100) and connected with two times 5 meter long patch cable LL142. In this location the samples experienced a gamma radiation shower generated by the decelerated electrons in the beam dump - so called „Bremsstrahlung“.

Since both samples have been installed in this location at same time they experienced the same radiative field.


Return Loss

One of the essential features of high-frequency components and systems is the adaptation of the ports. A vectorial network analyzer (VNA) is generally used to define them. With this, adjustments of 60 dB can readily be measured.

These high adjustment values are not measured because the VNA has even better values but rather because its systematic errors are calculated by calibration with a known calibration kit. However, the individual parts of the calibration kit also introduce uncertainties, which also limit the measurement accuracy. It is also questionable whether an adjustment of 60 dB is the true value.


Voice of cables

The optimum measuring cable is not found by accident but is chosen after precise consideration of all relevant criteria.
How to pose the right questions, whether „more expensive“ equals „better“, etc. – we have put this together for you. To our aid came Instrumentation engineer Stefan Burger. Born in Freiburg, Germany, he owns the Delta Gamma RF-Expert consultancy in Hampton, Australia, and travels the world to give support for RF-solutions. The lab has the most sensitive test-devices ready for use. Staff have been trained extensively on the specifics, and conscientiously prepared for diverse new testing possibilities. Precision and extreme accuracy are a natural for each and everyone of the involved test engineers. The expectations are considerable since a significant budget has already been invested.


Simply Better Measurements

Preface | The lab has the most sensitive test-devices ready for use. Staff have been trained extensively on the specifics, and conscientiously prepared for diverse new testing possibilities. Precision and extreme accuracy are a natural for each and everyone of the involved test engineers. The expectations are considerable since a significant budget has already been invested. To ensure the quality chain continues, the very best cables, adapters and connectors should be used.

This is why elspec has developed the portable Premium Measurement Kit for you. It includes all the commonplace connection parts such as microwave cable assembly for 18 GHz, adapter N/SMA und SUCOFORM® 86 jumper cables with protective cap, plus the matching torque wrench - all compact but easy to use


VSWR of Electrical Cables

When measuring pre-assembled coaxial cables with the network analyzer, it is important to understand that the matching read-out on the analyzer results from the superposition of the signals reflected by the connectors on both ends of the cable. The dB (short for decibel) is a unit we constantly encounter carrying out such procedures.

Let’s have a quick look at the „decibel“ and how the calculation using it works: the Bel (that is the original designation) presents power relationships on a logarithmic scale. Since the Bel is somewhat cumbersome to use, a tenth of a Bel was adopted as a more conducive unit. Thus the deci-„Bel“ (dB) has established itself as the common measure.


Coaxial Cables – Build and Use

To connect signals from A to B one may use, for example, two parallel wires. In fact, this does work up into the HF-range if we keep our eyes on the impedance of this two-wire line. Disadvantageous is, however, that the field propagates around the wires and that everything coming close will have an impact on the transmission properties.

Waveguides, on the other hand, carry the electro-magnetic field inside a metal tube. What happens externally has no impact on the field. The disadvantage here is that there is a minimum frequency from which the energy transport is in fact possible. Moreover, such a conductor is very stiff and may not be bent easily



Torque wrench

The N-connectors were developed in 1942 and are therefore one of the oldest connectors still in regular use. Many things were not known at that time and so the system has its shortcomings.

This Whitepaper discusses the issue of secure bolting and what can happen if this is not ensured.


Semi Rigid

Semi-Rigid cables are mainly used for applications in high frequency bands up to 110GHz. The cable is unique and it is easily bent to finished shape and still maintains its set after bending. This property makes it ideal for the use with automated bending equipment as well as hand forming. Our vendors are qualified to MIL-DTL-17 and is listed on the Qualified Products List of the U.S Defense Logistics Agency.
A very special advantage of it is the 100% shielding with low VSWR, available cable diameters range from .020inch to .500 inch and available impedance range from 10 Ohms to 100 Ohms.