Semi-rigid coax

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New for June 2021. Semi-rigid (SR) coax is a workhorse of the test equipment industry.  You can bend it to fit your application, and it stays bent, and if you keep it near room temperature, it can be phase stable. You can even add some hardened contacts to a dollar's worth of SR cable and sell it for $1000 as an RF probe!

Smaller diameter semi-rigid coax has copper inner and outer conductors, while larger cables may have steel inner conductors that are plated with copper. Steel is lossy but make the cable more rigid. It is easy to predict the loss of a pure copper cable, but you should rely on manufacturer's data for any cable that has a steel inner conductor.

SR cables often use PTFE dielectric (ER=2.02). In some cases some air is used to form tiny bubbles in the PTFE, which is called foamed PTFE. Speaking of foam, let's take a trip back in time to when National Lampoon was a magazine, during the formative years of the Unknown Editor, and way before the name was applied to any of the tragic "vacation" movies.  Timberland Tales was a comic feature that included an Indian boy named Maurice, and his dog Foamy.  You can see an image of Foamy, Maurice and Constable Tom (some say he had a bit of brain damage) on this web site. Why is this important? It's not.

Microwave cables are a huge industry.  Most people who work on cables know very little about RF, and it often shows in the data sheets.  We looked a pile of data sheets from a reputable vendor, and found that the cut-off frequencies reported are often wrong (in some cases, by a factor of more than 10). So we generated our own table, below. Some of the cables have foamed PTFE, which we estimated the dielectric constant to 1.71. You can use coax all the way to its cut-off frequency, but you will also need to pay attention to which connectors you intend to attach to it. We prefer to stick with the "85%" rule. We only listed cables that are nominally 50 ohms, but of course you can get other impedances, notably 75 ohms.

Velicity of Propagation Size ER Outer conductor inner diameter "D" Inner conductor diameter "d" Shell thickness Cut-off frequency Cut-off frequency 85% rule Calculated Z0
  Mils   inches Inches inches GHz GHz Ohms
70.4% SR-020 2.02 0.015 0.004 0.0025 278.25 236.51 55.74
70.4% SR-034 2.02 0.026 0.008 0.004 155.49 132.17 49.70
70.4% SR-047 2.02 0.037 0.011 0.005 110.14 93.62 51.15
76.5% RF-070F 1.71 0.059 0.02 0.0055 72.73 61.82 49.58
76.5% SR-086F 1.71 0.066 0.023 0.01 64.56 54.88 48.31
70.4% SR-'086 2.02 0.066 0.02 0.01 61.47 52.25 50.35
76.5% SR-118F 1.71 0.097 0.032 0.0105 44.54 37.86 50.83
76.5% SR-141F 1.71 0.118 0.04 0.0115 36.37 30.91 49.58
70.4% SR-141 2.02 0.117 0.036 0.012 34.55 29.37 49.70
70.4% SR-250 2.02 0.209 0.064 0.0205 19.37 16.46 49.90

Coax impedance equations are on this page, and a more accurate look is on this other page.  Coax cut-off frequency equations are available on this page. And of course, you can use our on-line coax calculator to solve for both of these coax parameters.

Next, we'll use the Microwaves101 coax spreadsheet to calculate the attenuation of these cables.  Oops, ran out of time, check back soon!



Author : Unknown Editor