Coax Cable Cutoff Frequency

This worksheet calculates the highest frequency at which a given coaxial cable should be used. This is called the "cutoff" frequency. Tom WA1MBA provided the formulae, one of which is 6th order, which provide 0.05% calculation accuracy. He also provided sanity checks along the way during development of this web page. It turns out that the accuracy of the results are more dependent upon the data entered than the accuracy of the equations employed (see the "EXAMPLE" at the bottom of this page).

Data required for entry includes either the dielectric constant er of the dielectric between the center conductor and the shield, or the velocity factor Vf of the coax type must be known. Additionally, since the diameter of the center conductor and of the shield are not generally published in the literature, these must be measured. Since the shield fits tightly around the dielectric, the outside diameter of the dielectric will suffice for the inside diameter of the coax shield which is the actual dimension of interest for this calculation.

NOTE:  The "cutoff" frequency calculated here is that frequency below which coax acts like coax and NOT like a combination of coax and waveguide. Coax will pass RF at higher than "cutoff" but does so in a nearly haphazzard way because of the waveguide and TEM modes traveling simultaneously but at different velocities.

The dielectric constant question MUST be answered, the units of the dimensions MUST be selected, and all entries made before the calculator will work.
Do you know the dielectric constant (er) of the dielectric between the center conductor and the coax shield?
YES     NO

If "YES" click on box and enter the dielectric constant er:   
If "NO" click on box and enter the cable velocity factor Vf:  

Select units of center conductor and dielectric dimensions:

inches     mm

Click on box and enter O.D. of dielectric:               
Click on box and enter O.D. of center conductor: 

Click on and scroll down for results!

The following are intermediate calculations useful for "sanity checks".

EXAMPLE:  Check out RG-400. This is an RG-58-sized coax, all teflon insulation and a good quality braid shield. The published velocity factor is 0.695, while the dielectric constant for the teflon used is 2.1. The center conductor diameter is 0.05"/1.25mm max, and of the dielectric, 0.118"/3mm max. These are all "published" figures. Run the worksheet and check the results:

  • Using er and inches, Fco = 31.651 GHz
  • Using er and mm, Fco = 31.771 GHz
  • Using Vf and inches, Fco = 31.877 GHz
  • Using Vf and mm, Fco = 31.999 GHz

So, don't use RG-400 above 31.5 GHz and you should be OK!

NOTE:  If one converts the published dimensions in mm to inches (divide mm by 25.4), the values become 0.11811" for O.D. of the dielectric and 0.04921" for O.D. of the center conductor, and the cutoff frequency using er is calculated as 31.772 GHz, only 0.001 difference from using mm. Clearly the number of significant digits the manufacturer chooses to use before rounding off the value of and publishing the parameters will impact the perfection of the calculation results.....

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