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 e_{r} of the dielectric
between the center conductor and the shield, or the velocity factor V_{f} 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.


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