# Re: [PEDA] Off topic / Need formula to calculate F in Mhz > Wave length.

```At 04:11 PM 11/07/01 +1000, you wrote:
>This is fine for free space, but if you're trying to calculate the
>wavelength in something like coax you have to take into account the change
>in velocity:
>
>
>Wave Lenght (in meters) = (VF.c)/f
>
>Where VF = velocity factor (about 0.6 for RG59)

VF is 1/Sqrt(Er) - so for microstrip lines you can determine VF from
Er(eff) - effective *relative* dielectric constant.  This is assuming that
the magnetic characteristics of the material are essentially the same as
free space (mu-zero) if not then the VF becomes 1/(sqrt(Ur*Er))

where Ur is the effective relative mu (permeability), and Er (epsilon) is
the effective relative dielectric constant (relative permittivity)

>         c = speed of light in vacuum in m/s

which is precisely (as in no error) 299 792 458 m/s.

>         f = frequency in Hz

There ... the pedant in me is sated for a few months...
Ian Wilson

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