I understand the formula for calculating the far field distance from an antenna is:

Distance = (2D2)/λ

Is the D in the above equation the length of the antenna under test (antenna under test is a small PCB antenna)? In which direction (1 or 2 in the image below)?

Far field test set up

Whilst trying to research this, I came across an image [Source]

Image showing formula found online

But I was sure the formula for converting between wavelength and frequency was:

Frequency = 1/λ

Are there two formulas?


1 Answer 1


Are there two formulas?


\$\large\frac{1}{f}=time\$ (of one period)

\$\large\frac{c}{f}=\lambda\$ (wave length in meters)

\$D\$ is the largest length on the antenna: So, on a typical monopole, \$\frac{1}{4}\lambda\$
src. wikipedia

Based on your DUT image, (1), is the largest length.

  • \$\begingroup\$ how could I forget 1/f was period and not wavelength! As for the Wikipedia page you linked regarding D in the equation, I read this before posting my question and it states: 'where D is the largest dimension of the radiator'. Am I misunderstanding? \$\endgroup\$
    – MRB
    Mar 8, 2022 at 16:05
  • \$\begingroup\$ My bad, you are correct. Distance is R. D is the antenna dimension. Updated post. \$\endgroup\$
    – Aaron
    Mar 8, 2022 at 16:15
  • \$\begingroup\$ in which direction should this dimension be in? For example if it is a small PCB antenna, is D the length in the direction of the measurement antenna (1) I'm my image above or is D always the largest dimension, no matter how the PCB antenna is orientated? \$\endgroup\$
    – MRB
    Mar 8, 2022 at 16:17
  • \$\begingroup\$ If the antenna is electromagnetically short, \$D\$ doesn't matter, use the centroid. If the antenna is electromagnetically long, then "\$D\$ is the largest physical linear dimension of the antenna" (or diameter if a dish) \$\endgroup\$
    – Aaron
    Mar 8, 2022 at 16:26
  • \$\begingroup\$ sorry I am not understanding, if I have a PCB mount antenna, 'use the centroid', where am I measuring from, the centroid (centre of then antenna I am assuming) to where? \$\endgroup\$
    – MRB
    Mar 8, 2022 at 18:04

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.