# How to calculate far field distance from an antenna

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)? Whilst trying to research this, I came across an image [Source] But I was sure the formula for converting between wavelength and frequency was:

Frequency = 1/λ

Are there two formulas?

Are there two formulas?

No.

$$\\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.

• 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?
– MRB
Mar 8, 2022 at 16:05
• My bad, you are correct. Distance is R. D is the antenna dimension. Updated post. Mar 8, 2022 at 16:15
• 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?
– MRB
Mar 8, 2022 at 16:17
• 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) Mar 8, 2022 at 16:26
• 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?
– MRB
Mar 8, 2022 at 18:04