# Calculating field strength mV/m at distance d - check me

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1. Jul 8, 2015

### Fran3

it's been awhile for me so check if I'm remembering this right... Thanks.

electric field strength is inversely proportional to distance squared... right?

So if the electric field strength is 1,000 mV/m at 1 meter then the following is true... right?

km______mV/m________ the math
1________1000 .00______=reference point
2_________250.00_______ =1000/(2^2)
3_________111.11________=1000/(3^2)
4__________62.50________=1000/(4^2)
5__________40.00________etc

thanks for any help.

2. Jul 8, 2015

### Staff: Mentor

...from a point charge. If the source of the field is a point charge, or at least small enough compared to the distances involved, that a point is a good approximation, then your calculations are correct.

3. Jul 8, 2015

### Orodruin

Staff Emeritus
This depends on your setup. It is true for a point charge or a spherically symmetric charge distribution.

4. Jul 8, 2015

### Fran3

In this case we are talking about field strength of a radio signal being radiated from an omni-directional antenna.

In the example the radio signal measures 1,000 mV/m at a distance of 1km and we want to calculate the field strength at different distances.

Ignoring any mitigating factors such as attenuation through structures or whatever... are our calculations correct for what we could consider near-field distances and also characterize as "ground wave" distinguishing from "sky wave" or "skip".

thanks for the help.

5. Jul 8, 2015

### Staff: Mentor

This is different from the electrostatic field of a point charge. In this case, in the far field the amplitude of the electromagnetic wave varies as 1/r, not 1/r2. It's the radiated power density (W/m2) that varies as 1/r2.

6. Jul 8, 2015

### tech99

A convenient formula for the electric field strength from a grounded monopole antenna in mV/m is (300 sqrt P) / D, where D is distance in km and P is power in kW. It is necessary to be very careful about ground losses, especially where the frequency exceeds about 1 MHz.

7. Jul 8, 2015

### Fran3

So if
P=4kW
D=2km
electric field strength = ([300*sqrt(4)]/2) = 600/2 = 300 mV/m

and for D=4km
electric field strength = ([300*sqrt(4)]/4) = 600/4 = 300 mV/m

correct?

and thanks !

8. Jul 8, 2015

9. Jul 8, 2015

### davenn

10. Jul 9, 2015

### Fran3

Woops! When you cut and paste you should complete all the edits in the pasted text. Yep, should have read...

So if
P=4kW
D=2km
electric field strength = ([300*sqrt(4)]/2) = 600/2 = 300 mV/m

and for D=4km
electric field strength = ([300*sqrt(4)]/4) = 600/4 = 150 mV/m

And Dave, thanks for the link. Very handy.