[TowerTalk] A fundamental antenna question

[Bob Nielsen]

On Sun, Jun 26, 2005 at 02:08:48PM -0400, Wes Cosand wrote:
> Folks:
> 
> I am embarrassed to ask a question I should be able to figure out but my 
> Handbook and Antenna Book aren't proving sufficient to overcome my ignorance.
> 
> How can I calculate the field strength in volts/meter as a function of 
> distance from an isotropic antenna emitting a given power in watts?  (Let's 
> ignore the near field/far field issue.)  Where does a ham go for this kind 
> of information?
> 
> Thanks
> 
> Wes, WZ7I

Wes,

I haven't come across any texts which discuss this, but you can look at 
the situation as a sphere with a radius equal to the distance from the 
antenna which has a uniform power density.  Calculate the surface area 
of the sphere (4 PI R^2) in square meters. Divide the power radiated by 
this value and you have the power density in watts/square meter.  Since 
E^2 = RP, you multiply the power density by the impedance of free space 
(377 ohms) and take the square root of that result to get the field 
strength in volts/meter.

73, Bob N7XY

-- 
Bob Nielsen, N7XY                          n7xy (at) n7xy.net
Bainbridge Island, WA                      http://www.n7xy.net