[600MRG] Definition needed - V/m

[Claude Baker]

Ken,

Volts per meter is a measure of field strength. The voltage you get out of an antenna is directly related to the effective AREA of the antenna. You may have noticed that the formulas for gain of a loop antenna is directly related to the area enclosed by the loop. For a given piece of conductor, you get more signal from one turn than from using the same conductor to form a multi-turn loop.

The notion of volts/meter goes back to radiated power and distance from the transmitting antenna. Engineering texts generally start with an isotropic antenna which doesn't exist anywhere except in one's imagination. The isotropic antenna, equivalent to a pin point so small that it can't be measured, radiates power equally in all directions, i.e. into a sphere.

If you put the isotropic radiator inside a sphere, the power falling on 1 sq meter of the surface is given by
Pr = Pt/(4*pi*R^2)
Pr = power received
pi = 3.14159...
Pt = total power transmitted
R = radius of the sphere or distance from the transmitting antenna in meters.
By the way, the surface area of a sphere is 4*pi*R^2

This formula gives you the watts/meter^2 at any distance from the antenna.

Now to get to volts/meter we use one of the formulas for power: P = E^2/R, or power equals voltage squared divided by the resistance.

The resistance of free space is 120*pi or approximately 377 ohms so:
Pr = E^2 / (120*pi)

Again, Pr = power present on 1 meter^2 at some distance from the radiator.
Solving for E results in
E = sqrt(120*pi*Pr) volts/meter

I have some experience with E-probes, they can be easily calibrated to produce an output directly related to signal strength in volts per meter. An E-probe antenna is a capacitor; for instance a 2 meter rod 3/8" diameter has a capacitance of approximately 20 pF to ground. If this "whip" is connected to the input of an amplifier with an input capacitance of 20 pF there is a capacitance voltage division by 2. With this setup, the effective input to the amp is a voltage equal to the the field strength in volts per meter. Add a little gain and you easily measure uV per meter. The amp input needs to be very high input impedance, generally achievable with a FET input. 
 
Forgive my pontification, don't know any other way to explain the topic - and I'm not sure I succeeded.


Claude Baker, AC0ZL