Near field maximum

[Ptitjean]

Hello,

In the farfield zone, we can approximate the electrical field by the formula E=sqrt(30*E.I.R.P)/distance
We also know that this value is a maximum of the field.

I was wondering whether this value could also be a maximum in the nearfield zone ?
If not, is there a way to approach the value of the electrical field in this zone ?

Thanks
Ptitjean

 

[logbook]

The problem with the near field is that the nature of the antenna then becomes critical to the field shape. I would think the field intensity heading into a parabolic antenna would at first increase as you get closer, but closer than the focal length wouldn't it go the other way?

 

[pstuckey]

Q1: No, the equations for electric field have terms proportional to: 1/d, 1/d^2, 1/d^3

In the far field all but the 1/d terms become very small. In the near field the other terms dominate. Also, E.radial is not ~0 in the near field as it is in far field.

Q2: I'm not aware of a simple equation, but perhaps someone else here has seen one.

Peter

 

[VEBill]

As an example, a dipole has much higher voltages at the tips than at the feedpoint. The peak voltage at the tips is critically dependant on many details (loading plates versus sharp points) and would also be affected by the local environment (even measuring instruments).

 

[Higgler]

The maximum E field is usually at 0.4D^2/Lambda for standard antennas. That can be used as an estimate if a high power antenna might overhead a radome or a human.

kch

 

[Ptitjean]

Thanks all for the explanation