RF Ground Connection Loss for Part 15 AM Systems

[R. Fry]

The link below leads to a chart showing typical r-f losses for various configurations of buried ground wires for various earth conductivities, as calculated by NEC 4.2 software.  A chart of earth conductivities in the continental U.S. is included at the bottom of that page.  Together they should allow a fairly reasonable estimate for the loss in the r-f ground connection of many systems.

This loss resistance is in series with the r-f current that can flow on the radiating conductors of a Part 15 AM system, so for best system performance it should be minimized.

http://i62.photobucket.com/albums/h85/rfry-100/Pt15AM_Ground_System_Losses.jpg

[druidhillsradio]

Here is a way to somewhat validate the map in the previous post:

http://www.ehow.com/how_7215160_out-ground-conductivity.html

Hope this helps.

[R. Fry]

As a followup illustration, the link below shows how the loss in the r-f ground connection at the base of the antenna affects the distance to a given groundwave field intensity, other things equal.

The calculations are based on a Part 15 AM transmitter mounted a few inches above the earth, with the top of its vertical antenna 3 meters above the earth.

http://i62.photobucket.com/albums/h85/rfry-100/Part15_AM_Fields.gif

[druidhillsradio]

Hey Rich,  Insulated or uninsulated wire? Or does it matter?  Lots of debate on this.  In know or at least I've been led to believe, that on HF freqs it does not matter.  MW icluding 160 meters may be another matter.

[R. Fry]

Insulated or uninsulated (radial) wire? Or does it matter? 

It doesn't matter to the radiation performance of a vertical monopole, as the r-f (a-c) current flowing in the earth due to the displacement current radiated from the monopole does not need a direct current path to enter and flow along a buried radial conductor.

Uninsulated wires used for buried radials can provide a path to bleed off static voltages on the monopole, although most AM broadcast stations use several vertical ground rods driven through the radial field close to the tower base, for this purpose.

[Offgridkindaguy]

How about the diameter of the wire. Electric fence wire is cheap and will accept soldering..

[R. Fry]

Generally 14 AWG diameter or larger conductors are most practical to install without kinking, and are large enough in OD for the application.

Broadcast stations often use 10 AWG copper-clad wire for buried radials, and many of those systems have survived for decades without much change in performance (assuming they haven't been stolen).

Other materials may tend to disintegrate a lot quicker, depending on soil characteristics.