Topband: Magic Antenna Land

[K4SAV]

Hugh Valentine wrote:
> Question:  When using a 160M inverted "L" antenna, he constructs a radial system of 20 radials, half 33' and half 133' in length and drives an 8' ground rod into the salt marsh he lives on and connects the radials and the Salt water ground Rod together.  
>
> Oval gets pretty good results but is wondering what effect his ground rod in Salt water is having on his antenna performance.  Oval disconnects the Ground rod and runs on Radials only.  He is wunderin....did I improve or degrade my signal???....and he wonders if he could measure current flow in the 1/4" ground bus to the ground rod...would it show any current flow and how would that compare to the current flow in the radials....????  Will his SWR change? ........
>   

There seems to be little information available on the subject of radials 
over/in salt water and ground rods.  Not many people have salt water to 
play with, and data is lacking.  I can give you my opinion of what will 
happen based on some known facts.  I don't have any measured data to 
support my conclusions.

Known facts:
1.  Depth of penetration of 1.8 MHz RF in average soil is about 37 feet.
2.  Depth of penetration of 1.8 MHz RF in salt water is about 7 inches.
3.  The conductivity of salt water is better than average soil but 
nowhere nearly as good as copper.

Obviously from the data above, ground rods and radials are going to 
operate a lot differently over/in salt water than they do over/in 
average ground.  It should also be obvious that a vertical over salt 
water is going to be very good compared to a vertical over average 
ground, regardless of how poor a ground/radial system you have.  So 
comments of "it works great" should be expected, but could it have been 
better?

To understand this, first consider what happens if you take a radial and 
submerge it in salt water.   RF radiated from the vertical will never 
reach the radial because it will not penetrate more than about 7 inches 
from the surface.  Similarly RF conducted down the radial will not 
affect the fields above the water line.  Since copper is a very good 
conductor and salt water is poor, RF conducted down the radial will 
continue to propagate down the radial, and the length of the radial will 
affect the antenna feedpoint impedance. Those radial currents will be 
dissipated in the salt water.  A similar analogy for a vertical over 
average ground would be a vertical using a well casing for the ground 
system.  As far as collecting fields from the antenna and conducting 
them back to the feedpoint, the radial does very little.  For that 
purpose the radial may as well have been only about 7 inches long 
(although you may not the happy with the SWR that a 7 inch radial gives 
you).  So the submerged radial may give you a nice feeling because it 
lowers the feedpoint impedance but it does little for improving the 
gain.  The same thing can be said for a ground rod driven into a salt marsh.

So for best performance where should the radials be?  They need to be 
located above the water.  That means you have elevated radials and if 
there are only a few, they should be resonant radials.   As for the 
question "does the ground rod improve antenna performance", my opinion 
is that it does not, and may contribute to a small decrease in gain 
(assuming the radials are above the water).  SWR, however, may change 
and that change can be in either direction.

Any contributions of measured data to support or to dispute the above 
opinion is appreciated.

Jerry, K4SAV