[Elecraft] OT: Elevated vs. Buried Radials

[Jim Dunstan]

At 08:37 AM 5/18/2013, Joel Black wrote:


>Is the only benefit portability?
>
>
>Thanks,
>Joel - W4JBB

Hi Joel

As you mentioned ... physical, portable, and safety issues aside the 
difference between raised and buried radials is the difference in how 
they perform their function;  that is how they effectively balance 
the antenna currents in the vertical radiating element and allow 
maximum radiation (usually vertically oriented).

Example:

imagine a 1/2 wave dipole horizontally oriented in free space above 
earth producing a horizontal oriented radiation pattern.  Now bend 
the 1/2 wave 90 deg so one side is vertical while the other side 
remains horizontal and you now have a combination of horizontal and 
vertical radiation while the radiation efficiency remains the 
same.  In order to eliminate the horizontal component install a 
second horizontal 1/4 wave element installed 180 deg opposite the 
first horizontal wire and the horizontal radiation component cancels 
leaving only the vertical component.  This configuration is a 
vertical ground plane antenna and is quite efficient even though 
approximately half the radiated power is lost in the cancelled out 
horizontal portion.

Now assume for whatever reason you want the feed point to be at 
ground level and you lower it more and more.  As you do so the 
efficient 2 element ground plane (1/4 wave each) comes closer and 
closer to ground level and the resonant efficiency of the ground 
plane becomes lower and lower due to the interaction with the earth 
until the resonant length of the ground plane becomes 
irrelevant.  Now in order to handle the RF current flow necessary to 
allow maximum current flow in the vertical radiating element a 
different method is required..... you now actually need to allow 
current to flow from the "ground System" to the earth 
itself.    Different radial properties are required and resonance is 
no longer required..  To a large degree RF current flow now depends 
on characteristics of the earth and the mass of the coupling material 
that you use to come in contact with it.

This is true of all vertical antennas.  Some tricks are employed to 
reduce this ground effect which is at maximum if the vertical element 
is 1/4 wave (low impedance feed point).  For example if the element 
length is increased the feed point impedance is increased and the 
current flow required for a given power is reduced. There are any 
number of articles on how to do this.

73  Jim, VE3CI