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
______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:Elecraft@mailman.qth.net
This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html
