In Dec, K5PC asked if the choke he uses on his elev radial system was effective enough. There
is a simple measurement that can made to evaluate the effectiveness of the 
countermeasures such as the use of a choke in the feed-line.
W7EL, in the ARRL ANT HANDBOOK, writes that the Field Strength, FS, can be written as
FS = 10 Log ( K / R )
where K is a constant (for a constant RF power into the ant), and R is the real
part of the impedance at the base of the ant. 
 
What one finds for a very limited elev radial system is that R increases when ground 
current is allowed to flow. This causes the current and the field strength to decrease, making 
the ant less efficient. One can measure the real part of the base impedance without ground 
current flow(R2), and with ground current flow (R1), and estimate the loss by
  Change in FS = 10 Log (R1 / R2 ) = (  Change in gain, for EZNEC modeling )
The good news is that as one increases the number of elev radials (symmetrically) the 
differences between R1 and R2 tends to disappear. At my location I could not detect
any differences between R1 and R2 at 6-8 elev 1/4w radials. One may not get the same
exact results but the trend should be the same.
The reason for the above effect is that the ground current and the elev radial return current 
has significant phase differences which results in a reduction in the total return current. (due to 
differences in the vel of propagation). Also the current flowing via ground is more lossy, and 
the net effect is that for a given excitation the current decreases and thus the value of the 
real part of the base impedance goes up. ( Pin = I^2 R, current goes down, R goes up)
As the number of elev radials is increased the amount of current
flowing via the ground connection is reduced while the percent of the total 
current flowing in the elev radials is increased, resulting in less cancellation. For
For those for whom antenna modeling is the all important thing, 
 delta FS = delta Gain = 10 Log R1/R2
models exactly as that.
 Measurement of the real part of the base impedance with or without ground current flowing
is a relatively simple way to evaluate the effects of a somewhat complex phenomena.
Rob, W1MK
 
 



________________________________________________________________________
FREE Reminder Service  - NEW from AmericanGreetings.com
Click HERE and never forget a Birthday or Anniversary again!
http://track.juno.com/s/lc?s=197335&u=http://www.americangreetings.com/products/online_calendar.pd