Hasan,

I think you are measuring the input impedance of an Inverted-L against
a system of ground radials.

You are trying to estimate the input resistance of the ground radials
by subtracting the CALCULATED radiation resistance of the Inverted-L
from the measured antenna input resistance.

Excellent, there is no better way of doing it!

First of all, the overall length of the antenna must be 1/4-wavelength
resonant at the testing frequency such that its input impedance is
PURELY RESISTIVE. The measured input resistance, of course, will be
greater than the calculated radiation resistance referred to its base.
The difference between them is the required input resistance of the
ground radials.

The hard part of the exercise is calculating the radiation resistance
referred to the base of the Inverted-L. The radiation resistance is a
very complicated function of the dimensions, overall length and
height, of the antenna.

However, for the purposes of estimating ground loss resistance, (it
changes with rainfall and temperature of the season), the following
approximation for radiation resistance is good enough.

RadRes = 18 * ( 1 - Cos( Theta ) ) ohms,

where Theta is an angle = 180 * H / ( H + L ) / Lambda degrees,

H = height of vertical portion of Inverted-L,
L = length of horizontal portion of Inverted-L
and Lambda is the free-space wavelength.

This formula applies ONLY when L+H is 1/4-wave resonant. Which is the
condition under which you are working if you are doing the job
correctly.

You will not find the formula in the books of bible-writer Terman.
Nor in any of the works of the other regular gurus. If you ask from
where it came from, it came from one of my old notebooks and I worked
it out for myself, years back.

Bear in mind it is only an approximation. It would take 6 months to
work out how precisely accurate it is and I don't have the time. But
it's as least as accurate as you can make impedance measurements. I
do hope I have copied it out correctly.

By the way, as the number of your radials increases and the ground
loss resistance gets very low, don't be surprised if you calculate
negative values of ground loss resistance.
----
Reg, G4FGQ