Topband: [Fwd: Field Strength Calculator]

[Tom Rauch]

> As shown in the chart below the 1/4 wave standard has 120 
> radials and
> produces 191.382 mV/m @ 1 mile.  My point here is that 
> measurements at 1
> mile of your 1800 kHz transmission with 1 KW into whatever 
> you have and
> using a good FSM can remove a lot of doubt when compared 
> with the
> theoretical and/or corrected values.

Unfortunately those values are normalized values of perfect 
conductivity earth. Anything you do to use them must include 
estimated attenuation of the real soil and a correction, so 
we cannot directly apply them to anything.

The procedure for using the FCC estimates are:

1.) You set up a test vertical and measure FS at many points 
along lines moving out from the antenna. (These lines are 
called radials, like the radials in a ground system, but 
they are just directions.)

2.) You then compare the SLOPE of the measurement points 
with distance along each radial to FCC published slopes for 
various soil conductivities. The idea is to match the 
attenuation rate with a soil conductivity value on a curve. 
That gives you an estimate of the soil losses to use in a FS 
correction. The worse the soil, the more you increase the 
FS.

3.) You then use a correction factor based on the soil 
conductivity, power, and distance to normalize the reading 
of your antenna to a kilowatt at a mile, or a kilowatt at a 
kilometer.

The actual reading is never what the perfect conductivity 
case indicates. When we read about BC engineers saying "the 
antenna had 192mV/m at one mile" what they are really saying 
is the FS they measured, allowing for the estimated ground 
losses and **normalized** through a series of estimates and 
manipulations, comes out to 192mV/M at a mile. It doesn't 
mean at all if you went out a mile and ran 1kW you would 
measure 192mV/m, or whatever they say. You might have 
measured 125uV/m.

This is why when I read articles claiming an antenna is 
almost 100% efficient based on a series of estimates and 
data manipulations, I just shake my head. The data goes 
through so many manipulations and estimates a person can 
make the FS almost anything he wants.

For our purposes FCC estimates have little meaning. They 
mean little enough when used correctly.

73,
Tom