[BC] am receiver for monitoring at a 20 KW site (add)

[Richard Fry]

Richard B. Johnson wrote:
> ....  So, you can see that at 1 meter from the antenna, the signal is 
> about 300 volts/meter for 1 kW and it drops off linearly with distance.

The E-field at 1 meter quoted above appears to be based on the reality that 
the groundwave E-field at 1 km for 1 kW radiated by a 1/4-wave monopole 
driven against 120 x ~1/4-wave buried radials is 300 mV/m, and the 
ASSUMPTION that the E-field at ALL distances from the radiator varies 
inversely with distance.  If that belief was true, then for a distance of 1 
meter from the radiator, that E-field would be 300 mV/m * 1,000 = 300,000 
mV/m = 300 V/m.

But that belief is NOT true, because the E-field when approaching within one 
radian distance of a radiator [1/(2*pi) lambda] rises much faster than its 
inverse distance value.  Even for a frequency of 1700 kHz, that radius is 
much greater than 1 meter (actually, >28 meters).

The link below is a paste on this subject from John Kraus' "Antennas for All 
Applications," 3rd edition, which illustrates the point.

http://i62.photobucket.com/albums/h85/rfry-100/KrausGraphicFields-.jpg

RF