[BC] This is Part 15 Radio :)

[Richard Fry]

Willie... wrote:
>Given a typical P15 Legal AM transmission system,
> what would the signal be at, say, 20 feet?
________________

Below are some calculated values for field strengths at several distances
from the Part 15 compliant r-f system described there.

These calculations are based first on a NEC-2 analysis to determine the
FCC efficiency for the antenna, and then using that value with the FCC's
propagation curves for the frequency and ground conductivity, to determine
the distances to the contours shown.  This is a proven process which
applies even at these low powers.

DATA:
Frequency = 1700 kHz
Applied Power = 80 milliwatts (~output power of Part 15 AM tx)
Radiator = 3-meter total length including the conducting path from the tx
  chassis to the ground plane (antenna is ground-mounted).  This length
  is the maximum allowed by FCC Rules.
Antenna System RF Resistance, Loading Coil plus Ground = 10 ohms
Ground Conductivity = 8.0 mS/m (typical value)
Radiation System FCC Efficiency = 21.12 mV/m at 1 mile for 1 kW applied

RESULTS:
Field Strength > Distance
2 mV/m > 0.09 miles (good signal)
1 mV/m > 0.18 miles (fair)
0.5 mV/m > 0.34 miles (noisy)
0.05 mV/m > 2.5 miles (very noisy)

Usable signal strengths at much greater distances than shown
may/probably indicate that Part 15 Rules are not being observed.

A common practice for "Part 15" AM is to install the tx and antenna on an 
elevated mount, and use a long conducting path to the ground plane at 
physical earth.  The ground conductor itself radiates, and greatly 
increases the net radiated field.  Paper 3 at the link below develops this 
situation in some detail.

   http://rfry.org/Software%20&%20Misc%20Papers.htm

As far as multiple Part 15 AMs on the same freq to better cover a 
community, the carriers of some commercial Part 15 txs can be synced 
together to remove the intercarrier beats, but the separate tx systems 
still act like the radiators of a DA array, and create net coverage areas 
with little or no signal in some sectors where it is expected to serve. 
The need for correct audio phasing at each tx also tends not to be 
recognized.

RF