Antennas for Part 15 AM Stations (US)
 

Under the line below is my response to the post of 'LH' on another website.
The subject started when someone else posted that legal Part 15 AM stations
can have usable groundwave coverage out to 5-10 miles.

Maybe some at r.r.a.a. might be asked to get involved with one of these
stations at some point, and find these comments useful. Or not.

RF

+ + +

\\ LH wrote:
Next time a non-D AM tower falls over, let's clean up the mess,
then install a Rangemaster or other high quality Part 15 transmitter
on the ground at the center of the radial system and see how far
it gets out, measure the field strength, etc. The official 3 meter
antenna to be used.
_____________

An interesting concept.

A 3-meter vertical radiator is ~3.6 electrical degrees at 1 MHz. Using
Figure 32 in George Brown's paper I referred to in an earlier post, the FCC
efficiency of such a vertical with a ground system consisting of 113 radials
of 0.27 wavelengths each is about 40 mV/m at 1 mile for 1 kW of radiated
power.

Using that efficiency with the FCC's AM curves for 1 MHz, and assuming that
the Part 15 AM tx could supply 80 mW to the radiator,* here are the
parameters for a ground conductivity of 8 mS/m:

2.00 mV/m 0.1751 miles
0.500 mV/m 0.6695 miles
0.100 mV/m 2.9125 miles

NEC-2 calculates a base impedance of ~ 0.04 -j6500 ohms for this set of
conditions (4mm constant OD radiator). No practical transmitter could
deliver its rated power into such an impedance, but we have ignored that for
these calculations. (The reactance term can be cancelled by using a loading
coil, but the required coil would absorb a large amount of the available
power due to I^2R loss).

Considering that Part 15 AM stations using intentional radiators neither
have the ground system described above, nor can cause the RF current
equivalent to 80 mW to flow in the radiating portion of a 3-meter antenna,
it is clear that any Part 15 "coverage" claimed that approaches or exceeds
what is shown in the calculations above must be related to the use of an
illegal antenna system, and/or illegally high tx power.

* AM Part 15 limits the tx to 100 mW of input power.
Output power will be less -- 80 mW was assumed.

RF //