[BC] LPAM (was Smallest AM Radiator/TIS)

[Richard Fry]

>15.219 allows a 3m ground lead and antenna TOTAL length.
>If someone uses the standard 102" Radio Shack whip, they
>are not going to be allowed to build a set of 100ft radials into
>a ground plane as well ;)

The applicable Part 15 AM rules are either 15.209, or 15.219. Rule 15.209 
does not define anything about tx power or antenna system gain/efficiency, 
only that the peak field strength in microvolts/meter when measured 30 
meters away from the antenna cannot exceed [24,000/Operating Freq in 
kilohertz].

As an alternate, Rule 15.219 can be applied in place of 15.209. It limits 
the DC input power to the final r-f stage in the tx to 100 mW, and the 
total length of the antenna, feedline and ground lead (not "ground system") 
to 3 meters.
Of these two rules, 15.209 is much more restrictive.

So Part 15 AM does not define or limit the r-f ground itself -- only that 
the length of the conductor(s) connecting the tx system TO an r-f ground, 
together with the lengths of the actual "antenna," and the wire connecting 
the antenna to the tx cannot exceed 3 meters.

Any conductor leading from a buried r-f ground through space and connecting 
directly (or with a short "ground lead") to a Part 15 AM tx chassis  DOES 
radiate, because r-f current is flowing through it between the r-f ground 
and the tx chassis. The flow of r-f current through a conductor generates 
radiation.  And anything that does radiate r-f is an antenna, or part of 
one.

A buried r-f ground itself does not produce useful radiation.  It only 
reduces ground losses so that a larger portion of the available tx power 
can be radiated by the antenna.

All other parameters equal, for a matched vertical monopole radiator of a 
length up to 5/8-wave, the longer the radiator, the greater the radiated 
field it produces (except at the zenith). This is why in 15.219 the FCC 
limited Part 15 AM radiator length in the first place.  And this also is 
why "elevated" Part 15 AM tx systems perform better than those installed at 
ground level, where the total length of the radiating conductors is 3 
meters or less.

The Rangemaster may have been FCC-certified with a 102" whip, but AFAIK the 
length of the exposed conductor leading to r-f ground is not part of the 
public record for that certification.  Radiation from that conductor can be 
many times more than the 102" whip itself is generating.  This reality of 
physics apparently has escaped some FCC field inspectors, and even the 
author of the system installation drawings shown for the Rangemaster on the 
Rangemaster web site.

A Part 15 AM tx operating with 100 mW DC input power still can be part of a 
"pirate" operation if it is used with an illegal antenna, even though the 
tx may have a label saying it is Part 15 certified, and an applicable FCC 
ID number.

If the tx was FCC-certified, that certification was based on the radiating 
hardware used in the certification process. Using that tx with a different 
antenna voids that FCC certification. Compliance with Part 15 then is the 
responsibility of the user.   Using a tx with 100 mW input power with an 
antenna system having a radiating length longer than the total 3-meter 
legal length can produce the same result as increasing the power output of 
a 100 mW tx used with a legal-length antenna.

For a benchmark, here is an estimate for a 3-meter vertical radiator with 
its base at ground level using a nearly perfect r-f ground, as based on the 
FCC's propagation curves. The more typical r-f ground used for Part 15 AM 
would reduce these distances significantly.  The efficiency of the radiator 
was calculated using NEC-2.

INPUT DATA:
Loading Coil Resistance ~2 ohms
Frequency = 1,600 kHz
Tx Output Power = 80 mW
Conductivity = 3.0 mS/m
RESULT:
Contour level > Distance to contour
5.000 mV/m > 0.0322 miles
2.000 mV/m > 0.0805 miles
0.100 mV/m > 0.9448 miles

Actual, or claimed coverage distances much greater than these probably are 
based on operator enthusiasm and/or operations not in compliance with Part 
15.

Here is a link to a paper with more information on the effects of mounting 
Part 15 txs and their "3-m" radiators on towers, flagpoles etc

http://www.freefilehosting.org/public/13167/Part%2015%20AM%20Antennas.pdf

RF