PhilB said:
It is possible to have a 30 meter ground wire and still comply with the field strength limit. The conspicuous absence of a field strength reading in this case doesn't compute.
FCC Part 15.209 states the maximum field strength permitted for unlicensed, radiating systems (see partial quote below).
In the case of the citation in question the maximum field permitted 30 meters from the transmit antenna is about 14.9 µV/m (microvolts per meter).
While it is true that any transmitter power and any antenna/ground path configuration may be used under Part 15.209, physics shows that, whatever the system configuration, its effective radiated power (ERP) may not exceed about 2.2 µW (microwatts) in any direction without exceeding the 15.209 limit for 1610 kHz.
The transmitter power output needed to produce that 2.2 µW ERP with the matched, ground-mounted system I analyzed and linked to earlier in this thread is about 310 µW. An elevated Part 15 AM system using a 30-meter ground lead (or even one of ~20 feet, as shown in the publications of some Part 15 AM transmitter manufacturers) is much more efficient than when installed at earth level, and the transmitter output power then would need to be many times less than 310 µW to remain compliant with 15.209.
So, even if the output power of a Part 15 AM transmitter with 100 milliwatts of input power (per 15.219) might be as little as 1 milliwatt, that 1 milliwatt can be
far above the power needed to comply with 15.209 -- even when applied to an antenna system strictly meeting 15.219 (3-m radiator at earth level, using a ground path of a few inches).
All of this doesn't address the question about the FCC not referring to field strength in this citation. One might speculate that if this station
had met 15.209 it would not have attracted the attention of the FCC. The reason for this is that field strength permitted under 15.209 is so low that probably no-one beyond a radius of 50 feet or so would be able to to receive it well, so as to complain about it. Certainly it could not provide the "community broadcast" service expected of it. And even if the FCC inspected it, it would not have been cited even if it used a 30-meter path to an r-f ground, as long as the 15.209 field limit was being met. T
Note that good, ground-mounted Part 15 AM systems using commonly available, commercial Part 15 AM transmitters, and operating strictly observant to 15.219 easily can produce fields that exceed the limit permitted by 15.209. And even moreso for one with a 30-meter ground conductor.
For this reason probably most Part 15 AM users would prefer to be FCC-evaluated using 15.219, rather than 15.209.
A Part 15 AM system with the base of its < 3-meter antenna installed a few inches above the earth unquestionably meets Part 15.219(b). Installing the tx/3-m antenna well above the earth while using a long conducting path to r-f ground does not meet 15.219(b) as a fact of physics -- and this is what may (and has) lead to FCC problems for some users.
__________
Sec. 15.209 Radiated emission limits; general requirements.
(a) Except as provided elsewhere in this subpart, the emissions from an
intentional radiator shall not exceed the field strength levels specified in
the following table:
------------------------------------------------------------------------
Measurement
Frequency (MHz) Field strength distance
(microvolts/meter) (meters)
------------------------------------------------------------------------
0.009-0.490...................... 2400/F(kHz) 300
0.490-1.705...................... 24000/F(kHz) 30
1.705-30.0....................... 30 30
30-88............................ 100 ** 3
88-216........................... 150 ** 3
216-960.......................... 200 ** 3
Above 960........................ 500 3
------------------------------------------------------------------------
//
