bill@flutter.tv.tek.com (William K. McFadden) (01/30/91)
Last week I posted a request to the net asking what the current FCC
regulations were for low-powered FM broadcasting. As you may recall, I
wanted to pipe CNN audio around a building.
I was primarily interested in what levels of intentional radiation were
allowed outside the property line of a privately-owned industrial
area. Unfortunately, there either does not exist such a regulation or
none has been found. I believe there must be such a regulation,
because our EMI test lab generates field strengths far in excess of the
limits in Part 15. These emissions are legal because they are
self-contained. I would still like to find out what the FCC says about
this. By the way, there is such a rule for AM stations on a school
campus. The emissions are not limited except at the perimeter of the
campus. (Hey, they teach college credit courses here; does that
qualify Tek as a school campus?)
The only regulations anybody could find that pertain to the FM band
apply to operation at any location, including a neighborhood. Hence,
these are quite restrictive. I wish to thank
gary_t_trindl@cup.portal.com for faxing me the pertinent pages of CFR
47 Ch. 1 (10-1-89 edition) Part 15. It reads:
15.239 Operation in the band 88-108 MHz.
(a) Emissions from the intentional radiator shall be confined within
a band 200 kHz wide centered on the operating frequency. The 200 kHz
band shall lie wholly within the frequency range of 88-108 MHz.
(b) The field strength of any emissions within the permitted 200 kHz
band shall not exceed 250 microvolts/meter at 3 meters. The emission
limit in this paragraph is based on measurement instrumentation
employing an average detector. The provisions in 15.35 for limiting
peak emissions apply. [I don't have a copy of 15.35, but since the
operation is FM, I doubt this limit would apply--WKM]
(c) The field strength of any emissions radiated on any frequency
outside of the specified 200 kHz band shall not exceed the general
radiated emission limits in 15.209. [see below--WKM]
(d) [omitted, only applies to custom-built telemetry equipment--WKM]
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:
Frequency (MHz) Field strength Measurement 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
Comments:
250 uV/m @ 3m is hardly any higher than the unintentional level most
computing equipment is allowed to put out. I believe this is not
enough to cut through the computer-generated hash in the building where
I work. It might be possible to install transmitters every 25 feet,
but this is impractical. I might try to set up something that would
cover a small portion of the building.
Some people have suggested carrier-current AM. This would not work
here for two reasons. First, the computer generated hash is much
greater at 1 MHz than 100 MHz, and AM is more susceptible to noise.
Second, the power distribution system here would not propagate the
signal beyond a small area.
Several people have pointed out that rebroadcasting CNN would violate
copyright law (however I doubt CNN would pursue it).
Thanks to everyone who responded. If anyone has an answer to my first
question (as stated at the beginning of this article), please let me
know.
--
Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-639 Beaverton, OR 97077
bill@videovax.tv.tek.com, {hplabs,uw-beaver,decvax}!tektronix!videovax!bill
Phone: (503) 627-6920 "SCUD: Shoots Crooked, Usually Destroyed"