Roy Lewallen wrote:
John - KD5YI wrote:

Hey, Roy -

My 3 mW answer was for 30 meters. I think that ratios correctly to your
30 microwatts at 3 meters. Yes?

Yep, that's right. And I finally got curious enough to look up the
actual regulation. According to section 15.225(a), intentional radiators
at 13.553-13.567 MHz are allowed 15,848 uV/m at 30 meters, which is
about 4.75 mW to an isotropic antenna in free space. There's also a
0.01% frequency stability requirement which would imply crystal control.
And any emissions extending outside the band have to be about 54 dB
lower, 30 uV/m at 30 m.

Roy Lewallen, W7EL

Do what??? The power output can vary depending on a number of things
including coax and antenna, you can run 1,000 watts and come up 10,000
uV/meter at a distance of 30 meters. As the FCC Office of Engineering
and Technology has stated in their bulletin which deals with Part 15.

"What is the relationship between "microvolts per meter" and Watts?
Watts are the units used to describe the amount of power generated
by a transmitter. Microvolts per meter (µV/m) are the units used to
describe the strength of an electric field created by the operation of
a transmitter. A particular transmitter that generates a constant level
of power (Watts) can produce electric fields of different strengths
(µV/m) depending on, among other things, the type of transmission line
and antenna connected to it. Because it is the electric field that
causes interference to authorized radio communications, and since a
particular electric field strength does not directly correspond to a
particular level of transmitter power, most of the Part 15 emission
limits are specified in field strength."

Todd N9OGL
OMEGA ONE BROADCASTING

N9OGL wrote:
Roy Lewallen wrote:

John - KD5YI wrote:

Hey, Roy -

My 3 mW answer was for 30 meters. I think that ratios correctly to your
30 microwatts at 3 meters. Yes?

Yep, that's right. And I finally got curious enough to look up the
actual regulation. According to section 15.225(a), intentional radiators
at 13.553-13.567 MHz are allowed 15,848 uV/m at 30 meters, which is
about 4.75 mW to an isotropic antenna in free space. There's also a
0.01% frequency stability requirement which would imply crystal control.
And any emissions extending outside the band have to be about 54 dB
lower, 30 uV/m at 30 m.

Roy Lewallen, W7EL


Do what??? The power output can vary depending on a number of things
including coax and antenna, you can run 1,000 watts and come up 10,000
uV/meter at a distance of 30 meters. As the FCC Office of Engineering
and Technology has stated in their bulletin which deals with Part 15.

"What is the relationship between "microvolts per meter" and Watts?
Watts are the units used to describe the amount of power generated
by a transmitter. Microvolts per meter (µV/m) are the units used to
describe the strength of an electric field created by the operation of
a transmitter. A particular transmitter that generates a constant level
of power (Watts) can produce electric fields of different strengths
(µV/m) depending on, among other things, the type of transmission line
and antenna connected to it. Because it is the electric field that
causes interference to authorized radio communications, and since a
particular electric field strength does not directly correspond to a
particular level of transmitter power, most of the Part 15 emission
limits are specified in field strength."

Todd N9OGL
OMEGA ONE BROADCASTING




C'mon, Todd. He qualified it by saying "4.75 mW to an isotropic antenna in
free space." It is just a way to get a "feel" for how much power might be
required based on the FCC uV/m limit under certain theoretical conditions.

I would start with a calculation like this if I were designing a transmitter
whose signal has to meet those limits. I think that would be better than
starting with a 1000W transmitter when only a few milliwatts is required.

When was the last time you used an isotropic antenna? How was your trip to
free space?

John