I see on another forum http://www.part15.us/modules.php?name=Forums
referring to this document http://www.fcc.gov/mb/audio/decdoc/scandoc/910724/1.jpg
Rich states:
Using the equation
Field Strength in dB above 1 microvolt/meter =
104.77 + ERPi - 20 log (D)
where
ERPi = Effective Radiated Power in dB referenced
to 1kW from an isotropic radiator
D = Distance in kilometers
and applying the necessary units conversions says that input power of ~11.5 nanowatts to a linear 1/2-wave dipole will produce the FCC's maximum field of 250 uV/m at a distance of three meters. A nanowatt is a millionth of a milliwatt!
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RadioheadC asks:
So, what accounts for the difference between the 10 or 11.5 nanowatts that the FCC and you respectively estimate and the 1 to 10 milliwatts that manufacturers seem to use?
Rich replies:
-----------------------------
The FCC Public Notice you linked to refers to the "Maximum Effective Radiated Power" permitted -- which is the peak directional power actually radiated by the antenna -- not the power input or output of the Part 15 FM transmitter itself.
The transmitter output power can be whatever value it needs to be to produce no more than 250 uV/m at a 3-meter distance in any direction from the antenna. So if the antenna has a peak gain of 1X, the power applied to its input connector could not exceed the nominal 10 nanowatt value.
I think the FCC Rules for "type acceptance" require commercial manufacturers of Part 15 FM transmitters to supply the antenna, and that it should either be permanently attached to the transmitter, or use a unique connector so that it can't be replaced with an antenna that wasn't tested and certified as meeting the Rules with that transmitter.
In any case, antennas for the FM band usually are very efficient, because even though they are short physically, they are long in terms of wavelength (unlike for Part 15 AM antennas). A 1/2-wave FM dipole, for example, is only 4 or 5 feet long, and has a peak gain of 1.64X compared to an isotropic radiator. A "whip" type antenna has somewhat less gain than that, but still far more gain than would permit using anything approaching 1-10 mW with it, and still meet Part 15 Rules.
Manufacturers quoting Part 15 FM transmitter power of 1 to 10 mW may say (if pressed) that these powers are for use only "where authorized." Or maybe some of them either haven't done the math, or are referencing inapplicable/outdated FCC documents. Good question to ask the ones who do this.
//
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Then EDM FAQ document says this:
"I am confused. I read that my local authority allows only 10nW of RF power, your weakest unit transmit 10mW of RF. In my book it is 1 Million times over the limit!?
Don't confuse RF power input to an antenna with actual radiated power received or measured at a point in space. To try and explain: Suppose you take a light bulb and suspend it in the middle of a big sphere or ball. If that is say a 100W of light output, it is obvious that if you can measure at a very small point on the inside surface the light intensity, and convert it to equivalent power it will not be 100W any more but, 100W divided by all the many millions of pin-points that makes up the surface area of this sphere. The light source can be your antenna radiating in all directions, and the point anywhere on the surface representing the receiving antenna some distance from the transmit antenna. Doing the math on a often enquired uV requirement, we calculated that:
250uV/m @ 3m ~ 12.5nW of received power intensity or in engineering terms -59dBm using a 1m sampling antenna 3m distance away from the transmit antenna. (This is only how WE see and interpret it as outsiders, you are advised to get a 2nd opinion)
Also remember that apart from an antenna not be able to transmit or receive without losses, there is free space attenuation that "eats" up some of the signal as it travels from the transmit antenna to the receiver antenna."
---------------------------------
Who is correct, or are they both correct? I wish the FCC would publish requirements for normal people (homebrewers) to understand. It is like trying to figure out a legal document. By the time you finished, you still don't know what the blaze it means! It would have been much simpler to just say 'P'mW out of the transmitter using a no gain, 'N' gain antenna. Why all this complications?
referring to this document http://www.fcc.gov/mb/audio/decdoc/scandoc/910724/1.jpg
Rich states:
Using the equation
Field Strength in dB above 1 microvolt/meter =
104.77 + ERPi - 20 log (D)
where
ERPi = Effective Radiated Power in dB referenced
to 1kW from an isotropic radiator
D = Distance in kilometers
and applying the necessary units conversions says that input power of ~11.5 nanowatts to a linear 1/2-wave dipole will produce the FCC's maximum field of 250 uV/m at a distance of three meters. A nanowatt is a millionth of a milliwatt!
-----------------------------
RadioheadC asks:
So, what accounts for the difference between the 10 or 11.5 nanowatts that the FCC and you respectively estimate and the 1 to 10 milliwatts that manufacturers seem to use?
Rich replies:
-----------------------------
The FCC Public Notice you linked to refers to the "Maximum Effective Radiated Power" permitted -- which is the peak directional power actually radiated by the antenna -- not the power input or output of the Part 15 FM transmitter itself.
The transmitter output power can be whatever value it needs to be to produce no more than 250 uV/m at a 3-meter distance in any direction from the antenna. So if the antenna has a peak gain of 1X, the power applied to its input connector could not exceed the nominal 10 nanowatt value.
I think the FCC Rules for "type acceptance" require commercial manufacturers of Part 15 FM transmitters to supply the antenna, and that it should either be permanently attached to the transmitter, or use a unique connector so that it can't be replaced with an antenna that wasn't tested and certified as meeting the Rules with that transmitter.
In any case, antennas for the FM band usually are very efficient, because even though they are short physically, they are long in terms of wavelength (unlike for Part 15 AM antennas). A 1/2-wave FM dipole, for example, is only 4 or 5 feet long, and has a peak gain of 1.64X compared to an isotropic radiator. A "whip" type antenna has somewhat less gain than that, but still far more gain than would permit using anything approaching 1-10 mW with it, and still meet Part 15 Rules.
Manufacturers quoting Part 15 FM transmitter power of 1 to 10 mW may say (if pressed) that these powers are for use only "where authorized." Or maybe some of them either haven't done the math, or are referencing inapplicable/outdated FCC documents. Good question to ask the ones who do this.
//
--------------------------------
Then EDM FAQ document says this:
"I am confused. I read that my local authority allows only 10nW of RF power, your weakest unit transmit 10mW of RF. In my book it is 1 Million times over the limit!?
Don't confuse RF power input to an antenna with actual radiated power received or measured at a point in space. To try and explain: Suppose you take a light bulb and suspend it in the middle of a big sphere or ball. If that is say a 100W of light output, it is obvious that if you can measure at a very small point on the inside surface the light intensity, and convert it to equivalent power it will not be 100W any more but, 100W divided by all the many millions of pin-points that makes up the surface area of this sphere. The light source can be your antenna radiating in all directions, and the point anywhere on the surface representing the receiving antenna some distance from the transmit antenna. Doing the math on a often enquired uV requirement, we calculated that:
250uV/m @ 3m ~ 12.5nW of received power intensity or in engineering terms -59dBm using a 1m sampling antenna 3m distance away from the transmit antenna. (This is only how WE see and interpret it as outsiders, you are advised to get a 2nd opinion)
Also remember that apart from an antenna not be able to transmit or receive without losses, there is free space attenuation that "eats" up some of the signal as it travels from the transmit antenna to the receiver antenna."
---------------------------------
Who is correct, or are they both correct? I wish the FCC would publish requirements for normal people (homebrewers) to understand. It is like trying to figure out a legal document. By the time you finished, you still don't know what the blaze it means! It would have been much simpler to just say 'P'mW out of the transmitter using a no gain, 'N' gain antenna. Why all this complications?