Following in this post is an exchange of e-mail between John A. Reed, Senior Engineer, Technical Rules Branch at the FCC Office of Engineering and Technology in Washington, DC and me, and is quoted here with his written permission.
I re-arranged them to be read in chronological order from the top down. I don't know why the time on John Reed's e-mail responses are earlier than the times on my e-mails they answer, but I left them alone.
This isn't really easy to read and maybe to understand fully, but for those who persist, Mr Reed's responses should go a long way at dispelling some of the beliefs commonly held by Part 15 AM users, and even some manufacturers of Part 15 AM transmitters -- whether or not they are FCC-certified for Part 15 use.
From: Richard Fry
Sent: Tuesday, August 26, 2008 8:39 AM
To: John Reed
Subject: Part 15 AM Rules
Hello again, Mr Reed -
I posted the text below the line in response to a query on a "Part 15" website. Does my analysis and commentary appear to be accurate as far as the technical and FCC reality?
The FCC has two areas of Part 15 applying to the MW AM broadcast band.
Under 15.209 the power of the transmitter and the length of the transmit antenna, feedline and ground are undefined. The only requirement is based on the field strength that such a system can produce at 30 meters.
Under 15.219 the field strength is undefined, but transmitter input power,and the overall length of the antenna, feedline and lead to ground are limited.
The FCC advises anyone who asks that these two forms of Part 15 rules were intended to provide very nearly the same performance, and that 15.219 was included as an alternate to 15.209 to eliminate the need for Part 15 AM users to be able to measure field strength accurately -- which requires expensive test equipment.
In the FCC document linked at the bottom of this post, the approximate maximum coverage radius they expect for a Part 15 setup is about 200 feet (for both AM and FM).
But the realities of physics allow a system operating strictly under 15.219 to produce a greater radius than 15.209 does, when referring to the same field strength (other things equal).
For example, the analysis in this link http://i62.photobucket.com/albums/h8...sed1500kHz.gif shows that, for ideal conditions a system operating strictly under 15.219 could produce a field strength at 30 meters that in this case is about 250 times greater than is permitted under 15.209. Probably such realities were unrecognized by the FCC when 15.219 was added to Part 15.
From the text of some of the FCC NOUO documents, it appears that they will physically inspect an unlicensed AM system they are investigating to see if it complies either with 15.219 or 15.209. Whether or not the transmitter itself has been certified by the FCC for Part 15 use has no bearing on this, and is never mentioned in the NOUO.
If the system obviously violates 15.219 such as when using an "elevated" installation with a long, radiating conductor to an r-f ground, then they assume that 15.209 applies. Note that the ground conductor includes more than the short "ground lead" connecting the transmitter to a conducting path to r-f ground (a metal flagpole, tower, "lightning ground" conductor, etc). Physics shows that an r-f ground does not exist at the top of any of those structures, and that the entire length of the conducting path to a true r-f ground radiates as part of the antenna system.
In practice, the radius to a given field strength is less for 15.209 systems than for fully legal 15.219 systems (other things equal). But if the inspected system does not meet the installation/operational requirements of 15.219, then any field strength that the FCC measures showing that the limit at 30 meters is excessive under 15.209 can/may lead to an NOUO. And any system intended to operate under 15.219 (even strictly so) almost certainly will exceed the limits of 15.209.
So the safest approach to avoid an NOUO is to insure that the Part 15 AM transmitter is installed within several inches of the physical earth, and that the top of the radiator connected to the r-f output connector of the transmitter is not more than 3 meters above the physical earth. Then, using a transmitter that is FCC certified for Part 15 AM should assure that the transmitter input power limit of 15.219 is met, and that using such an installation should not result in an NOUO if inspected by the FCC.
FCC LINK: http://www.fcc.gov/mb/audio/decdoc/scandoc/910724/1.jpg
From: John Reed
To: Richard Fry
Sent: Tuesday, August 26, 2008 8:18 AM
Subject: RE: Part 15 AM Rules
There are a few problems here.
1. The expected range for unlicensed FM is considerably less than that of unlicensed AM - probably on the order of about 30 feet or so. Yes, I know that a Public Notice was released saying that unlicensed AM and FM transmitters have 200 feet of range. On the practical side, however, you're lucky to even get 30 feet in the FM band. I did see one experiment achieve 400 feet of range when operated in a remote area with a low background noise level and no other FM stations any where near the selected frequency, using a very sensitive FM receiver with a good antenna, and transmitting in a mono mode with 75 kHz of deviation applied to the modulation. This is why our rules do not specify a range - it's a relative term that is completely dependent on the environment. The same applies to range estimates for operation in the AM band. (That same Public Notice also incorrectly stated that you can have 50 mW ERP in the AM band and 10 uW in the FM band. The 250 uV/m at 3 m limit in the FM band translates to an ERP of 11.4 nW or 0.000,000,014 W. A field strength level of 24 uV/m at 30 m, as permitted under Section 15.209 at 1000 kHz, translates to an ERP of 10.5 nW. The non-technical author of the notice should have checked with the engineers before writing this. Note that the numbers in this Public Notice are not binding - the equipment must meet the standards in the actual regulations.)
2. The paper you cite in http://i62.photobucket.com/albums/h8...sed1500kHz.gif uses an RF loading coil to match to the impedance of the shorter antenna, thereby considerably increasing the resulting field strength. When a loading coil is employed, we consider its electrical length as part of the antenna. If a loading coil of the type described is installed, the antenna (plus connecting cable and ground lead) will far exceed the permitted 3 m length. When the antenna and ground meets the requirements in Section 15.219, you will have a much lower field strength.
3. A transmitter is certified under Section 15.209 or 15.219. If it was certified under Section 15.209, the radiated emissions must comply with the limits in that section. If it was certified under Section 15.219, the input power and the combined lengths of the antenna, connecting cable and ground must comply with the limits in that section. There is no need to check for compliance under Section 15.209 when the transmitter was certified under Section 15.219. If the 15.219 system exceeds the standards in that section, it is in violation of the rules. Of course, if the 15.219 system exceeds those specifications the radiated emissions also will exceed the 15.209 limits. The field inspection folks may check for both simply to show that the system is not compliant under any of the possible standards.
From: Richard Fry
Sent: Tuesday, August 26, 2008 11:54 AM
To: John Reed
Subject: Re: Part 15 AM Rules
Thank you for your reply.
Could you please comment further on these quotes from it, with the respect the questions they prompted?
"1. That same Public Notice also incorrectly stated that you can have 50 mW ERP in the AM band and 10 uW in the FM band."
I believe the notice states "0.010 microwatts" for FM, rather than 10 uW. A value of 0.010 µW is 10 nW. Also it can be noted that 10 nW is a little less than the power given per the equation in FCC OET Bulletin 63 to produce the maximum Part 15 FM field from a 1/2-wave dipole, which is 11.43 nW, although the equation in OET 63 applies to the power radiated by an isotropic antenna, and yields an answer of 18.75 nW. That 18.75 nW must be reduced by the peak gain of the 1/2-wave dipole, which is 1.64 (18.75 / 1.64 = 11.43, approx).
"2. A field strength level of 24 uV/m at 30 m, as permitted under Section 15.209 at 1000 kHz, translates to an ERP of 10.5 nW."
This is true for far-field conditions, but according to Figure 6-5 and the accompanying text of Kraus' Antennas, 3rd Edition, a distance of 30 meters lies well within the near-field boundary -- which exists at about 47.7 meters for this setup (please see the clip of this at http://i62.photobucket.com/albums/h8...hicFields-.jpg ). As such wouldn't the ERP needed to produce 24 µV/m at a 30-meter distance be less than 10.5 nW?
"3. When a loading coil is employed, we consider its electrical length as part of the antenna. If a loading coil of the type described is installed, the antenna (plus connecting cable and ground lead) will far exceed the permitted 3 m length."
Is this an administrative interpretation? Wouldn't a "pure" application of physics show that, while there could be a relatively small amount of radiation from a loading coil, that radiation is far less than is produced when the amount of wire comprising the coil is analyzed separately in the form of a linear radiator?
Also, do you agree that the entire conducting path to an r-f ground buried in the earth constitutes the ground lead used on Part 15 systems, and that the ground lead is not limited to the length of a short conductor connecting the transmitter chassis to a grounded flagpole or tower?
From: John Reed
To: Richard Fry
Sent: Tuesday, August 26, 2008 11:27 AM
Subject: RE: Part 15 AM Rules
Re 1: You are correct; the PN references 10 nW, not 10 uW. However, we go by the field strength limit which works out to an EIPR of 18.75 nW or an ERP of 11.4 nW.
Re 2: Free space propagation equations are less than reliable under most real world situations. At 1000 kHz, assuming a small antenna, the near field/far field would occur at approximately 48 meters. That means it's easy to play games with the field strength measurement, moving the receiving antenna by a small increment to obtain a large change in measured signal level. I realize that the equation doesn't work at 30 m but wanted to provide a relative example.
Re 3: This is how we've always interpreted 15.219. The entire point of limiting the antenna length and the length of the connecting lead and ground lead is to ensure that the resulting antenna is extremely inefficient. It's not just the radiation from the loading coil; it's the much higher field strength that results from the system after you match impedances.
As to the ground lead, this is exactly what we've been saying all along. If you attach your ground lead to a drainpipe, billboard, or other structure, that structure becomes part of the ground lead and is included in the 3 m limit. Similarly, if you install a ground plane or otherwise change the ground efficiency.
*** End of e-mail sequence