Part 15 AM Coverage - High vs. Low Carrier Frequency

A NEC4.2 analysis shows this, for the system described:

• 2.8-meter, Base-fed, Base-loaded, Vertical Monopole
• Monopole Base Elevation = 0.2 meters above the Earth
• R-F Ground = 16 Buried Radials, 3 meters in length (each)
• Radial Common Point Concentric with Vertical Axis of Monopole
• Earth Conductivity = 5 mS/m, d.c. 13 (about average)
• Applied R-F Power at Loading Coil Input = 80 mW, unmodulated

At 1650 kHz...

• Loading Coil R-F Resistance at System Resonance = 20 Ω (assumed)
• Groundwave Field Intensity at 1 mile = 84 µV/m

At 570 kHz...

• Loading Coil R-F Resistance at System Resonance = 30 Ω (assumed)
• Groundwave Field Intensity at 1 mile = 34 µV/m

More details:

For the 1650/570 kHz systems:

• Radiation resistance (R[SUB]r[/SUB]) = 0.1/0.01 Ω
• Ground Connection R-F Resistance (R[SUB]g[/SUB]) = 7/7.2 Ω
• Loading Coil R-F Resistance at System Resonance (R[SUB]c[/SUB]) = 20/30 Ω, assumed

Antenna system radiation efficiency = R[SUB]r[/SUB] / (R[SUB]r[/SUB] + R[SUB]g[/SUB] + R[SUB]c[/SUB]).

The reduced, groundwave propagation losses at 570 kHz are not able to compensate for the lower ERP that system produces.

§15.221 Operation in the band 525-1705 kHz.

(a) Carrier current systems and transmitters employing a leaky coaxial cable as the radiating antenna may operate in the band 525-1705 kHz provided the field strength levels of the radiated emissions do not exceed 15 uV/m, as measured at a distance of 47,715/(frequency in kHz) meters (equivalent to Lambda/2Pi) from the electric power line or the coaxial cable, respectively. The field strength levels of emissions outside this band shall not exceed the general radiated emission limits in §15.209.

(b) As an alternative to the provisions in paragraph (a) of this section, intentional radiators used for the operation of an AM broadcast station on a college or university campus or on the campus of any other education institution may comply with the following:

(1) On the campus, the field strength of emissions appearing outside of this frequency band shall not exceed the general radiated emission limits shown in §15.209 as measured from the radiating source. There is no limit on the field strength of emissions appearing within this frequency band, except that the provisions of §15.5 continue to comply.

(2) At the perimeter of the campus, the field strength of any emissions, including those within the frequency band 525-1705 kHz, shall not exceed the general radiated emission in §15.209.

(3) The conducted limits specified in §15.207 apply to the radio frequency voltage on the public utility power lines outside of the campus. Due to the large number of radio frequency devices which may be used on the campus, contributing to the conducted emissions, as an alternative to measuring conducted emissions outside of the campus, it is acceptable to demonstrate compliance with this provision by measuring each individual intentional radiator employed in the system at the point where it connects to the AC power lines.

(c) A grant of equipment authorization is not required for intentional radiators operated under the provisions of this section. In lieu thereof, the intentional radiator shall be verified for compliance with the regulations in accordance with subpart J of part 2 of this chapter. This data shall be kept on file at the location of the studio, office or control room associated with the transmitting equipment. In some cases, this may correspond to the location of the transmitting equipment.

(d) For the band 535-1705 kHz, the frequency of operation shall be chosen such that operation is not within the protected field strength contours of licensed AM stations.

§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 (microvolts/meter) Measurement distance (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



(b) In the emission table above, the tighter limit applies at the band edges.

Drilling Down

Kelly:

The system I described and analyzed in my opening post is different than a system permitted either by FCC §15.221 or §15.209 that you referenced. But it is legal, nonetheless.

The configuration and operating parameters in my OP are not limited as to the field intensities they legally may produce, as long as they strictly meet sub-paragraphs (a), (b), and (c) of FCC §15.219 (quoted below) -- which is true for my OP.

§15.219 Operation in the band 510-1705 kHz.

(a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100 milliwatts.

(b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.

(c) All emissions below 510 kHz or above 1705 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be demonstrated by measuring the radiated emissions.

rfry said:

A NEC4.2 analysis shows this, for the system described:

• 2.8-meter, Base-fed, Base-loaded, Vertical Monopole
• Monopole Base Elevation = 0.2 meters above the Earth
  [*]R-F Ground = 16 Buried Radials, 3 meters in length (each)
• Radial Common Point Concentric with Vertical Axis of Monopole
• Earth Conductivity = 5 mS/m, d.c. 13 (about average)
• Applied R-F Power at Loading Coil Input = 80 mW, unmodulated

Click to expand...

16 three-meter radials is 48 meters of "ground lead." Isn't that a no-no?

Ground Lead?

KeithE4 said:

16 three-meter radials is 48 meters of "ground lead." Isn't that a no-no?

Click to expand...

The r-f ground lead of a Part 15 AM transmit system consists of (and is limited to) the total length of the physical conductor(s) connecting the r-f ground terminal or chassis of that system to a true r-f ground.

The true r-f ground in this case uses sixteen 3-meter radials buried in the earth with their common point located directly under the vertical axis of the monopole.

Those buried wires are not "ground leads," they are just part of the r-f ground reference, itself. They add no useful radiation of their own to the far-field radiation of that transmit system.

rfry said:

The r-f ground lead of a Part 15 AM transmit system consists of (and is limited to) the total length of the physical conductor(s) connecting the r-f ground terminal or chassis of that system to a true r-f ground.

The true r-f ground in this case uses sixteen 3-meter radials buried in the earth with their common point located directly under the vertical axis of the monopole.

Those buried wires are not "ground leads," they are just part of the r-f ground reference, itself. They add no useful radiation of their own to the far-field radiation of that transmit system.

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I understand what you're saying, but I remember something from years ago that the FCC considered buried radials -- indeed, any physical wire or cable that is part of the antenna system, including non-radiating coax -- as part of the 3 meter limit. I like your idea better, but...

In other words, if I built a system like you describe, feed it with a length of coax from the transmitter, I would have to make sure that the field strength at 30 meters is 24000/f (kHz), regardless of the power output, in order to be legal under 15.209, and keep the FCC off my back. 570 kHz would mean a field strength of 42.1 µV/m at 30 meters, and at 1650 kHz it would be 14.5 µV/m. I have no idea how that would translate to ERP. It could not be legal under 15.219 from what I understand, unless the radial field were not there, and the antenna was connected directly to the transmitter output (where the loading coil would be).

The rule is poorly worded by 2017 standards, but IIRC, the intention was to keep the Part 15 station's range to be about 100 yards or less. It'd cover a few houses in a neighborhood, but that's about it.

KeithE4 said:

... In other words, if I built a system like you describe, feed it with a length of coax from the transmitter, ...

Click to expand...

Just to note that the systems I described did not include that length of coax.

Certainly they could be included in systems complying with FCC §15.209.

But a system fully compliant with §15.219 can produce much higher groundwave fields than those limited by compliance to §15.209.

rfry said:

Just to note that the systems I described did not include that length of coax.

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True, but if the transmitter is mounted outside, it'll still require coax to deliver power and audio. It's still necessary, plus filtering would be required to keep the noise off the power and audio. Much easier to use the coax for RF, especially given the low loss if the antenna is properly matched (less than 1 dB at 1.7 MHz, even with a 5:1 VSWR). Too bad it's technically not legal under §15.219.

Certainly they could be included in systems complying with FCC §15.209.

But a system fully compliant with §15.219 can produce much higher groundwave fields than those limited by compliance to §15.209.

Click to expand...

§15.209 specifies about 14.5 µV/m at 30 meters for 1650 kHz. You state a field strength of 84 µV/m at one mile (1609 meters) with your setup -- a lot higher at 30 meters. You don't think the FCC will flag this if they get wind of it? That has to be much, much more power than what would be generated with a 70-80 mW transmitter output with a 10 foot wire hanging off of it (most likely inside) and no real ground other than maybe the AC power line the transmitter is plugged into -- something more like what the FCC intended when they came up with these rules decades ago.

KeithE4 said:

... §15.209 specifies about 14.5 µV/m at 30 meters for 1650 kHz. You state a field strength of 84 µV/m at one mile (1609 meters) with your setup -- a lot higher at 30 meters. You don't think the FCC will flag this if they get wind of it? ...

Click to expand...

The FCC would have no legal basis in 47CFR Part 15 to cite an unlicensed operator strictly meeting §15.219(a)(b)(c), no matter what field intensity exists 30 meters from that transmit system.

The FCC citations (NOUOs) that have been issued to such operators usually do refer to the field intensity the FCC measured at some distance, and note that it exceeds the value permitted by §15.209.

Then the citation notes that the tested system did not meet §15.219, either, often due to the use of a long, radiating "ground" conductor that made the antenna system non-compliant with 15.219(b).

Those two findings form the basis for the NOUO.

No doubt the FCC has inspected some installations that did not meet §15.209, but did meet all of §15.219. Of course then no violation occurred, and no NOUO would be issued.

... no real ground other than maybe the AC power line the transmitter is plugged into ...

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Unfortunately, the "ground pin" receptacle of an a-c outlet does not provide a useful ground reference for radio frequencies. The conductor(s) connecting it to the ground rod at the a-c service entrance will be part of the radiating length of the antenna system until reaching the ground rod, itself.

Below is an illustration of this:

rfry said:

Kelly:

The system I described and analyzed in my opening post is different than a system permitted either by FCC §15.221 or §15.209 that you referenced. But it is legal, nonetheless.

Click to expand...

Really? So you're saying that a system like this would comply with the emission distance measurements limit of 30 meters as listed? A couple thoughts:

1. No, I don't think this would comply.
2. Who friggin cares? The AM band is slowly aging out. Building a Part 15 or, one could argue, an illegal pirate MW station on a dying broadcast band is a complete waste of time and effort. You can play with antenna design legally with amateur radio. Why screw around with this?

Kelly A said:

Really? So you're saying that a system like this would comply with the emission distance measurements limit of 30 meters as listed? ... No, I don't think this would comply. ...

Click to expand...

\\ §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: etc... //


The transmit system I defined and analyzed in my opening post complies with §15.219(a)(b)(c), which is permitted as an exception to §15.209 by the clip next above.

FCC §15.219 does not limit the field intensity that a transmit system complying with it can produce at 30 meters, or any other distance.

rfry said:

\\ §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: etc... //


The transmit system I defined and analyzed in my opening post complies with §15.219(a)(b)(c), which is permitted as an exception to §15.209 by the clip next above.

FCC §15.219 does not limit the field intensity that a transmit system complying with it can produce at 30 meters, or any other distance.

Click to expand...

All well and good, but I think it would be very smart to heed the FCC's advice on how much range a Part 15 broadcaster should get: About 200 feet on AM and FM.

FCC said:

Part 15 Devices

Unlicensed operation on the AM and FM radio broadcast bands is permitted for some extremely low powered devices covered under Part 15 of the FCC's rules. On FM frequencies, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR (Code of Federal Regulations) Section 15.239, and the July 24, 1991 Public Notice Opens a New Window. (still in effect). On the AM broadcast band, these devices are limited to an effective service range of approximately 200 feet (61 meters). See 47 CFR Sections 15.207, 15.209, 15.219, and 15.221. These devices must accept any interference caused by any other operation, which may further limit the effective service range.

For more information on Part 15 devices, please see OET Bulletin No. 63 ("Understanding the FCC Regulations for Low-Power, Non-Licensed Transmitters") Opens a New Window. . Questions not answered by this Bulletin can be directed to the FCC's Office of Engineering and Technology, Customer Service Branch, at the Columbia, Maryland office, phone (301) 362 - 3000.

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https://www.fcc.gov/media/radio/low-power-radio-general-information

FCC Advice

KeithE4 said:

All well and good, but I think it would be very smart to heed the FCC's advice on how much range a Part 15 broadcaster should get: About 200 feet on AM and FM.

Click to expand...

Following in this post is part of 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.

The FCC Public Notice referred to below by John Reed is linked at https://apps.fcc.gov/edocs_public/attachmatch/DOC-297510A1.pdf .

From: John Reed
To: Richard Fry
Sent: Tuesday, August 26, 2008 8:18 AM
Subject: RE: Part 15 AM Rules

... 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. ...

The Public Notice says 0.01 microwatts on FM -- close enough for government work. But the AM number is clearly wrong. And 30 feet is about what I get on my FM transmitters that are plugged into PCs (designed for in-car use, but they're perfectly adequate for inside the house).

Followup

Below is a plot of the groundwave fields produced from 0.1 to about 0.9 miles by the Part 15 AM setup defined in my opening post.

The field at 1/4 mile for this set of conditions is 0.43 mV/m.

A field of 0.15 mV/m might be useful for a good receiver setup in a low-noise area. In this case that field occurs about 0.62 miles from the transmit antenna.

I still don't get why you're so excited about this. Doing part 15 on the AM band, is like inventing carbon fiber buggy whips.

Perhaps he just enjoys the engineering challenge of seeing what he can get
out of a tiny fraction of a watt.

FreddyE1977 said:

Perhaps he just enjoys the engineering challenge of seeing what he can get out of a tiny fraction of a watt.

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Or, in case of myself and others who use Part 15 FM devices to play our music throughout the house, the proliferation of translators and LPFM stations is pretty much wiping out FM frequencies in many metro areas. We'll have no choice but to use AM in the not-too-distant future.

KeithE4 said:

Or, in case of myself and others who use Part 15 FM devices to play our music throughout the house, the proliferation of translators and LPFM stations is pretty much wiping out FM frequencies in many metro areas. We'll have no choice but to use AM in the not-too-distant future.

Click to expand...

Here in the 21st Century there's this thing called Bluetooth. For $35 dollars someone can purchase a Bluetooth transmitter and not much more for a receiver that will not only cover your entire house with 50Hz-20Khz audio bandwidth, but with overall, much better sounding than some Part 15 mono AM lash up.

Kelly A said:

Here in the 21st Century there's this thing called Bluetooth. For $35 dollars someone can purchase a Bluetooth transmitter and not much more for a receiver that will not only cover your entire house with 50Hz-20Khz audio bandwidth, but with overall, much better sounding than some Part 15 mono AM lash up.

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I am quite aware of Bluetooth, Mr. Know-it-All. I own a couple of those BT speakers, as a matter of fact. I also have nothing but contempt for them. Too much of a pain to set up. A radio is Turn it on, tune it in, done. No pairing. No headaches.

If you don't like Part 15 devices, then that's all just fine and dandy. That's your choice. But don't bother wasting your time or mine complaining about them.