Part 15 AM Coverage Capability: Reality Check

[rfry]

Antenna engineering textbooks, NEC calculations, and thousands of field strength measurements made in the broadcast industry over the last 75+ years show that the maximum field strength in the horizontal plane that is produced by a vertical, 1/4-wave monopole radiating 1,000 watts over an almost perfectly conducting, flat ground plane is about 300 millivolts/meter (mV/m) at a radius of 1 km (0.62 miles).

A legal Part 15 AM tx that was 100% efficient, and used with the above antenna would generate a field at 1 km that would be reduced by the square root of the power difference, or to a field of 3 mV/m in this case.

But a 3-meter, ground-mounted Part 15 antenna system is only about 1% as efficient as the 1/4-wave radiator is. So instead of radiating 100 mW, the Part 15 antenna system radiates around 1 mW. That leads to a further reduction in the field at 1 km by the square root of 100, bringing it to about 300 microvolts/meter (µV/m).

Note that all of these fields assume an almost perfectly-conducting ground over the propagation path. Typical ground conditions are far from perfect, so the fields at 1 km would not be even this high.

By broadcast standards, a 300 µV/m field is very marginal in providing a usable signal to a typical, cheap AM receiver located inside a home. And every doubling of the distance decreases the received field by more than 50% (including ground losses).

From this information it can be seen that claims of "legal" Part 15 AM coverage extending for a radius of 2, 3 and 4 miles cannot be realistic, unless the system is not meeting Part 15 limits.
//

 

[sparky_marky]

Is that based on one tx?
What about the claim of more distance with 3 - 5 tx's ganged up?
Thank you for the inlightment.

sparky_marky

 

[Black_Shire]

Antenna engineering textbooks, NEC calculations, and thousands of field strength measurements made in the broadcast industry over the last 75+ years show that the maximum field strength in the horizontal plane that is produced by a vertical, 1/4-wave monopole radiating 1,000 watts over an almost perfectly conducting, flat ground plane is about 300 millivolts/meter (mV/m) at a radius of 1 km (0.62 miles).

A legal Part 15 AM tx that was 100% efficient, and used with the above antenna would generate a field at 1 km that would be reduced by the square root of the power difference, or to a field of 3 mV/m in this case.

But a 3-meter, ground-mounted Part 15 antenna system is only about 1% as efficient as the 1/4-wave radiator is. So instead of radiating 100 mW, the Part 15 antenna system radiates around 1 mW. That leads to a further reduction in the field at 1 km by the square root of 100, bringing it to about 300 microvolts/meter (µV/m).

Note that all of these fields assume an almost perfectly-conducting ground over the propagation path. Typical ground conditions are far from perfect, so the fields at 1 km would not be even this high.

By broadcast standards, a 300 µV/m field is very marginal in providing a usable signal to a typical, cheap AM receiver located inside a home. And every doubling of the distance decreases the received field by more than 50% (including ground losses).

From this information it can be seen that claims of "legal" Part 15 AM coverage extending for a radius of 2, 3 and 4 miles cannot be realistic, unless the system is not meeting Part 15 limits.
//

...Or unless the transmitter is battery-powered, with no AC power lead or audio line (if the transmitter's built-in auto-repeat digital audio recorder is being used instead) running up the pole to the transmitter box. The Part 15 AM transmitters' AC power leads, audio input cords, and microphone cords all radiate, but their lengths are not counted as part of the 3 meter antenna + antenna feed line (if any) + ground lead total combined length. Why? Because they are incidental radiators that are not connected to the transmitters' tank circuits and any RF they radiate is capacitively coupled into them. These conductors don't radiate much, but their radiation in combination with the stronger line-of-sight signal from the 3 meter antenna is how these transmitters achieve broadcast ranges of 2 - 3 miles.


-- Black Shire

 

[rfry]

Is that based on one tx? What about the claim of more distance with 3 - 5 tx's ganged up?

The numbers I posted are based on a single transmitter, with no co-channel interference. If 3-5 Part 15 AM transmitters on the same frequency were far enough apart geographically so that their useful coverage areas did not overlap, then yes, that system would cover 3-5 times more area than a single system (other things equal).

But if the separate stations had overlapping areas where their signal strengths were about 10% or more of the geographically-adjacent station(s), then they would interfere with each other in some coverage sectors, even if their carrier frequencies and modulation polarities were synchronized -- which would correspondingly reduce the total coverage area of the "network."
//

 

[Black_Shire]

Is that based on one tx?
What about the claim of more distance with 3 - 5 tx's ganged up?
Thank you for the inlightment.

sparky_marky

The AM1000 Rangemaster www.am1000rangemaster.com has provision for synchronizing several geographically-separated transmitters together to cover a larger area, without the annoying "beat" in the received audio that the listeners would otherwise hear if several un-synchronized transmitters were operated on the same frequency.

While the scattered transmitters do produce an aggregate pattern that has lobes and nulls, the users select the transmitter sites so that the nulls "fall" on areas where the signal isn't wanted or needed.


-- Black Shire

 

[rfry]

Part 15 AM transmitters' AC power leads, audio input cords, and microphone cords all radiate, but their lengths are not counted as part of the 3 meter antenna + antenna feed line (if any) + ground lead total combined length. Why? Because they are incidental radiators that are not connected to the transmitters' tank circuits and any RF they radiate is capacitively coupled into them.

But note that even though a "ground lead" connected to the ground terminal of a Part 15 AM tx chassis is not directly connected to the output of the tank circuit of a Part 15 AM tx (assuming it has one), if that conducting path to an r-f ground plane is long it can have a profound effect on the net fields radiated by the Part 15 AM antenna system.

The same is true of any other radiating wires (no matter how energized) in a Part 15 AM system, whether or not they are acknowledged by a Part 15 operator or even an FCC inspector as doing so.
//

 

[sparky_marky]

if in the process of using 3-5 transmitters you find 1 of your repeaters
is in a bad place and you get the 'overlap zone" with that annoing pulse. Can you lower the broadcast power level in the bad location to adjust the size of the
overlap zone? Or does the sinc'ing remove that no matter if one is too close or not ?
thanks guys

sparky_marky

 

[Black_Shire]

Part 15 AM transmitters' AC power leads, audio input cords, and microphone cords all radiate, but their lengths are not counted as part of the 3 meter antenna + antenna feed line (if any) + ground lead total combined length. Why? Because they are incidental radiators that are not connected to the transmitters' tank circuits and any RF they radiate is capacitively coupled into them.

But note that even though a "ground lead" connected to the ground terminal of a Part 15 AM tx chassis is not directly connected to the output of the tank circuit of a Part 15 AM tx (assuming it has one), if that conducting path to an r-f ground plane is long it can have a profound effect on the net fields radiated by the Part 15 AM antenna system.

The same is true of any other radiating wires (no matter how energized) in a Part 15 AM system, whether or not they are acknowledged by a Part 15 operator or even an FCC inspector as doing so.
//

In the Part 15 AM transmitters with metal cabinets, the cabinet is indirectly connected (through the RF chassis ground on the board) to the "cold" end of the tank circuit inductor (the antenna goes to the "hot" end, of course) and serves as part of the RF ground. The AC ground lead and the ground lead of the unbalanced audio input jack are connected to the RF chassis ground on the board as well, but not directly to the "cold" end of the tank circuit inductor. These connections are also used in Part 15 AM transmitters that don't have metal cabinets (except the Rangemaster, which uses a balanced 600 ohm audio line).

Do the AC and audio lines radiate? Of course they do. Does the FCC care? As long as the radiating conductors are AC power and/or audio leads for the transmitter, no.


-- Black Shire

 

[Black_Shire]

if in the process of using 3-5 transmitters you find 1 of your repeaters
is in a bad place and you get the 'overlap zone" with that annoing pulse. Can you lower the broadcast power level in the bad location to adjust the size of the
overlap zone? Or does the sinc'ing remove that no matter if one is too close or not ?
thanks guys

sparky_marky

The synchronization of the Rangemaster transmitters prevents the audio "beat" from occurring -- Black Shire

 

[rfry]

if in the process of using 3-5 transmitters you find 1 of your repeaters
is in a bad place and you get the 'overlap zone" with that annoing pulse. Can you lower the broadcast power level in the bad location to adjust the size of the
overlap zone? Or does the sinc'ing remove that no matter if one is too close or not ?

The synchronization of the Rangemaster transmitters prevents the audio "beat" from occurring -- Black Shire

Accurately synchronizing the r-f carrier frequencies of nearby co-channel Part 15 AM stations will avoid the single-tone audio beat(s) that can result from not synchronizing them, but that will not remove the pattern interference that they can generate -- which can produce geographic zones where none of the stations can be clearly heard due to a net cancellation of their r-f fields.

Neither will it cure the effects of non-matching program audio modulation polarities, which can cause audio distortion in receivers even though all of the separate Part 15 txs are frequency- and phase-locked.
//

 

[Black_Shire]

if in the process of using 3-5 transmitters you find 1 of your repeaters
is in a bad place and you get the 'overlap zone" with that annoing pulse. Can you lower the broadcast power level in the bad location to adjust the size of the
overlap zone? Or does the sinc'ing remove that no matter if one is too close or not ?

The synchronization of the Rangemaster transmitters prevents the audio "beat" from occurring -- Black Shire

Accurately synchronizing the r-f carrier frequencies of nearby co-channel Part 15 AM stations will avoid the single-tone audio beat(s) that can result from not synchronizing them, but that will not remove the pattern interference that they can generate -- which can produce geographic zones where none of the stations can be clearly heard due to a net cancellation of their r-f fields.

Neither will it cure the effects of non-matching program audio modulation polarities, which can cause audio distortion in receivers even though all of the separate Part 15 txs are frequency- and phase-locked.
//

In your world, I guess...

 

[jcolletta275]

No, he's right... it's the same effect that allows any AM station to have a directional signal. If you transmit from more than one antenna, each in a different place, the placement of each antenna will have an effect on the coverage pattern of your signal. I'm no expert as to how that's done and how to control it, but that's how it works. There is, of course, a difference between different antennae connected to the same transmitter and different antennae connected to different transmitters, but the effect will still appear to some degree.

 

[Black_Shire]

No, he's right... it's the same effect that allows any AM station to have a directional signal. If you transmit from more than one antenna, each in a different place, the placement of each antenna will have an effect on the coverage pattern of your signal. I'm no expert as to how that's done and how to control it, but that's how it works. There is, of course, a difference between different antennae connected to the same transmitter and different antennae connected to different transmitters, but the effect will still appear to some degree.

As I said in an earlier posting in this thread, all of the geographically-dispersed Rangemaster transmitters *do* create an aggregate pattern that has lobes and nulls, but the transmitter users select the transmitter sites so that the nulls "fall" on areas where the signal isn't wanted or needed. Also, this system has an important advantage over a directional tower array: unlike a directional tower array, there are no feed line losses or phasing line losses because each transmitter has its own directly-connected 102" whip antenna (with no feed line) atop its weatherproof enclosure.


-- Black Shire

 

[rfry]

Do the AC and audio lines radiate? Of course they do. Does the FCC care? As long as the radiating conductors are AC power and/or audio leads for the transmitter, no.

The FCC does care about limiting the interference capabilities of unlicensed stations in the AM broadcast bands. That's why power and antenna length limits were set up in Part 15 AM rules in the first place.

Believing that it is acceptable to the FCC to improve Part 15 AM coverage by radiation from a ground, power or audio wire in addition to the 3-meter radiator would be a difficult position to defend, from an engineering and conceptual viewpoint. Have you (or anyone) ever seen any documented support for this idea from the FCC?

These conductors don't radiate much...

On the contrary, physics shows that in an installation where the Part 15 tx and 3-m radiator are installed ~5 feet or more above the earth, those wires can radiate more r-f than the 3-meter section above them that is considered to be the complete antenna.

. .. but their radiation in combination with the stronger line-of-sight signal from the 3 meter antenna is how these transmitters achieve broadcast ranges of 2 - 3 miles.

The first post in this thread shows that a field strength of about 300 µV/m at 1 km would be the upper limit for a fully compliant Part 15 AM operation. That value assumed nearly perfect conditions, including a clear line of sight between the tx and rx. So the field at 1 km would not change even if the 3-m antenna was elevated above the earth, unless that additional field was generated by a higher power tx and/or by using an effective antenna length greater than 3 meters (by radiation from ground leads, etc).

The r-f power radiated by the nearly perfect Part 15 installation in this example would have to be increased at least 1600% in order to move its 300 µV/m contour out to 2.5 miles.
//

 

[Black_Shire]

James R. Cunningham was a licensed broadcast engineer who designed and built AM, FM, and Short Wave radio stations in the USA and all over the world. He also produced and sold his own line of several different Part 15 AM transmitters, ATUs, and Carrier Current AC line couplers that conformed to the Part 15 rules governing all three categories of Part 15 AM broadcasting systems (100 milliwatt intentional radiators using 3 meter antennas, Carrier Current systems, and Part 15 AM campus rules intentional radiators).

In his publications he provided real-world measurements of his systems' performance using professional-grade Field Strength Meters and oscilloscopes. He also reproduced the relevant FCC rules in his publications, and his equipment and installations (including his elevated 100 milliwatt Part 15 AM transmitter systems) complied to those rules *to the letter*.

When you design and produce your own line of Part 15 AM equipment and publish your measured (not theoretical or computer-modeled) data that identifies the test equipment you used to prove your assertions (so that others can test them) as James Cunningham did, I'll take your pronouncements seriously...but not until then.


-- Black Shire

Do the AC and audio lines radiate? Of course they do. Does the FCC care? As long as the radiating conductors are AC power and/or audio leads for the transmitter, no.

The FCC does care about limiting the interference capabilities of unlicensed stations in the AM broadcast bands. That's why power and antenna length limits were set up in Part 15 AM rules in the first place.

Believing that it is acceptable to the FCC to improve Part 15 AM coverage by radiation from a ground, power or audio wire in addition to the 3-meter radiator would be a difficult position to defend, from an engineering and conceptual viewpoint. Have you (or anyone) ever seen any documented support for this idea from the FCC?

These conductors don't radiate much...

On the contrary, physics shows that in an installation where the Part 15 tx and 3-m radiator are installed ~5 feet or more above the earth, those wires can radiate more r-f than the 3-meter section above them that is considered to be the complete antenna.

. .. but their radiation in combination with the stronger line-of-sight signal from the 3 meter antenna is how these transmitters achieve broadcast ranges of 2 - 3 miles.

The first post in this thread shows that a field strength of about 300 µV/m at 1 km would be the upper limit for a fully compliant Part 15 AM operation. That value assumed nearly perfect conditions, including a clear line of sight between the tx and rx. So the field at 1 km would not change even if the 3-m antenna was elevated above the earth, unless that additional field was generated by a higher power tx and/or by using an effective antenna length greater than 3 meters (by radiation from ground leads, etc).

The r-f power radiated by the nearly perfect Part 15 installation in this example would have to be increased at least 1600% in order to move its 300 µV/m contour out to 2.5 miles.
//

 

[rfry]

When you design and produce your own line of Part 15 AM equipment and publish your measured (not theoretical or computer-modeled) data that identifies the test equipment you used to prove your assertions (so that others can test them) as James Cunningham did, I'll take your pronouncements seriously...but not until then.

My numbers and conclusions can be verified simply by referring to the FCC rules for AM broadcast stations, and elementary mathematics.

There is a requirement in 47 CFR Part 73.189 (2)(ii) that licensed AM broadcast stations of certain classes must be able to produce a minimum field strength of 282 mV/m at 1 km for 1 kW of radiated power: Anyone can confirm that using the FCC web site.

The FCC web site also has an on-line applet where field strength at 1 km for 1 kW of radiated power can be determined based on the height of the tower, the frequency, and the configuration of the ground system. This data is based on real field strength measurments of real hardware given in the benchmark 1937 paper by Dr. George Brown and others of RCA as published in the Proceedings of the I.R.E, and which has been confirmed by real field strength measurements of real hardware by qualified broadcast engineers and consultants literally thousands of times since.

Plugging the numbers from paragraph one of my opening post in this thread into the FCC applet gives a result of about 306 mV/m (see link at bottom of this post). I rounded the value to 300 mV/m in my first post.

The rest of the conclusions in my first post are based on standard engineering concepts and simple math, flowing from this reference value of 300 mV/m at 1 km for 1 kW of radiated power.

It should also be noted that Numerical Electromagnetics Code (NEC) is a proven means to evaluate r-f radiation systems, and is commonly used by broadcast consultants to design directional arrays that must strictly meet FCC pattern requirements as proven by field strength measurements after installation. And they do.

Richard Fry, CPBE (Life Certification)

- FCC 1st Class Radiotelephone Operator's License first issued in 1959
- Staff and Chief Engineer of AM and FM broadcast stations
- RCA Broadcast Division Field Engineering Supervisor, 1965-1980
- Harris Broadcast Division Senior RF Applications Engineer, 1980-1999 (retired)

LINK: http://i62.photobucket.com/albums/h85/rfry-100/FieldforQtrWaveMonopole.gif

 

[Black_Shire]

If computer modeling and theoretical calculations were totally sufficient, the FCC would not require proof measurements of new stations and antenna modifications to existing stations (nor would the FAA require flight testing of new aircraft designs and modifications to existing aircraft designs). Q.E.D.


-- Black Shire

When you design and produce your own line of Part 15 AM equipment and publish your measured (not theoretical or computer-modeled) data that identifies the test equipment you used to prove your assertions (so that others can test them) as James Cunningham did, I'll take your pronouncements seriously...but not until then.

My numbers and conclusions can be verified simply by referring to the FCC rules for AM broadcast stations, and elementary mathematics.

There is a requirement in 47 CFR Part 73.189 (2)(ii) that licensed AM broadcast stations of certain classes must be able to produce a minimum field strength of 282 mV/m at 1 km for 1 kW of radiated power: Anyone can confirm that using the FCC web site.

The FCC web site also has an on-line applet where field strength at 1 km for 1 kW of radiated power can be determined based on the height of the tower, the frequency, and the configuration of the ground system. This data is based on real field strength measurments of real hardware given in the benchmark 1937 paper by Dr. George Brown and others of RCA as published in the Proceedings of the I.R.E, and which has been confirmed by real field strength measurements of real hardware by qualified broadcast engineers and consultants literally thousands of times since.

Plugging the numbers from paragraph one of my opening post in this thread into the FCC applet gives a result of about 306 mV/m (see link at bottom of this post). I rounded the value to 300 mV/m in my first post.

The rest of the conclusions in my first post are based on standard engineering concepts and simple math, flowing from this reference value of 300 mV/m at 1 km for 1 kW of radiated power.

It should also be noted that Numerical Electromagnetics Code (NEC) is a proven means to evaluate r-f radiation systems, and is commonly used by broadcast consultants to design directional arrays that must strictly meet FCC pattern requirements as proven by field strength measurements after installation. And they do.

Richard Fry, CPBE (Life Certification)

- FCC 1st Class Radiotelephone Operator's License first issued in 1959
- Staff and Chief Engineer of AM and FM broadcast stations
- RCA Broadcast Division Field Engineering Supervisor, 1965-1980
- Harris Broadcast Division Senior RF Applications Engineer, 1980-1999 (retired)

LINK: http://i62.photobucket.com/albums/h85/rfry-100/FieldforQtrWaveMonopole.gif

 

[rfry]

If computer modeling and theoretical calculations were totally sufficient, the FCC would not require proof measurements of new stations and antenna modifications to existing stations (nor would the FAA require flight testing of new aircraft designs and modifications to existing aircraft designs).

The point you seem to be missing is that the measured data I referred to in my last post is the basis for my statements about the best-case coverage capabilities of compliant Part 15 AM stations.

The FCC computer applet I referred to only makes it easy to access that measured data for the conditions on which my first post was based, and to show the basis for the credibility of my conclusions.

If you don't believe that a legal 1 kW AM broadcast station as in 73.189 (2)(ii) develops at least a 282 mV/m field at a radius of 1 km, please research this belief further in the engineering resources available to you.

Better yet, measure one for yourself with your own NIST-traceable field strength measuring equipment, and report back to us with your test equipment lineup, methods, and results.
//

 

[Black_Shire]

Er--you have that backwards. James Cunningham designed and built his Part 15 AM equipment (100 milliwatt intentional radiators, Carrier Current systems, and Part 15 campus rules intentional radiators) and made measurements of their performance using professional-grade test equipment, and then published his test results so that anyone could duplicate the tests for themselves.

Until and unless you do the same, your charts and plots are inferior to his real-world data that he obtained on actual, physical transmitters that he built, installed, and tested.


-- Black Shire

If computer modeling and theoretical calculations were totally sufficient, the FCC would not require proof measurements of new stations and antenna modifications to existing stations (nor would the FAA require flight testing of new aircraft designs and modifications to existing aircraft designs).

The point you seem to be missing is that the measured data I referred to in my last post is the basis for my statements about the best-case coverage capabilities of compliant Part 15 AM stations.

The FCC computer applet I referred to only makes it easy to access that measured data for the conditions on which my first post was based, and to show the basis for the credibility of my conclusions.

If you don't believe that a legal 1 kW AM broadcast station as in 73.189 (2)(ii) develops at least a 282 mV/m field at a radius of 1 km, please research this belief further in the engineering resources available to you.

Better yet, measure one for yourself with your own NIST-traceable field strength measuring equipment, and report back to us with your test equipment lineup, methods, and results.
//

 

[rfry]

James Cunningham designed and built his Part 15 AM equipment (100 milliwatt intentional radiators, Carrier Current systems, and Part 15 campus rules intentional radiators) and made measurements of their performance using professional-grade test equipment, and then published his test results so that anyone could duplicate the tests for themselves. Until and unless you do the same, your charts and plots are inferior to his real-world data that he obtained on actual, physical transmitters that he built, installed, and tested.

Do you really think that the beliefs of James Cunningham on this topic are superior to those upon which the FCC bases its AM allocations and coverage policies that, for decades of provable field experience have been shown to be accurate?

If your perceptions are correct, and show with scientific accuracy that my statements about Part 15 AM coverage capabilities are wrong, I will publicly say so here with credit to you.

And if your statements are wrong, are you willing to do the same?

(Later) Also... why do you use a screen name that obscures your true identity? And what are your workplace and academic credentials that might encourage your readers to believe what you write?
//