Signal Strength/Distance Question for Part 15 AM

[Carmine5]

Most of us are aware of the (in)famous Long Beach, CA pirate who was operating on 1500 and whose signal was heard over about a three mile radius. In the FCC's NAL it said this:

"Your operation on frequency 1500 kHz was measured at
30,000 microvolts per meter (uV/m) at 30 meters. This exceeds the
allowable unlicensed limit of 16 uV/m at 30 meters established in Section
15.209 of the Rules."
http://www.fcc.gov/eb/FieldNotices/2003/DOC-284465A1.html

However I can find no published rule for a specified uV/m at 30 meter strength for Part 15 AM. The published rule for AM according to the FCC's latest OET Bulletin (July 10, 2008) for Part 15 is this:

"Section 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."

http://www.fcc.gov/oet/info/rules/part15/PART15_07-10-08.pdf

So who's zoom'in who?

C5

 

[Carmine5]

As a follow up, 15.209 of the rules state this:

Frequency Field Strength Measurement Distance
(MHz) (microvolts/meter) (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
_______________________________________________________________

So what kind of an impact are we looking at for part 15 AM hobbycasting?

C5

 

[rfry]

As a follow up, 15.209 of the rules state this:

Frequency Field Strength Measurement Distance
(MHz) (microvolts/meter) (meters)

0.490 - 1.705 24000/F(kHz) 30

So what kind of an impact are we looking at for part 15 AM hobbycasting?

C5

The legal groundwave field intensity allowed at a distance of 30 meters from an unlicensed AM setup on 1500 kHz is 16 microvolts per meter, per FCC 15.209 (and the quoted NOUO). Clearly the FCC measurement showed non-compliance with 15.209.

The link below leads to a calculation using standard engineering equations for about the maximum groundwave field intensities that could be expected at the two stated distances from a fully legal setup operating under Part 15.219.

Even then, the field intensity measured by the FCC at 30 meters is considerably higher than is practically possible from a legal Part AM setup under 15.219 (about 7-1/2 times greater, for these values).

So the "Part 15" setup cited by the FCC in this NOUO did not meet either the requirements of Parts 15.209 or 15.219 for legal operation, even given ~the most efficient parameters that could be expected under 15.219.

The excess field intensity measured by the FCC possibly was the result of using an "elevated" 3-m antenna, whose radiating structure (including the conducting path to an r-f ground) exceeded 3 meters, the use of excess transmitter power, or some combination thereof.

LINK: http://i62.photobucket.com/albums/h85/rfry-100/FCCAction-Unlicensed1500kHz.gif

//

 

[Tom Wells]

Why would the FCC have chosen 30 meters, when all other AM BC band services specify measurement at 1000m?
If I recall correctly.... or correct me if 1km is not one of the first required points.

Is there any establishment or implication that coverage "shall" not extend to 1 km? Seems so.

Otherwise it should be measured at the same standard reference point as any other AM BC band signal,
and an appropriate standard established at THAT distance.

As extreme location variablility exists, wouldn't specifying compliance at the greater distance have been a lot more
helpful insuring a usefulness to Pt 15 operation rather than making sure it couldn't?

Still the 7.5 factor would seem to be a lot of indulgence to risk.
I am reassured when I hear my pt 15 fade out in a block or 3, depending on rain, wind, moon, etc and gosh knows what.
At fleapower, there's no room for anything but perfect tuning, and any losses in efficiency means the next-to-nothing radiated power
is only half of that.

 

[Carmine5]

The Long Beach pirate admitted that he used 400 milliwatts feeding a top-loaded 32 ft. antenna.

My point is that with the FCC's published field strength for Part 15 AM, trying to achieve a mile or two of coverage would put one in danger of being shut down, even with a type-accepted TX.

Or am I wrong?

C5

 

[radiocph]

There is an alternate provision for 100 mW into a 10' antenna. Theoretically, if you're operating
strictly within those parameters, the FCC shouldn't hassle you about uV/M (unless you're causing
interference to a licensed station)

By placing the transmitter right at the antenna, and setting up a good ground system (among other things)
you could probably get *some* signal out at about a mile. Whether it would be strong enough that a normal
radio listener (not a DXer or other geek) would bother to listen to is another story....

 

[rfry]

Why would the FCC have chosen 30 meters, when all other AM BC band services specify measurement at 1000m?

The FCC probably does not consider Part 15 systems to be a broadcast band service for the general public, but one intended for the private use of the operator with his/her own nearby AM broadcast receivers.

Otherwise it should be measured at the same standard reference point as any other AM BC band signal, and an appropriate standard established at THAT distance.

The maximum field from a Part 15 AM setup operating legally under 15.209 would be buried in the noise at a distance of 1 km. For example, the maximum legal field on 1500 kHz would be about 0.48 microvolts per meter at 1 km. And this assumes a clear, reflection-free path to the receive location. The urban noise level on the AM broadcast band is much higher than that due to power line hash, SCR light dimmers etc. Typically it takes a field of at least 1 millivolt/meter to provide fairly quiet reception on a cheap radio in an urban home.

//

 

[rfry]

My point is that with the FCC's published field strength for Part 15 AM, trying to achieve a mile or two of coverage would put one in danger of being shut down, even with a type-accepted TX. Or am I wrong?

No, you are right. A transmitter with Part 15 AM FCC certification can be used in systems that do not meet 15.219, even though the r-f input power of that transmitter meets the 100 mW maximum limit under 15.219.

This results from installing the transmitter and the 3-meter conductor assumed to be "the antenna" some 10 or 20 feet above the earth, and then connecting the transmitter chassis to an r-f ground in the earth (grounding rods, etc). That long ground conductor will radiate more than the 3-m conductor at the output of the transmitter, and such an installation means that the functional antenna length is greater than the 3-m length permitted under 15.219.

//

 

[WCWalker]

What transmitter was this pirate using?

 

[Carmine5]

What transmitter was this pirate using?

He didn't say. It was a hobby/experiment. So it's possible he used a home built TX.

C5

 

[carlvenorden]

I think Mr. Hobb's is basically correct here, after reading the posts.

I've built over 130 antenna/ground system/coil:ATU' systems for the sstran and similar transmitters.

Now, there is no way I can control how customers of mine have used these units, but my thoughts have always been that the antennas should be installed in the ground, not on top of a building or pole/tower.

Other than the fact they may not be legal, people tend to forget that AM is a ground based radio system. The closer the antenna is to the ground the better the signal, especially with a radial system installed to help the signal travel. Hoisting an AM antenna 20/30 feet in the air is an invitation for an FCC inspection or inquiry. Without a ground system, this installation could indeed pass inspection. But I like to side on the safe with a nice ground system and antenna shorter than the 108" allowed.

You NEED to send 100mW into the ground, not thru the air.

Since this comes up more often than not, I would like to address the question, "How much range will I get?"

I expect that question from people who are radio novices, for the most part, most part 15'ers are newbies and ask questions like this, and they are ok.
But if you have radio experience you know that 100mW at 10 feet max is not going to put out a monster signal, no matter if your transmitter is a $1000 Rangemaster, or a $100 sstran. It's all the same.

The best thing to do is mount your antenna in the ground, install a radial ground system that will counterpoise your air signal,(that's 10 feet max each way), keep your antenna and ground wire to a minimum under maximum allowed length and you will have no problem with the FCC regarding your range. (In the case of the sstran antenna, 9 out of 10 installations will have the antenna a good foot shorter than 108" allowed, and the ground wire is only inches long).

Certainly you need to stay 30kHz away from licenced broadcast stations (which will only elongate your range, and minimize interference), and broadcast on a frequency which is the clearest in a one mile range of your antenna. THAT in and of itself will give you the most range while staying completely legal.

I respond to at least one email a week (sometimes 10) whose first question to me is: "If I use your antenna, what range will I get?"........That question leaves a lot of questions to me unanswered! And it is unanswerable if I don't know the antenna will be in the woods, between two steel buildings three feet apart, on an apartment concrete breezeway or even tucked into a closet in the middle of a hotel! I've heard them all now. It is very few customers who have a 20x20 foot lot in which to situate an antenna in the middle of a plot of land like that.

The best way to think about a part 15 AM (or FM!) station is to think this way:

"I may range out a block. If I get out better, I did a good job of tuning my antenna, installing the ground, and I kept it all legal."

Generally what I read (not from customers but here) is, "I get out a block, and at night, less". Right. It is part 15 1/10th of a watt. It's not Radio Cuba's transmitters by any means.

Some of the very best part 15er's did it that way, and in highly conductive areas do range in the several mile range. But they have no on air competition either.

It is a Tenth of a Watt: Commercial stations with 1/4wave antennas at 100 watts at night often travel a whole mile or less. With a lot less, you are doing about what they are, so enjoy it.
Just my 2!!!
(FYI: in about the worst soil conductivity area outside of the shores, l installed my antenna and ground system in a rock/clay area.....surrounded by metal encased mobile homes I had distances from 1/4 of a mile to over two miles because my antenna was properly tuned, was legal size and had a nice ground system. By rights, I should have gotten out a block, but I took the time to tune and retune it til it was right.) It CAN be done, but it is not an One Hour JOB.
Carl the Antenna Guy.
sstran antennas/gound systems and tuning units.

 

[Neil E.]

Thank you, Carl, for the nice description of the practicalities of part 15 and antennas, especially for your emphasis on legal operation. We all wish to squeeze as much range from our systems as possible but we also need to be realistic as you are in your post.

In response to a poster's question about the transmitter, there is one kit with which I am familiar which can be adjusted to specification and produce power input close to 400 mW. With a simple modification, this unit can produce 1 watt input power. High power operation (>100 mW input) could be either intentional or not. The not situation is problematic in that a person could be in violation without knowing it by trusting that the kit was designed to legal specification. It is a good idea to measure the input power of any AM part 15 transmitter. My SSTRAN measured 93 mW for example. I do not know the details of the cited station but from the description of the antenna system involved it could be concluded that legal operation was not a priority.

Neil

 

[Carmine5]

Actually, my question was strictly one of legality.

Now that the FCC has published a field strength/distance figure for a Part 15 AM intentional radiator, is it still legal to broadcast a mile or two, even with a type accepted device like a Rangemaster? This assumes, of course, that the installation itself is legal.

C5

 

[Neil E.]

What transmitter was this pirate using?

Understood, but I was responding to the post quoted above.

In response to your last question, the legality of part 15 AM is described as either a maximum field strength at a distance or the maximum input power with antenna, feedline, and ground lead length restrictions. The transmission range is not regulated.

Neil

 

[rfry]

...is it still legal to broadcast a mile or two, even with a type accepted device like a Rangemaster? This assumes, of course, that the installation itself is legal.

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/h85/rfry-100/FCCAction-Unlicensed1500kHz.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

//

 

[Carmine5]

Thanks, R. Fry. This is what I was looking for. In fact, the above information is good to keep in mind for anyone contemplating building a part 15 station.

C5

 

[carlvenorden]

Thank you for the kind words Neil.
In fact, I was told by the manufacturer of sstran that indeed, the power will be less than 100mW ON Purpose. It is "about" 93mW which is what you said you measured.
Meantime the antenna I build usually tops out at about a foot below the maximum of 108" when tuned. With it's superb sound, that makes the sstran a bargain and a legal one at that!
Carl

 

[Hamilton]

Of course as long as the radiator is keep to the 3 meters by shielding or other wise limiting any radiation from the ground, then a system can be placed anywhere and comply with 15.219.
The main item of concern is limiting radiation to/from a 3 meter long area.

Also some transmitters can operate without a ground, however ours needs lighting protection.

They are just trying to keep people from doing insane things with the ground to get a lot more range, the normal, next to a pole, wire straight to dirt just will not radiate as an wire (antenna) in free space.

If the ground looks like it is designed to radiate, screens, ect, then the agent will probably have an issue with it.

This is the policy FCC agents will look at:

"The 3-meter combined length specified in Section 15.219(b) refers to the length of all radiating elements. Attaching the ground lead to an unshielded radiating object, or the addition of a ground screen, will cause the effective length of radiating elements to exceed 3 meters, in violation of Section 15.219(b)."

Seems clear to me.

 

[Hamilton]

One thing I forgot to mention, don’t forget this rule applies to inside transmitters too, not just outside transmitter on poles or rooftops.

If you operate an indoor transmitter that has it’s radio ground connected through the power supply cord to the wall socket ground, the same situation applies, strictly speaking by the letter of FCC law and published policy, the radiator is supposed to be confined to 3 meters. And there are probally a million tabletop AM transmitters out there.

Every Part 15 tabletop transmitter I have ever seen connects it’s radio ground straight through to the house box ground, effectively turning the entire house wiring system into a radiator (a bit longer then 3 meters).

Any thoughts about this Rich?

 

[Tom Wells]

The complex coupling with the house wiring will be greatly attentuated by the power transformer.
The wiring is an unknown factor. It may or may not be in conduit or shielded flex armor.
Then there's whether or not the house AC ground is a good RF ground.
The random capactive/inductive coupling could be anything.

In my experiments, coupling of any signal into unintended radiators/absorbers gives odd lobes in coverage.

Don't forget the effects of any longwire antenna you may have.
Try grounding them to your RF common when not using them for receive.
That is the proper practice, anyway.

I would expect more radiation from the other wires if ony beacause there's no iron in the way.
Still wouldn't expect any help from such "extra" radiation" since it's not the main RF circuit, and my experiments
show that this radiation if often subtracting from the desired radiation from the intended radiator.

My trannsmitter is 5 feet below grade level. Do I get to disregard the underground -to-above ground section from
consideration as a radiator? I would have to think so..

If you want to try do a directional, I think the effects should be pronounced at 100 mw.