FCC's John Reed Comments on Part 15 AM Rules and Interpretations

[WCWalker]

Daniel, by his own admission Fry is NOT a part 15 broadcaster. He has never even used a Type Accepted part 15 transmitter. He claims to have at one time experimented with part 15 broadcasting but that obviously was many years ago.

And yes, the FCC will quite possibly change policy or take action against both licensed broadcasters and policy and unlicensed if they get pestered enough by people that have nothing better to do than waste the valuable time of the FCC offices which are already over worked and understaffed. I know of one case personally where this did happen to several licensed stations in a single market.

 

[rfry]

Rich, So about the study you mentioned, the difference between the unshielded and shielded essentially being the same, It appears that the large radiation gain comes from the height increase, not so much from added radiation from the increased ground length.

Keith, radiation is produced by r-f current flowing along a conductor. Other things equal, longer conductors produce greater radiation. The height of the radiator above the earth has nothing to do with this.

You may be confusing your belief about this with the coverage performance of antennas operating above 30 MHz, which use the space wave to transmit a line-of-sight signal to a terrestrial receive location. The height of the transmit antenna is important then, but note that the transmit antenna doesn't radiate any more power due to that height. The only reason coverage improves for higher elevations on VHF and above is that the line-of-sight terrestrial path lengths are increased.

This certainly does not apply to 3-m monopoles used in the AM broadcast band, whether or not they are elevated. The radio horizon from the center of a 3-m whip mounted with its base at earth level is over 3 miles away -- which is a greater radius than a legal Part 15 AM system could possibly provide a useful signal to a typical radio.

All of this may be confirmed in any good antenna textbook.

To Wm C. Walker - I do own and operate a Part 15 certified FM transmitter.

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[Timewarp]

I have been a broadcast engineer for 30 years as well as a ham. The public has a word for guys
in my profession. The name is geek. It means bight in technical matters but pretty damn dumb about everything else.

From reading these posts, I understand why the people feel this way about us. The FCC rules may not have changed on paper but the interpretation of these rules has now changed. This is nothing new. The FCC changed it's interpretation on the rules about city of license as well. No nothing changed on paper, just
the interpretation. This is why radio stations are now allowed to move and leave your town with no radio service. But, it wasn't a rule change on paper, but a change of the rules did occure when the way the
rules were interpreted was changed.

With Part 15 AM broadcasting there were loopholes that made it legal. These loopholes have now been closed. One of the unwritten rules was pre-existing versus new construction. If you attached an antenna
to a pre-existing tower they did not consider the length of the tower. But if you erected the tower
for the purpose of doing the Part 15, then the length of the tower was counted.

So, now many Part 15 AM community stations that were legal last year are in violation today.
Now guys, the issue here isn't your ego or my ego. The issue is the public good. And the
people having access to their own public airwaves. Making it more restrictive is a step in the wrong direction.

And just because it doesn't affect you or me does not make it right. We need more community radio stations
not less.

 

[WCWalker]

Timewarp. I suspect that if these alleged changes are enforced that we'll likely see sales of Type Accepted AM transmitters halt and a spate of new pirate stations. People are not going to spend hundreds of dollars on a transmitter that will travel 300 feet when they can buy a kit that will do the same thing at a fraction of the cost.

 

[WCWalker]

If people use a Choke or Surge Arrestor to tie off their ground lead from the transmitter it is very unlikely that anyone will have a problem with FCC Enforcement. In fact, if these alleged changes have been in place for the last year, to date, we've not seen or heard about a single Part 15 AM station using a Type Accepted transmitter being asked to make any changes to their existing setup and installation. This seems to indicate that most stations are already in compliance and have not merited any action being taken by the FCC.

 

[Hamilton]

Hi Rich,
I think in the field Part 15 transmitters do get more range as they go higher possibly because they clear surrounding obstructions.

____________________________________________

Here is something I wrote some time ago:

A viable low power radio service would be greatly beneficial to the general public. There are thousands of small towns across the country that have no local voice, and need and want such. Local stations bring communities together and provide a closer sense of family or belonging. Also there is often a need to promote local businesses are disperse information to tourists, helping the local economy.

As a rule, small towns have had local service bought out and removed by the large companies, which have turned their programming over to nationwide canned programming. We hear from towns all over the country, “The only news and information we get is from “Big Town” 100 miles away, which means nothing to us.

Small towns need a local service that can provide local news, school information, programming made for the local needs. Almost all small towns have an individual, business or government agency who will step up to the plate to provide this service.

The problem is the small towns have no emergency service broadcast network. A low power radio service could provide this critical and valuable service for the town.

In the past there have been many proposals for an AM band low power service of different sorts. I feel the problems that have existed are:

-The FCC is already overtaxed with it’s current workload and would not be enthusiastic about adding thousands of licensed lower power stations, all of which would demand almost as much resources, or as much resources as full power stations.

-Writing and deciding the rules and regulations for such a service.

-The current proposals tend to be a minor versions of full power station, with mostly all the rights and licensing requirements thereof,

Instead of trying to make a “minor version of a full power station” what I propose is to expand on the Part 15 service already written. The advantages of considering / doing this are many:

-The FCC should see almost no increase in workload, since these devices are license free.

-Since the primary mandate of Part 15 is not to cause interference to a licensed station the NAB “full power broadcast industry” should not be concerned. I hope that the “full power broadcast industry” would not want to deny small towns a local service to fill a need that is not being met in the sole interest of “choking potential competition”. I feel that small town radio has a place and the “full power broadcast industry” has a place and both can coexist gracefully.


-Small towns could then easily add an informational small town radio (emergency and/or otherwise) service that would be effective, with minimal costs and no FCC licensing requirements (ask any small town manager if they would like this) greatly benefiting the public good.

Small towns in today’s economy are often against the wall, trying to survive. The FCC could greatly contribute to the “public good” by allowing these entities a way to provide these downtown areas with tourist information, information on local businesses, and local programming as they wish.

 

[rfry]

Hi Rich, I think in the field Part 15 transmitters do get more range as they go higher possibly because they clear surrounding obstructions.

Consider that most of the radiation from an elevated "Part 15" AM system with a 20 foot ground conductor originates from that ground conductor, and not the 3-m whip above the transmitter. So little to none of the structure generating most of the radiation has a clear line of sight path over house-sized obstructions to receivers near the earth, anyway.

The increased field strength from these elevated systems is the result of their added radiating length (above the 3-m length permitted by 15.219), and not because the 3-m whip part of the antenna system may have better clearance over obstructions.

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[Hamilton]

Rich,
I still think it helps to get the antenna above anything around it that will shield the signal, I think the filter choke is an effective solution to a keep a ground from radiating.
See
http://www.am1000rangemaster.com/pdf/Installation guidelines to comply with FCC Part 15 Rules A.pdf

Also it is possible to disconnect the lighting safety ground from the RF tank circuit.

 

[Hamilton]

Rich,
Oops sorry, one other thing, about the study you mentioned, was there any data on the shield used? As a shield gets very long it can couple to the inner conductor and become a radiator itself, depending on diameter, ect. What sort of data was used here.

 

[Carmine5]

Keith,

I'm glad you're hanging in here to discuss this issue. It looks as if it's turning out to be a watershed discussion on the subject of ground length and radiation.

Hopefully, when all is said and done we can put the matter to rest and those who are doing so, can run their part 15 station in peace.

C5

 

[rfry]

... was there any data on the shield used?

The shield consisted of a grid of interconnected wires. Their spacing is such that the grid would be essentially the same as a solid surface to 1.7 MHz r-f.

The wire model of the shielded system used in the study is shown in the link I posted earlier.

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[Hamilton]

Rich,
I am trying to understand the shield used, any idea what the capacitive coupling to the ground was?
What I am getting at is that any shield, if the coupling is high and as the length gets longer, becomes a radiator itself.

Is it assumed in the model the shield is some sort of perfect shield? That there is no radiation from the ground?

 

[rfry]

Is it assumed in the model the shield is some sort of perfect shield? That there is no radiation from the ground?

NEC (and cgbrock's experience per his quote below) both show that it doesn't much matter whether or not a long ground wire is shielded. Either way the system has about the same amount of gain -- which in my NEC study resulted in about 4.5 times more field strength than the same system with a 1" ground wire. The added field is due to the radiation set up by the r-f current flowing along either the unshielded ground wire, or along the outside of the shield.

cgbrock « Reply #31 on: September 01, 2008, 09:28:12 pm » wrote in this thread:

"If anyone is interested, I replaced the wire from my elevated transmitter to ground with a shielded coax cable. I did not notice any appreciable difference in signal quality or range."

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[Hamilton]

Rich,
Sounds like what I thought, basically a (non-shield)

Obviously if there is enough coupling and length the shield will become a radiator.

A better designed shield (shorter, little, no coupling between shield and ground) would work, or a filter between the transmitter ground lead and ground, correct?

Or disconnecting the lighting safety ground from the RF tank completely, ?

If something is not radiating there is no reason to consider it part of the 3 meter limit.

 

[cgbrock]

Rich,

When I replaced the ground wire with shielded cable, let's assume my total radiation remained exactly the same. This means either 1) the unshielded ground wire was not radiating, or 2) the shield had exactly zero effect. Is it possible for a shield to have exactly zero effect? I need educating - this is not my field (no pun intended).

Chuck

 

[rfry]

...either 1) the unshielded ground wire was not radiating, or 2) the shield had exactly zero effect.

The answer to this can be seen in the link below, which shows the r-f current distribution along the system with the 20-ft, unshielded ground conductor from my NEC models posted earlier in this thread.

Notice that the r-f current is highest, and is uniform along the 20-ft ground conductor. At the elevation where the 3-meter whip connects to the r-f output connector of the transmitter, the r-f current tapers in a nearly linear manner to a very low value at the top of the 3-m whip.

The r-f current flowing along these conductors is responsible for the EM radiation from this antenna system. The longer the conducting path and the more current flowing along those conductors, then the greater the r-f radiation from that system.

From this it is clear that unshielded ground conductors in elevated Part 15 AM systems radiate, and in fact as in this case, they can radiate much more than the 3-m whip considered to be "the antenna."

This is the reason that this elevated system had about 13 dB higher antenna gain than when mounted near the ground, using a 1" ground conductor. It is not due to the elevation of the 3-m whip above nearby obstructions (which obstructions were not even included in the NEC models).

Your experience, Chuck, as well as the NEC model of such that I posted both show that shielding the ground conductor does little or nothing. In the shielded version the r-f current travels along the outside of the shield, which produces radiation.

LINK: http://i62.photobucket.com/albums/h85/rfry-100/CurrentDistributionOnPt15AMSystemWi.gif

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[Hamilton]

Chuck,
Rich is probably right about your shield being a non-shield, the (faraday shield) will only work if the coupling between radiator and shield is minimum. I think you mentioned you bought your coax at the hardware store, it probably has to much capacitance per foot, (to much coupling between radiator and shield).

So at lengths beyond a few feet, the coax you have basically won’t work. The outer braid becomes a radiator.

Sorry my web site wasn’t more clear, I have some notes in there about using large diameter coax, and about keeping the length short, but I need to make those notes more obvious.

You could consider using a filter, or some sort of effective shield.

 

[rfry]

...You could consider using a filter...

However if all r-f current was filtered from all the conductors leading away from the transmitter (except the 3-m whip) then there would be no increased radiation from mounting the Part 15 AM tx and whip at some elevation above the earth, and little point in doing so.
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[ironbear]

Hamilton included a clamp-on the wire style ferrite with the loose parts of a Rangemaster I installed for a client a few years ago.
It was apparently included to decouple RF from the audio and power cable, at the transmitter. After reading this thread, I believe I'd be inclined to use one on the ground lead as well (maintaining DC or lightning ground) in any elevated installation.

Separating myth from reality is hard, as this thread painfully demonstrates. The reality is that people who seriously want to do community AM stations need to become effective activists for a genuine, licensed or unlicensed LPAM service.

...You could consider using a filter...

However if all r-f current was filtered from all the conductors leading away from the transmitter (except the 3-m whip) then there would be no increased radiation from mounting the Part 15 AM tx and whip at some elevation above the earth, and little point in doing so.
//

 

[Hamilton]

...You could consider using a filter...

However if all r-f current was filtered from all the conductors leading away from the transmitter (except the 3-m whip) then there would be no increased radiation from mounting the Part 15 AM tx and whip at some elevation above the earth, and little point in doing so.
//

Really the issue that is important to me here (the point) is practicality, not the extra radiation as you may think. When I install a transmitter I need to be able to install it where the customer (or building landlord) will allow. Not to many would allow a unit on the front lawn (really that is a joke) They all want it with the rest of the radio stuff, on the roof. I don't care about radiation from the ground, I just want to get the unit installed in a practical manner. I would install a ferrite on both the control and ground wires if the ground was otherwise unshielded.

Here is an interesting question, since sometimes it is practical to use the building wiring ground, should a filter be used there? I would probably do it just to be sure, but pretty much all Part 15 AM transmitters connect their grounds directly to the building wiring, so there may be some allowance there.