Part 15 AM antennas

[rfry]

Didnt you have a chart at one time that showed how better Part 15 systems run from second floor of a 2 story house?

The plot in red for a 5-meter height in the chart I posted shows that, approximately (at least relative to the other two plots in that chart).

It doesn't matter what the Part 15 AM transmitter and 3-m whip are supported by.

The better performance is not due to the height of the 3-m whip above the earth. It is the result of using a long ground conductor, which makes the system functionally non-compliant with 15.219(b).

//

 

[LibertyNT]

Didnt you have a chart at one time that showed how better Part 15 systems run from second floor of a 2 story house?

The plot in red for a 5-meter height in the chart I posted shows that, approximately (at least relative to the other two plots in that chart).

It doesn't matter what the Part 15 AM transmitter and 3-m whip are supported by.

The better performance is not due to the height of the 3-m whip above the earth. It is the result of using a long ground conductor, which makes the system functionally non-compliant with 15.219(b).

//

You have to rule in Physics with Part 15. At such low power levels im certain being higher has to have some advantage. Due to lack of obstructions to the weak signal. which should result in increased range. Kinda Like FM.

 

[rfry]

... At such low power levels im certain being higher has to have some advantage. Due to lack of obstructions to the weak signal. which should result in increased range.

Below are some comments I posted about this on another site a few years ago. The statements I commented on are paraphrased, and shown in bold face.

There is little radiation from a long ground lead. The height of a 3-m whip above the earth produces better signals than if it is lower. The 3-m whip is a point source of radiation on medium wave frequencies

These beliefs are popular among Part 15 AM equipment suppliers and operators, but they are not supported either by theoretical physics, or decades of field experience.

Here is a quote from RADIO ENGINEERS' HANDBOOK by F. E. Terman, 1st Edition, page 799, in the section about grounded vertical radiators:

"When the length of the grounded antenna is of the order of one-eighth wave or less, the radiation is almost exactly proportional to the cosine of the angle of elevation."

The 3-m whip portion of a Part 15 AM antenna system is much shorter than one-eighth of a wavelength even at 1700 kHz, so we can construct a short table showing its relative field in the vertical plane:

Elevation Angle = Relative Field [cos(angle)]

0 deg = 1.000 (always maximum radiation in the horizontal plane)
15 deg = 0.966
30 deg = 0.866
45 deg = 0.707
60 deg = 0.500
75 deg = 0.259
90 deg = 0.000 (always zero radiation toward the zenith, directly above the radiator)

These characteristic patterns also can be seen in the paper at this link (figure on page 3) http://filebay1.home.comcast.net/~filebay1/Elevated_Part_15_AM_Antennas.pdf

Conclusion: these electrically short antennas are far from being point sources.

Adding a long, vertical ground lead/conductor to a Part 15 AM tx+whip extends the radiating length of the antenna system. All of the r-f current that exists at the base of the elevated 3-m whip flows along that "ground" path at about the same value as at the base of the 3-m section. The flow of r-f current along the ground wire makes it radiate, just as that current makes the 3-m whip radiate. This can be seen in the figures on pages 1 and 2 of the paper referred to above.

So it is the added radiation from the ground conductor of an elevated Part 15 AM system that produces the increase in signal strength/coverage radius, not because the 3-m whip is an isolated point source, which when elevated has better line of sight paths to nearby receivers. The majority of the radiation from such systems occurs from the ground lead, not the whip. A well-designed and impartial measurement will prove this.

Concerning the radiation from the ground lead, I have read a couple of reports on other boards where people claim to have shown that the field strength near the ground conductor is very small compared to that near the antenna. My take on this is there is possibly an error in the measurement technique and present it here to ask if it makes sense.

Yes, it is very reasonable to question it. The accuracy of such measurements will depend on the sensitivity of the typical field strength meter (FSM) to radiated fields, and the horizontal distance from its antenna to the Part 15 AM antenna components.

The field of a radiating antenna system should not be measured very close to it, because that does not give usable results with a FSM. Such is possible only with exotic and expensive near-field probing techniques, where the complete radiation pattern can be synthesized from the near-field data measured at various locations along the antenna aperture.

A better approach for Part 15 AM would be to locate a sensitive FSM a few hundred feet horizontally distant from the Part 15 antenna system, over level ground. The meter should be tunable, to be sure that only the frequency under test is being measured. The total radiated field observed from a Part 15 AM system with a long ground lead will be higher than when the 3-m whip is mounted with its base at/very near ground, using a short conductor connected to the same r-f earth ground as used in the elevated system (and other things equal).

As the 3-m whip in the elevated Part 15 antenna system then will be virtually the same distance away from the FSM, and with no better line-of-sight path to the FSM than from the earth-based system, this will conclusively show that it isn't the height of the 3-m whip that is important, it is the total length of the radiating conductors used (3-m whip plus the long conducting path from the tx chassis to the earth r-f ground).

//

 

[LibertyNT]

Ah I see. What if the Ground was Insulated to not radiate? Then would it have the same results as an Antenna on the ground?

 

[rfry]

Ah I see. What if the Ground was Insulated to not radiate? Then would it have the same results as an Antenna on the ground?

Sorry, but no, because insulated wires radiate/receive r-f energy just as well as bare ones.

Some have suggested using a coax cable to prevent radiation from the center conductor, which center conductor serves as the "ground" conductor to an elevated Part 15 AM transmitter and 3-m whip. But the r-f current on the center conductor couples into the shield, and then travels down the outside of the shield to r-f ground. So using coax for this application radiates just about as well as a single, unshielded wire.

//

 

[LibertyNT]

Ah I see. What if the Ground was Insulated to not radiate? Then would it have the same results as an Antenna on the ground?

Sorry, but no, because insulated wires radiate/receive r-f energy just as well as bare ones.

Some have suggested using a coax cable to prevent radiation from the center conductor, which center conductor serves as the "ground" conductor to an elevated Part 15 AM transmitter and 3-m whip. But the r-f current on the center conductor couples into the shield, and then travels down the outside of the shield to r-f ground. So using coax for this application radiates just about as well as a single, unshielded wire.

//

R. Fry
rimember

Offline Offline

Posts: 666

RF Systems Engineer (retired)

Uh oh This was your 666th Post ;D

I have to ask this though,
Wouldnt something be wrong with your transmitter if the RF was going into the ground? seems like the Antenna is wired to be the antenna. and the ground wired to be the ground. Or is the radiation leaking into the ground?

 

[rfry]

Wouldnt something be wrong with your transmitter if the RF was going into the ground? seems like the Antenna is wired to be the antenna. and the ground wired to be the ground. Or is the radiation leaking into the ground?

A fair question.

The ground terminal of a Part 15 AM transmitter is labeled and used correctly IFand only if it connects directly to a true r-f ground.

But note that a true r-f ground does not radiate. That is the definition of an r-f ground.

As I have posted in text and shown using various links, a true r-f ground does not exist at the top of "massive" (or minimal) ground wires, and the grounded metal flagpoles, masts, towers, and the steel frames of billboards and high-rise buildings. Such conducting paths to a true r-f ground (something buried in the earth) DO radiate.

Therefore whatever conducting path to a true r-f ground that is connected to the "ground" terminal of an elevated Part 15 AM transmitter becomes part of the radiating length of the antenna system, along with the ~3-m whip -- which results in a functional non-compliance with Part 15.219(b).

//

 

[LibertyNT]

Ah i understand now. Thanks Fry for clearing that up [never thought id ever say that ]
But for some Part 15 Broadcasters you MIGHT not need to worry about this rule. some Offices
if you treat them real nice will let this slide by.

Sometimes [and once again i cannot believe im saying this] listen to Fry just a little bit.
He seems to know stuff about antennas. Although his rules seem harsh and terrible they will keep
your bum out of trouble with the FCC. Under some circumstances some rules will be bent.
Just really depends on that Agents mood

Alright then.
Just keep your Stuff under or at 10 feet in Antenna length [and ground] and you'll be safe.

 

[LibertyNT]

And To Follow up on this antenna deal.

Multiple Part 15 setups mounted on poles have been inspected and Passed by FCC agents.
So in reality here,
You will be just fine if you mount a transmitter on a huge pole. Electrician codes will protect you here
because you need a lightening ground.

So tell your friend all you have to do is mount that puppy on a pole. ground it and your set.

Hope he has fun with it 8)

 

[rfry]

The FCC's view on clustering or synching transmitters like the Rangemaster follows:

http://fjallfoss.fcc.gov/oetcf/kdb/forms/FTSSearchResultPage.cfm?id=20973&switch=P

Mr Walker's post was interesting, but with due thought begs questioning his statement about synchronizing or clustering multiple "Part 15" AM transmitters.

Mr Walker apparently believes that such is condoned and permitted by Part 15 and FCC policy.

I wondered what the FCC meant in Mr Walker's reference by the phrase "uncoordinated transmitters."

So I asked them (see below).

The FCC response to my query might be of interest to those manufacturers, distributors, and prospective users who are considering, or now using such systems.


----- Original Message -----
From: [email protected]
Sent: Thursday, July 09, 2009 5:44 PM
Subject: Response to Inquiry to FCC (Tracking Number 908142)

FCC Home | Search | Updates | E-Filing | Initiatives | For Consumers | Find People

Office of Engineering and Technology

Inquiry:

In http://fjallfoss.fcc.gov/oetcf/kdb/forms/FTSSearchResultPage.cfm?id=20973&switch=P, it states, "In paragraph 137, the Commission stated that it concurred with the comments that multiple devices should be permitted provided, the individual transmitters comply with the rules and any emission resulting from the simultaneous operation of the individual, non-coordinated transmitters complies with the rules."

Could the OET please define what is meant by "non-coordinated" transmitters in the above quotation? If two or more Part 15 AM intentional radiators are installed near each other, and all use the same, synchronous carrier frequency and modulation then their radiated fields would produce an extended coverage area compared to a single, compliant system.

Is such a multiple system considered to be compliant under Part 15?

Response:

The term non-coordinated in practice has been used to mean independent transmitters so that the transmitters do not employ some method to cooperate so that simultaneous transmissions result in being non-compliant or not within the scope of the Part 15 rules. The scope of part 15 is to permit a low power independent device to radiate with out a licensee in a localized area. The specifics will depend what rule parts are being used.

In your case the part 15.219 devices using multiple transmitters transmitting on the same frequency and carrier phased synchronized to effect larger coverage area with the same information (are) considered coordinated and not permitted.

The effective air-signals would be equivalent to a signal transmitter exceeding a rule part in you refers to. Something similar to this may be applicable to a confined area under 15.221, but would require a detail review by the FCC before it could be certified.

//

 

[Hamilton]

Rich,
Keeping up with you could be a full time job! To bad for me I am not retired like you!
First of all people please don't let Rich be your only source of information, while he can be useful for formula solving, it has been my experience he tends to "spin" things to his way of thinking.
For example Rich says you can only use a true rf ground for a part 15, this is hogwash. He knows that even a groundrod in the dirt isn’t a “true” rf ground. For example there are over a million part 15 AM units that are table top transmitters that use the electrical ground as a ground which is by no means a “rf” ground. So user beware!

About Rich most recent attacks:

Rich,
By carrying this on you demonstrate an intent beyond a casual interest, but as has been the case for years, an intent to harm me and in general Part 15 business. Your questioning begs the disallowance of the procedures you question, you knowing the response you are looking for, it being well apparent what you are looking for, to harm me and my business.

The FCC inquiry system is like asking the IRS a tax question, they just check the know rules and answer your question according to their best understanding of your question. My question is will you ever stop trying to harm me and my (and the whole) part 15 business. As in the ground situation you begging the FCC to make/change policies to make part 15 more difficult for those that enjoy it and find it useful for their needs.


I hesitate to explain anything, Rich because the history is just a picked apart attack on anything I write, so this is for others. These transmitters were designed to be used as a “highway system” and were certified as such. That is to say they were certified to be connected to each other, multiple (synced) devices, That is why there is two terminal blocks, so they can be serially connected.

Each unit will comply with FCC rules.

There are many such systems in use, not only with AM but with FM also.

I have received many communications and clarifications over the years concerning this, I send them to customers. I won’t post them publically to be picked apart by enemies of the Part 15 world, but I am attaching some I received from Raymond Laforge, head of Enforcement.

“Could the OET please define what is meant by "non-coordinated" transmitters in the above quotation? If two or more Part 15 AM intentional radiators are installed near each other, and all use the same, synchronous carrier frequency and modulation then their radiated fields would produce an extended coverage area compared to a single, compliant system.

Is such a multiple system considered to be compliant under Part 15?”

Rich you have worded this inquiry to get the response you wanted, just like a poll question can be “spun” to get the responses a pollster prefers.

This wording is intended to make it sounds like it might violate Rich, but you know that, first of all, the Part 15 AM intentional radiators are not necessarily installed near each other, and the carriers are not really coordinated, but synchronized so there is no heterodyning.

Rich I assume this Brown section came from the FCC, or was it you? It says “in your case the Part 15.219 devices, ect,” when the inquiry doesn’t mention a specific case OR Part 15.219 transmitters, something fishy there.

My take is this is a “spin” to destroy the multiple transmitter option. Rich will Spin this so the FCC may think, (well maybe we should not allow any kind of multiple transmitter), or we have not addressed this, now we should, type of thing.

To those I have sent this too, multiple transmitters is in a battle, while it has been fine in the past, Richs questions and determination, as with the ground situation, can make the FCC relook at what they are doing with multiple transmitters, we need to rally here. The FCC reads the boards too, Rich knows that.

I don’t suppose we will ever see an end to proclamations by Rich that my system or others violate his FCC rules in one way or another. Rich, I just don’t understand why you don’t teach kids science, or some useful endeavor.

 

[RadioMan203]

In spite of what Richard Fry has written, there are documented cases of Part 15 stations operating with multiple transmitters which have been inspected (I know of one having been multiple times) by the Commission and have passed inspection. This very conclusion suggests Rich's prime motivation is to discredit the use of multiple transmitters in spite of the fact the very agency which Rich is provoking responses by his leading questions is the very same government body which has stamped a passing grade on such installations that are currently and have been allowed to operate for some time. What are we missing from these obvious facts?

One does have to question the motives of Fry's posts. While Rich has stated ad nauseum that "The goal of my posts is to provide accurate technical information about systems that actually meet Part 15." (taken directly from his R-I post September 20, 2008, 07:24:56 am), destroying a business by disseminating misleading and questionable statements, not to mention negative conjecture and intimating such equipment is illegal could very well bring a lawsuit against a person for potentially damaging the business reputation of the marketer or manufacturer of such product(s) which are proven to be legal to operate, in this case, under the Part 15 regulations.

 

[rfry]

One does have to question the motives of Fry's posts. ... by disseminating misleading and questionable statements

Everything following the word "Response" in my July 24th post in this thread was sent to me by the FCC in response to my inquiry. I changed the font color and underlined two words in the middle paragraph to call attention to that part of the reply.

The FCC comments there are rather clear, but if they seem misleading and questionable then perhaps someone can ask the FCC to clarify them further. Personally I don't see the need.

The FCC's reply appears to differ from the beliefs of some in the Part 15 AM community.

People are free to read what I and others write here, compare it to the written statements of the FCC and to the principles of physics, and decide how they wish to operate.

//

 

[RadioMan203]

Rich, as usual you glossed over the fact that there are installations the FCC have inspected and passed! Is this because it doesn't fit your agenda? Completely ignoring that fact sure places your personal objectivity into questionable merit as far as I'm concerned.

And while your statement of "Everything following the word "Response" in my July 24th post in this thread was sent to me by the FCC in response to my inquiry." is qualified by your statement of "The FCC comments there are rather clear, but if they seem misleading and questionable then perhaps someone can ask the FCC to clarify them further." You don't address the nature of your question. Just as a judge tells an attorney "don't lead the witness", the manner of which you asked your question by adding "If two or more Part 15 AM intentional radiators are installed near each other, and all use the same, synchronous carrier frequency and modulation then their radiated fields would produce an extended coverage area compared to a single, compliant system".

Of course, the response from the OET "The term non-coordinated in practice has been used to mean independent transmitters so that the transmitters do not employ some method to cooperate so that simultaneous transmissions result in being non-compliant or not within the scope of the Part 15 rules". The scope of part 15 is to permit a low power independent device to radiate with out a licensee in a localized area. The specifics will depend what rule parts are being used.

If your black and white narrow-minded interpretation of what was told to you was true, every college and university in the United States who had multiple on-campus transmitter installations would be cited as breaking the law, regardless of being carrier current, leaky coax or free radiating. Since there are multiple carrier current and free radiating transmitters on some college campuses to feed dormitories and other campus buildings it appears your interpretation of the law is flawed. Since each transmitter feeds a "localized area" as dictated by your response from the OET it appears to be in compliance.

Perhaps, if your definition of "near each other," you mean a handful of feet and by "use the same, synchronous carrier frequency" you mean utilizing one distinct oscillator circuit to generate the carrier frequency, you possibly have a point. Then again, this would mean every single carrier current transmission system in a college or university in the US using one transmitter and multiple amplifiers would be breaking the law!

In the case of a free radiating system, if these transmitters all contain discrete carrier generating oscillators operating via a separate frequency defining crystal along with the fact the example stations that have passed inspection usually have their transmitters spaced greater than 1/2 to 1 mile apart, I personally feel you're just out to stir the pot, yet again. Mounting five transmitters on a fence with a common oscillator is not compliant, plain and simple. Units operating miles apart? Seems to be fine by definition from what I have seen ... or not seen.

 

[Flying-Dutchman]

In spite of what Richard Fry has written, there are documented cases of Part 15 stations operating with multiple transmitters which have been inspected (I know of one having been multiple times) by the Commission and have passed inspection.

This statement may frighten off more people than anything else written.
Most Americans don't wish to engage in something that may bring police
or Federal Agents into their bedroom, even if it's legal.

My wish for LPAM with the FCC is more power and lift the antenna length restrictions. We also need a licensed LPAM service.
LPAM could be a valuable public service.

Perhaps instead of arguing about the rules, some of us need to petition
to change these restrictive rules. I think the current FCC might be open
to such suggestions.

 

[rfry]

Rich, as usual you glossed over the fact that there are installations the FCC have inspected and passed!

So you write, Bill, but are we all to believe that such is true on "hearsay" without valid, documentary support?

You don't address the nature of your question. Just as a judge tells an attorney "don't lead the witness", the manner of which you asked your question by adding "If two or more Part 15 AM intentional radiators are installed near each other, and all use the same, synchronous carrier frequency and modulation then their radiated fields would produce an extended coverage area compared to a single, compliant system".

I gave this information in my inquiry because I was unsure of what the FCC meant by their term "non-coordinated" transmitters, and wanted to present a clear inquiry that they could answer with little chance for confusion.

If anyone believes that the operating conditions stated in my inquiry to the FCC inadequately represent current practices utilized by synchronized AM systems believed to be permitted by Part 15, then s/he is free to contact the FCC for clarification or correction of their reply to me.

The FCC had the option and ability to re-define the conditions of their reply, but did not do so.

Of course, the response from the OET "The term non-coordinated in practice has been used to mean independent transmitters so that the transmitters do not employ some method to cooperate so that simultaneous transmissions result in being non-compliant or not within the scope of the Part 15 rules". ...

Note that Part 15 AM transmitters that employ some means to synchronize their operation with another transmitter some distance away are not "independent transmitters."

If your black and white narrow-minded interpretation of what was told to you was true, every college and university in the United States who had multiple on-campus transmitter installations would be cited as breaking the law, regardless of being carrier current, leaky coax or free radiating. ...

Not unless they are in violation of the applicable rules about the radiation level at some defined distance from the radiator(s), and just beyond the perimeter of the campus.

//

 

[Flying-Dutchman]

Synchronized transmitters is a cool concept. I remember the BBC using
this for Radio One when I lived in England 20 years ago. You could hear
Radio One from Scotland to the South of England with no fading.
They covered the whole UK on AM with one frequency this way.

Now, for this to work you must zero beat the frequency of each transmitter.
The other trick is the audio must be identical going into each transmitter.
Or, you will make a real mess.

Phone lines to deliver audio would be cost prohibitive for most part 15er's.
Long Range WIFI might work to deliver audio. But, digital encoding and
decoding causes audio delays.

I know one part 15er who uses 2 frequencies from 2 locations 8 miles apart
due to digital delay.

But, this concept does sound interesting. It would be practical for a University
that has it's own wires and cables.

 

[Hamilton]

Looks to me like Rich didn’t write the FCC out of curiosity, read his inquiry:

“Could the OET please define what is meant by "non-coordinated" transmitters in the above quotation? If two or more Part 15 AM intentional radiators are installed near each other, and all use the same, synchronous carrier frequency and modulation then their radiated fields would produce an extended coverage area compared to a single, compliant system.

Is such a multiple system considered to be compliant under Part 15?”

This is clearly an assumed made up inquiry that is not the whole truth, designed to get a negative response, Rich did you seriously expect to get any response but the one you got? Rich is looking for a possible “explode button” he can push to cause trouble for RangeMaster. It is not normal for people to go around doing this sort of thing of curiosity. I have asked for Richs help many times about this and that, but I only hear from him when he thinks he has RangeMaster on something, or if he wants information or documentation on a project, related to getting RangeMaster on something. This has been the history of the ground issue, now Rich has found another issue to work on.

I seriously am mystified as to the real motivation.

The Rangemaster is designed to be (synchronized) as are other FM Part 15 certified systems I have seen. This synchronization is designed to prevent heterodyning and phase flutter. As in FM part 15 systems I have seen, one example being installed in a shopping mall, our units can also be installed around a shopping mall or other large area/building. This concept has been discussed with FCC enforcement and allowed. Note also the transmitter was designed and certified with this function.

I would assume all systems to perform this function would have to be synchronized to work.

The problem is the Rich, really knowing nothing about our system and how it works, has implied to the public that it is not legal. This would not so much be a problem for, say , a grocery clerk doing this, but Rich has a public perception of being a expert in this area, while the public is not an expert. So Rich tends to be believed.

Flying Dutchman, what you say is true, some have an unfounded fear of the (feds) FCC, but they are typically professional, there to help you. They will help you comply if they is an issue.

Rich I see you did phrase the question to achieve a negative response, the FCC doesn’t rephrase questions.

It is clear the agent writing the response was not aware you were referring to an already certified system.

Synchronizing transmitters is very helpful to many folks , have a large area you need to cover? Multiple transmitters can do the job!


It is important to synchronize the audio closely to prevent echoes between the units, and synchronize the carriers if it is feasible to provide a link between them. Rich does not know (he never asked) but most multiple transmitter installations do not even synchronize the carrier, just the audio. Most of the time it is not feasible to synchronize the carriers, and is not needed if the transmitters are far enough apart. If the transmitters get close enough the carriers should be synchronized to prevent phase flutter.

 

[rfry]

...The Rangemaster is designed to be (synchronized) ... Note also the transmitter was designed and certified with this function.

Keith, did the FCC certification process include testing and documentation of two or more units operating while synchronized?

The last paragraph of the FCC reply in my post states about synchronized transmitters, "Something similar to this may be applicable to a confined area under 15.221, but would require a detail review by the FCC before it could be certified."

From that FCC statement it appears that they would require specific tests on synchronized systems before granting certification to them, doesn't it?

//

 

[LibertyNT]

The way i see it...
The FCC part 15 codes let you operate any Transmitter on the given band with the rules following that band use. I dont think it matters if you have more then 1 transmitter airing the same programming. All that matters to them is you have more then one. and to them as long as each one is compliant. thats just my 2 cents worth.