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Fm power and certification issues...

R

radiopilot

First, I apologize for opening what seems to be a rift over who has the proper power in FM transmitters, who is legal and who isn't!

It was not my intention to discuss what legalities are open for FCC scrutiny, but rather to open discussion on differnt setups, techniques, what works, etc.

Seems it opened distrust, envy, legal discussions on who has more power, etc. but in reality I don't care if you have 1 watt, 10 watt, .001 microwatt or whatever, it's not important to me but how important it is to you should you get visited by the FCC.

I know what I have is going to meet the rule of the FCC and others telling me I'm over the FCC is just blowing in the wind, I'll answer that later.

What seems to me is all discussion on this board seems to be of a negative nature no matter what the issue. The post below is based on info by the FCC and you can bear it or move on.

"Q - I asked how much output power? How is that measured ?

A - First, for clarity the measured power is not based on output power from the transmitter but rather on received energy at the test receiver antenna..This test must be performed for the manufacturer at an FCC accepted Contract Test Site or the FCC office in Maryland to receive certification..The transmitter is placed on a table that rotates, 3 meters(10feet) from a certified antenna, then the antenna is raised and lowered and the table is rotated to find the strongest possible signal. After that factor is found then the test receiver attached to the test antenna measures the received power level (it must be less than 47dbm/v per meter using an average detector not Quasi peak..

Q - Gosh, I'm getting a nose bleed, just tell me why in simple terms.
A - Because this insures that the signal will only travel a specific distance and will quickly die out past that. Some designs will have higher power and small antennas (internal), while others will have less designed output power with larger antennas (telescoping), or on some designs there is an attached wire you can string out. The 250micro volts is the amount of power received at the test antenna at 10 feet(3meters), not the amount of power transmitted from the device... Remember, these are not broadcast licensed transmitters and are certified under strict rules."

I'm not going to clarify what is said other than it states power output of the FM transmitter is not the issue but rather how well your antenna put out the signal in terms of the 250uv/3m per part15n rules.

Now I mentioned the Pico measured less than the 47dbmV output putting it at the legal power per part15 rules and is certified as such... I need to say nothing more!

Please understand I will continue to use the equipment I KNOW is legal knowing full well I am NOT breaking any laws. Now those that believe they are transmitting with whatever equipment they have are doing what they feel is best for their situation knowing that any visit from the FCC, they will have to prove THEY are legal.

Radiopilot
 
Here we go again.

Hi,

I need to respond. You wrote:

"Now I mentioned the Pico measured less than the 47dbmV output putting it at the legal power per part15 rules and is certified as such... I need to say nothing more!"

First of all dBmv is not a measure of power! It is a measure of voltage.

Second, the part 15 FCC rules for broadcast band FM operation do not mention power.

Third, and I SHOUT because I have seen so many posts about the legal FM power and I am tired of wasting my time reading about legal FM power:

PART 15 RULES FOR THE FM BROADCAST BAND LIMIT THE FIELD STRENGTH OF THE TRANSMITTED SIGNAL. THE RULES SAY NOTHING ABOUT THE POWER OF THE TRANSMITTER. Got it?? When I see links to sites that discuss this and they mention legal power for part15 FM, I know the author is incompetent and I distrust all that they say.

You may interpret the rules any way you wish to suit your situation and are free to do so at your risk, but you cannot change the physics and engineering that were consided when the rules were written. The FCC and their consultants were pretty smart when they tried to construct the rules so that the limitations were understandable in engineering terms, but they failed because almost no one can measure the FM field stength at 3 M. accurately. What they should have done
is write the rules according to the intent, which to me is obviously not to permit broadcasting. They should have just said "If your FM signal can be heard beyond 200 feet with a car receiver, it is not legal." That was the effect, whether you like it or not.

Anyone who posts discussions about FM power limits according to the FCC part 15 rules just does not know what they are talking about.

Neil
 
A simple picture (Re: Here we go again.)

I get it, Neil.

To give an extreme (not to mention unlikely) but illustrative example, I could run a 50 kilowatt FM transmitter feeding its signal directly into a (very) slightly "RF-leaky" dummy load, and yet be Part 15 FM legal as long as the field strength at 3 meters was at or below the FCC Part 15 FM limit. -- JasonW

> Hi,
>
> I need to respond. You wrote:
>
> "Now I mentioned the Pico measured less than the 47dbmV
> output putting it at the legal power per part15 rules and is
> certified as such... I need to say nothing more!"
>
> First of all dBmv is not a measure of power! It is a
> measure of voltage.
>
> Second, the part 15 FCC rules for broadcast band FM
> operation do not mention power.
>
> Third, and I SHOUT because I have seen so many posts about
> the legal FM power and I am tired of wasting my time reading
> about legal FM power:
>
> PART 15 RULES FOR THE FM BROADCAST BAND LIMIT THE FIELD
> STRENGTH OF THE TRANSMITTED SIGNAL. THE RULES SAY NOTHING
> ABOUT THE POWER OF THE TRANSMITTER. Got it?? When I see
> links to sites that discuss this and they mention legal
> power for part15 FM, I know the author is incompetent and I
> distrust all that they say.
>
> You may interpret the rules any way you wish to suit your
> situation and are free to do so at your risk, but you cannot
> change the physics and engineering that were consided when
> the rules were written. The FCC and their consultants were
> pretty smart when they tried to construct the rules so that
> the limitations were understandable in engineering terms,
> but they failed because almost no one can measure the FM
> field stength at 3 M. accurately. What they should have
> done
> is write the rules according to the intent, which to me is
> obviously not to permit broadcasting. They should have just
> said "If your FM signal can be heard beyond 200 feet with a
> car receiver, it is not legal." That was the effect,
> whether you like it or not.
>
> Anyone who posts discussions about FM power limits according
> to the FCC part 15 rules just does not know what they are
> talking about.
>
> Neil
>
 
Re: Here we go again.

> Hi,
>
> I need to respond. You wrote:
>
> "Now I mentioned the Pico measured less than the 47dbmV
> output putting it at the legal power per part15 rules and is
> certified as such... I need to say nothing more!"
>
> First of all dBmv is not a measure of power! It is a
> measure of voltage.
>
> Second, the part 15 FCC rules for broadcast band FM
> operation do not mention power.
>
> Third, and I SHOUT because I have seen so many posts about
> the legal FM power and I am tired of wasting my time reading
> about legal FM power:
>
> PART 15 RULES FOR THE FM BROADCAST BAND LIMIT THE FIELD
> STRENGTH OF THE TRANSMITTED SIGNAL. THE RULES SAY NOTHING
> ABOUT THE POWER OF THE TRANSMITTER. Got it?? When I see
> links to sites that discuss this and they mention legal
> power for part15 FM, I know the author is incompetent and I
> distrust all that they say.
>
> You may interpret the rules any way you wish to suit your
> situation and are free to do so at your risk, but you cannot
> change the physics and engineering that were consided when
> the rules were written. The FCC and their consultants were
> pretty smart when they tried to construct the rules so that
> the limitations were understandable in engineering terms,
> but they failed because almost no one can measure the FM
> field stength at 3 M. accurately. What they should have
> done
> is write the rules according to the intent, which to me is
> obviously not to permit broadcasting. They should have just
> said "If your FM signal can be heard beyond 200 feet with a
> car receiver, it is not legal." That was the effect,
> whether you like it or not.
>
> Anyone who posts discussions about FM power limits according
> to the FCC part 15 rules just does not know what they are
> talking about.
>
> Neil
>


First of all, my post is not referring to what the power needed to be for part15 FM requirements, I was quoting the difference between what Rfry is suggesting that ~11.4 microwatts is what is REQUIRED to get 250uv/3m.... I am saying this is wrong on his part!

There is no power requirement and you confirm that as well! My post said that the Pico puts out 35-47 dbmV as the output of the transmitter, I never said it was the wattage! If you go to the Pico website and read about the PCFM... it does not mention power output in terms of microwatts, milliwatts, or watts... period!

No one here is disputing any knowledge anyone has here, simply that anyone can use whatever they want to use to trasnmit, however should you get a visit from the FCC, be prepared to document proof of your transmitter and it's legality!

No one is asking anyone to 'waste' their time here, you may post or reply to any postings here whether you agree or not.

Radiopilot
 
Re: Here we go again.

> First of all, my post is not referring to what the power
> needed to be for part15 FM requirements, I was quoting the
> difference between what Rfry is suggesting that ~11.4
> microwatts is what is REQUIRED to get 250uv/3m.... I am
> saying this is wrong on his part!
______

You misunderstand. I wrote that it only TAKES ~11.4 nanowatts (not microwatts) applied to a simple 1/2-wave dipole to generate the maximum legal field of 250 uV/m at 3 meters' distance.

That is not the same as saying that ~11.4 nanowatts is the maximum tx power level allowed by the FCC. The FCC doesn't specify Part 15 FM tx power, just field strength 3m from the antenna.

But the reality is that this maximum legal field can be generated by transmitters using thousands of times less power than the Pico, EDM and Ramsey units are rated for, with even the simplest antenna.
//
 
Re: Here we go again.

> If you go to the Pico website and read about the PCFM...
> it does not mention power output in terms of microwatts,
> milliwatts, or watts... period!
_______________

And what it does mention is full of errors. Below is the text of an email I sent to them this morning about some of their Part 15 FM statements.

I don't know if they are catering to "Pirates," or just uninformed about Part 15 FM systems.

+ + +

Hello -

Your URL http://global-cm.net/distribu.html has some good information on it. But it also includes some information that is incorrect, and which might lead your customers/readers into problems with the FCC. This text is incorrect as to what Part 15 allows for systems in the FM broadcast band. Part 15 does NOT allow a "100 mW maximum limit" for Part 15 FM systems. That is a Part 15 AM Rule.

Also your text states that the output level of the PCFM tx is 45 dBmV, and then infers that this "translates to just under 100 microwatts of power." As the PCFM appears to be designed for cable applications, probably the output load it is designed for is 75 ohms, and 45 dBmV (177.8 mV) across 75 ohms is a power level of ~422 uW, not 100 uW.

But here is the point: a radiated power of only ~11.4 nanowatts is needed to generate the maximum legal Part 15 FM field from a simple 1/2-wave dipole, and not much more than that even when using a short "whip" type antenna. Therefore the Pico, Ramsey, and EDM units all are rated for thousands of times more power than usable for Part 15 FM operations at the maximum legal field strengths, when using even the simplest of transmit antennas.

Your text includes, "How to get the PCFM up to near the legal power level? The cheap and 'dirty' way would be to connect an inexpensive VHF distribution amplifier to the output of the PCFM, and then to an antenna system." As stated above, the PCFM already is capable of far more power than is needed for legal Part 15 FM operations. But in any case, using an external amplifier with a certified Part 15 FM transmitter is not permitted by Part 15.

Do you know if the PCFM unit is FCC certified under Part 15 FM? And if it is, which antenna was used with it during the certification measurements?

Regards,

Richard Fry
Broadcast Engineer

Visit http://rfry.org for FM transmission system papers.
 
Re: Here we go again.

> > If you go to the Pico website and read about the PCFM...
> > it does not mention power output in terms of microwatts,
> > milliwatts, or watts... period!
> _______________
>
> And what it does mention is full of errors. Below is the
> text of an email I sent to them this morning about some of
> their Part 15 FM statements.
>
> I don't know if they are catering to "Pirates," or just
> uninformed about Part 15 FM systems.
>
> + + +
>
> Hello -
>
> Your URL http://global-cm.net/distribu.html has some good
> information on it. But it also includes some information
> that is incorrect, and which might lead your
> customers/readers into problems with the FCC. This text is
> incorrect as to what Part 15 allows for systems in the FM
> broadcast band. Part 15 does NOT allow a "100 mW maximum
> limit" for Part 15 FM systems. That is a Part 15 AM Rule.
>
> Also your text states that the output level of the PCFM tx
> is 45 dBmV, and then infers that this "translates to just
> under 100 microwatts of power." As the PCFM appears to be
> designed for cable applications, probably the output load it
> is designed for is 75 ohms, and 45 dBmV (177.8 mV) across 75
> ohms is a power level of ~422 uW, not 100 uW.
>
> But here is the point: a radiated power of only ~11.4
> nanowatts is needed to generate the maximum legal Part 15 FM
> field from a simple 1/2-wave dipole, and not much more than
> that even when using a short "whip" type antenna. Therefore
> the Pico, Ramsey, and EDM units all are rated for thousands
> of times more power than usable for Part 15 FM operations at
> the maximum legal field strengths, when using even the
> simplest of transmit antennas.
>
> Your text includes, "How to get the PCFM up to near the
> legal power level? The cheap and 'dirty' way would be to
> connect an inexpensive VHF distribution amplifier to the
> output of the PCFM, and then to an antenna system." As
> stated above, the PCFM already is capable of far more power
> than is needed for legal Part 15 FM operations. But in any
> case, using an external amplifier with a certified Part 15
> FM transmitter is not permitted by Part 15.
>
> Do you know if the PCFM unit is FCC certified under Part 15
> FM? And if it is, which antenna was used with it during the
> certification measurements?
>
> Regards,
>
> Richard Fry
> Broadcast Engineer
>
> Visit http://rfry.org for FM transmission system papers.
>


Rfry,

I don't know about the website: http://global-cm.net/distribu.html
and only seen it yesterday when it was posted here on this board, but who are you to discuss whether they are catering to 'pirates' or any other use of any product they advetise?

I mention the Pico Macom website, there is where you'll find any answers to this product and or any other devices they produce!

It's outragous for you to come here on this board to infer your ideas as to how much power is required to produce the radiated 250uv/3m... for any equipment manufactured and certified by engineers in various companies and their submittals to the FCC labs for certification. Now perhaps if you want to work for these certifying labs or consult for these labs you can get your point across, but to say that a certified product one buys from a reputable company selling certified products conforming to FCC regulations, is out of legal bounds is just pure fantasy on your part. I'm going to use that product and not one that conforms to your whims as to how much power it needs to be!

I think your letter to the above companies is arrogant on your part not your buisness as you do not work for the FCC and or any of it's compliance offices!

Radiopilot
 
Reply to Radiopilot

> > Neil
> >
>
>
> First of all, my post is not referring to what the power
> needed to be for part15 FM requirements, I was quoting the
> difference between what Rfry is suggesting that ~11.4
> microwatts is what is REQUIRED to get 250uv/3m.... I am
> saying this is wrong on his part!
>
> There is no power requirement and you confirm that as well!
> My post said that the Pico puts out 35-47 dbmV as the output
> of the transmitter, I never said it was the wattage! If you
> go to the Pico website and read about the PCFM... it does
> not mention power output in terms of microwatts, milliwatts,
> or watts... period!
>
> No one here is disputing any knowledge anyone has here,
> simply that anyone can use whatever they want to use to
> trasnmit, however should you get a visit from the FCC, be
> prepared to document proof of your transmitter and it's
> legality!
>
> No one is asking anyone to 'waste' their time here, you may
> post or reply to any postings here whether you agree or not.
>
>
> Radiopilot
>
Hello Radiopilot,

I think we are pretty much in agreement, I was reacting to the mention of "legal power" in your post and on some of the expert websites I have visited. I perhaps awkwardly stated what I said, and if it is misuderstood, that was not my intent.

I think JasonW gave a good example in this thread about power. Obviously, power out of the transmitter determines the field strength, but I attempted to point out that the part15 rules do not limit FM range by limiting power and it is mistaken to assume that a given power is legal unless it is certified with a given antenna, and even then mitigating factors can change the strength (such as reflections and reradiation).

My advice is limit FM range to 200 feet, and if I am on the jury, I will accept that as a good faith attempt to operate legally.

Neil
 
Re: Reply to Radiopilot

> > > Neil
> > >
> >
> >
> > First of all, my post is not referring to what the power
> > needed to be for part15 FM requirements, I was quoting the
>
> > difference between what Rfry is suggesting that ~11.4
> > microwatts is what is REQUIRED to get 250uv/3m.... I am
> > saying this is wrong on his part!
> >
> > There is no power requirement and you confirm that as
> well!
> > My post said that the Pico puts out 35-47 dbmV as the
> output
> > of the transmitter, I never said it was the wattage! If
> you
> > go to the Pico website and read about the PCFM... it does
> > not mention power output in terms of microwatts,
> milliwatts,
> > or watts... period!
> >
> > No one here is disputing any knowledge anyone has here,
> > simply that anyone can use whatever they want to use to
> > trasnmit, however should you get a visit from the FCC, be
> > prepared to document proof of your transmitter and it's
> > legality!
> >
> > No one is asking anyone to 'waste' their time here, you
> may
> > post or reply to any postings here whether you agree or
> not.
> >
> >
> > Radiopilot
> >
> Hello Radiopilot,
>
> I think we are pretty much in agreement, I was reacting to
> the mention of "legal power" in your post and on some of the
> expert websites I have visited. I perhaps awkwardly stated
> what I said, and if it is misuderstood, that was not my
> intent.
>
> I think JasonW gave a good example in this thread about
> power. Obviously, power out of the transmitter determines
> the field strength, but I attempted to point out that the
> part15 rules do not limit FM range by limiting power and it
> is mistaken to assume that a given power is legal unless it
> is certified with a given antenna, and even then mitigating
> factors can change the strength (such as reflections and
> reradiation).
>
> My advice is limit FM range to 200 feet, and if I am on the
> jury, I will accept that as a good faith attempt to operate
> legally.
>
> Neil
>

Neil,

Thanks for the clarifying your statement and I was not inferring that you were in disagreemnt with what I was trying to point across... JasonW is correct in his statement as well.

I believe that anyone here foolish enough to get 'miles' from there FM transmitter is playing dice with the FCC and I am not going to be one for example. My PCFM goes out perhaps 3-4 city blocks, maybe less but that is what I want, I don't need it for miles around... What I wanted was a quality product with the best sound around without the interference and the PCFM does it for me.

Radiopilot
 
Re: Here we go again.

> I mention the Pico Macom website, there is where you'll find
> any answers to this product and or any other devices they
> produce!

This morning I called Pico MACOM, and spoke to a Terry Jones there in their engineering group. I asked him if their PCFM was FCC-certified for use as a Part 15 device. The answer was "No." He said it was intended and designed for a cable distribution network, not for Part 15. Check this for yourself: 800 421-6511.

> It's outragous for you to come here on this board to infer
> your ideas as to how much power is required to produce the
> radiated 250uv/3m...

The laws of physics prove what is necessary, not beliefs, websites and emotion. And the numbers I have posted can be confirmed by anyone who wishes to "do the math."

> but to say that a
> certified product one buys from a reputable company selling
> certified products conforming to FCC regulations, is out of
> legal bounds is just pure fantasy on your part. I'm going
> to use that product and not one that conforms to your whims
> as to how much power it needs to be!

Part 15 FM certified products used as configured when they were originally certified (including the same antenna) obviously are compliant. Use of equipment NOT Part 15 FM certified is not compliant, don't you agree? This includes the Pico MACOM PCFM, by their own statements, so anyone using it (at ANY power level) is technically operating a non-compliant system.

This doesn't bother me in the least, though, particularly if their operators don't write that using non-certified eqpt, and/or using power levels far in excess of what is needed to generate the maximum Part 15 FM field with even a short whip antenna is legal under Part 15. It isn't.

> I think your letter to the above companies is arrogant on
> your part not your buisness as you do not work for the FCC
> and or any of it's compliance offices!

Just to note that my email to GLOBAL COMMUNICATIONS produced a cordial response from Mike Kohl there, thanking me for my input, and saying that their Part 15 FM text was based on old information. He said that he thought it had been removed from their site, and now he would be sure it was.

I asked him for permission to quote his response here, and will do that if I receive it.
//
 
> ... test receiver attached to the test antenna
> measures the received power level (it must be less than
> 47dbm/v per meter using an average detector not Quasi peak..
...
> Now I mentioned the Pico measured less than the 47dbmV
> output putting it at the legal power per part 15 rules and is
> certified as such... I need to say nothing more!

1) It isn't possible to equate the power output from a Part 15 FM tx to the field present 3 meters from its transmit antenna. It is also necessary to know the peak gain of the transmit antenna.

2) The Pico PCFM is NOT "certified" as a Part 15 FM tx.

//
 
> > ... test receiver attached to the test antenna
> > measures the received power level (it must be less than
> > 47dbm/v per meter using an average detector not Quasi
> peak..
> ...
> > Now I mentioned the Pico measured less than the 47dbmV
> > output putting it at the legal power per part 15 rules and
> is
> > certified as such... I need to say nothing more!
>
> 1) It isn't possible to equate the power output from a Part
> 15 FM tx to the field present 3 meters from its transmit
> antenna. It is also necessary to know the peak gain of the
> transmit antenna.
>
> 2) The Pico PCFM is NOT "certified" as a Part 15 FM tx.
>
> //
>


Rfry,

I just got the answer from Pico Macom's engineering staff and Yes you are correct, the PCFM is not certified to Part15 because since the unit is made for the cable industry it does not need to be certified, however, this does not negate that the PCFM must still be engineered to meet part15 requirements as far as the 250uv/3m rule, one must adjust if you will to insure this does happen, but considering that Ramsey, EDM, Veronica, etc. are all non certified transmitters and in some cases poorly design, I don't see any other certified engineer from your society taking the challenge as you have to whittle everyone down on this board or other boards with your very negative attitude regarding anyone but licensed broadcasters to posess radio transmitting equipment! I take satisfaction in knowing the PCFM is engineered correctly and not over powered even though we know power is not the issue here.

The above caption cited (1) was taken from the Info-Radio site:

http://www.gate.net/~advradio/fcc.html

of course they produce the Part15 certified FM transmitter, so are they erroneously giving false info?... Want to take them on also?

Radiopilot
 
> I don't see
> any other certified engineer from your society taking the
> challenge as you have to whittle everyone down on this board
> or other boards with your very negative attitude regarding
> anyone but licensed broadcasters to posess radio
> transmitting equipment!

Your viewpoint is incorrect. My comments are not negative; they only report the realities of this situation. They are negative just to the extent that they are viewed that way by some "Part 15" operators. And clearly I can't, and won't, have anything negative to say about anyone operating ANY kind of radio transmitter in a way permitted by applicable law.

The real problem is that some "Part 15" operators have some invalid, if well-intentioned ideas about what constitutes legal operation. There is a lot of invalid information being circulated about this, so I decided to offer some comments from an engineering viewpoint, that could be confirmed independently by anyone wanting to do so.

> I take satisfaction in knowing the
> PCFM is engineered correctly and not over powered even
> though we know power is not the issue here.

Please explain how you are satisfied with the power rating of the PCFM for (non-certified) use on Part 15 FM.

> The above caption cited (1) was taken from the Info-Radio
> site:
>
> http://www.gate.net/~advradio/fcc.html
>
> of course they produce the Part15 certified FM transmitter,
> so are they erroneously giving false info?... Want to take
> them on also?

There are statements in that quote that need further research. I'll do that, and post the results later.
//
 
> What seems to me is all discussion on this board seems to be
> of a negative nature no matter what the issue. The post
> below is based on info by the FCC and you can bear it or
> move on.

There is a natural tendency to believe that such info (quoted below) must be correct because it is attributed to "the FCC," and it appears on a commercial website. But as we've seen, such sources may not be completely correct, and can lead readers into risky situations with respect to their Part 15 operations. I'll repeat Radiopilot's quote, and then add some comments below them:

> "Q - I asked how much output power? How is that measured ?
>
> A - First, for clarity the measured power is not based on
> output power from the transmitter but rather on received
> energy at the test receiver antenna..This test must be
> performed for the manufacturer at an FCC accepted Contract
> Test Site or the FCC office in Maryland to receive
> certification..The transmitter is placed on a table that
> rotates, 3 meters(10feet) from a certified antenna, then the
> antenna is raised and lowered and the table is rotated to
> find the strongest possible signal. After that factor is
> found then the test receiver attached to the test antenna
> measures the received power level (it must be less than
> 47dbm/v per meter using an average detector not Quasi peak..
>
>
> Q - Gosh, I'm getting a nose bleed, just tell me why in
> simple terms.
> A - Because this insures that the signal will only travel a
> specific distance and will quickly die out past that. Some
> designs will have higher power and small antennas
> (internal), while others will have less designed output
> power with larger antennas (telescoping), or on some designs
> there is an attached wire you can string out. The 250micro
> volts is the amount of power received at the test antenna at
> 10 feet(3meters), not the amount of power transmitted from
> the device... Remember, these are not broadcast licensed
> transmitters and are certified under strict rules."
>
> I'm not going to clarify what is said other than it states
> power output of the FM transmitter is not the issue but
> rather how well your antenna put out the signal in terms of
> the 250uv/3m per part15n rules.
>
> Now I mentioned the Pico measured less than the 47dbmV
> output putting it at the legal power per part15 rules and is
> certified as such... I need to say nothing more!

Presumably the "dbm/v per meter" in the quoted text above means decibels referenced to 1 microvolt per meter. The correct form of abbreviation for that term is dBµV/m. The lower case "m" preceding the abbreviation for volts (V in correct form) in these abbreviations means "milli," which is a multiplier of 1/1000. Probably that is not what is intended. But again, this is a unit of voltage measurement, not power. Also, a voltage that is 47 decibels above a 1 microvolt reference level is ~223.9 microvolts, not 250.

This Q&A was used trying to show that a Part 15 tx output "power" that is at, or below the 47 dBmV spec of the Pico tx must be legal, because it does not exceed the "47dbm/v per meter" value identified as legal in the quote for a received signal 3 meters away. Of course, these units don't match. And in any case, they are units of voltage, not power.

The Info-Radio website includes a table showing field strength versus distance for Part 15 FM systems. But the data in that table is very incorrect. According to the FCC, a Part 15 FM system cannnot exceed 250 µV/m 3 meters from its antenna. But the table starts at 90 feet showing the 250 µV/m meter value. Also, every time the distance in the table doubles, the field strength in the table goes UP by 2X, not down by 1/2 -- the opposite of reality.

So these are examples of how the operation of Part 15 systems gets misunderstood, unless care is used in "doing the homework."
//
 
Re: A simpler picture (PDF)

That's interesting. It seems to further muddy the waters, though, because it states that no unlicensed TV broadcasting is permitted at all, yet TV modulators (usually for channels 3 and/or 4) are widely available to transmit video and audio from DVD players to older TV sets or from video cameras to TV sets. -- JasonW

> Here's a "simple picture" (a PDF) that might add clarity or
> confusion.
>
http://www.fcc.gov/ftp/Bu> reaus/Mass_Media/Databases/documents_collection/pn910724.pdf
>
>
> The FCC states "loosely" .01 microwatts of power.
 
Re: A simpler picture (PDF)

> The FCC states "loosely" .01 microwatts of power.
____________

And 0.01 microwatts = 10 nanowatts, which is a little less than the ~11.4 nanowatts calculated for a 1/2-wave dipole to generate the maximum Part 15 FM field. Maybe they "rounded."
//
 
Re: A simpler picture (PDF)

> > The FCC states "loosely" .01 microwatts of power.
> ____________
>
> And 0.01 microwatts = 10 nanowatts, which is a little less
> than the ~11.4 nanowatts calculated for a 1/2-wave dipole to
> generate the maximum Part 15 FM field. Maybe they
> "rounded."
> //
>


Rfry,

I was tired of arguing any facts on the above posts as I have a 40 hour work week and I run three part15 stations so I don't have the luxury of debating countless issues with you regarding my preferences of equipment, etc.

But if the .01 microwatts as you and the FCC described is the final and valid definition of power for FM part15 devices, then the FCC must be at fault for certifying the RadioSign Part15 FM transmitter, and undoubtly others as well, quote and the website below:


RadioSign

RS-ACC100 FM TRANSMITTER Only
The RS-ACC100 sold by RadioSign® is a FM transmitter that is FCC part 15 certified. Output power is limited to part 15 regulations. The output power is **approximately 5 to10 mW or jumper upgrade for export to 400 mW**. This transmitter uses a fixed wire antenna and includes the metal enclosure, wall adapter, audio cable and operating instructions. The RS-ACC100 comes with a 90 day warranty. It can also be used with car power 12 Volts DC with cigarette lighter adapter* for mobile or portable applications. The only thing else that is needed is your choice of audio input devices (extra cost), such as a 74 minute audio CD player. Just think... audio announcements that you or a professional announcer records on a computer, then that fle is burned to a CD that will run in rotation 24 hours a day with up to 74 minutes of audio.

http://www.radiosign.com/products.html

Radiopilot
 
More talk about dB.

Hi Rich and others,

I agree with Rich's comments about the Info-Radio FAQs. I lost some confidence when they mentioned dBuV as being a measure of power.

Their comment "> Q - Gosh, I'm getting a nose bleed, just tell me why in
> simple terms. " really tells the story. There is general attitude everywhere that people don't want technical explainations, yet they want to use technical information and terminology. Thus, in an attempt to simplify, the rules of physics are bent and the wrong information is conveyed, as Rich pointed out.

For those who want to risk a bloody nose here is some background information:

The definition for the decibel is a POWER RATIO, but if the resistance does not change or is the same for multiple systems, then it is sensible to use dBV as a relative measurement of voltage. Since dB is always a ratio, we can state the reference for the measurement by noting the denominator, such as 1 microvolt, which gives dBuV. This becomes an expression of a real voltage which is referenced to 1 microvolt, but it is not a power unless the resistance is known. We seldom see the complete equation for dB as applied to voltage. It is:

dB = 20 log (V1 / V2) - 10 log (R1 / R2)

If R1 = R2 then the last term is 0. That is why it is seldom seen.

Without illustrations and more talk, the above is not a very good lecture on the subject. I just wanted to convey that dB is a power ratio and decibels have to be applied carefully to each situation and attempts to simplify can mislead.


Neil
 
Re: A simpler picture (PDF)

> That's interesting. It seems to further muddy the waters,
> though, because it states that no unlicensed TV broadcasting
> is permitted at all, yet TV modulators (usually for channels
> 3 and/or 4) are widely available to transmit video and audio
> from DVD players to older TV sets or from video cameras to
> TV sets. -- JasonW
>

Hi Jason,

These devices are for direct connection through a transfer switch to the antenna terminals and do not broadcast. They are closed circuit and part15 applies to radiation leaked. They are not intentional radiators.

Neil
 
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