Fm power and certification issues...

[radio8z]

Re: A simpler picture (PDF)

Hi,

If you recall, the limitation on FM is based on field strength. With a given antenna and a given output power, you will get a given field strength. If you choose an antenna, connect it to a transmitter, adjust the transmitter power to get a legal field strength, then you can go back and measure the transmitter power. There is really no point in doing this since the limits are field strength.

Note in their writeup they said "...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..."

Apparently with their fixed wire antenna they get the legal field strength with the power stated. Things would be better if they left the power description out.

Neil

 

[radiopilot]

Re: 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
>

Neil,

I think something like this makes it easier to estimate what types of power one will expect to get in terms of db to watts.

www.minicircuits.com/dg03-110.pdf

Radiopilot

 

[rfry]

Re: More talk about dB.

> 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.

Thanks, Neil, but to expand -- a decibel can be used to express the ratio between two levels of voltage, current or power. The use of decibels isn't restricted to power ratios.

For voltage, dB = 20*log(E1/E2). For current, dB = 20*log(I1/I2). For power, dB = 10*log(P1/P2). The logarithms are to the base 10 in each case.

A decibel relationship of voltages or currents cannot define an absolute power level unless/until the circuit resistance where the voltage or current existed is known, and a reference value is defined (a milliwatt, a microvolt, an ampere, etc).

Using decibels simplifies some mathematical operations, and expresses large ratios between any two values without using "awkard" numbers.
//

 

[radiopilot]

Re: A simpler picture (PDF)

> Hi,
>
> If you recall, the limitation on FM is based on field
> strength. With a given antenna and a given output power,
> you will get a given field strength. If you choose an
> antenna, connect it to a transmitter, adjust the transmitter
> power to get a legal field strength, then you can go back
> and measure the transmitter power. There is really no point
> in doing this since the limits are field strength.
>
> Note in their writeup they said "...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..."
>
> Apparently with their fixed wire antenna they get the legal
> field strength with the power stated. Things would be
> better if they left the power description out.
>
> Neil
>


Neil,

This is the similar argument between a FCC certified Rangemaster and the SStran or Ramsey AM transmitter? Is not 100mw the same no matter?

So if 250uv/3m using a short wire and .01 microwatts, per the FCC and Rfry then why would 10-15mw of power be ok with a telescoping antenna and FCC certified be any different or worse? It cannot be the same!

Radiopilot

 

[radio8z]

On dB, rfry and radiopilot are correct.

Hi,

Thanks to both for your responses. True, dB can be used to express many ratios. One of the most useful is gain.

I pointed out that the fundamental definition is a power ratio. This was developed by Alexander Graham Bell who was an audiologist and invented the Bell as a unit of measure of relative sound power. The decibel was later introduced when the resolution of the Bell proved inadequate. The decibel, one tenth of a Bell, is abbreviated dB with a capital B because of this.

We just have to be careful when using dB for expressions other than power. Used properly, the dB is a very convienent way to characterize signal processing.

You can use dBm or dBu to express power because the notation is understood to mean relative to 1 milliwatt or 1 microwatt respectively. Thus 13 dBm is actually a power. The problem is when one uses dBuV if the resistance is not known then the power is not known; only the voltage is known. I can calculate the output of an amplifier with a gain of 15 dB and an input voltage of 3dBuV to be 15 + 3 = 18 dBuV. This tells me the voltage, but not the power.

For those who are interested, the calculation is:

volts = 1uV * 10^(dBuV/20)

The output in the above example is = 1uV * 10^(18dBuV/20) = 7.94 uV.

Now, if I know that this voltage appears across a 50 ohm resistor, I calculate

power = (V^2)/R = ((7.94 uV)^2)/50 ohms = 1.26 picowatts

Notice that I did not have the power until I used the resistance.

One of the diseases of old professors is that they overanswer questions. I guess I did that.

Neil<P ID="edit"><FONT class="small">Edited by radio8z on 12/01/05 11:05 PM.</FONT></P>

 

[radiopilot]

Re: 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
> >
>
> Neil,
>
> I think something like this makes it easier to estimate what
> types of power one will expect to get in terms of db to
> watts.
>
> www.minicircuits.com/dg03-110.pdf
>
> Radiopilot
>

Neil,
Nonsense, you are correct! Anyway if you look at the table in the minicircuits site, you'll see that 47uV equates to -74dbm approx. and the power output is somewhere around .05nw, does this sound familiar in my arguments in the previous posts about how much power the PCFM transmitter outputs! Not very much power but even with that amount of power on a well installed antenna will give you 200-300 feet of coverage, maybe more but for a high school radio broadcast system it's not too bad, as I said and it should comform to the FCC as far as compliance, I just don't understand sometimes why people make harsh issues of things when all they have to do is sit back and have a cup of Jo...

Radiopilot

 

[radio8z]

Re: More talk about dB.

>
> Neil,
> Nonsense, you are correct! Anyway if you look at the table
> in the minicircuits site, you'll see that 47uV equates to
> -74dbm approx. and the power output is somewhere around
> .05nw, does this sound familiar in my arguments in the
> previous posts about how much power the PCFM transmitter
> outputs! Not very much power but even with that amount of
> power on a well installed antenna will give you 200-300 feet
> of coverage, maybe more but for a high school radio
> broadcast system it's not too bad, as I said and it should
> comform to the FCC as far as compliance, I just don't
> understand sometimes why people make harsh issues of things
> when all they have to do is sit back and have a cup of Jo...
>
>
> Radiopilot
>
Hi,

Thanks for your comment.

Yes, -74dBm is a power. My point is that dBuV is not.

I don't know if I am one of those you referred to as making harsh issues. It is not my intent to be harsh. What I saw was misapplication and misinterpretation of data on cited manufacturers websites, and I tried to be educational in my comments.

You stated in one of your posts that you are going to run your transmitter to achieve a range of a few blocks. I think this is a reasonable approach and therefore discussions about output power are not relevant. Unfortunately, for design and enforcement purposes the FCC has to put numbers on part15 FM, and the average Joe cannot ensure compliance base on these numbers. It's like a police officer citing you for going "too fast". Well, what is too fast? 50 in a 35 zone is putting numbers on it.

For the engineering types among us, just once I would like to hear Captain Kirk say "Scotty was right, the engines couldn't take it."

Neil

 

[radio8z]

Re: A simpler picture (PDF)

> > Hi,
> >
> > If you recall, the limitation on FM is based on field
> > strength. With a given antenna and a given output power,
> > you will get a given field strength. If you choose an
> > antenna, connect it to a transmitter, adjust the
> transmitter
> > power to get a legal field strength, then you can go back
> > and measure the transmitter power. There is really no
> point
> > in doing this since the limits are field strength.
> >
> > Note in their writeup they said "...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..."
> >
> > Apparently with their fixed wire antenna they get the
> legal
> > field strength with the power stated. Things would be
> > better if they left the power description out.
> >
> > Neil
> >
>
>
> Neil,
>
> This is the similar argument between a FCC certified
> Rangemaster and the SStran or Ramsey AM transmitter? Is not
> 100mw the same no matter?
>
> So if 250uv/3m using a short wire and .01 microwatts, per
> the FCC and Rfry then why would 10-15mw of power be ok with
> a telescoping antenna and FCC certified be any different or
> worse? It cannot be the same!
>
> Radiopilot
>


Hi,

All I can offer is that it depends on what the antenna does with the power it is fed. I didn't notice what antenna the FCC was referencing in their article.

I agree that 100mW on AM is the same regardless of the transmitter, but the antenna makes the difference. The Rangemaster was certified compliant with a particular antenna. Use another antenna and it may be compliant, but not certified.

Neil

 

[rfry]

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)

> Rfry,
> ...
> 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: (omitted here)
____________

An appropriate question, radiopilot, and I have puzzled over it for a long time.

The FCC defines a compliant Part 15 FM operation as one having not more than 250 µV/m field strength in any direction 3 meters away from its transmit antenna. That's pretty simple.

Calculating the amount of applied power needed by a 1/2-wave dipole to generate that field strength is relatively straightforward. That number is ~11.4 nanowatts, or ~0.0114 microwatts (proof on request).

So a Part 15 FM transmitter capable of 10 milliwatts of power has literally thousands of times more power capability than is needed to produce a legal Part 15 FM field with even a simple 1/2-wave dipole. That, also, is a simple conclusion.

Then how can a Part 15 FM transmitter using an intentional radiator produce 10 mW or more output power, while at the same time generating a legal Part 15 FM field? THAT is the good question.

To do so would mean that the Part 15 FM antenna system would need to be extremely inefficient -- MUCH MUCH worse than the 3-meter antenna systems used for Part 15 AM (which are notoriously poor radiators).

The length/configuration of the transmit antenna used for Part 15 FM is not defined or limited, as it is in Part 15 AM. And, because of the much shorter wavelengths used for FM, a physically short antenna is a much more efficient radiator on FM than it is on AM. In real numbers, the gain of a short FM antenna system is roughly 100X higher than it is for a short AM antenna system. Or to put it another way, Part 15 AM antenna systems are only ~1/100th as efficient as Part 15 FM antenna systems

These realities mean that 10 mW, or even 1 mW applied to the short "whip" type antenna commonly included with commercial Part 15 FM transmitters would produce field strengths far above the Part 15 limit.

Apparently the field strengths radiated by these commercial Part 15 FM tx+antenna systems did not exceed the Part 15 limit during the FCC certification measurements. This being true, one would have to conclude that the transmitters were not adjusted to produce their maximum rated power -- if that exceeds whatever it takes to generate the legal Part 15 FM field with the antenna in use during certification.
//

 

[JasonW]

Re: A simpler picture (PDF)

I have a channel 3/4 TV modulator (purchased from All Electronics) that was designed to broadcast wirelessly to older TV sets that don't have coax connectors. The range is only a few feet, but it transmits over the air. -- Jason

> > 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
>

 

[rfry]

Re: A simpler picture (PDF)

> I have a channel 3/4 TV modulator (purchased from All
> Electronics) that was designed to broadcast wirelessly to
> older TV sets that don't have coax connectors. The range is
> only a few feet, but it transmits over the air. -- Jason
___________

But they must have some kind of RF input connector. The picture probably would improve if you wired the RF output of the modulator directly to the TV set through an A/B RF switch of some kind.

Most home VCRs also have a Ch 3 or 4 RF output for direct connection to a TV set RF input. One could hook an antenna to the VCR RF output, but that isn't its intended use, and the picture would probably suffer.

Maybe that's the situation with your All Electronics modulator.

//

 

[JasonW]

Re: A simpler picture (PDF)

Nope. It has only one input connector (a 75 ohm "F" connector) and a coiled wire antenna in one end of the case, and no output connectors of any kind. -- Jason

> > I have a channel 3/4 TV modulator (purchased from All
> > Electronics) that was designed to broadcast wirelessly to
> > older TV sets that don't have coax connectors. The range
> is
> > only a few feet, but it transmits over the air. -- Jason
>
> ___________
>
> But they must have some kind of RF input connector. The
> picture probably would improve if you wired the RF output of
> the modulator directly to the TV set through an A/B RF
> switch of some kind.
>
> Most home VCRs also have a Ch 3 or 4 RF output for direct
> connection to a TV set RF input. One could hook an antenna
> to the VCR RF output, but that isn't its intended use, and
> the picture would probably suffer.
>
> Maybe that's the situation with your All Electronics
> modulator.
>
> //
>