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Real Part 15 power at transmitter!

R

radiopilot

There have been posts here regarding the ~11 nanowatt power the Part15 FM transmitters are to have... Why do I and others as well should have a problem with this amount of power?

First my receiver has a sensitivity of 45uv for the 'stereo pilot' to activate and the mono mode it is 4.5uv to receive that same signal... now at 45uv that equates to -74dBm (which is a little less than 1nw), the mono signal would be
-94dBm (which is approx .001nw), BUT that is at the receiver side and not at the transmitter... and why do I know this.. because the Belkin Tunecast II is a Part 15 certified FM transmitter and is rated for -22dbm at 107.9 MHz and
-15dbm at 88.1 MHz ... so at -22dBm that power is approx. .01mw and at -15dBM that power is approx. less than 1/10 of 1mw.... or .1mw....

Anyone can measure this signal... it's a measured signal right from of the O'scope (picture available) and an unmodified Belkin Tunecast! These Belkin units are certified for 150uv/3 meters and not 250uv/3 meters so using that logic one can see that the 'true' Part-15 FM transmitters do have more power at the output end than ~11nw as some suggest here...

To prove the above go to your 'tuner/recveiver' specifications and see what you need for the signal to be present at the antenna.

Radiopilot
 
> First my receiver has a sensitivity of 45uv for the 'stereo
> pilot' to activate and the mono mode it is 4.5uv to receive
> that same signal... now at 45uv that equates to -74dBm
> (which is a little less than 1nw), the mono signal would be
> -94dBm (which is approx .001nw), BUT that is at the receiver
> side and not at the transmitter... and why do I know this..
> because the Belkin Tunecast II is a Part 15 certified FM
> transmitter and is rated for -22dbm at 107.9 MHz and
> -15dbm at 88.1 MHz ... so at -22dBm that power is approx.
> .01mw and at -15dBM that power is approx. less than 1/10 of
> 1mw.... or .1mw....
_____________

Sorry, but these numbers don't compute. Are they based on that Mini-circuits table you linked to?

As I stated, that table can't be used convert field strength to power.

//
 
> > First my receiver has a sensitivity of 45uv for the
> 'stereo
> > pilot' to activate and the mono mode it is 4.5uv to
> receive
> > that same signal... now at 45uv that equates to -74dBm
> > (which is a little less than 1nw), the mono signal would
> be
> > -94dBm (which is approx .001nw), BUT that is at the
> receiver
> > side and not at the transmitter... and why do I know
> this..
> > because the Belkin Tunecast II is a Part 15 certified FM
> > transmitter and is rated for -22dbm at 107.9 MHz and
> > -15dbm at 88.1 MHz ... so at -22dBm that power is approx.
> > .01mw and at -15dBM that power is approx. less than 1/10
> of
> > 1mw.... or .1mw....
> _____________
>
> Sorry, but these numbers don't compute. Are they based on
> that Mini-circuits table you linked to?
>
> As I stated, that table can't be used convert field strength
> to power.
>
> //
>

Sorry but this db to watts scale is accurate for conversion and till someone else does the calculation we can put this on hold... but I suggest as before someone else take on this challenge stated in the below post!

The table is used to see what the dbm of the RECEIVED power is to convert to watts and or uV (microvolts)... it was not used in the calculation itself!

Radiopilot
 
> The table is used to see what the dbm of the RECEIVED power
> is to convert to watts and or uV (microvolts)... it was not
> used in the calculation itself!
____________

But the issue is that the table you linked to cannot be used to support the points you are trying to make.

That chart deals with some conversions within a 50 ohm system, but does not allow relating received power in an antenna system to the arriving field strength that produced it.

If you don't believe me, ask Mini-Circuits, or reference some electrical engineering textbooks, including those covering fields and waves.

//
 
> But the issue is that the table you linked to cannot be used
> to support the points you are trying to make.
>
//



It's just a 'conversion table' that says: 1 VOLT = 1 WATT = 1 AMP = 1 OHM, etc.

Though the above is just an example... the table is just that... a conversion of dBM to Volts to Watts....

> That chart deals with some conversions within a 50 ohm
> system, but does not allow relating received power in an
> antenna system to the arriving field strength that produced
> it.
>

Most transmitters in my world are either 50 Ohm or 300 Ohm... but for transmitting it's usually 50 Ohms... using an SWR to tune....

Receiving antennas are either 75 Ohms or 300 Ohms.... but the 'field strength' at the receiving end... is what ever strength regardless if you measure it with a field strength meter or tuner of known sensitivity....


> If you don't believe me, ask Mini-Circuits, or reference
> some electrical engineering textbooks, including those
> covering fields and waves.
>

You can look up any conversion scale or calculator in any book or website and the numbers are exactly the same as in the table for a 50 Ohm scale....

Radiopilot
 
> Receiving antennas are either 75 Ohms or 300 Ohms.... but
> the 'field strength' at the receiving end... is what ever
> strength regardless if you measure it with a field strength
> meter or tuner of known sensitivity....
_________

Sorry, but your understanding of this subject is inaccurate.

For example, a field strength of "X" mV/m would produce different amounts of power at the feedpoints of receiving antennas having different gains. So how then can you use your table to convert power in dBm to the field strength in mV/m arriving at the antenna?

Answer: you can't.

//
 
> It's just a 'conversion table' that says: 1 VOLT = 1 WATT =
> 1 AMP = 1 OHM, etc.

Of course, that's not the case, at all.

Where did you learn your electronics theory, if I may ask? Your understanding of electronics is so wacked, I thought for a long time you're making parody posts. No kidding.
 
> > It's just a 'conversion table' that says: 1 VOLT = 1 WATT
> =
> > 1 AMP = 1 OHM, etc.
>
> Of course, that's not the case, at all.
>
> Where did you learn your electronics theory, if I may ask?
> Your understanding of electronics is so wacked, I thought
> for a long time you're making parody posts. No kidding.
>


You if you read between the lines...

The above reference was an example to prove a point not that it's electronics THEORY!!!

What the topic of reference was that 'so much dBM = so much Volts = so much Amps = so much Watts, etc.' Kapish?

Anyone reading a dBm to Volts or Watts scale should have no problem....

Well if you have so much greater knowledge in electronics or anything YOU can really offer, let's get the knowledge out and do the math yourself....

Radiopilot
 
> > Receiving antennas are either 75 Ohms or 300 Ohms.... but
> > the 'field strength' at the receiving end... is what ever
> > strength regardless if you measure it with a field
> strength
> > meter or tuner of known sensitivity....
> _________
>
> Sorry, but your understanding of this subject is inaccurate.
>
>
> For example, a field strength of "X" mV/m would produce
> different amounts of power at the feedpoints of receiving
> antennas having different element lengths. So how then can
> you use your table to convert power in dBm to the field
> strength in mV/m arriving at the antenna?
>
> Answer: you can't.
>
> //
>

Look 22uV at the receiver is just that 22uV... not 22uV/m... and if the signal strength at 1/2 mile is say -75 dBM, that can be converted to a uV or a microwatt, etc. not that it has to be 'XX/M' strength.... If I took a sensitive meter and measured the antenna coming into the tuner it measures 22uV... it's not measuring that in meters... just a reading and that's is all... now using the dBm scale, you can then read what the wattage and or dBM of the signal is... simple!


Let's leave the receive antenna out of any equations you have and only calculate the field strength at some distance only 1 mile, no antenna at the transmitter, just the transmitter alone outputting 100mw and the answer will still be different than you suggest!

Radiopilot
 
> Look 22uV at the receiver is just that 22uV... not 22uV/m...
> and if the signal strength at 1/2 mile is say -75 dBM, that
> can be converted to a uV or a microwatt, etc. not that it
> has to be 'XX/M' strength.... If I took a sensitive meter
> and measured the antenna coming into the tuner it measures
> 22uV... it's not measuring that in meters... just a reading
> and that's is all... now using the dBm scale, you can then
> read what the wattage and or dBM of the signal is... simple!

It may be a simple concept for you, but it isn't true. The receiving antenna system has a certain amount of gain, and will develop a certain amount of power (dBm) at the receiver input terminals as a result of that gain. Other things equal, an antenna with higher gain than another will produce more power in dBm at the receiver. The performance of the receiving antenna cannot be ignored.

> Let's leave the receive antenna out of any equations you
> have and only calculate the field strength at some distance
> only 1 mile, no antenna at the transmitter, just the
> transmitter alone outputting 100mw and the answer will still
> be different than you suggest!

Sorry, but that is absurd. If the transmitter has no antenna then there will no radiated power, and no field generated to propagate to the rx antenna.

Your replies indicate such a lack of fundamental understanding about the transmission, propagation and reception of radio waves that it appears pointless to pursue this subject with you on this forum. But I am willing to carry on with you in private about it as it appears productive, if you wish. My email address is shown in my profile.

//
 
>>Sorry, but that is absurd. If the transmitter has no antenna then there will no radiated power, and no field generated to propagate to the rx antenna<<

Funny but there are plenty of FM transmitters on the market with no antennas and if they do it's just couple of inches and YES there can be 'field strength' at the rx antenna!

But you are right it's pointless to continue the discussion... perhaps someone else may perhaps in the future look into this... as far as I'm concern let's put it to rest...

Radiopilot
 
Clarification I hope

> "Anyone reading a dBm to Volts or Watts scale should have no
> problem....".


dBm is a measure of power and is watts. (remember dB has no units).

dBm = 10 log (power/.001 W.)

Power = .001 W. (10^(dBm/10))

dBm is not a measure of voltage. However if the resistance is known then one has to do the math to find the voltage. The table you referenced is for a 50 ohm system, thus the voltage can be calculated from the power for a 50 ohm load:

V = SQRT(PR)


Neil
 
> You if you read between the lines...
>
> The above reference was an example to prove a point not that
> it's electronics THEORY!!!
>
> What the topic of reference was that 'so much dBM = so much
> Volts = so much Amps = so much Watts, etc.' Kapish?
>
> Anyone reading a dBm to Volts or Watts scale should have no
> problem....
>
> Well if you have so much greater knowledge in electronics or
> anything YOU can really offer, let's get the knowledge out
> and do the math yourself....
>
> Radiopilot

No, Rfry has done a done a fine job of creaming your pretzel logic over and over and over with the facts. Why repeat the same formulas and facts over and over? The patience that well-qualified engineer has shown with you is astonishing. Obviously, he cares enough to try to keep some real boneheads out of trouble. As a guy who works with transmitters for a living, I can tell you everything I've read of his checks out.

Now, I'm not OCD and I don't have all day to debate your flawed theory and facts. I am about at the point of questioning your motives in the face of the facts that are repeatedly presented to you. That's because people who follow your lead can get (and perhaps have gotten) themselves into serious trouble with the FCC, including heavy fines. I think it's almost getting to be a moderation issue on this board, because of the serious damage that can be done. Kapish?
 
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