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FM Transmitter from a PC?
http://www.pcs-electronics.com/en/products.php?sub=pc_fm_trans
Has anyone worked with this? You can scale the power back to .010 watts, but can be cranked to .300 watts. Would said transmitter be legal in the FCC eyes? was thinking of getting it in the future.
Mark Shannon
AM 980 WOQ/Omaha’s Independent Radio
Total posts : 45366Mark,
While I have not personally tried that unit, I would be worried that if you stick a PCI card with RF inside your computer, that you would be subjecting it to all kinds of undesireable noise (like the fans, disk drive motors, etc.). Generally, you want to have your RF components shielded (by metal enclosures) and to try to keep them away from noise that is generated by other electronics and electrical stuff as much as possible, especially given the low power outputs of Part 15 FM.
Very strictly speaking (since that would apply to the FCC, since you asked), FM standards are expressed in microvolts per meter, as measured by some very expensive field test equipment that most of us don’t have or can’t afford, rather than milliwatts of output (like Part 15 AM is). The ancient part 15 FM standard (circa 1970s) was 100 milliwatts (mw) for FM, but chances are if you are using even close to 50 mw now, you would probably be in violation. Obviously the exact power output to push beyond the standards will depend on antenna, how and where it is installed, feed lines and a whole bunch of other variables.
Frankly, I would also be curious about that unit if anyone has worked with it.
Total posts : 45366I used the early one.
Somehow, this thing picks up no additional noise – and it even has open specs so you can write your own software for it 😉 which I kinda dig.
Total posts : 45366mlr,
I stand corrected on my fears about noise. Thanks for the insight.
Now of course you have piqued my curiosity:
What can you program on this thing? I would imagine that the board’s own software would control output power an dfrequency setting, but I am not sure what else.
What else does the board’s software control and what proprietary stuff have you done (or thought about)?
AM980WOQ:
The exact FCC specification is for Part 15 FM signal strength: 250uV/M at 3 Meters. With the range of power that you described (10 to 300 mw – milliwatts), most manufacturers seem to set their Part 15 “certified” devices at or below 10 milliwatts to be very conservative, not knowing ahead of time what conditions the end user would encounter. Legality would be based on the field signal strength, not power output like AM.
Total posts : 45366It was a couple of years back – I don’t even have the stuff anymore. What I always thought was cool is that SAM2 has the built in module for the PCIMAX…
Total posts : 45366[quote:8ba329bb39=”RadioheadC”]The ancient part 15 FM standard (circa 1970s) was 100 milliwatts (mw) for FM, but chances are if you are using even close to 50 mw now, you would probably be in violation. Obviously the exact power output to push beyond the standards will depend on antenna, how and where it is installed, feed lines and a whole bunch of other variables.[/quote:8ba329bb39]
How true. Using the equationField Strength in dB above 1 microvolt/meter =
104.77 + ERPi – 20 log (D)where
ERPi = Effective Radiated Power in dB referenced
to 1kW from an isotropic radiator
D = Distance in kilometersand applying the necessary units conversions says that input power of ~11.5 nanowatts to a linear 1/2-wave dipole will produce the FCC’s maximum field of 250 uV/m at a distance of three meters. A nanowatt is a millionth of a milliwatt! Physically small transmit antennas for VHF are MUCH more efficient than for MW.
BTW, for equal conditions this equation produces the same numbers that the FCC “Curves” program does for free space conditions.
If you want to experiment with a PC based, legal Part 15 FM station you might look for a “LineX” FM tx that plugs into, and is powered by a computer USB port. It also takes its stereo audio feed from the USB port, and transmits it on one of several PLL-selected freqs in the FM band. I’ve used one, and it sounds pretty good.
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Total posts : 45366So, since this is a truely finite thing (250mV/3m), in a perfect condition, what kind of range do you get in legal p15 FM?
Total posts : 45366[quote:3d626eaa5f=”mlr”]So, since this is a truely finite thing (250mV/3m), in a perfect condition, what kind of range do you get in legal p15 FM?[/quote:3d626eaa5f]
The table below was generated using the equation I posted earlier, and the fact that every time distance doubles, field strength is reduced 6 dB. The column showing +6 dB values covers the case where there is a unity, in-phase reflection of the direct signal (unlikely in the real world).From the table it can be seen that any signal that can give useful service to a good car radio over a line of sight path of more than about one mile probably does not meet FCC Rules for unlicensed FM stations.
Signals for boomboxes, Walkmen etc inside houses in areas of “urban clutter” might need to be 12 dB greater than for a car radio in clear, open country. That would limit legal service to home receivers to a radius of 1/4 mile, more or less.
Miles / Field, uV/m / +6 dB
0.002 / 250.00
0.004 / 125.00 / 250.00
0.007 / 62.50 / 125.00
0.015 / 31.25 / 62.50
0.030 / 15.63 / 31.25
0.060 / 7.81 / 15.63
0.119 / 1.95 / 3.91
0.477 / 0.98 / 1.95
0.954 / 0.49 / 0.98
1.908 / 0.24 / 0.49
3.817 / 0.12 / 0.24
7.633 / 0.06 / 0.12
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Total posts : 45366Rich,
Thank you very much for your analysis. You are a true font of knowledge and resource to appreciate. (I enjoyed trying to follow your calculations for the elevated AM antenna, where I lurked…)
I am a bit over my head here. (I had some technical training in the ancient past, although RF was not one of my strengths.)
When you talk about the car radio at a mile away, what is a reasonable signal strength (based on your table) that a car radio should be able to pick up (a) with decent listenablilty and (b) as a marginal but recognizable signal (i.e. threshhold before disappearing into the static of background noise, or even another station)? Would that be like 2 mv/M for (a) or another number?
Also, what does that third (rightmost) column mean? Is that the number of mV/M which is 6 dB above the middle column?
Finally, what (if any) is the dB gain of a half-dipole anetenna? And what kind of signal loss can one expect feeding it, say with 25 feet of RG-8/U or RG-58 and common Radio Shack style (but soldered) PL-259 connectors?
Many thanks (in advance) for your time and wisdom.
Total posts : 45366[quote:2433e61d8c=”RadioheadC”]…what is a reasonable signal strength (based on your table) that a car radio should be able to pick up (a) with decent listenablilty and (b) as a marginal but recognizable signal (i.e. threshhold before disappearing into the static of background noise, or even another station)? Would that be like 2 mv/M for (a) or another number?[/quote:2433e61d8c]
A good car radio has an FM sensitivity of about 1 uV for monaural signals, in the absence of interference. Full stereo sensitivity is much higher, maybe around 500 uV, below which a typical auto receiver “blends” progressively back to its monaural mode. Without interference (ie, electrical noise and/or other stations), a 1 uV/m field will produce fairly quiet reception for monaural signals — maybe an audio S/N of around 30 dB. The receiver needs maybe 10X that to drive it into full limiting and best S/N (maybe 60dB).
[quote:2433e61d8c]Also, what does that third (rightmost) column mean? Is that the number of mV/M which is 6 dB above the middle column?[/quote:2433e61d8c]
Yes. This should be considered an upper limit for received signals at that distance, which can occur only when the direct wave from the antenna and a reflection of it from some nearby surface are equal, and in phase. This rarely happens in reality, though. And if the two are out of phase, they will cancel, giving the receiver nothing to receive.
[quote:2433e61d8c]Finally, what (if any) is the dB gain of a half-dipole anetenna?[/quote:2433e61d8c]
If you meant a 1/2-wave dipole, its peak gain is 2.15 dB above an isotropic radiator. An isotropic radiator is a theoretical, reference antenna with equal gain in all directions. It doesn’t exist in the real world, but is a useful reference when evaluating other antennas.
[quote:2433e61d8c]And what kind of signal loss can one expect feeding it, say with 25 feet of RG-8/U or RG-58 and common Radio Shack style (but soldered) PL-259 connectors?[/quote:2433e61d8c]
The efficiencies of 25-foot lengths of RG-8 and RG-58 are both 90% or better at 100 MHz. But the line loss really doesn’t matter — it is the field radiated by the antenna that counts. Even if the line loss was 90%, the tx power could be raised to compensate for it, and the peak field from the antenna would be the same 250 uV/m at a distance of 3 meters as if there was no line loss at all.But this means you must be able to accurately measure the maximum field in any direction 3 meters from the antenna, which takes some good test equipment and skills, not to mention the difficulty of doing that while the antenna is 25 feet or more off the ground.
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Total posts : 45366Rich,
Again, thank you so much. You are good!
I also noticed another posting on another forum that referenced the following FCC Public Notice (dated July 24, 1991):
http://www.fcc.gov/mb/audio/decdoc/scandoc/910724/1.jpg
This still shows the 250 uV/m at 3 meters standard, it also gives an equivalence of 0.01 microwatts as a maximum ERP (Effective radiated power), which is 10 nanowatts or “close enough for government work” to your “~11.5 nanowatt” calculation.
Where I still scratch my head is it is my understanding that the so-called certified P15 FM transmitters (like some of the ones referenced in the resources section of this website, or some of the mp3 and iPod FM transmitters generally show output powers that range from 1 to 10 milliwatts. (Admittedly, many just specify the 250 uV/m at 3 meters in their specs, but some have 1 to 10 mW and I have come across some reviews and other postings where power outputs fall in this range.) It was also my understanding that manufacturers that sold certified productis tended to be conservative in their power outputs in order to protect themselves against all potential uses.
So, what accounts for the difference between the 10 or 11.5 nanowatts that the FCC and you respectively estimate and the 1 to 10 milliwatts that manufacturers seem to use? It is just having a miniscule anltenna (like a very short whip or wire that is 12 inches or less) versus the theoretical isotopic dipole? Or are there other factors as well (including my feeble understanding)?
Total posts : 45366Well, you’ve given me some food for thought. I would like to try it out one at some point down down the road.
I wonder if the TV station I work for would have a meter to make sure I’m not in violation of FCC rules. I’d doubt it, but it’s worth a try. In my location, I can hit about 100 houses and appartments under Part 15 FM whereas I can only hit 1/4 of that with my AM broadcast.
Thanks for all your help
Mark Shannon
AM 980 WOQ/Omaha’s Independent Radio http://www.980woq.net
Total posts : 45366[quote:b064c02119=”RadioheadC”]So, what accounts for the difference between the 10 or 11.5 nanowatts that the FCC and you respectively estimate and the 1 to 10 milliwatts that manufacturers seem to use?[/quote:b064c02119]
The FCC Public Notice you linked to refers to the “Maximum Effective Radiated Power” permitted — which is the peak directional power actually radiated by the antenna — not the power input or output of the Part 15 FM transmitter itself.The transmitter output power can be whatever value it needs to be to produce no more than 250 uV/m at a 3-meter distance in any direction from the antenna. So if the antenna has a peak gain of 1X, the power applied to its input connector could not exceed the nominal 10 nanowatt value.
I think the FCC Rules for “type acceptance” require commercial manufacturers of Part 15 FM transmitters to supply the antenna, and that it should either be permanently attached to the transmitter, or use a unique connector so that it can’t be replaced with an antenna that wasn’t tested and certified as meeting the Rules with that transmitter.
In any case, antennas for the FM band usually are very efficient, because even though they are short physically, they are long in terms of wavelength (unlike for Part 15 AM antennas). A 1/2-wave FM dipole, for example, is only 4 or 5 feet long, and has a peak gain of 1.64X compared to an isotropic radiator. A “whip” type antenna has somewhat less gain than that, but still far more gain than would permit using anything approaching 1-10 mW with it, and still meet Part 15 Rules.
Manufacturers quoting Part 15 FM transmitter power of 1 to 10 mW may say (if pressed) that these powers are for use only “where authorized.” Or maybe some of them either haven’t done the math, or are referencing inapplicable/outdated FCC documents. Good question to ask the ones who do this.
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Total posts : 45366Do any FCC type-accepted / certified / verified part 15 FM transmitters exist that were built < 1970s? What would the FCC do if they saw you using one (one that was made before the 250uV/m @ 3m limit) that had their label on it?
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