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Portable FM transmitters

J

justthenumbers

Sorry if this is the wrong forum for this question, but I wasn't sure where else to ask...

What is the wattage for a standard portable FM transmitter - the type you'd hook up to an iPod to transmit on an open frequency in your car?
 
These devices are unlicensed, but still covered by FCC Part 15. The Rule does not define the power of the transmitter, or the antenna to be used. Instead it limits the field intensity that can be radiated to a maximum of 250 microvolts per meter in any direction 3 meters away from the antenna.

The radiated power it takes to do that with even a simple antenna is extremely small -- on the order of 0.000 000 011 watts (11 nanowatts).

Here is a link to an FCC bulletin with more detail:

http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet63/oet63rev.pdf

RF
 
What is legal is so puny that any interference will mess with it unless it is injected directy into your antenna port on the radio. That's why XM and Sirrius had stuff made that was "over power". The truth is that the FCC's part 15 rules are nothing short of ridiculous, sort of like that agency as a whole. If I wanted to get a modulator I'd look into at least a Ramsey with the strap in the high power setting. 100mw isn't going to be enough where you'll ever see an FCC inspector if you are even close to careful about what channel you use, etc.
 
Everyone is free to decide if they want to observe Part 15 or not, but below is a link to an FCC "NOUO" issued for operating with a lot less FM power than 100 milliwatts. There are many other examples.

In this particular case the free space field strength measured by the FCC at 219 meters would be produced by a 1/2-wave dipole radiating 0.12 milliwatts.

http://www.fcc.gov/eb/FieldNotices/2003/DOC-286303A1.html

The chart below shows what fields to expect for systems of various powers.

http://i62.photobucket.com/albums/h85/rfry-100/6417f684.gif

RF
 
OKCRadioGuy said:
<snip>...100mw isn't going to be enough where you'll ever see an FCC inspector if you are even close to careful about what channel you use, etc.
Click to expand...

I have information which indicates that this may not be true. Below I have copied an article I wrote a while back which you can believe or not per your choice:

I was curious about the FM transmitter power levels involved in FCC NOUO (notice of unlicensed operaton) actions so I did a bit of research. The FCC states in the NOUO the measured field strength at a distance. From the FCC site, I selected some NOUO reports for FM and noted the field strength and distance. I then calculated the approximate ERP involved with each of these.

I used the following assumptions to calculate the ERP:

1. The peak free space field strength produced by 11 nanowatts ERP at 3 meters is 250 uV/m.

2. The antenna is a resonant dipole. (Actual antennas may have more or less gain than a dipole so these numbers do not represent the exact transmitter output power.)

3. The field strength increases linearly with distance upon approaching the antenna.

4. The distances and field strengths reported by the FCC were accurate.

I derived the equation

ERP(mw.) = (19.6 x 10^-12) x [ FS(uV/m) x distance(meters) ]^2

to give the approximate ERP. Using this, I calculate the ERP numbers below.

FCC NOUO resulted from these calculated estimates of ERPs expressed in MILLIWATTS:

93., 357411., 6.7, 0.001, 289., 0.703, 577., 120., 0.187
Click to expand...

Neil
 
Oh you are 100 percent right, technically. What I am stating is the FCC stands for the Federal Check Cashers. If they can't make a lot of money on the deal somehow then they won't bother. Unless the 100mw bothers someone I wouldn't look for them to get too excited on a per-person basis. They eventually got excited with XM and Sirrus because collectively there were enough of their devices out there wiping out non-comm listening near them. Otherwise it's no big deal. The end user was and will likely never be bothered for those devices too.
 
OKCRadioGuy said:
Oh you are 100 percent right, technically. What I am stating is the FCC stands for the Federal Check Cashers. If they can't make a lot of money on the deal somehow then they won't bother.
Click to expand...

OKCRadioGuy: Your comment above is quite cynical. Do you believe that the FCC made ANY money by issuing the NOUO I linked to, and the many other NOUOs they issued for excess field strength from unlicensed FM transmit systems?

Quite the opposite, these actions cost the FCC quite a bit of money in test equipment, field engineering, and office time to process and pursue -- which is money out of the pockets of all US taxpayers.

RF
 
Exactly. Thats why they generally dont bother. They like things like auctions and nailing broadcasters and cell companies for paperwork errors for thousands because they can pay it. Its all about the money and politics with the fcc,
 
OKCRadioGuy said:
Oh you are 100 percent right, technically. What I am stating is the FCC stands for the Federal Check Cashers. If they can't make a lot of money on the deal somehow then they won't bother. Unless the 100mw bothers someone I wouldn't look for them to get too excited on a per-person basis. They eventually got excited with XM and Sirrus because collectively there were enough of their devices out there wiping out non-comm listening near them. Otherwise it's no big deal. The end user was and will likely never be bothered for those devices too.
Click to expand...
You may refute my post and analysis if you wish but for the good of the order here it is beneficial that both sides of this equation can be evaluated. On my side, I have presented hard data based on fact and on your side you responded as quoted above. You say it is "no big deal" yet the data I presented indicate otherwise. I admit that by reading the NOUOs we cannot know what prompted the FCC actions but if my analysis is correct one can definitely get into trouble with only fractions of milliwatts of power on FM.

Thanks for engaging in this dialogue for I think it is very informative for all of our readers.

Neil
 
OKCRadioGuy said:
Exactly. Thats why they generally dont bother.
Click to expand...

Maybe "generally" is good enough for you.

But for those who take your advice, and as a result receive an FCC NOUO -- ?

RF
 
I use the Ramsey model 30, and it works well. If you want to take your sound card out to the transmitter you need a 1/8" stereo plug to RCA plugs.

This is strictly hobby stuff, so don't expect it to sound like a Class C.

I have mine on my desk, and when I play audio files on my computer, I can walk around in the backyard with my walkmans on to listen. But it fades at about 250 feet from the device. At 100 feet it's picketing pretty good.

I think the last time I measured the output with my service monitor it was about 37 mw. Not a lot of power, and the whip is sort of an 1/8 wave antenna, so don't expect miracles.

Still, if you want something to listen to stuff off your computer, it's not that bad.
 
I did some experiments with Breakaway Broadcast Processor and a cheap low power FM transmitter with built in stereo encoder and pre-emphasis.

I set BBP to 75us pre-emph, de-emphasised on. I used the L/R PEQ to compensate for the transmitter's less than perfect 75us pre-emphasis curve.

The result?

Quite acceptable, actually! A million times audio quality and stability than running the transmitter barefoot. Plenty of overmod, little treble spikes up to 130% due to the de-emph -> re-pre-emph process, but for a low power transmitter such as this, that's basically irrelevant.

Granted, at $199, BBP is too expensive for a setup like this. However, the only limitation in the free demo version, is a single commercial every 45 minutes of audio. The countdown does not run during dead air. So, if you can live with that, go ahead and use it free -- it should prove a major boost to sound quality. :)

///Leif
 
I bought a cheapo digital tuned fm transmitter from biglots. Using a 10 foot audio cable (It uses this as the antenna) slung up in a tree I can pick it up till about a half a mile away on my vehicle stereo. I was amazed, it seems really powerful for its size. Just for kicks, using sound solution and winamp I can whoop the processing of some of our local stations with this thing.
 
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