Inconvenient Truths about Part 15 FM

[rfry]

It is quite common to read posts on various "Part 15" boards about the unlicensed use of FM transmitters having 25 mW output power (and more) with 1/2-wave dipole antennas. But if compliance with FCC Part 15.239 is needed/desired, then such configurations are unlikely to provide it. Here's why.

The FCC limit in Part 15.239 is a 250 µV/m field measured 3 meters in any direction from the transmit antenna. Physics shows that a 1/2-wave dipole will radiate that peak legal field when a matched power of only 11.43 nanowatts is applied to its feedpoint.

A matched system using a transmitter delivering 1 milliwatt or more at the far end of a coax cable connected to a 1/2-wave dipole will radiate a peak field that is at least 295 X greater than permitted by FCC 15.239. Here is the math:

SQRT(1 mW / 0.00001143 mW) = 295.78...

The reason that the square root applies is because field intensity varies as the square root of the power.

The loss in 100 feet of RG-6 coax cable is about 2 dB at 108 MHz, when the cable is properly terminated. For a matched system this means that if 25 mW is applied by the transmitter to one end of 100 feet of that coax, about 15.8 milliwatts is delivered to the other end.

That 15.8 milliwatts produces a peak field 3 meters from a 1/2-wave dipole that is 1,175 times greater than permitted by FCC 15.239.

With such performance from unlicensed systems in the 88-108 MHz band in the US, FCC action is not an impossibility -- and it would be easy for them to detect non-compliance.

Of course those who operate where they aren't subject to Part 15.239 have nothing to worry about from the FCC.

 

[pianoplayer88key]

How would you calculate how much power is necessary with an electrically short antenna? I'm thinking wavelengths below 0.05λ, below the 19° the AM Figure 8 calculator can use. Also we're talking FM (88-108MHz), not AM (0.53-1.7MHz), although I'm also interested in figuring out how to calculate for ~13.56MHz. Also, the ground would, at best, be the chassis of the battery-operated device. Is the math the same for inverse field at the distances specified in the various rules, regardless of the frequency?
For example, what power might be necessary to produce 250µV/m @ 3 meters, if your transmitter was the size of an iPod Shuffle, and the antenna was entirely contained within the unit's case? Or, if it was the size of an iPhone or a typical Android phone (again with a self-contained antenna), how would you calculate what power would produce 15,848µV/m @ 30 meters on 13.56 MHz?

 

[rfry]

How would you calculate how much power is necessary with an electrically short antenna? I'm thinking wavelengths below 0.05λ, below the 19° the AM Figure 8 calculator can use.

If you are asking about medium-wave antennas, they almost always consist of a monopole driven against a ground plane. A monopole of 0.05λ and less has almost the same directivity and radiation pattern as one that is 0.25λ high. But the radiation resistance of a 0.05λ monopole is extremely small, which means that most of the available power from the transmitter is dissipated in losses in the ground system, and in matching loss.

The radiation resistance (Rr) of a short monopole can be calculated, but the other resistances usually are unknown. If they are known, then radiation efficiency equals Rr / (Rr + R loss), which will lead to the effective radiated power from the system. From that the inverse distance field intensity can be calculated.

I'm also interested in figuring out how to calculate for ~13.56MHz. Also, the ground would, at best, be the chassis of the battery-operated device. Is the math the same for inverse field at the distances specified in the various rules, regardless of the frequency?

Yes, although the same issues exist as given in my response above.

For example, what power might be necessary to produce 250µV/m @ 3 meters, if your transmitter was the size of an iPod Shuffle, and the antenna was entirely contained within the unit's case? Or, if it was the size of an iPhone or a typical Android phone (again with a self-contained antenna), how would you calculate what power would produce 15,848µV/m @ 30 meters on 13.56 MHz?

Sorry, but none of that is possible without having details of the exact antenna configuration and the other losses in the antenna system.

 

[druidhillsradio]

Hi Rich, Hope all is well.

 

[darklife]

Let me ask this... Who gives a rats ass?
Honestly unless you really have that many people interested down to the very specs why does one go to the extreme of giving a shit?
Yes rules are rules but in reality we all speed sometimes when no one is around. We indulge in things that aren't exactly legal. We do it with respect though for our loved ones. As in everyone plays but we play within the rules and also our own rules.
Pirate radio exist in spirit. Some people are so sick of public radio that is licensed that somewhere someone must break the law.
I for one as a listener am glad for it. In fact most people I know are tired of legal radio. Not in the sense that they are tired of legality but the crap that goes on the air because of. They want to turn on a radio and get in touch with the studio on a personal level, they want fun. Something that the radio no longer provides.
We the people miss alternative anything. ANYTHING.
Why are we going to internet broadcasts and XM radio (BTW I hate XM also, just making a point) it's because it all sucks on the FM dial. We have idiot A or idiot B, and most of those idiots are voice tracked.
Do you think we like that? Hello would you like it?
Real radio has DJs. Believe it or not the radio as we know today for all its professionals who supposedly know what is going to make money for the station forget that real people listen, they forget that ratings are local, they forget that I listen. They forget me.
Hello?
This medium is a large part of our culture. People love radio, you all think we tune out to our ipods and our computers, but little do you know we still love the one thing that is free and always around, the one thing that will be here when all else fails, it's radio.
I love radio still but sometimes as an average listener I wonder why when you idiots are worried about crap like ratings yet you don't take the time to learn where those ratings come from, they are self profited. They are a self lubricated **** toy. Yet here we are on the outside wondering why a great medium we love is failing. Well please fix that.
Until then I will continue to show educated the way of pirate radio. Hello?

 

[druidhillsradio]

Nice Web Site Dark. Keep up the good work!

 

[darklife]

LOL sorry about the rant. I was tired and ticked off at some other things and took it out here. Oops ;D

 

[darklife]

Nice Web Site Dark. Keep up the good work!

Thanks for the compliment. I work hard to make those forums what they are. Always looking for new interesting members to join.
BTW the direct link is here...
http://www.darkliferadio.proboards.com
I hope linking it is okay. The link in my signature is a shortened version of it but it just redirects to the above anyways so set your bookmark to the above URL instead because it's quicker.
Once again thanks. I will try not to drink a few and rant on here ever again hehe.

Besides that I am all about running things legally. Pirate radio is what it is and if people wish to take the plunge to operate such a transmission then it's on them. I'm not even going to get into a discussion about it here because it would end in disaster. I'm just saying it's a sign of the times. When I can turn on an FM or Shortwave radio and hear so many pirates today compared to a few years ago it tells me that people are fed up with traditional licensed outlets and are looking to get their voices and music heard. I can personally vouch for this as I am in the same boat

 

[druidhillsradio]

I understand. Radio in the Tampa Bay area pretty much sucks.

 

[Ermi Roos]

I wonder what signal quality is typically used to determine the "range" of a Part 15 FM transmitter. I suspect that it is the minimum readable signal, which would be mono, and noisy. But why would anybody tolerate such a signal using FM? For full-quieting stereo, which is more like what is needed for acceptable FM listening, the range for a minimum-readable signal would have to be reduced by a factor of 10, or so. When I read about someone getting 1000 foot range using (say) a CCrane transmitter, I suspect that the real range is in the vicinity of 100 feet.

 

[rfry]

... But why would anybody tolerate such a [mono, and noisy signal] using FM? etc...

Casual listeners to such a noisy signal probably would not tolerate that noise for more than a short time (a few minutes, max?).

Most likely the operators of such unlicensed setups have a much higher tolerance for the noise level in the received signals they transmit than do casual listeners -- which is natural.

Whether or not an unlicensed AM/FM system truly meets Part 15, might such tolerance by such operators also be part of the reason for their claims of unexpectedly large coverage areas by some of these systems?

 

[audioguy]

I wonder what signal quality is typically used to determine the "range" of a Part 15 FM transmitter. I suspect that it is the minimum readable signal, which would be mono, and noisy. But why would anybody tolerate such a signal using FM? For full-quieting stereo, which is more like what is needed for acceptable FM listening, the range for a minimum-readable signal would have to be reduced by a factor of 10, or so. When I read about someone getting 1000 foot range using (say) a CCrane transmitter, I suspect that the real range is in the vicinity of 100 feet.

I tested a CCrane transmitter and I wasn't even able to get a usable signal across the living room at a distance of 10 feet, in mono, on a very sensitive portable FM radio (Grundig G8). This radio receives out-of-market stations that I can't even hear on my Technics ST-9030. Now I don't know if I just got a bad transmitter, but in my opinion, the Commission erred and set the limits for Part 15 FM at least 10 dB too low for legitimate around-the-house applications. The American people deserve to have some access to the broadcast bands. 50 uV/m at 15m is not usable.

 

[Tom Wells]

I tried a CC Crane FM unit and the only way it was usable was connect a wire between it and the FM radio with the speakers out on the patio.

Not exactly radio by my standards. :-\
When the audio died on it, I left it lying dead. Not worth fighting to fix.

 

[LibertyNT]

I wonder what signal quality is typically used to determine the "range" of a Part 15 FM transmitter. I suspect that it is the minimum readable signal, which would be mono, and noisy. But why would anybody tolerate such a signal using FM? For full-quieting stereo, which is more like what is needed for acceptable FM listening, the range for a minimum-readable signal would have to be reduced by a factor of 10, or so. When I read about someone getting 1000 foot range using (say) a CCrane transmitter, I suspect that the real range is in the vicinity of 100 feet.

Ah the love able C Crane...
I actually do get about 1000+ feet or so of signal from my C Crane,
however the in home reception is probably 300 feet at best, clear stereo car reception ends at about 700 feet. 700+ plus its hit and miss reception, sometimes clear, but always detectable.
With my portable Grundig S350 DL I can pick out sweet spots in the 1,000 foot range. But it is really spotty that far out. and very, very, very subject to atmospheric conditions.

When I do my range tests I usually pick a day with good conditions.

I've found on days where the air is real heavy, is when the C Crane does best.

I guess I got lucky with my C Crane, I've heard from several others they get nuttin out of em.

 

[ironbear]

Mono is your friend. I run an unmodified, FCC Certified, Progressive Concepts ACC100 mono transmitter in a PVC tube mounted topmost on a 30' Rohn. It's 1000'+, fairly solid, to a car radio.

 

[druidhillsradio]

Mono is your friend. I run an unmodified, FCC Certified, Progressive Concepts ACC100 mono transmitter in a PVC tube mounted topmost on a 30' Rohn. It's 1000'+, fairly solid, to a car radio.

Hi Ironbear,

What are you using for an antenna?

 

[ironbear]

Mono is your friend. I run an unmodified, FCC Certified, Progressive Concepts ACC100 mono transmitter in a PVC tube mounted topmost on a 30' Rohn. It's 1000'+, fairly solid, to a car radio.

Hi Ironbear,

What are you using for an antenna?

The ACC100's wire antenna, as-is, taped to the inside of the PVC. Vertical polarization. In a concession to dipole-ness, the power and audio leads run through a ferrite bead, 31" from the box, also inside the PVC. It's purposely mounted above the Rohn tower, to avoid distortion to the pattern. This rig has been designed for easy retrieval and disassembly, should the FCC stop by for a look. I think that's unlikely, though, as coverage is about what you'd expect...it's just more reliable due to placement above roof level.

 

[Ermi Roos]

Mono is said to have a 21 dB S/N ratio advantage over stereo, but I don't think that the advantage is as great as that at the limit of detectability of the signal. A noisy stereo signal is received as mono because the pilot tone is not detected, and the stereo reception circuitry in the receiver is not activated. The L+R signal (which is the mono signal) will be somewhat stronger in the mono transmitter with the same output power as stereo because it is not necessary to transmit the pilot tone and the L-R signal.

Just as a wild guess, I would not expect the maximum range of a mono transmitter to be increased by much greater than about 1/4 of the maximum stereo range. I haven't been able to find an engineering analysis of this topic. Perhaps one of the more gifted contributors to this discussion board (R. Fry, maybe?) will be able to either find or generate one.

 

[ironbear]

Ernie, I'm not even sure about extended overall coverage in mono, but forced mono at the receiver makes the audio far more listenable throughout the (small) coverage area. If ever there was a broadcast signal that could have benefited from companding, FM stereo's L-R is the one.

 

[Ermi Roos]

Unfortunately, some receivers only have automatic stereo/mono selection, and you can't specifically select mono reception.