Rich,
Thank you for the interesting comments. I won't disagree with you. I don't have any professional grade testing equipment beyond a simple volt-ohm meter, so I am not able to verify the 250 uV/meter at 3 meters standard.
However, unless you or someone else can correct me (and I really would like to know the answer to this), I believe that the Crane FMT even tweaked up still puts out closer to 7 - 10 mw (possibly less) and not 25.
I assume (again, please correct me if I am wrong) that your calculations on a half dipole are based on a free radiating one with the signal going into the dipole having no attenuation and the dipole standing in a theoretical free space with nothing to attenuate the signal.
In my specific case, I think that there is a lot of attenuation. First off the RG-58 terminates in a PL-259 which then goes through a PL-259 to BNC adapter and a BNC to F Connector adapter before the TV RF Amp and then the balun after the TV Amp. All connectors and cables and components after the Crane are Radio Shack "quality" (hobbyist rather than pro quality), since efficiency was not high on the priorty list.
Most of the attenuation comes from my particular environment. The antenna is on an inside first floor wall in the corner of the house that is farthest from the road and closest into a hillside that goes uphill from the house and effectively blocks all RF transmissions to and from that direction. I also think that there is a lot of attenuation of the signal by the house itself (several interior walls, with big old metal heating ducts, wires and plumbing going to/from the floor above, windows, etc.) before the signal can come out the side of the house that is closest to the road and the side of the house that would "face downhill." In fact, there is a corner of the kitchen where the signal is really scratchy and sometimes intermittenent even on a fairly good radio. (Although the signal is good in other parts of the kitchen and outside the house.) Location was based on expediency and not any desire for efficiency.
Finally there are some woods below the meadow/lawn that is downhill from the house which probably contribute to signal attenuation. (The trees in those woods are taller than the roof of the house, despite being a little downhill.)
I thank you for your caution. At this point this is more experimental and "personal use" rather than a proper P15 Community Radio operation. If I ever do one, I would be careful about the rules.
RadioheadC: I believe that the Crane FMT even tweaked up still puts out closer to 7 - 10 mw (possibly less) and not 25.
Rich: That is still 1000s of times too much. It takes only ~11.4 nW (nanowatts) radiated from a 1/2-wave dipole to generate the peak, legal FCC field of 250 uV/m, 3 meters from the antenna.
RadioheadC: I assume (again, please correct me if I am wrong) that your calculations on a half dipole are based on a free radiating one with the signal going into the dipole having no attenuation and the dipole standing in a theoretical free space with nothing to attenuate the signal.
Rich: Yes, and that is what the FCC specification is based upon. The cables and connectors you listed will have an insignificant loss compared to what it would take to apply ~11.4 nW from even a 1 mW tx to its matched antenna.
RadioheadC: Most of the attenuation comes from my particular environment. The antenna is on an inside first floor wall in the corner of the house that is farthest from the road and closest into a hillside that goes uphill from the house and effectively blocks all RF transmissions to and from that direction. (etc)
Rich: FCC Rules don't care about this. All that matters is that your Part 15 FM antenna does not produce more than 250 uV/m field strength at distances beyond 3 meters in any direction from your transmit antenna.
RadioheadC: At this point this is more experimental and "personal use" rather than a proper P15 Community Radio operation. If I ever do one, I would be careful about the rules.
Rich: The purpose of your operation doesn't really matter, in a legal sense. The FCC Rules for unlicensed transmitters apply in all cases.
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RadioheadC: I believe that the Crane FMT even tweaked up still puts out closer to 7 - 10 mw (possibly less) and not 25.
Rich: That is still 1000s of times too much. It takes only ~11.4 nW (nanowatts) radiated from a 1/2-wave dipole to generate the peak, legal FCC field of 250 uV/m, 3 meters from the antenna.
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Rich, I have read your interesting 11 nanowatt analysis elsewhere. I may well be over the P15 limits. However, the property here is of such a size and topography that, while not being an expert myself, I would expect that other than the NSA an agency would need a search warrant to come close enough to the transmitter to get a measurable result. The point is that if no one else can hear the transmission, it is unlikely to draw complaints and thus enforcement action.
My question still stands, because I would like to know its answer purely from a hobbyist's theoretical perspective, as opposed to its relative position as to whether it is legal or not: What is the ouput power of the Crane FM T either out of the box or tweaked up? Do you or does anyone else know?
RadioheadC: I assume (again, please correct me if I am wrong) that your calculations on a half dipole are based on a free radiating one with the signal going into the dipole having no attenuation and the dipole standing in a theoretical free space with nothing to attenuate the signal.
Rich: Yes, and that is what the FCC specification is based upon. The cables and connectors you listed will have an insignificant loss compared to what it would take to apply ~11.4 nW from even a 1 mW tx to its matched antenna.
//
So you are saying that the FCC will base its case on a theoretical, laboratory environment and not on actual field measured conditions?
RadioheadC: Most of the attenuation comes from my particular environment. The antenna is on an inside first floor wall in the corner of the house that is farthest from the road and closest into a hillside that goes uphill from the house and effectively blocks all RF transmissions to and from that direction. (etc)
Rich: FCC Rules don't care about this. All that matters is that your Part 15 FM antenna does not produce more than 250 uV/m field strength at distances beyond 3 meters in any direction from your transmit antenna.//
There are places inside my house, 3 meters from the transmitter where you can not pick up its signal. Nada, niente. That tells me that there is a lot of attenuation in the signal's path to those spots. (There are also other spots where there is a good signal quite a way's out.)
Again, my question is not so much pertaining as to the rules, but as to the physics of radio propagation. Any scientific or engineering insight (as opposed to regulatory constraints) here?
RadioheadC: At this point this is more experimental and "personal use" rather than a proper P15 Community Radio operation. If I ever do one, I would be careful about the rules.
Rich: The purpose of your operation doesn't really matter, in a legal sense. The FCC Rules for unlicensed transmitters apply in all cases.
//
Then just call me a pirate.
I really just a hobbyist in a location that happens to be rural enough to be off the screen.
RH: What is the ouput power of the Crane FM T either out of the box or tweaked up? Do you or does anyone else know?
Rich: I don't, anyway.
RH: So you are saying that the FCC will base its case on a theoretical, laboratory environment and not on actual field measured conditions?
Rich: No, they will measure it. What I'm saying is that you can't expect cable and connector loss to reduce the power of a tx able to supply as little as 1 mW of output power enough to produce a legal field when radiated by a matched, 1/2-wave dipole.
RH: There are places inside my house, 3 meters from the transmitter where you can not pick up its signal. Nada, niente. That tells me that there is a lot of attenuation in the signal's path to those spots. (There are also other spots where there is a good signal quite a way's out.)
Rich: Receiver performance depends in part on the radiation patterns of the transmit and receive antennas, obstruction losses, and signal reflections from other conductors and surfaces along the radio path. Together they can produce the results you report.
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