I looked at the CCrane site. It apears that they too violate the rules by suggesting that you attach a 31 inch wire to the pre-existing antenna.
That would certainly put the Whole House FM Transmitter somewere around 25 to 50 Nanowatts and the SainSonic (Supposedly 500mW at High Power @ 50nW-1mW (No where near 500mW as that would cover more than a mile. So again this proves that these transmitter companies lie about power output as this graph shows it. It also shows that the Whole House FM Transmitter certainly puts out way more than 250mV per meter. Or does this show that the 150 foot rule is debunk and you can actually get 1/4 mile on a certified part 15 FM Transmitter? I'd think the latter. I've seen plenty of folks getting out 1/4 mile without an incedent. Now some of the frequencies people were cited for were right where NPR Radio is and I'm willing to bet Some (not all) citations were because some jerk decided because he or she didn't find a frequency with pure static they'd transmit aneyway. Makes me wonder why they don't come up with portable AM Certified part 15 transmitters that would cover the range people are looking for on FM. Car Radio's have AM Stereo so that could be an alternative for car transmitters. Not trying to be a jerk just trying to understand why or how a Whole House FM Transmitter is getting out close to 1/4 mile on a ghetto blaster? On a car Radio I could see that. Now I remember in Michigan Frankinmuth where they had a Christmas light show and transmitted it on 101.3 I think. It got out about 1/4 mile from the house or stores and restaurants? This is something that interests me cuz if this is true I can certainly run certified part 15 FM and get plenty of listeners around here for my Internet station to promote it. It does shed a light on the so called 500 mW transmitter however. I didn't think it was anywhere near that power as I had a 100mW transmitter years ago on FM and I can tell you it went 3 miles in Stereo all day long. This was when I was in high school and didn't know about part 15. It was rated at 100mW and I had a friend with FM meters that did test it @ 100mW out. So this is why I knew the SainSonic transmitter in no way was at that power.
Not much field strength is lost looking at 88 vs 108MHz for a 11” antenna.
To be complaint, if the power is not adjusted for frequency, the TPO needs to be set so the field at the top of the FM band meets part 15 limits. Then the field at the bottom of the FM band can be calculated as:
E_88=E_108 * sqrt (G_88 / G_108)
Where:
E_88 is the field strength at 88MHz
E_108 is the field strength at 108MHz
G_88 is the antenna gain at 88MHz
G_108 is the antenna gain at 108MHz
I do not have the gain for an 11” antenna over the FM band, but at 88MHz an 11” antenna is 0.08 wave lengths and at 108MHz an 11” antenna is 0.1 wave lengths. (I am sure someone will have better numbers than what I have next, but it is the best I have on hand). Based on a regression for gain vs vertical antenna over ground for HF, a 0.08 wavelength antenna has a gain of -2.1dB or a power gain of 0.61 and a 0.1 wavelength antenna has a gain of -1.7dB or a power gain of 0.671. So roughly if a transmitter with a 11” antenna puts out a field of 250uV/m @3m for 108MHz, I would expect it to put out a field of 238uV/m @3m for 88MHz, or a 5% loss in field strength.
CCrane is probably on the edge of “violating the rules” by suggesting attaching wire to the end of their antenna in the web site. If CCrane put that suggestion in the manual that is in the packaging with the product, the part 15 certification would probably not have happened. As is, when a CCrane FM or FM2 is taken out of the package, if it is used as the included instructions tell the user to use it, it will meet the field strength requirement. The suggestion to add wire to the antenna is on the website which is not part of the packaging of the retail product.
For some perspective...
Free-space NEC antenna models using an 11" linear whip driven against a 1" long PCB d-c common trace in the transmitter show this:
Antenna Feedpoint Z =
2.15 -j 2460 ohms at 107.9 MHz
1.4 -j 3000 ohms at 88.1 MHz
If the antenna system could be resonated on those frequencies with a loading coil or network having a resistance of 5 ohms, then the power output from the transmitter at the input to the coil/network to produce a maximum field of 250 µV/m 3 meters from the antenna would need to be about 40 nW at 107.9 MHz, and about 55 nW at 88.1 MHz.
How closely these NEC models might describe a given commercial product is unknown, of course.
You only have to do the math to debunk the notion that a U.S. Part 15 compliant FM transmitter can reach 1/4 mile (approx. 1450 feet).
At 192 meters (about 600 feet), the maximum field strength would be 4.2 uv/m approx. Now, that doesn't mean that that is the signal strength induced at the antenna terminal of the receiver - posts in the past here have shown that it would be about 2uv or less. The most sensitive car receivers have a sensitivity of between 1.5 - 2 uv with some quieting.
So, those at the top end of that range will get about 600 feet range, those at the bottom might get another 150 feet or so. And you might hear the signal with noise for a bit beyond that (but I've found that FM signals drop off pretty quickly, unlike AM). But no way 1450 feet with a clear signal (about double that distance again).
My guess is that if that kind of range is being obtained, there's something else going on. Perhaps the transmitters were tweaked (such as using that hidden screw adjustment in the C Crane, or maybe adding on to the antenna or even some other mod). Or perhaps there's just some variability in the specs of the finished product (you would need a field strength of about 500uv/m at 3 meters for the 1/4 mile range, and a very sensitive car radio).
That's the problem with these ancedotal accounts of range. You have no idea if you are dealing with a pristine, out of the box, transmitter. And you also have to way of knowing what the measure is for the quality of the signal.
Finally, I've used a Canadian compliant FM transmitter under ideal circumstances on Bowen Island. It was placed on a rooftop on a building at the top of a hill, directly overlooking the intended coverage area. Thus, there were few obstructions, and for the most part, almost line of sight to most car antennas. My very best range with a very sensitive receiver was just under 1km, or a little over 3000 feet (the car positioned for direct line of sight, not moving). Most of the time, range was less than that to the average car - between 1/4 to 1/2 km (with no dropouts, after that, dropouts would occur due to trees and such).
And that's with 4 times the allowed field strength of the U.S.
The paperwork for my C.Crane FM1 Transmitter now includes a print-out of Rich's Post # 20 above.
Some things to think about:
- Totally regardless of any transmitter manufacturer's claims or available data, the operator is 100% responsible for its operation...no one else. If you are unlucky enough, or in violation enough, to end up with a NOUO, you can say all you want to the FCC about mfr's claims of certification and/or legality...they will basically tell you what I just wrote.
- Power to the antenna system is almost never the same as ERP. You can tune the system, trim the output, whatever you like, and get more range, but I can guarantee that if you can receive a signal on a car radio or decent FM receiver, i.e., somethng other than a $3000 ham all band receiver with a plethora of preamps and filters, at 1/4 mile away, you will be in violation...it's just physics.
- Along with that, remember reception is almost as dependent on the receiver as it is the transmitter. I didn't look at the Whole House video, but if it was during the football season, the field was likely wet, providing somewhat better ground (far more important with AM transmission), and there is nothing in between...line-of-sight is how FM bcb works at those freqs. Combine that with, say, a C Crane High Performance AM/FM radio (very good low priced receiver), it isn't surprising the signal made it well at the opposite end. OTOH, what do you think it would do if the receiver were some sort of cheap '70's model clock radio or "transistor" radio?
- It takes a lot less power to make the same distances with AM transmission...the caveat being AM is significantly less susceptible to interfence.
...Just some things to think about...
@Ken Norris: you brought a good point that I should have thought of about the wet football field. Plus during foggy conditions a transmitter would go further. Plus there is no real way of telling if the user didn't use the "secret" high power mode of the transmitter for the 20-30 second demo hardly long enough to get a NOUO. Not sayhing it did happen that way. The receiver is very important as one could soup up the receiver with notch filters and RF amps inside the receiver itself. There are mods for boom boxes to make them receive further if your willing to add the amps and filters. Even the claims of the SainSonic AX-05B could be from a modified receiver as well. I guess the only thing I could do if I win the Whole House FM Transmitter 3.0 is to report the range I get. In order to not break any copyright laws I could use a Stereo mic have one stepson yell one watt in the left channel and the other stepson yell World Wide over and over while I take my boom box and connect the Audio into a camera and walk around to prove it's actual range. When the signal goes Mono and you hear both boys without separation that is the limit of the range. This way I could put it right on Youtube. If and when I go to Canada put the transmitter on "Secret" high power mode and do something similar to demo the High Power Mode. It would have to be while in Canada and maybe not longer than 60 seconds just to demo the difference. Than one could figure out whether the transmitter was in the high power mode or not. Just an idea.
I'll look into a used AM Transmitter with an indoor antenna in hopes I could go a few blocks. Only thing is I don't know if I'd get listeners. It would work for going out on the front porch and listening to my station. But I really don't know if it would work as well as FM. could go both eventually. Even a few blocks would be nice.