If you know the power, and the antenna characteristics, along with the receiver sensivity, you can calculate the approximate range. That will give you the potential for interference. You did that yourself, Rich, when you did those NEC plots.
I don't believe that 1 watt, or anything close to that, will fly. With one watt and a good, elevated, outside antenna (how are you going to regulate that the antenna be inside, and just what exactly does that mean), you can get miles of coverage (as evidenced by unlicensed New Zealand broadcasters).
I don't think that you can hope to get much more than making Part 15 FM equivalent to Part 15 AM (and likely not that either, with interference concerns) - so for the best antenna, with the best receiver, you would maybe get up to a mile range. Much less with typical antennas and receivers - Part 15 AM transmitters typically get 1/4 to 1/2 mile. I don't know what kind of power that would work out to, but it can be approximated.
If we assume a field strength at 5280 feet (approx 1 mile) of 2.5uv, that works out to a field strength of 4400uv/m at 3 meters. That then works out to be about 200 nanowatts - so take it up to 1 microwatt to allow for all sorts of inefficiencies and the other factors that seem to affect low power FM signals so much. I wonder what a NEC plot would say about the 2.5uv contour for a microwatt of power, and a dipole antenna (say, installed at a 10 meter height). That would give you pretty much the maximum possible range for such a transmitter.
There's pro's and con's to everything....every idea has a negative and positive. Key is to find a middle ground that works for all involved.
Rich, even you have to admit the FM rules in the USA are a "little" to restrictive and a slight relaxation in the allowed field strength won't cause any interference that would be a problem as long as the transmitters are clean.
Mark
We are at an impasse based on what Tim has demonstrated.
His field tests find conclusively that a certified FM transmitter produces significantly more power than 15.239 allows, with added strength caused by connecting cables.
I have run tests on two other certified transmitters, FM 1 & FM2 from C.Crane, and found that connecting cables increase the field strength output as does the addition of a metal surface (ground plane) under the transmitter.
Because certification does not guarantee compliance for the end user, we are left with no means whatsoever to assure compliance with 15.239.
Take this further... if we can't determine the present field strength as allowed by 15.239, we will also be unable to calculate any higher field strength, if allowed.
Nothing is gained by talking about anything but this basic point.
... If we assume a field strength at 5280 feet (approx 1 mile) of 2.5uv, that works out to a field strength of 4400uv/m at 3 meters. ...
Note that a field intensity of 2.5 µV/m arriving at the receive antenna of most FM receive systems would not provide a very useful output for the users of those receive systems -- especially for analog or digital FM stereo transmissions.
Also note that your statements in the clip above apparently are based on free-space path loss, which does not apply for transmission paths near the earth.
I agree - that's why I bumped up the power. 2.5uv is the absolute minimum that you can receive a signal with on a car radio with a sensitivity of 1uv (or possibly under). I chose it because the FCC won't care about the typical user and range - they'll want to know the maximum range and the possibilities of interference.
Here is what we do know. As I have talked to Brian we know that the Whole House FM Transmitter is just under 100mW at its low power (Out of the box) setting. We also know that the field strength at 3 meters with cables plugged into this unit is 1775 uV/m at 3 meters. This is far above the strength of 1000 uV/m at 3 meters for Canada. With this in mind it would not be hard to ask for a multiplier of 2-3 times that strength which is 3550 for twice the power and if you want 4 times that you get 7100. This would get you close to a 500mW transmitter into a rubber duck (Same as the Whole House 3.0 or Sainsonic AX-05B). To further test my theory of someone has a Patomac FIM-71 and tests the SainSonic AX-05B or FailSafe CZH-05B transmitter at its 500mW setting I am sure you will come up with this reading onto a Rubber Duck antenna (The one SOLD with this unit). If the law sated Any use of any other antenna is in violation of the rules and made 500mW the legal limit I don't see what Rocket science it would take to understand this. We have the Data for a Rubber Duck. So talking about a di pole, Ground Plane, Beam or any other antenna except Rubber Duck and 500mW is irreverent at this point. Now after we have the tests for a telescoping antenna at 100mW we can compare that as well. SainSonic does sell a telescoping antenna that is suppose to go with their 7 Watt transmitter. However you can buy the antenna separately. What we would have to do is test that field strength assuming the TNC connector is the same as the TNC for the rubber duck. Something tells me that the strength would quadruple for the telescoping antenna. There is also a Signsteck CZE-T200 transmitter which has a Rubber Duck and 4 power levels that we can test for field strength. It is not certified but would also prove my theory because it has 4 levels and 4 ranges in reception to a car Radio. 1MW (10 ft), 10mW (150 Ft), 100 mW (¼ mile), 200 mW (½ Mile). This was to a Car Radio. If we test the field and range we have the exact proof on a Rubber Duck antenna to present to the FCC. The already illegal strength the Whole House puts out has not made planes fall from the sky, Ambalances miss their patients, Police to miss their dispatches, Public Safety has not been harmed, and life continu3ed as normal. So though you have organizations with the Chicken Little syndrome we have data to prove otherwise. We know SainSonic's output, FailSafe's output and Signsteck CZE-200's output and we will say Whole House's output at 100mW. All have Rubber Duck antennas so lets start with that for forging the new rules for FM. Put up an outside antenna – GO TO JAIL!!! That is it for FM.
In August the computer that operates KDX Worldround Radio froze solid three times, having never done so before.
During the same period I'd been conducting transmitter/antenna tests with two C.Crane FM transmitters in close proximity to the computer, and wonder if that was causing the freezes.
Since September 1 the transmitters have been moved to a distance and no compter crashes have happened, so things are looking good for naming the RF from the transmitters as being potentially hazardess to computers at close range.
Please report your experiences of this kind.
The computer "freeze" problem has continued to occur at random, and Youtube has a few videos about the "XP freeze problem," but the main point is, it wasn't being caused by the proximity of the C.Crane FM transmitters.
In signal-level tests with the spectrum analyzer, I previously reported gaining a little output by placing a CD disc under the C.Crane FM Transmitter, giving a kind of capacitance-coupled ground-plane.
Which brought the thought... perhaps if the RF output of Part 15 FM transmitters was higher than allowed under 15.239, the radiation might start to interfere with nearby computer components, since many people place their transmitters on top of or alongside computers.
Then I realized, a ground plane is only good for vertical polarization, but I find that horizontal works better indoors. Instead of a "virtual ground," placing metal under the transmitter, perhaps we can come up with a "virtual di-pole" by placing wire or a CD on the side of the transmitter facing away from the antenna ??
There are two main changes in the C,Crane FM Transmitter 2 compared to the earlier C.Crane FM 1.
Several published reviews have noted that a detectable noise floor can be heard in the FM1 when listening closely to the carrier, which is gone from the FM2.
And while FM1 has a single red LED that flickers to display overmodulated sound, the FM2 has 2 LEDs, the green lights during average audio and the red showing overmodulation.
Other than those two differences, the two transmitters sound exactly alike and have the same range.
Using Stereo Tool or any good processor for raising the average audio level, the modulation heard from an FM1 is excellent and the audio will conceal the relatively low level of the noise floor.
Doing A/B comparison, FM1 on 89.5 MHz driven by Stereo Tool and FM2 at 89.9 MHz carrying unprocessed audio, the FM1 sounds stronger and more dominant due to the diffence in audio handling.
The conclusion shows that the C.Crane FM1 remains a viable and useable transmitter and there is good reason to own one if it comes at a good price.