Just disgusting! and way to early.
Mark
On the graph is the hoizontal blue bar showing 3340uV/m at 3 feet resulting in 97uV/m at 100ft? or is that not it at all?
That bar is not blue at a horizontal distance of 3 feet and an elevation of 6 feet AGL from that transmit system.
The color code for that field is shown by the color spectrum along the left vertical axis of that graphic (red tending to purple)
So if a transmitter is certified for a given field strength that is good only where the measurement was taken.
Certification is based on the electrical/physical/environmental conditions of the system under test at the time of measurement by a non-FCC facility, and FCC acceptance of those tests.
It would be unlikely that many/most users of such certified systems could or would even want to duplicate the conditions under which those systems were certified -- especially when such conditions might reduce the coverage radius they want to achieve.
Yes, there are many variables indeed, as we know. Distance above ground to work with the Potomac is either 7 or 30 feet. I used 7 because 30 is far too much work! Accuracy is better at 30 feet. I can't remember off hand, I believe it's 12% at 30 feet, 18% at 7 feet. Still my tests were so far over it didn't matter much.
Look up some of the certification test reports submitted to the FCC and it will help you see how certification testing is done.
That Potomac FIM that was on ebay is a FIM-72. It does NOT cover the FM broadcast band. That listing has been there a while and contradicts itself in a couple places, but it's clearly a FIM-72 that is listed. I see there is a '71 listed, but bidding starts at 2495.00 if I remember right. Actually not a bad price but a lot if you want to test a $200 transmitter one time! I use my equipment in my work so the cost is not so much a factor.
TIB
Based on your measurements the CM10 exceeds Canadian rules as well. I can't remeber where I found it but the CM10 is made in China for Decade by one of the notorius factories known for saying one thing and doing something else when it comes to compliance. Also, what you experienced moving the power cable is exactly what is done during compliance testing. Typically a spectrum analyzer in the peak hold mode is used. next you move all the cables around, rotate the device using a turntable and the receiving antenna is raised and lowered 1 to 4 meters. This is done in both the vertical and horizontal plane. My guess is Decade is dumping these for two reasons: 1 is the compliance issue, and 2 is they cannot compete price-wise with imports directly from the Chi-Coms. Just a guess.
WDCX: Mark from Canada has shown that the Decade CM-10 is indeed a CZH-05B Transmitter with the external connector replaced with a telescoping antenna and (supposedly switched and locked to the Low power -48dbm) mode like the SainSonic AX-05B transmitter the one Bill absolutely HATES even though they have fixed their newer one to be more compliant. In fact other than the possible illegality of the accidental carrier current FM feature its compliant and as of yet no one has told me I'm illegal with my accidental carrier current FM transmitter. But that said I'd still LOVE for Bill to do an unbiased test of this $54 transmitter. If I were to move where there was an 88.1 and the whole FM ban d was full I'd have to sell it or maybe donate it for testing. I'm not going to ask an engineer of a Radio station to test it that would be suicide!!
Actually, if you take a look at one of Rich's graphs, it is likely that the Decade CM10 Tim received was Canadian tuned - depending on the height of the transmitting antenna, the field strength at 30 meters would be somewhere between 97 to 140+ (I can't remember exactly, somewhere between 140-150uv) / m.
But the problem with measuring FM compliance with field strength is that field strength varies greatly depending on a number of factors, such as height of the transmitting antenna, height of the receiving antenna, never mind obstructions.
So a transmitter could be (Canadian) compliant at one height, but way over (by 50% according to Rich's graph) at another height. And yet it's still certified. And the typical end user has no way to tell.
In those new rules being proposed by the FCC for wireless microphones, they talk mainly about watts, not field strength. That would be the way to go for Part 15 FM - I doubt they would ever allow even 1 milliwatt, but perhaps 1 micro watt to the antenna? The Canadian standard specifically states under 1 microwatt, in addition to the field strength requirement.
Comments made at Hobbyblaster are often derogatory toward Part 15 equipment and the people active in Part 15 activity.
Over a long period of time PhilB, inventor of the excellent AMT3000 and AMT5000 transmitters has been bashed as a site policy and most recently a false claim was posted and belabored about an FCC fine against Phil which never took place.
Around the same time one of them expressed his disgust for the audio quality of the C.Crane FM transmitters, a claim without support anywhere, anytime, by anyone. Plain fact is the C.Crane's have outstanding sound indistinguishable from licensed FM transmitters.
I've never bothered to count how many Hobby Trolls exist at Hobbyblaster, let's just say there are six of them, so we can refer to them as The Six Stooges.
Nyet Nyet Nyet.
In a tightly regulated Forum, such as that one, the webmaster (along with the right hand man/gunsel) sets the tone. There are a lot of really good and knowledgeable people over there (I'm ignoring a few of the simpering sycophants) and I have a lot of respect for those guys. So it's really too bad that it all has to be overlaid with that overpowering condescending attitude. I lay all the blame for that attitude at the top.
I've yet to see detailed data published from any of their tests over there. I wonder if they'll do it this time.
Good point, Artisan, there are some fine people posting at Hobby who tend to get upstaged by the brash band.
Looking back at Post # 33 Tim in Bovey said: "Distance above ground to work with the Potomac is either 7 or 30 feet. I used 7 because 30 is far too much work! Accuracy is better at 30 feet. I can't remember off hand, I believe it's 12% at 30 feet, 18% at 7 feet."
Those are the standards used by Tim for testing the FM transmitters, and he explained that certifying labs use those same distances.
What I don't recall reading is an explanation of why those particular distances are chosen... 30" would be more convenient, being the average height of a table... so why either 7' or 30'?
Good question Carl.
And yes, it's really only a few that I have issues with. I think it's great that we seem to be getting more from 'over there' posting here. Perhaps we can just make that condescending attitude I referred to become irrelevant (and those having it will either have to change, or just fade away).
... There are a lot of really good and knowledgeable people over there (I'm ignoring a few of the simpering sycophants) and I have a lot of respect for those guys. ...
I especially miss any new posts there from Ermi Roos.
IMO his "public" posts there (and earlier on Part15us) were very worthwhile.
When the FCC comes to your neighborhood and finds your FM Transmitter past the compliance level are they going to be sure to wait until ideal conditions to flag your station? I'd think not. Yes they may take 3 measurements once they pick up your signal on the dial. At best they will drive around and park at different spots close to you and take a reading. They'll try and be inconspicuous to you so you don't hurry and make adjustments. One could pretend to be a tourist looking at historical areas near you and say their parking in your driveway to see the area near you. Meanwhile they take out the Potomac FIM 71 and do a reading. Now doing this without your knowledge is something that is going to take some real skill. Could they fly over your house in an airplane or helicopter to do it? I don't know how they could fly 10 feet above your house maybe use a drone with a Potomac FIM71 and camera tied to it but that too would be far fetched. Now they could park at a neighbors house somehow so you don't see them. I'm just thinking how people receive letters unknowingly broadcasting while they took a reading? Around where I live it would be easy for this to take place. I could think of many places the FCC could hide and I'd never know that they were even there. Even with a van with the FCC logo I'd probably not notice if they parked at one of the businesses nearby. I should not give them any ideas but I am sure this is really how it does take place. When parked they would have to know at the time where you are and with those huge direction finding antennas on the roof you'd think they would get noticed driving by. Maybe they simply have someone in a car drive by and take a reading first just to do a rough guess first then come back later and do the test the right way. Sort of like the video on Youtube. It was rough and raw but did show how you could use a crude way to track a signal and too get a dbu reading of the signal.
7 feet and 12 feet are used simply because those are the heights for which calibrations have been done for the Potomac meter. The Potomac manual includes the data necessary to use these heights including determing the antenna factor which must be applied to the readings taken. If you get a chance to look at a FIM-71 manual there's a section dedicated to how the National Bureau of Standards used a calibrated field to determine these criteria.
If you'd like to read how you use them there's a manual posted here:
http://www.pi-usa.com/pdf/71-instr.pdf
It doesn't include the NBS testing section with some interesting photos as well that I have in my original printed manual. In fact it appears a lot of the science included in the big actual book isn't in this PDF but it's still interesting.
TIB
Nowadays the FCC will most likely be using an R-506. It interfaces to a computer with automatic GPS coordinates, computes antenna factor automatically, doesn't need regular calibration, outputs the data of readings to a computer, is smaller and easier to use. I verified my FIM-71 readings with my R-506. I don't have a computer/GPS interface for it, as I don't need it for those purposes.
TIB
The FCC only needs one reading that shows you're over. The rule isn't to see out a signal that meets the rule, but to find one that doesn't.
The reason for calibrating a field intensity measuring set with its supplied antenna elevated 30 feet above the earth is related to the fact that the FCC defines the coverage contours (fields) of licensed VHF/UHF broadcast stations at that elevation.
The Potomac FIM-71 also was calibrated with its antenna at 7 feet AGL, but measurement accuracy is not as good as when 30 feet elevation is used.
In the far field, a measurement made with the FIM antenna at 7 foot elevation is about 23% of its value at 30 foot elevation (subject to the accuracy spec of the FIM).