Suddenly it dawned on me last night that I had another Part 15 FM transmitter. About 10 years ago we bought one of those little transmitters for the sole purpose of plugging it into the stereo in my home office, where I could play Christmas records on the turntable and transmit them to the radio in the kitchen while we were making cookies! And sure enough, the little transmitter was in the drawer where I left it back then.
It's a General Electric EWT-950 FCC ID NZTSF-140. I'll attach a photo if I can figure out how to do it again.
I thought, what the heck, I'll take this out to the test field and set up all my gear again and see what kind of poop this little bugger puts out. BEsides, I wanted to do tests again on the Whole House 3.0 simply because I have a hard time believing it's THAT far over the limits.
Anyway, got everything set up "by the book" as before in a wide open field. This little GE unit runs on two "AAA" batteries. Has no jacks, connectors, or any sockets in it, just a 15 inch built in audio cable with a 1/8" stereo plug on the end. It's got the power button and a frequency select switch with thess "zones" 88-92, 92098, and 98-108. AFter you set the range switch there's a tiny tuning dial on the side. I used the Tecsun to lock it on on frequency.
So, what's the field strength you say? At the standard 3 meter test range output was 61 uV/m. As you know the limit is 250 uV/m at three meters. This is so legal it's ridiculous! This does explain why it had a hard time getting clear music to the kitchen from my office, through one wooden wall, and a distance of maybe 20 feet. LOL.
As long as I had the gear out I thought I'd double check the results for the Whole House 3.0. Yup. It's just as illegal as it was a couple weeks ago. More so, actually. I tested at 99.5 (I used 92.7 last time) another very dead spot on the dial. Readings were consistantly higher. I'm not figuring and typing them all out, but they were all higher. Which I guess makes sense as with the little antenna on this transmitter, the higher you go in frequency the closer the match for the small antenna.
I was still going over in my head how the two certified transmitters I tested were so far over the limit, and trying to figure out if there was some anomoly that would have made my tests inaccurate so I was itching to do it again. Same results.
So, FWIW, TYG.
TIB
I wonder why it was only 61 uV/M @ 3 meters. Its a wonder it even go out that far.
Just to note that VHF (FM) field intensities measured even by perfectly-calibrated instrumentation such as an FIM-71 and/or R-506 with known receive antenna factors are dependent on the heights of the transmit and receive antennas above the earth, path length, the polarization of the transmit and receive antennas, and earth conductivity along such propagation paths.
The graphic below illustrates this, with respect to the fields existing from zero to 10 meters above a level ground plane for the other conditions stated there.
Tim, The same as everyone here at part15, I am very grateful to you for making such a total project out of analyzing the mini-transmitters.
More than anything else we could do you have given us a vivid look at the invisible world we are dealing with under 15.239.
Some guy sitting there with his certified transmitter can be happily way above limit and not ever know it... until a visitor arrives to bust his confidence balloon.
My four certified FM transmitters all agree with each other on the spectrum analyzer, but I don't know if they're all too high, just right, or under-power.
There's some kind of an ancient oriental philosophy I read about somewhere called "Floating World," suggesting that reality is just one big puddle of lives and events floating around with nothing solid. That's where we are with these little FM things.
The FIM-71 includes graphs used in determining antenna factor at heights for the receive and transmit antennas at 7 or 30 feet above ground. Needless to say I test at 7 feet. If I remember right the margin of error at 7 feet is +/- 18% and 12% at 30 feet. At the three meter distance between antennas there's not a whole heck of a lot of propogation I wouldn't imagine. The FIM-71 antenna is calibrated for horizontal polarization. Of course there can be many more variables -- one reason I go to a large, flat open field.
I have seen in FCC documents posted here and other places that the FCC always seems to mention in complaints that the margin of error was always figured in the operators favor.
There are clearly a ton of variables especially in testing at these low power levels. With the audio cable plugged into the WH 3.0 and hanging free straght down from the transmitter I could watch the field strength vary in synchronization with the slight breeze making the cable sway!
TIB