So I received the CM-10 today. Quick delivery and pleasant communications with the Decade folks over the past week or so, so that was all good. In the box the transmitter with antenna attached, power supply and a stereo RCA to 1/8" stero jack cable. That was it. No instructions, no documentation, nothin'.
I plugged it in and it turned on to 98.5, so I left it there as that's completely blank around these parts. Plugged in audio from my iPhone (after finding a stereo 1/8 to 1/8 cable) turned on the kitchen radio, and turned up the audio input level until I heard distortion and backed it off till it sounded fine. That was the extent of setup.
Took it out to my driveway. This is on a small town street, and I'm right in the middle of the block. We have sidewalks. I set it up on a board set across two plastic sawhorses (nothing metal around for to screw with signals). I had previously marked out the sidewalks at 3 meters, and then at 20 feet, 40 feet, 60 feet, etc all the way to 200 feet. Acutally measured, to the inch. Keep in mind today my plan wasn't for any sort of careful scientific testing, but here's what I know with my first experience ever with a Part 15 Fm transmitter (except for one of those little goofy things for in the car).
I grabbed my Tecsun PL310 just because it has a handy Dbu scale on it. So here are the readings (no, i'm not going to try to make a pretty table of figures). At 8 inches from the transmitter I had 60Dbu. at 3 meters I had 41 Dbu. I also had 41 Dbu at 20 feet. At 40 feet it dropped to 34 dBu, at 60 feet it was 26 dbu, at 80 feet it was 17, at 100 feet it was 11, also 11 at 120 feet, at 140 feet it was down to 7 dBu, at 160 feet it was at 4dBu as well as at 180 feet still at 4dBu and at 200 feet the meter read 00 dBu but by carefully turning the radio I could still hear the signal. Beyond 200 feet, basically nothing. I could find spots where manipulating the radio antenna and spinning myself around slowly I could find spots where I could hear the signal but with noise.
Then I grabbed my Grundig S350DL and made the same trip. This radio just has a signal strength meter of no real consequence, so I just walked down the line listening, and again by 200 feet it was basically gone. In all this testing this is actual measured 200 feet and straight in line, open space from the transmitter antenna to the receiver in my hand. There is nothing at all between me with a radio and the transmitter antenna, and I'm holding the radio with the antenna vertical, just like on the transmitter.
Then I hopped in the car, the handy parked outside car -- a 2010 Ford Escape. Drove down the block. I'll say one thing, the audio from this transmitter is quite good, especially considering I did nothing to set the level except listen on a portable radio, but it really did sound good. Obviously in the car it sounded great as I drove by the transmitter, but when you're driving it doesn't take long to get past 200 feet, and again it was getting noisy by 200 feet and not much more than 4 car lengths past my marker for 200 feet I basically lost it, the noise took over and although the sound was there it wasn't listenable and in a few more feet it was gone. I also drove around the block to the east and west and reception was marginal but I could hear it. When I got down to the end of the parallel streets it was gone. But remember the transmitter was about 3 feet off the driveway on sawhorses, so the signal was going through houses to get to my car.
Then, just to see if it made a difference, I took two 24" wire test leads with alligator clips on the ends, hooked them together for 48 inches, clipped this to the top of the transmitter antenna and clipped it up to the top of a 2x4 to get it up in the air. Did the same thing with the Tec Sun. Readings were higher, especially near -- e.g. the three meter mark went from 41 to 43 dbu, at 40 feet the 34 went to 40 dbu, the 11 at 100 feet was up to 21, the 7 dbu at 140 feet was up to 11, but again, by the time I got out to 200 feet it was pretty much gone -- I *maybe* got ten feet more before it was gone.
Now I know that dbu from a meter such as this can't be used to determine actual field strength but maybe Rich can put this into perspective a bit for us? Hopefully in the next few days I'll get out there with the R-506 and get an actual accurate field intensity reading at 3 meters and see what we're actually working with. I just didn't have the time and ambition to get that setup today. But if field intensity readings at 3 meters are near 250 uV/m this will pretty much show a legal transmitter, far as I can tell is solid to about 200 feet as predicted. And that was outside, line of sight with nothing but air between me and the transmitter!
When I have more actual numbers I'll post the into in this thread.
Tim in Bovey