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
Interested to see the actual field strength you measure with the CM-10.
Mark
... 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. ...
Just to note that if both the transmit antenna, and the fully-extended HF/VHF whip antenna on the PL-310 were vertically polarized, and the receiver itself was rotated horizontally around the same physical centerpoint in space -- then there would be negligible change in the signal strength received/displayed/heard on the PL-310 as it was turned/rotated.
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.
Looking forward to reading your results, Tim.
200 feet on a portable radio is pretty good.
200 feet and a little bit on a car radio is pretty poor. If the sensitivity on the car radio is about equal to that of a portable, it's pretty bad, as far as car radios go (I have a 2013 Ford Flex and the radio in it is awful). Generally portables can be anywhere between 2 to 10 uv sensitivity with 20db SNR, while most good car radios are between <1 to 1.5uv (bad ones can be the same as portables). You'll get double the range even if the car radio sensitivity is half that of the portable.
I don't know what the sensitivity of the Tecsun is with a given SNR.
The only way to really measure the potential range is to take field strength readings at various distances, line of sight. A good car radio (under 1uv with 20db SNR) should be able to receive a 2.5uv/m signal (which for a legal maxed out transmitter, is about 300 meters or close to 1000 feet).
I still have 3 cars to try out 🙂
We shall have field strength readings soon enough.
I was quite expecting reception in the car to be much better, so I was surprised to lose is just past the 200 foot mark. And I did do all this line of sight, absolutely nothing between me with the radio or the car.
But even if it's better in a car you can't expect your potential audience to go sit in cars to listen! And at 30 mph you drive 1000 feet in about 23 seconds, so if you had a 1000 radius giting you a diameter coverage of 2000 feet, a listener passing by could hear you for about 46 seconds.
Were you the one who mentioned covering the area where parents sat in cars to pick up their kids from school? Now that's a pretty good plan, actually!
Tim in Bovey
Somehow a duplicate and now deleted response to Reply 4. See the post next below.
Response to Reply 4 above:
... I don't know what the sensitivity of the Tecsun is with a given SNR. ...
The Tecsun PL310 has a published sensitivity of "<2 µV for a 30 dB SNR."
RE Post 1 above:
... 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. ...
Tim - Seeing that you have the R-506, you will be able to compare the actual field intensity as measured by a calibrated FI meter to whatever reading you get on your Tecsun PL-310 at the same measuring locations.
If you get a chance to post that data for several distances from the transmit antenna it would be helpful. That comparison for a few strong and weak AM stations on different frequencies would also be very interesting to see.
Tim in Bovey wrote "That was it. No instructions, no documentation, nothin'."
That is odd, did you call Decade to see if they maybe forgot to send the instructions?
I believe some where on their website they mentioned adjusting the telescopic antenna for the frequency you intend to transmit on. This could have limited your range.
Although you said,you added wire to the antenna. Just wondering aloud.
"The Tecsun PL310 has a published sensitivity of <2 µV for a 30 dB SNR."
There's something off kilter there. At 200 feet, a maximum Part 15 legal field strength of 250uv/m at 3 metes should be 12.5uv/m. If that was really the sensitivity of the Tecsun, it should easily have been able to hear that signal.
The only thing I can think of is either 1) the Decade was not tuned to 250uv/m at 3 meters or 2) the sensitivity of the Tecsun is a bit liberal, to say the least.
The actual field strength results will be very interesting in a number of ways, to say the least.
I did find a download for the manual on their website. Didn't tell me anything I didn't know. Three pages half of which was logos and a picture of the transmitter. I thought there may be some info on antennae length based on frequency too but not a word about it.
Tim in Bovey.
For The Love Of God I hope that Decade CM-10 is Way Under the legal power. If its full legal power we all in a heep of crap!! I read this last night and about sh&t myself. Brian however from Whole House FM Transmitter did make a sound statement tht their transmitters are under the legal power and they do not put them at max. So we'll just have to see before I make any assumptions. I'm trying to keep an open mind here. I'd probably have to go AM Really Fast if this is the case because you know the deal and I think a lot of us part 15ers would be very unhappy running to AM if that is all you can legally get out of your FM Transmitter. So I still have hope this thing is under especially now that they stopped making the CM-10. There is something off there. Is the FCC raising a stink with all the makers of FM Transmitters? Something tells me they may be something we don't yet know. I know Brian made mention of issues with the FCC in the past so it does raise some question there as well. I just hope our FM days aren't numbered for any real range.
Quote from post 12 "I know Brian made mention of issues with the FCC in the past so it does raise some question there as well."
Some information about this can be found in this thread:
http://www.part15.us/forum/part15-forums/general-discussion/whole-house-fm-transmitter-0
Manufactures and distributors need to be careful.
Neil
I'm hoping to be able to get the Decade CM-10 out with the R-506 this weekend to get the real field strength numbers. My wife works this weekend so I can concentrate on the work at hand without having a lot of other projects going on at the same time. Small chance I might get to it before the weekend as well.
I did read a review on the CM-10 over at HB which said this "Our test sample performed quite well but slightly lower than expected. In speaking with the folks at Decade we discovered their decision was to provide an amount of cushion to their early production to insure that they were well within FCC Part 15 limits. Currently produced transmitters are being calibrated much closer to the legal maximum RF standard to maximize potential coverage." Now, I don't know when that was written but he was using a Potomac FIM-71 to take field strength, which is also a unit used by broadcast engineers and the FCC. The main advantages of the R-506 are a bit more accuracy, digital readout, much wider frequency range, ability to output to a computer, etc. I do note that over on HB they never publish any numbers.
I just have a feeling that if a guy had a handful of nearly any of these Part 15 transmitters of the same make and model and checked them all one by one in the same setting, with the same test gear, with the same surroundings, that the outputs would vary. Just me gut feeling.
The wierd thing is, I was losing the signal just past 200 feet in the Ford Escape, as well. And on the Grundig receiver too. I could get a bit further with the Grundig, but maybe 50 feet tops (it also has a much longer receiving antenna) And once I got out to where I was losing it I tried to reposition the radio to get a better signal with very limited success.
I'd just run out in the front yard and do quick setup and test, but I'd rather get into the open field a couple blocks north of me and get setup where there isn't a building to be seen, nor anything else except grass, so I'm more certain I avoid enviromental factors or potential reflections. I doubt I'll get much reflection off a Part 15 transmitter, but just to play it safe.
Tim in Bovey
Oh no! Now you've got me worried! The CM-10, supposed to be at BETS power here in Canada is doing alright for me here but don't want to find out it could be better! I may call Micheal at decade and ask about this.
Waiting anxiously for timinbovey's measurements with a real field strength meter.
Thelegacy, is it possible we could be converted to AM ?
Mark