Posted on November 26, 2007
I found this unit for sale on the consumer electronics rack of the corner drugstore for about 20$. I noted the notice of FCC part15 compliance on the back of the package, and looking carefully through the side of the clamshell it was in, I noted it had an FCCID #. Oh boy! Another affordable FCC compliant FM transmitter. And this time not quite from the toy department (for those who remember my testing and experiments with the “003” this summer).
The FCCID# for this unit is: SCZPSLFMTX1, and it was easy to find the reports on the unit via the FCC OET generic search using the first 3 letters to locate the manufacturer’s products. It is sold by Maxell, but the actual manufacturer appears to be Phitek Systems Ltd.
I put in fresh batteries (it takes 3 AAA), checked the 4 channels the unit can transmit on and determined 88.5 was the clearest by listening with my stereo, and then set the unit for 88.5. I plugged it into a battery operated cd “walkman” type player, turned the player’s volume all the way down, powered up the cd and turned on the transmitter. The stereo 15 ft away immediately “blanked”, stereo light came on, and I faded up the volume. A very nice sounding little transmitter. I’ve seen a lot of negative comments on various of these little “play your portable music players through your car or home stereo” type devices, so I was pleasantly surprised that the sound was clear and the separation sounded good. I walked to the other end of the house and other than the occasional “dropout” typical with part15 devices as you pass through distances equal to the wave nodes, it was nicely solid. At approx 60 ft indoors, it sounded good.
Ok, time for a better test. I disconnected my kit-built transmitter and hooked the FMT-1 to the “audio out” of my computer. This bypassed my usual eq and brick wall limiter to see how it would do just as it came out of the package. I turned my computer’s volume all the way down, turned on the transmitter and sneaked up the volume. Sounded quite nice on the stereo at the other end of the house. So I hooked a boombox up to a computer on the other end of the house with winscope on it and played a 100% normalized test tone mp3 on the studio machine. Ran back and forth for a few minutes getting it set so there was no distortion showing on the winscope trace of the test tone. Back to some music. It sounded quite good, though a little quieter then the stations near it on the dial. No surprise there, since I don’t use compression. Signal was clean and clear, a bit less than 100% modulation, but that’s fine.
I was about half an hour from doing my usual midnight saturday night show, so I settled down to look over the test info on the FCC OET site. I noted a few points of interest. First, the unit has pre-emphasis built in. The next thing I noticed was that in the test, they plugged the unit into an obviously grounded (it had an AC cord) cd player for taking their measurements. Logically then, any “advantage” due to the presence of a ground via connection to a 110v device was part of the testing and the device was still found compliant.
Then I noticed that the tests mentioned the unit having an internal and external antenna. I did not note any external antenna, as you can also see if you check the pictures on the FCC site. After a bit of puzzling, I noted that there was a wire visible in the battery compartment and measured it with a ruler and yup, it was approximately 9 cm. The OET pages say the antenna is 200 mm total, 110 internal and 90 mm external. I am assuming they counted in the battery compartment as external, and the internal part would be in the circuit board compartment. I am not taking a screwdriver to it to find out, since I currently plan to make use of this unit and prefer to keep it in unmodified condition.
Knowing that some small units like the Belkin use the audio cable as the antenna, and a common (and probably less than legal) mod with the Belkin is to use an audio extension cable to gain more antenna and thus boost more range. Logically it would not work with this unit since it has an internal antenna, but I decided to try it anyway. I tuned it in with my multiband radio (which has an s-meter) which was about 15 ft away and shortened the receiver’s antenna to get an “s6”. I added a 6 ft extension cable between the transmitter and the computer. Moving the cable did not change the meter reading. Getting the transmitter about 4 ft from the computer *did* improve the signal to about an s8, though. So I repeated the experiment with the battery cd player I’d tried first. Keeping the transmitter sitting on a wooden shelf and moving the portable cd player up and down, then towards the receiver and away, no noticeable difference on the receiver’s s-meter. So the unit does not seem to benefit by using an audio cable as an antenna. Time to get the boombox again. The FCC test was done with the unit plugged into a boombox type cd player that had an ac cord and as such would have been electrically grounded. So.. Put the boombox on a shelf about 4 ft over the computer. Plug the transmitter into the boombox. S8. Remove boombox and set the transmitter on the shelf with the 6 ft audio extension cable connecting it to the computer. Still S8. Also a small bit of whine/hum I’d noted during seconds of dead air between songs was gone. Move the unit (still on the cable) near the monitor and the computer. Monitor was the source of the hum, the computer was adding the whine. Tried stopping the cooling fan with a thin screwdriver. Bingo, the whine was fan noise. Both problems solved by putting the FMT-1 unit on the shelf over the computer and using a connecting cable.
To put all that more simply, it does seem to benefit from the presence of a decent ground, and the FCC documentation shows it was tested while connected to a grounded device. However, the length or positioning of the ground wire (via the audio cable shield) does not seem to be a factor that makes a noticeable difference. Plugging it directly into the back of a metal case desktop computer induces noise and cost some signal (very possible due to the receiver being in front of the computer while the transmitter was plugged into the back).
At this point, it was about 5 min from the beginning of my usual midnight show and so I quit experimenting for the time being. I called the neighbor who I knew was home and had him check for the station on the new frequency. Our houses are quite near to each other, and his bedroom clock radio is about 40 ft away. His stereo (which picks up better) is about 60 ft away. Good sounding signal on both. A pleasant surprise considering that is through aluminum siding and house wiring and drywall and who knows what else in the walls of the buildings. He was the only listener I was certain I had at the moment and I had a show to do. So the only other test was how it held up under battery power. 8 hrs later the signal was still good when I went off the air.
In summary, the Maxell FMT-1 did quite well for an inexpensive device. The sound is quite good *except* when the receiver is less than 3 ft from the transmitter. If they are very close the signal sounds distorted, though it is very sharp and clear on a radio any reasonable distance away. I am guessing at saturation. If you’ve ever graphed the equation often used here for the field intensity of an FM transmitter of a given power into a dipole over a range of distances from 0 to 1k meters or so, you’ll note that while it is for all practical purposes linear at any realistic distance, it is a sharply climbing curve as it goes into very short distances like less than 1 meter. Even a tiny transmitter can swamp a reciever at very close range. But since the distance inside a car is so small, it might be why this general type of unit seems to have a reputation for poor sound. The other possibility that comes to mind is they might be overdriving the unit into distortion, since I found that adjusting the unit with a test tone so it didn’t show any “flats” on the waveform resulted in the volume level on the source being lower than I’d usually use for say, headphones. This unit likes a rather low level of audio input.
It’s crystal based PLL, so the frequency is very stable and tunes easily with a digital receiver. It has pre-emphasis, and as such not a lot of eq (if any) is necessary. It’s about the size and shape of those little round plastic containers kids get “bubble tape” bubblegum in, and as such is very portable and takes almost no setup time.
I haven’t checked for range outdoors yet, but indoors it does about as well as the “003” did. I’ll be interested in seeing what it could do in a waterproof housing a ways off the ground, when I figure out a good way of going about it for a long-term installation. My house is about 35 ft X 65 ft and 3 floors counting the attic, and it was easy to pick up the signal over the whole house on anything at least as good as a table radio with a small telescoping antenna.
Last words on the FMT-1 for this post.. If you’re looking for a very inexpensive FCC compliant device for trying part15 FM, you could do a *lot* worse than this unit. This thing cost *less* than it did to kit-build my first transmitter and homebrew a simple antenna. I spent 30$ “saving money” when I could have spent 20$ for this and not had to wonder if I was actually compliant or not. I learned a lot building my first transmitter and getting it to sound decent and put out a fairly clean signal.. But if there had been complaints and the FCC had decided my little home-made-from-a-kit transmitter had been way out of compliance, I could have learned about large fines too. In that sense, this is a far more “newbie friendly” way to get a start in the possible “very near neighborhood” range of legal part15 FM with a compliant transmitter. It doesn’t actually sound bad even if you *do* plug it directly into the back of a computer. The little bit of hum and whine that approach has are only noticeable at moments of full silence and so this little unit is probably as close as anyone can get to “on the air right now” for 20$ (probably less if you shop around a bit). So if someone has been asking for hints of what you might like in your xmas stocking this year, you might consider one of these units as a possible option.
I think this will do what I need quite adequately, and as such the rather “rude and crude” kitbuilt transmitter and homebrew antenna I put together over a year ago are now formally retired from duty. Not sure yet if I’ll strip it down for parts or make a “historic” wall display of it for my station as “our first transmitter”.
I’ll probably post more on it as I figure out the best placement, figure out some of the challenges of mounting, determine if an external power supply is workable or if perhaps a small solar charger would be better and etc.