By a stroke of luck, today we were blessed with a sunny 58 degree day, just what I needed to schlep out into the test field once again with the C. Crane FM 2 transmitter that arrived over the weekend while we were in Kansas visiting my Son and his family. The rest of the week is forcast for 30's and rain. So I had to jump at the chance.
Same idendical test setup as the previous tests. Same field, same location, same calibrated (by Potomac) Potomac FIM-71 field intensity meter. Same setup, 3 meters between receive and transmit antennas, both antennas exactly 7 feet above ground as outlined in the Potomac manual, etc.. etc.. Tests were run operating the transmitter on brand new "AA" alkaline batteries. Here are the results.
The C.Crane website, other sellers websites, and the included manual state that the coverage for this transmitter is "between 30 and 75 feet, line of sight with no obstructions". Well, this is exactly right. As delivered, in " out of the box" stock condition, the field strength at three meters was 110 uV/m. This is of course EXTREMELY LEGAL. In fact this is less than half the legal limit of 250 uV/m. This was with both antennas horizontal. If I changed the transmitter antenna to vertical it dropped to 43 uV/m, but that is to be expected. I brought the Tecsun PL-310et radio with me to see what sort of actual distance I got, and the receivers dBu scale dropped to zero at a measured 35 feet but I was still able to hear the signal by carefully orienting the radio antenna for best reception, although with noticeable hiss. By 40 feet away I couldn't hear the signal at all. So, quite literally, the transmitter as shipped covered to about 35 feet. This is in wide open space, line of sight.
You may know (or not) that inside this transmitter there is a power output control potentiometer. You can find all sorts of websites and even YouTube videos showing how you can turn up the power for "better results". Of course none of these posters have any clue if their transmitters remain legal once they turn this control. So, there I was, standing in a field wondering..... I didn't bring any tools with me at all. But I did have the small Swiss Army knife that I always carry, and darned if the screwdriver blade didn't fit just right to remove the screws to access the inside of the transmitter! There are two screws under the little rubber "feet" on the transmmitter bottom, and one inside the battery compartment. Remove the back carefully making sure to clear the input level control and the power adapter jack. Now, I didn't want to get into a complicated testing routine here, I just wanted to turn it up to full power and see what she'd do. I watched the FIM as I turned the control and verified that COUNTERCLOCKWISE increases the power. I gently turned it all the way CCW, put the back on, and returned it to test position and manned the meter. With the output turned to full, the field strength at three meters was now at 5850 uV/m! That's a heck of a jump and over 23 TIMES the legal USA limit.
Naturally I thought.... wonder if this would be legal in Canada! So I hoofed the FIM out to 30 meters (98.4 feet) hoping it would come in under 100 uV/m. Nope. Full power at 30 meters gave me a solid 270 uV/m, nearly three times the legal limit in Canada.
Just for the heck of it I did a quick reception test with the Tecsun with the transmitter on full power -- I was out well past 250 feet with clear signal and about 4 dBu on the meter. I got tired of walking. I'm sure I could have gone quite a bit further.
So, the result is... The C.Crane FM 2 is clearly legal as shipped, and by a wide margin. Access to the "secret" power output control is pretty easy and if you had access to an accurate FIM you could indeed readily adjust it to full legal US output level limits.
The audio input cable is built into this transmitter. So any testing or use would have to include the audio cable. You've seen how plugging the cable into other transmitters greatly increased their output. This one is clearly built with setup done with the built in cable at the factory. It also tunes down to .5 steps, e.g. 92.75 mHz. Which seems silly to me unless you're receiving it on an old school "dial" type radio.
I didn't do any serious testing for audio quality -- what I heard on the FIM and the Tecsun sounded just fine. I also didn't set up the Spectrum Analyzer to test either. Perhaps when I have more time and I'm sutck indoors over our 6 month winter :) I rather like this transmitter. It's solid, sits well on a table, comes with an AC adapter and is easy to operate. I just might hang on to this one to use to distribute station audio around the studio.
TIB