It has come to my realization that I am well equipped to do an "FM Transmitter Shootout". I have owned and operated a Potomac FIM-71 unit for more than 20 years, and just this week through a fluke of being in the right place at the right time scored a ZTechnology R-506 complete with the GPS interface and many other fine accessories. These two devices are those actually used by the FCC when checking for illegal FM stations. I don't own a vintage PC to run the software for the R-506 so I couldn't do on screen mapping like the FCC does when they're busting a perp, but I'm actually quite over equipped to take precise field intensity measurements from FM transmitters. Would be quite simple to set up a few and do the measurements at 3 meters and see what they actually generate.
I have zero experience with FM Part 15 transmitters. However it appears most of them look pretty much like a small transistor radio and have small supplied antennas that are required for their certification. I don't know that any of them talk about grounding systems or any other complex and varied installation quirks so a test should be pretty straightforward. Unlike the AM transmitters where there are a lot of variables and modulation ability can have an effect on coverage.
Two problems I see. I don't HAVE any of these, and doing an accurate test would require new, stock units purchased (not provided) for the test. If you ask for one they just may tweak it before sending it out. And the other issue is once I get setup, I'd probably waste a lot of time doing audio tests, etc to see which ones sound best in stock configuration.
Hmm. I just get these thoughts when I buy new toys.
Damn my curiosity.
So, if I were to do this, which certified FM Part 15 transmitters would be the choices for testing? Nothing that's a kit, as that certainly would be a unit with variables just by the nature of assembly.
I might even publish actual numbers.
Tim in Bovey
Since there was so much concern both on HB and here I'd like to see what the SainSonic AX-05B puts out as far as field strength. Id like to see how it compares with the Whole House FM Transmitter 3.0 and the Decade CM-10 (Since we've talked about these two FM Transmitters) and wanted to compare them). I'd like to see both power levels tested on the Sainsonic AX-05B and since you have a FIM you can quickly tell if its over the limit and shut down FAST if it is. And if not, we'll know what it is at both levels. If the low power is under I'd like to know. Lets bust some more myths out there about these units and stop guessing. Thanks Tim I'm glad how have the same stuff the FCC uses.
I noticed on the Decade website the CM-10 will be discontinued when the existing stock is sold. I didn't see antying there about amodel to replace it.
The HB site has a pretty good review/lab test of the Decade CM-10. Also using a Potomac FIM-71, which is indeed the standard used by most engineers even today. But the ZTechnology R-506 and R-507 are the more modern digital/computer savvy units. Both the Potomac and the ZTech are in use by the FCC as can been seen in the evidence filed with complaints and seizures.
I'm somewhat surprised to see how inexpensive the FM units are compared to the certified AM units! If we can come to a consensus on which to test I'd probably buy them at retail, test them, then offer them for sale here at a discount, as I myself don't really have a need for one.
Tim in Bovey
Welcome to Tim's considering the maybe possibility of doing certified FM transmitter measurements.
Doubt whether the list of certified FMs is very long... starting with
Decade CM-10
SainSonic AX-05B
C.Crane FM2
Wholehouse 3.0
?? What else is there?
Panaxis AC-100(Progressive concepts)
Also Decade MS-100(but very expensive!)
All the others don't have certification.
There is another that comes to mind....Artisan mentioned it in a post but can't remember now.
Mark
Mobile Black Box is one but not sure any mention of certification.
The Black Box website shows four FM transmitter models, with no mention of certification.
http://www.mobileblackbox.com/
V6000 is marked "up to 200mW"
Eclipse 4000 - 500 mW SOLD-OUT
Orion 5000 - 5 Watts
V3000 - USB 7-frequencies - "up to 150-feet."
If the V6000 is variable you could adjust it so it goes 750 feet in Stereo to a Car Radio. Or if you have a spectrum analyzer you could also adjust that unit. The V3000 certainly is great for NextKast and could be within the part 15 rules if it goes 150 Feet just like the Whole House FM transmitter 3.0. It would have great sound too without any hum as its a direct connection to your computer and has its own sound card. I'm gonna have to see if I can have a look at the V3000 but only 7 frequencies will put this unit in the useless category for most folks.
The spectrum analyzer is only one way to set up a part 15 FM transmitter for compliance.
Two other ways are...
An FM radio with an S-meter that shows signal strength;
An FM radio with field strength indicator such as TECSUN PL-310 and others.
If you have one certified FM transmitter in manufacturer's condition, just compare other transmitters to it, all from the same distance.
When Time gives us the actual readings of some of these we could do that. I don't have another fully powered certified transmitter on my possession to compare mine with. I sure wish I did. I'm trying to win the Whole House FM Transmitter 3.0 at least I know that one will do 800 feet to a car Radio and around 600 feet to a Digital boom box. Still plenty of power to broadcast to some homes and businesses near me. I could also rest that no harmonics are out there. For someone on a limited income like myself any part of this hobby is a miricle in itself to have completed. I struggle at times for food on the table and power and rent. So we'll see how this goes.
That your original transmitter in new condition is actually operating at the maximum allowed power. What if your transmitter used as a "standard" was really at 350uV at 3 meters? or 122uV?
I would hope, in this plan, to test more than one (especially of the lower priced models) and see if their output is even close between samples of the same transmitter.
And keep in mine that a signal strength meter on a radio isn't nearly as sensitive as a FIM measuring in uV.
It seems, however, that availability of new certified units is actually somewhat limited. There don't seem to be that many to choose from.
It would also be easy enough to check for spurs and harmonics as well.
I don't know of a spectrum analyzer that actually tells you the field intensity. You could use it to compare but not to determine absolute value. Otherwise engineers wouldn't be buying $10,500 units with $1200 calibrated dipoles to check FM braodcast transmitters.
Carl what spectrum analyzer do you have? I have a huge, heavy monster HP that I haven't dug out in years but I see all sorts of new smaller units available today.
Tim in Bovey
Tim asked: "Carl what spectrum analyzer do you have?"
Brand: ATTEN
Model: AT5005
I've been away for a few days, and almost missed this thread.
I think that an FM transmitter shootout is a great idea. I would include the Whole House 3, the C Crane 2 (I believe that's the version), the Sainsonic (more on that later) and the Decade MS-100 (if one can be obtained cheaply enough). The CM-10 would also be an obvious choice, but since it's been discontinued I'm not sure it's worthwhile.
macdev's experiments with the Whole House 3 do disprove the statement that Part 15 FM signals can only legally travel 200 feet. However, Tim is right, in that even though it's likely that that transmitter was Part 15 legal (purchased certified, new, and used unmodified out of the box), the output was never measured and it is possible that it was not entirely compliant. The chances that it was significantly over are not high (even if it was 500uv/m at 3 meters, that would only reduce the measured range by half, to about 400 feet or so, and the 200 foot range statement is still disproved). And those results do match up with what others using different but certified transmitters have obtained. However, it is still a question.
I would include the Sainsonic (on its low power or -48dbm setting), as it appears to operate in a gray area of the rules, and that would definitively answer a lot of questions about its use. If you're compliant, you don't have to be certified.
It would be nice to use 2 samples of each transmitter (probably the most inexpensive ones) as they are mass produced, and that will allow us to see the variations between samples. It's not practical with a transmitter such as the Decade MS-100, but that transmitter is hand tuned at the factor before it goes out, and is pretty much guaranteed to operate at the maximum field strength allowed. The MS-100 could also be left out of the initial testing if cost becomes a significant factor.
I would test the transmitters out with several receivers of preferably known sensitivity. I would make sure that one of those was a good car radio with close to 1uv sensitivity with 20db SNR (which will match up pretty closely to the one used in macdev's experiments).
I would avoid, however, relying solely on the receivers. One experiment that was proposed at another side had the testers measuring the field strength of a signal at a point when that signal faded out on a receiver - all that measures is the sensitivity of the receiver. And there's no guarantee that the claimed sensitivity of a receiver is the actual sensitivity.
I think it's important to create a measured contour map of field strength for a Part 15 signal - i.e., what is the field strength of a legal signal at 400 feet range? 800 feet? 1000 feet? Conducting it in a field where there is perfect line of sight would be ideal, but it would also be useful to measure the affect known obstructions have on that field strength.
It's a lot of work. I'd offer to help out but geography gets in the way. I've been on the lookout for a relatively inexpensive FIM that has been calibrated to conduct my own tests, but I'm not sure that those exist anymore.
If you do decide to go ahead with it, Tim, please let us know how we can help. I'm sure that you'll get a lot of support, and I am not concerned about bias with you doing it (as I would be with others).
Has anyone ever used this site to determine broadcast range?
Coverage Prediction Map
I realize we ARE talking about testing transmitters with special equipment, you might want to plug some of those numbers into this site and get a second opinion. Wouldn't hurt.
Bruce.
I just registered and tried it out. I plugged in 0.0000002 for power (176 nanowatts - 11 (approx) * 16, which is the BETS ERP for a dipole) and it came back with 'Invalid power'. So it's obviously not set up for the power levels that we use on Part 15 or BETS.