Illegal Transmitters

@mark - I typed out a rather lengthy reply to you, before deciding not to post it. This is turning into a rabbit hole that I'm not sure I want to go down! As I think I said to you in the other thread though, I do agree that the issue of all these overpowered FM transmitters being sold to people, most of whom don't realize are illegal to operate, is a far bigger issue than some relatively minor details concerning the Decade transmitters.

I have read that a lot of the cheap Chinese FM transmitters generate spurii, and are not at all clean in their output. This really bothers me. The FM broadcast band is not too far away from the aircraft band, raising the potential for these illegal transmitters to be interfering with important communications. Just from an engineering standpoint, I don't like to see transmitters that put out dirty signals; it's just shoddy design. That they are allowed to get away with selling these inferior products is pretty darned irksome to me.

@rugster I thought aircraft still used AM..  Didn't Mark recently post a couple videos explaining AM capture effect, and why they aircraft don't use FM? 

@richpowers - yes, AM is still in use on the aircraft band. It's true that AM is not subject to the capture effect, so the potential for a low power FM signal to interfere with it is reduced. I still don't like the idea of spurious RF being emitted on the aircraft band though.

@rugster I wouldn't be so quick to drink the koolaid being spouted by other Part 15 sites about "cheap" Chinese transmitters.  Many have vested interests in trouncing FM, while promoting AM (such as having colleagues distributing AM transmitters).

Overpowered, yes, but many of the Chinese transmitters are surprisingly well engineered, as well or better than the certified transmitters available.

For example, the Decade CM-10 (now discontinued) was just the famous CZE-05 with a few resistors at the output to reduce power (otherwise it was identical).  It was certified in both Canada and the U.S.

The Retekess TR-508 is/was RSS123 certified in Canada at 400 milliwatts, and tests showed that it was as clean as the Decade MS-100 at all power levels.  It's also Part 15 certified, although how they got it is beyond me.

I suppose the myth of cheap Chinese transmitters does serve a purpose in that it may steer users away from them due to fears of interference.  But the major issue with these transmitters is, as I stated earlier, their potential range due to excessive power levels.

I had a feeling that using the phrase "cheap Chinese transmitters" probably wasn't the best idea @artisan-radio!  I'm open to the idea that it's unfair to paint them all with the same brush, though I do remember seeing an online article somewhere (and not on any of the Part 15 sites) where a spectrum of one of the more generic brands from overseas had a dirty signal. Of course, I can't find the article now, and it may well have applied to just one particular brand/model.

In addition to the potential for causing interference to services in other bands, another danger of spurii is that they might cause interference to nearby FM BC band listeners on adjacent channels (but you all know that!)

But yes, the RF output power of these non-certified transmitters is most likely the biggest drawback with them. I'll be more careful with my statements and characterizations in the future.

This thread and the one about the Decade citation really illustrates that something is wrong with the certification and compliance issues in the U.S.

In Canada, all devices used must have a Technical Acceptance Certificate, or TAC, i.e., be certified.  That's the first thing that is checked if and/or when an ISED agent shows up at your door.

While there's a big hullabaloo made about certification in the U.S., there's also the issue that the transmitter must be compliant during operation.  And that's a big problem for FM.

First, there's really no way to know if your FM transmitter is compliant unless you own a very expensive field strength meter.  Most users don't.  And while there's that document about expected range that the FCC produced once upon a time, it bears no resemblance to reality.  Depending on environment, installation particulars and most importantly the quality of the receiver and antenna, I guarantee you that I can come up with a scenario to make almost any transmitter fall within that 200 foot range estimate.

Certification is yet another story.  It's measured under strictly controlled conditions in a lab, with no audio cable attached (just a straight carrier).  In the real world, there are a myriad of factors that can influence field strength.  Add an audio cable (which is, after all, why these things are bought) and you get an increase in field strength.  Add height and you get increased field strength.  Signals reflect and scatter based on obstructions, weather, etc. and that can either increase or decrease field strength.  The list goes on and on.  So it's highly unlikely that a transmitter showing the permitted field strength in the lab will show the same field strength outside that lab.  It likely will not be compliant, and most users will not recognize this.

Surely the FCC also is aware of this fact.  So what's the point of insisting on compliance after the sale for a certified transmitter?  The fact that it is certified is enough for it to be sold; it should be enough for it to be used, unmodified, without fear, in the field.

To be fair, it seems that it's only other hobbyists with an axe to grind against FM that insist on strict compliance and yap about it all the time.  I suspect that the FCC, like ISED, won't just shut a station down for a non compliant field strength if the transmitter is certified and unmodified, unless there are other factors involved (such as interference with a licensed station).

@rugster You just described the engineering and undesirable RF by the typical ceiling fan sold at your Big Box hardware store. I know a big shot engineer in STL from Emerson who visited the factories in China regarding fans, and he just rolls his eyes when it comes to quality control.