Even fm frequencies

The rules only specify the band of frequencies on which you can operate. You can operate on any carrier frequency in the band. If operation were restricted to the standard channels, the rules would also have to specify frequency tolerance.

Thanks Ermi,

I was sure that was the case but wanted a second opinion.
Better safe than sorry. I will be moving my studio around sometime this week and did not want to string cables or wires around the house.

Since I wont be using the FM to broadcast to the public this will work as a temporary stl to feed audio from the mixing board to the TH Transmitters in another part of the house.

The Behringer mixing board I use has a ultra-bright blue led and colored led's for the VU meter which is a little annoying when my wife and I want to sleep. The studio is in our bedroom, so what I will be doing is moving the studio gear into a closet while leaving the two AM transmitters right where they are.

Moving the transmitters is not an option since they seem to be working pretty well right where they are.

Thanks again

you are limited to 150uV/m @ 3m when using even channels.

No problem o,
I have everything moved to the closet and have shortened the telescopic antenna that came with my Ramsey 25B so that the signal only covers inside the home.
I am lucky if i can pick it up on the other end of the house and my mobile home here is 80 feet long!

My neighbor behind me might be able to hear it but only after they hang outside their window with a radio in one hand and a hat made of aluminum foil. 🙂

The sound is pretty good too, the fm transmitter is set to stereo but the radio feeding audio to the talking house tx is set to mono, processing is provided by sound solution. Again this will be a temporary fix.

Just to be sure that my copy of Part 15 is not out-of-date about this subject, I printed out the latest version of Section 15.239 from the Government Printing Office, which is linked from the FCC website.

As far as I can tell, there is nothing in 15.239 that states where the operating frequency must be, provided that the permitted 200 kHz band centered at the operating frequency is wholly within the frequency range of 88-108 Mhz [15.239(a)].

Maybe what kc8gpd meant was Section 15.239(c), which specifies Section 15.209 limits outside of the 200 kHz band centered at the operating frequency. Section 15.239(c) also does not say that the operating frequency must be on a standard FM channel.

So long as the signal you are sending by FM is ultimately destined to be monaural, I suggest disabling stereo in the transmitter. This will improve its range and the reception on the outskirts will be less noisy.

In 1987, what is now Section 15.239 was called 15.162. Low-power FM transmitters were called "wireless microphones" at the time.

As Part 15 was updated, the preface to section 15.162 was deleted. What the preface said was:

"A wireless microphone may operate on any frequency in the band 88-108 MHz providing that it follows all of the following conditions."

The conditions that followed were similar to what is now in Section 15.239. If something like the preface were retained in 15.239, It would have eliminated any questions about where the operating frequency may be.

works good for my needs.

I only needed to cover 16 feet from the closet the studio is stuffed into, to the talking house transmitters on the other side of the room.

RF was lowered on the FM 25b to nearly nothing and the antenna was shortened to limit the signal to the one room and maybe the kitchen.

I cannot pick it up on the other side of the house or even outside so it is serving it's purpose, which is to eliminate the need to string cables from one end of the room to the other.

Serving the public with the FM was not really what I wanted to do.
I will probably look for a audio/video sender to replace the FM 25b since it is not fcc certified.

After thinking about it for a couple days I now think using even FM frequencies for STL is a brilliant idea. Most publicly owned receivers these days are digital, which means most people have no way of discovering your presence. Since most trouble starts with a complaint, people can't complain about what they don't know. Just never tell anyone about the FM. I think I might try it here.

would work good for anyone that might have their transmitter in a separate building away from the studio.

Imagine if your transmitter was 80 to 100 feet away from the studio in a barn or some kind of building. Use a fm transmitter with a yagi antenna pointed towards the tx where you have another yagi to receive the fm signal.

I guess the only limit's here are your imagination and the distance between the studio and the tx.

I'll defer to Ermi here, who has been checking on the current regs, but I seem to recall directional antennas being prohibited with part 15 devices. I don't feel like that's a huge concern for the STL application, but it might bear keeping in mind, if that's still true.

Other than that I definitely agree that low power FM is a great STL, in part because the pre-emphasis is already handled and there is a better fidelity than the final use.

The regulations deal with transmitter field strength, so a Yagi at the receiver would not be a problem. There are Yagis for FM reception that give about 7 dB gain over a dipole, and will roughly double the range compared to when a dipole is used.

A Yagi at the transmitter is also OK, but would not be very useful. The field strength limit still has to be met, and there would be no improvement in range.