Last year I did a little write up on the how to's of broadcasting your theater sound. Someone recently asked me again about it, I was about to point them in the direction of my old post, but I thought some of it was a bit out of date, plus it seemed like I left out a few things. So I have freshened it up a bit and I'm re-posting it with additional information. I have a lot of low power experience with broadcasting, so if anyone ever wants to pick my brain (what I have left of it anyway) about low power broadcasting pertaining to either AM or FM that is fine and I would welcome it.
Broadcasting your sound may look difficult if you have never done it before, but with a little knowledge it is honestly quite simple. Though some setups are more complex than others. First there are FCC rules about unlicensed broadcasting, and if you want to read all about it do a web search for FCC part 15 rules. However the most important thing is that you choose a frequency that is not in use by a licensed station in your area, as interfering with a licensed broadcast could get the FCC’s attention. Also you're not supposed to broadcast with more than .1 watts of power, though some folks get away with a little higher output. Most importantly your signal shouldn't be interfering with a licensed station and you shouldn't be broadcasting miles beyond your theater.
Setup. Most broadcast units have a 1/8” or 1/16” line in plug, and also most have 1/8” or 1/16” microphone plug, and some use RCA type inputs, you can run your sound directly out of your media source to the broadcast unit. Or you can plug the broadcast unit in after any sort of processing you might have. After that you plug the power in and select the frequency you want to broadcast on and that is it. One note of caution some units can be damaged if they are plugged in without an antenna hooked up.
If your going to buy a new broadcasting setup you want to buy one that is an FM unit. There are AM units available but as we all know AM is more prone to interference FM and most AM units are mono sound, where as FM units will transmit in stereo. Also AM is prone to humming issues and is much harder to setup, though if set up properly a good AM transmitter setup can give you that old school drive in sound. The only other reason I can see to pick an AM unit is cost, as AM units sometimes come up on E-bay for cheap like $25.00 cheap or perhaps you already have access to one. (Ramsey still offers a AM kit for $35 but the way). I still use an AM transmitter in my theatre, but that is because I had the setup prior to my getting in to BYT. In addition to my AM setup this year I'm also transmitting on FM with a Ramsey FM100B, that is rack mounted in a carry case with a Bringer processor and let me tell you that FM system... WOW it produces some awesome sound when played on a good radio. If your hesitant about broadcasting because you don't think it could possibly sound as good as your speakers, you should think again. Sound broadcasted over a good quality transmitter picked up on a good radio can sound darn good.
So basically you need two things. A broadcast unit and some sort of antenna. AM and FM antennas are a little different, so I will focus more on FM here because that is more than likely what you will be using. If someone wants more information about AM antennas let me know, but I will say that AM antennas require a load coil, and it is the load coil that makes setting up an AM system a bit more tricky. A lot of FM transmitters come with some sort of antenna built right on to the transmitter, most commonly it will be metal or rubber. For folks with a small city size lot that is probably fine (or if your just trying to cover you deck area), but for greater coverage in a larger yard and for better sound quality with less distortion you can get an external antenna that you set at a specific length which varies depending on the frequency you’re broadcasting on, that is called antenna matching or trimming. Some low cost FM transmitters don't have anyway to even attach an external antenna, so that is something to keep in mind when looking at transmitters. A good well tuned antenna can really increase range as well as improving sound quality. I have found that a ¼ wave antenna which generally runs $35 - $75 range is sufficient. Trimming or tuning the antenna consists setting the mast to a certain length which will vary depending on the frequency and the antenna you have purchased, specific instructions are included with every new antenna I have ever dealt with.
In my setup I have both my AM and FM transmitters located with my other equipment and then my antennas are mounted to the top of my screen. While these antennas are visible during the day you can't see them at all during the movie so it is not a distraction, but you could also mount an antenna on a pole any where in your yard or perhaps off your fence if you have one. It just needs to be tall enough so no one can touch it.
Some ideas of potential setups. One of the transmitters I recommended in my original write up was one of the Rangestar transmitters, further investigation shows those units can have problems with something called channel bleed. It is essentially where the unit causes unwanted interference on other frequencies, which is a pity because those are good units otherwise, but excessive channel bleed can affect broadcasts on other channels, that the user isn't even aware of, which in turn could get the FCC's attention. A few known good units are the SSTrans units and pretty much all the Ramsey units plus many more depending on your budget. The trouble with the Ramsey units is they can be hard to find already assembled, as the company is focused mostly on kit production. However they can be found already assembled though third partys as well as Ramsey does offer some of there upper end units already built. If you can budget for a $300 transmitter I would strongly recommend the Ramsey FM100 or the FM100B, they are the closes thing you can find to a professional unit in terms of sound quality. I have had the opportunity to hear the sound out of a number of different units and the Ramsey FM100 series seems to be the best, plus it has built in filtering and small amp. Also the FM100 series has a built in antenna as well as the ability to use an external antenna. If you need a less expensive unit perhaps try looking at the Ramsey FM 25 or 35 also SSTrans would be an option.
One last thing on transmitter selection. I see a few people here that have mentioned that they use a wholehouse transmitter. While I am not personally a fan of the wholehouse transmitter, there is no doubt that it is a decent entry level transmitter, for those with smaller yards. It is low cost and FCC certified. But in terms of sound quality and coverage you really could do better.
Another thing to consider in a transmitter is how the frequency is set. Some transmitters it is as simple as pressing a button on the control panel, a few require opening up the case and flipping dip switches or worse yet some old AM units have crystals that have to be swapped out to change frequency. Also finding a good clear frequency can be an issue for some, but this link will be of great help in finding one. http://www.radio-locator.com/cgi-bin/vacant
Also you can use your radio and simply search up and down the dial. I recommend looking at the website on the link, writing down a few open frequency they list and then test them on your radio to make sure they are clear.
I know all of this sounds like a lot, but really it is fairly simple. If your using an external antenna then the initial setup of the antenna will take a little bit of work but once it is up, it can stay up 24 / 7 and won't need to be adjusted unless you dramatically change your broadcast frequency or if something hits it. And once that initial setup is done, I think you will find (depending on your current speaker set up) that it is easier than the speakers and might even give better sound. So for me it is this simple. For my FM unit the transmitter is mounted in a case with a compressor, the two units are always connected to each other so all I have to do is plug power in on the box, plug the audio input line into the unit and plug the antenna cable in, (since I use and external antenna) and turn the power switch to on. Less than 30 seconds. It's simple once the initial setup is done and it sounds great. My AM system is a little bit more complicated, because AM is much more picky, and it needs to be adjusted every time I turn it on because small changes in weather and humidity affect the transmission quality of AM. However if you really want to get into broadcasting your sound and you can't find a clear frequency on the FM dial, going with AM is an option. For me I don't have any other way of putting my sound out other than broadcasting so I will be keeping my AM unit around as a backup should I have a problem with my FM unit.
In conclusion broadcasting your sound can be a lot of fun and can add a whole new element of excitement to your backyard theater. I hope that folks find this post informative and helpful. Thanks.