That's a nice page about matching short antennas for efficiency that I've sent people to a few times. Even though it's a tube stage being matched, the explanation is solid in my view, and worth studying.
The pre pre power of a legal part 15 signal makes a good STL not much else.
On the other hand there is a certified wireless headphone TX that goes 500 ft on 84.925 Mhz stereo and that would definitely cover your yard and then some.
looking at the current rules for part 15 FM it's a wonder anyone use it for anything other than broadcasting MP3s to your car radio or an otherwise stereo system without an AUX.
Now Sean Cuthbert made a FM to am transmitter for antique radios and car radios.
I'm wondering if we're going to start seeing am transmitter made for cars to play MP3s on with a 12-volt cigarette lighter adapter. You wouldn't need much for an antenna just a short piece of wire and you could put that wire next to the windshield or the side of the windshield close enough that you're am antenna with pick it up.
I know this could have some good use and many ways. I'm just saying the landscape for FM I'm part 15 is not good.
It's possible that more AM circuits will start to be made, and maybe not by companies deeply connected with Part-15 as in the past with the likes of SSTRAN, Panaxis or Ramsey, but by electronics manufacturers who pick up already circulating designs and manufacture them.
There are a couple of good examples in AM, and I think the clearest one is the product called a 50 milliwatt AM transmitter, a small non-PLL tunable kit that's been on the market since last year. It's well thought out for stability, and the modulator is simple and good, and it uses common parts, nothing special or expensive.
I just found out that the design is from the book, 'Build Your Own Low-Power Transmitters' by Graf and Sheets, 2001. That info is indirectly thanks to Station8.
Another is a PLL design by Otto Tuil that I've seen on the 'Netherlands Forum For Old Radios' for some years, both in a DIP switch tuning version and an updated micro-controller tuned version with display. His PLL section is standard, like a lot of low power PLL units have, but the modulated RF stage is unique, using an analog multiplier IC instead of a class-C transistor.
Two kit versions of that have started to be made, the original as on Otto's page, and a version with an amplifier stage after the modulator. Like Otto's transmitter, they're 9 khz Euro-dial channel spacing.
It goes to show that if you publish a simple circuit design using standard parts, someone may make a kit of it to fill a need.