AM radio is just one of many tools in the tool kit to use for mass communications systems, an assembled-on-site moderate power AM transmitter (provided by FEMA) seems like it wouldn't be very efficient (transmitter site not properly prepared to provide the strongest possible signal), but it might be the only way to communicate emergency info (to free FEMA AM radios).
The temporary FEMA AM transmitter (possibly a design that could be transported and assembled/operated using a flatbed truck) could be operated by the National Guard as needed (maybe only a few hours per day).
Kirk Bayne
If you're really interested in exploring this type of idea, which, as others have pointed out, has many flaws, there are other, more simple ways of doing something similar, with technologies and resources that are already in place and available.
The idea of having small, battery-powered radios and transmitters ready for deployment in an emergency would mean that you'd need a ton of them staged every 20 or 30 miles so they could be deployed quickly, or there would potentially be a very long delay, possibly of several hours or a day of having them deployed and the transmitters set up if you only have, say, 5 or even 10 sets for the entire country. Keep in mind that, for metro areas, you'd need at least hundreds of thousands of radios. The logistics of dropping the radios is also difficult. Do you drop them into parking lots which means residents in a disaster-ravaged area somehow need to find a way to go to a distribution area to get them, or do you drop them randomly which means the risk of radios being dropped and caught in trees, landing in bodies of water or where they cannot be retrieved. You've then got to stock fresh batteries, then restock the end users with batteries after the first group dies, you must test all parts of all those systems at least annually, etc.
Alternatively, wind-up AM and FM radios already exist. I know, I have one and I used it during the last larger-scale natural disaster I was involved in. The thing can run on A/C power, batteries or you can just wind up a crank on the side and use it for hours. Mine also has a light, an emergency blinking light and can also charge my cell phone using the wind-up power. No batteries needed. You don't need to keep fresh batteries in stock, you don't need to replace batteries when they die. I got mine as a "gift" when I pledged to my NPR radio station. I was so impressed that I then bought additional wind-up radios from Amazon for friends and family.
So why not simply have everyone include a wind-up radio with their storm preparedness kit? If you want to go further, take the money the government would need to spend storing battery powered radios and the means to deploy them, and use that $$ to have an annual distribution day where you'd hand out a ton of these things in various cities and towns, especially in areas where tornadoes, hurricanes and similar are likely to affect residents. Or better yet, if people really want or need them, they can register on the internet and one will be sent to them if they've not already asked for one within a certain period of time.
Regarding transmitters, rather than having portable ones that would need to be assembled, along with inadequate portable towers in the field, instead just fund the use of existing stations with adequate transmitting facilities. So if a disaster hits in Atlanta, the government would have a contract in place saying they takeover whichever station(s) reach the most amount of people and the largest overage area(s) and voila! People tune to those stations using their wind-up radios.
Again, still lots of questions and things to be worked through, logistically, financially and otherwise, but some of these ideas I think are much simpler and more efficient than the original one proposed.
NOTE: I edited my original post in a few areas, as when I saw
@DavidEduardo's comment about shutting this thread down and putting it out of it's misery, I quickly posted my comments, then went back and edited/updated.