One might think being located far out in the rural Midwest, there should be no problem finding clear frequencies for 47 CFR 15.219 operations. That is, unlicensed AM MW broadcasts in the 510-1705 kHz range with 100 mW input power and 3 meter antenna (inclusive of ground lead).
However, to maximize range one has to consider more than ground conductivity and wavelength. For licensed broadcasters, there is a presumption (even on the heavily populated local “graveyard” channels) that there is some standoff distance between you and the next transmitter. Of course, the reason local AM MW channels are useless (low SNR) beyond 10-15 miles from the transmitter at night is the extremely high congestion (too many close transmitter) on these channels. The long range of clear channel transmitters comes largely due to the implicitly very sparsely populated channels they exist on. Of course, that assumes all transmitters have reasonably efficient antennas and ground systems to meet the minimum efficiency requirements of 47 CFR 73.182 and 73.189.
AM MW transmitter circuit
Complex $100 transmitters can have design limitations that are constricting due to the 100 mW INPUT power regulation. The VEC-1290K is cheap and simple enough to allow for example replacing the inductors with higher efficiency (lower loss) coils. Add bypass caps to the power supply and check the trapezoidal (X-Y) waveform for proper modulation depth. You can turn the LM386 modulator into a low-pass filter by changing the capacitor, or add another stage with dual opamp and dead-bug wiring.
Selecting AM Part 15 license free broadcast frequency
I run it at full bandwidth, which used to mean ~ 20 kHz, but now due to recent encroachment by second adjacents means at night I need to reduce my audio transmit bandwidth to 10-15 kHz. You have to modify AM receivers to get that bandwidth, and non-CQUAM car radios might have only 3 kHz of bandwidth. This means my center frequency is 1640 kHz, and the modulation covers from 1620-1660 kHz at day, and about 1630-1650 kHz at night. I don’t believe my puny signal would bother them, but I am throwing away power transmitting in their channel bandwidth, despite their distance. SNR α bandwidth.
To get the best frequency, you need to look two channels (20 kHz) up and down from your intended frequency. You will already know if a third adjacent is in town (30 kHz); you might need to go an extra channel away from them for poor selectivity receivers. You’ll find at night it’s tricky to find a full bandwidth clear channel, even in the expanded band, which is the only place for license free AM that gets decent range day and night. You might have to do like a couple commercial stations do if you want to have full 20 kHz at day: switch to a lower bandwidth 10 kHz broadcast at night on a different channel, maybe one channel up or down.
Note! Unless you are running less than 3-4 kHz audio bandwidth, you CANNOT use 1700 kHz center frequency since the cutoff for Part 15.219 operations is 1705 kHz–that’s absolute, not center frequency of 1705 kHz.
Another trick is for the receiver to deliberately tune off frequency, such that the carrier and one sideband are captured. This only works for analog receivers of course, and the trend is to digital frequency receivers. So I can’t slide my transmitter too far off center for the digital car radios.
Improving Part 15 AM broadcast SNR and range
At nighttime, audio compression would help make apparent SNR higher by increasing the loudness of quiet passages. To go all out, adding an expander circuit to the receivers akin to Dolby B noise reduction would add nearly 10 dB apparent SNR. That goes beyond casual enjoyment, and for now given the excellent ground conductivity of the Midwestern soil I’ll enjoy covering my 1/2+ mile radius. Just be sure you’re modulating nearly 100% or if with negative peak limiters 130% or so and you’ll be fine.
High quality audio sources
The fidelity of even 14.4 kbps RealAudio well exceeds that of conventional AM. If legally permitted, you can enjoy distant AM radio stations on your AM radio–via your AM transmitter connected to your computer internet streaming audio. The usual phonograph and CD sources (remember licensing for public performance issues) can be enjoyable as well.
Where SNR is adequate and with receivers modified to pass 20 kHz, your Part 15 AM MW broadcast system can exceed the fidelity of FM radio.