Clear frequencies are vital for 47 CFR 15.219 unlicensed AM MW broadcasts in the 510-1705 kHz range with 100 mW input power and 3 meter antenna (inclusive of ground lead). 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 to 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.
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. 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: recent encroachment by second adjacents means at night calls for audio transmit bandwidth 10-15 kHz. Non-CQUAM car radios might have only 3 kHz of bandwidth. Center frequency is 1640 kHz has modulation from 1620-1660 kHz at day, and about 1630-1650 kHz at night. To get the best frequency look two channels (20 kHz) up and down from the intended frequency. If a third adjacent is nearly (30 kHz) go an extra channel away from them for poor selectivity receivers. 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. Some commercial stations have full 20 kHz at day but switch to a lower bandwidth 10 kHz broadcast at night on a different channel, maybe one channel up or down.
Note! Unless running less than 3-4 kHz audio bandwidth don’t 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. The transmitter can’t slide 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. Adding an expander circuit to the receivers akin to Dolby B noise reduction would add nearly 10 dB apparent SNR. Keep modulation clean and near 100% peak to maximize coverage.
High quality audio sources: the fidelity of even 14.4 kbps RealAudio well exceeds that of conventional AM. Hear distant AM radio stations on an AM radio–via AM transmitter connected to computer internet streaming audio. The usual phonograph and CD sources (remember licensing for public performance issues) work as well.
Where SNR is adequate and with receivers modified to pass 20 kHz, a Part 15 AM MW broadcast system can exceed the fidelity of FM radio.