AM two-way radio channel spacing
Since 1972, standard VHF AM used 25 kHz channel spacing generally worldwide. In 1999, European regulations began a mandatory phase-in of 8.33 kHz channel spacing for VHF AM aviation communications to increase the number of available channels. The rest of the world generally remains on 25 kHz spacing for aviation. One factor driving the 8.33 kHz spacing in Europe was the number of relatively close proximity airports and large number of small nations each having their own aviation communication needs. Other countries like China and the United States also have regions with many airports and a lot of air traffic, but each have a unified regulatory framework managing all VHF airband communications.
Anyone who has used AM CB radio or AM broadcast radio would understand why 25 kHz channel spacing is reasonable to avoid adjacent channel interference with real-world AM transmitters and receivers in most reasonable proximities. Even without overmodulated, splattering transmitters, an AM receiver can experience noticeable adjacent channel interference when within about 1 km of another AM transmitter operating on a channel just 10 kHz away. If the CB radio transmitter is operating properly, 20 kHz spacing is adequate when not closer than about 1 km.
This observation can be loosely derived from a typical AM CB radio (say the Cobra 19 mini) receiver’s selectivity performance of say 6 dB at 7 kHz spacing and 60 dB at 10 kHz spacing. Consider also the dynamic range of the receiver, akin to Sherwood Engineering’s Receiver test data, which doesn’t include CB radios. Assume the CB radio transmitter ERP is 36 dBm (4 watts) and the receiver sensitivity for 20 dB SINAD is -113 dBm (0.5 µV). At 1 km distance, free-space path loss at 27 MHz is about 61 dB. This first order approximation neglects important factors like radiation pattern distortion from ground reflections. Assuming 0 dB receiver antenna gain, received signal strength would be about -25 dBm, which can exceed the maximum dynamic range of the receiver front end, if we arbitrarily assume a receiver dynamic range of about 80 dB. However, the typical environmental noise floor on 27 MHz - neglecting skip or other anthropogenic interference - may be ~ -110 dBm in the AM receiver bandwidth, so the dynamic range impacts of a legal CB radio transmitter 1 km away may not be noticeable. An adjacent channel signal 10 kHz away (neglecting modulation bandwidth) would be attenuated by about 60 dB, resulting in a received adjacent channel signal strength of about -85 dBm. The adjacent channel signal would be strong enough to cause noticeable interference on the desired channel, and unwantedly open the squelch if no desired signal is present. With 20 kHz spacing, the adjacent channel signal would be attenuated by over 80 dB, approaching the general receiver’s dynamic range limit vs. the noise floor, and would be much less likely to cause noticeable interference, assuming the transmitter is clean.
The emissions mask in FCC part 95.979 requires better than -35 dBc in the band 8 kHz to 10 kHz from the carrier, and better than -59 dBc beyond 10 kHz from the carrier. The radio transmitter will often do better than -59 dBc at 20 kHz from the carrier and increasingly better beyond that.
Switching to AM VHF airband radio, a typical installed receiver such as the Icom IC-A220 has receiver selectivity at the 25 kHz step of 6 dB at 3 kHz spacing and 60 dB at 22 kHz spacing, and at the 8.33 kHz step of 6 dB at 2.8 kHz spacing and 60 dB at 7.4 kHz spacing. A typical handheld airband transceiver such as the Yaesu FTA-250L RX selectivity in the 25 kHz step of 6dB at 8 kHz spacing and 60 dB at 25 kHz spacing, and at the 8.33 kHz step of 6dB at 2.8 kHz spacing and 60 dB at 8.3 kHz spacing. Thus we see how at airband, where the frequency plans would be made to slice up former 25 kHz step frequencies into distant frequency reuse at 8.33 kHz step, while avoiding close-by overload, the 8.33 kHz step is quite useful.
To put this back into CB radio terms, suppose CB radio is popular in a town. The typical use I’ve seen is for local groups to be at least 20 kHz apart to avoid adjacent channel interference. For example, CB channel 20 (27.205 MHz) is more popular as a “local” channel than channel 18 (27.175 MHz) to avoid adjacent channel interference to/from channel 19 (27.185 MHz). Since CB radio use by definition is entirely ad hoc, users generally space themselves 2 channels (20 kHz) apart in real-world observations. For airband VHF AM in Europe, channels of 8.33 kHz spacing are successfully used with proper frequency planning considering physical separation to avoid adjacent channel interference and overload.
Given the above, I think President should have make like 8 P-channels and not have had “P4” as CB channel 18. They should have instead done like P1 = CB1, P2 = CB3, P3 = CB12, P4 = CB14, P5 = CB22, P6 = CB23, P7 = CB27, P8 = CB29. This avoids CB channels 6, 9, 16 (SSB use), 19, and 20 and the SSB channels 30-40. However those are only hypothetical channel assignments, and President did not in fact do that. Given the existing President P-channel assignments, I think President should consider adding P6-P9 like: P6 = CB14, P7 = CB22, P8 = CB23, P9 = CB27. Again that’s hypothetical. First we need to get more P-channel or at least FM + CTCSS users on CB radio.