Radio Propagation, Antennas and Electromagnetic Compatibility
Propagation basics
VHF is essentially line-of-sight, with range extended by antenna height and atmospheric conditions. MF ground-wave gives reliable short-to-medium range; skywave at night can extend it but is less predictable. HF relies on ionospheric reflection, and the usable frequency depends on time of day, season and solar activity: higher frequencies work better by day, lower frequencies by night. Traps include assuming a fixed HF frequency works around the clock, and confusing ground-wave with skywave range.
Antennas
Antenna height and location matter. VHF antennas should be mounted as high as possible with a clear horizon. MF/HF antennas need adequate length and a good ground or counterpoise. Keep antennas away from metal structures that distort the radiation pattern. A common trap is assuming any wire will do for HF; tuning and grounding are essential.
EMC
Electromagnetic compatibility means ensuring that transmitters do not interfere with other equipment and that other equipment does not disrupt radio reception. Keep transmitting antennas separated from receiving antennas and from sensitive electronics, use screened cables, and avoid transmitting near personnel. Questions may test that transmitting on one set can cause interference on another, and that correct installation and screening reduce it.
Worked example
A vessel reports poor HF communication at night on a high frequency but good communication by day. The likely cause is propagation: the higher frequency is not reflected at night, so a lower frequency should be selected. If the same vessel reports interference on VHF when the HF set transmits, the cause is likely EMC and antenna separation or screening.
Rule of thumb
For HF, choose the highest frequency that gives a reliable path by day and a lower one at night; for VHF, height is range.