Anything which conducts can be coupled at any frequency, the efficiency may be very low however. For a simple aerial, it is usual to drive a half wave at the centre. At 14MHz, the aerial will be about 10m long and will show a drive resistance of about 50 ohms which modern amateur radio transmitters are designed for.
Longer or more complex aerials can provide directivity (more signal) but may need matching circuitry. Many persons have spent many years playing with the design of aerials and there are computer programs (I use MMANA) to try them out without climbing trees!