F2 propagation (F2-skip) is the reflection of FM broadcasting signals off the F2 layer of the ionosphere. The phenomenon is rare compared to other forms of propagation (such as sporadic E propagation, or E-skip) but can reflect signals thousands of miles beyond their intended broadcast area, substantially farther than E-skip. F2-skip affects the upper ends of the high frequency (HF) spectrum and the low ends of the very high frequency (VHF) spectrum; only a small portion of F2's effective range overlaps frequencies used by consumer broadcast reception, also contributing to the phenomenon being rarely encountered.
Solar activity has a cycle of approximately 11 years. During this period, sunspot activity rises to a peak and gradually falls again to a low level. When sunspot activity increases, the reflecting capabilities of the F1 layer surrounding earth enable high frequency short-wave communications. The highest-reflecting layer, the F2 layer, which is approximately 200 miles (320 km) above earth, receives ultraviolet radiation from the sun, causing ionisation of the gases within this layer. During the daytime when sunspot activity is at a maximum, the F2 layer can become intensely ionized due to radiation from the sun. When solar activity is sufficiently high, the MUF (Maximum Usable Frequency) rises, hence the ionisation density is sufficient to reflect signals well into the 30-60 MHz VHF spectrum. Since the MUF progressively increases, F2 reception on lower frequencies can indicate potential low band 45-55 MHz VHF TV as well as VHF amateur radio paths. A rising MUF will initially affect the 27 MHz CB band, and the amateur 28 MHz 10 meters band before reaching 45-55 MHz TV and the 6 meters amateur band. The F2 MUF generally increases at a slower rate compared to the Es MUF.
Since the height of the F2 layer is some 200 miles (320 km), it follows that single-hop F2 signals will be received at thousands rather than hundreds of miles. A single-hop F2 signal will usually be around 2,000 miles (3,200 km) minimum. A maximum F2 single-hop can reach up to approximately 3,000 miles (4,800 km). Multi-hop F2 propagation has enabled Band 1 VHF reception to over 11,000 miles (17,700 km).
Since F2 reception is directly related to radiation from the Sun on both a daily basis and in relation to the sunspot cycle, it follows that for optimum reception the centre of the signal path will be roughly at midday.
The F2 layer tends to predominantly propagate signals below 30 MHz (HF) during a solar minimum, which includes the 27 MHz CB radio, and 28 MHz 10-meter amateur radio band. During a solar maximum, television, amateur radio signals, private land mobile, and other services in the 30-60 MHz VHF spectrum are also propagated over considerable distances. In North America, F2 is most likely to only affect VHF TV channel 2, in Europe and middle east channel E2 and E3 (and the now deprecated channel itA) and in eastern Europe channel R1.
Television pictures propagated via F2 tend to suffer from characteristic ghosting and smearing. Picture degradation and signal strength attenuation increases with each subsequent F2 hop.