Editing TV and FM DX (section)

==Sporadic E propagation (E-skip)==
[[File:SporadicE-NPS.gif|thumb|Ray diagram of sporadic E event]]
[[File:LUXMAN L-235 & TUNER T-230L.jpg|thumb|upright=0.9|right|Luxman T-240L stereo FM tuner (top) and L-235 amplifier (bottom)]]
{{main|Sporadic E propagation}}

Sporadic E, also called E-skip, is the phenomenon of irregularly scattered patches of relatively dense [[ionization]] that develop seasonally within the [[E region]] of the [[ionosphere]] and reflect TV and FM frequencies, generally up to about 150 MHz. When frequencies reflect off multiple patches, it is referred to as multi-hop skip. E-skip allows [[radio wave]]s to travel a thousand miles or even more beyond their intended area of reception. E-skip is unrelated to tropospheric ducting.

Television and FM signals received via Sporadic E can be extremely strong and range in strength over a short period from just detectable to overloading. Although [[Polarization (waves)|polarisation]] shift can occur, single-hop Sporadic E signals tend to remain in the original transmitted polarisation. Long single-hop ({{convert|900|-|1500|mi|km|disp=or}}) Sporadic E television signals tend to be more stable and relatively free of multipath images. Shorter-skip ({{convert|400|-|800|mi|km|disp=or}}) signals tend to be reflected from more than one part of the Sporadic E layer, resulting in multiple images and ghosting, with [[Phase (waves)|phase reversal]] at times. Picture degradation and signal-strength attenuation increases with each subsequent Sporadic E hop.

Sporadic E usually affects the lower [[very high frequency|VHF]] band I (TV channels 2 – 6) and band II (88 – 108 MHz FM broadcast band). The typical expected distances are about {{convert|600|to|1400|mi|km}}. However, under exceptional circumstances, a highly ionized Es cloud can propagate band I VHF signals down to approximately {{convert|350|mi|km}}. When short-skip Es reception occurs, i.e., under {{convert|500|mi|km}} in band I, there is a greater possibility that the ionized Es cloud will be capable of reflecting a signal at a much higher frequency – i.e., a VHF band 3 channel – since a sharp reflection angle (short skip) favours low frequencies, a shallower reflection angle from the same ionized cloud will favour a higher frequency.

At polar latitudes, Sporadic E can accompany auroras and associated disturbed magnetic conditions and is called Auroral-E.

No conclusive theory has yet been formulated as to the origin of Sporadic E. Attempts to connect the incidence of Sporadic E with the eleven-year [[Sunspot cycle]] have provided tentative correlations. There seems to be a positive correlation between sunspot maximum and Es activity in Europe. Conversely, there seems to be a negative correlation between maximum sunspot activity and Es activity in [[Australasia]].