[600MRG] 630m WSPR Propagation Over a Typical Day: Six Regimes

[James Hollander]

Six-Regime 630m.  Third post.  73,  Jim H   W5EST   
 
IV. PRE-SS REGIME
     Ionization and recombination of positive ions in the D-region establish a daytime equilibrium of high electron concentration and accompanying D-region absorption of 630m signals.  This equilibrium level of electron concentration declines as the sun's ionizing radiation descends in elevation angle toward sunset SS. This is because the D-region becomes more obliquely oriented and more nearly edge-on to the arriving solar ionizing radiation.  Less-distant and stronger stations' signals increasingly survive the absorption in the D-region.  SNRs rise above the WSPR decodability threshold (about -33 dB) at a receiving station with a good antenna system and receiver.  Ionospheric convection and patches of varying electron concentration can cause SNR variations and reception gaps.  A switch from D-layer reflection to F-layer reflection around SS may occur and is still under study.

V.  POST-SS REGIME
    The sun's ionizing radiation shuts off at SS for 630m signal purposes. Equilibrium of D-layer ionization and recombination is lost. The recombination rate of electrons with positive ions in the D-region diminishes the electron concentration and its D-region absorption.  The sun's visible light still reaches eastward into the post-SS  D-region and inhibits negative ion formation.  But the sun then progressively fades from East to West even there.   A negative ion formation rate progressively joins the positive ion recombination picture. D-region absorption falls and 630m WSPR SNR consequently rises as a result. This post-SS regime extends for about an hour to several hours after SS.  The pre-SS and post-SS regimes together deliver an S-shaped SNR pattern.  This pattern is subject to SNR variations and cutoff of SNR-varying weaker signals by the WSPR decodability threshold.  As the post-SS regime continues, additional WSPR signals emerge into decodability. Timing of such signal emergence is approximately bounded by an opportunity time window in the night that depends on geographic locations of stations and time of year (see Sept 19 reflector posts).  Ionospheric convection and patches of varying electron concentration cause SNR variations as much as 15 dB or more.  Multipath self-interference of signal in the F-region contributes to the SNR variations.

(Next post:  NIGHT-TIME REGIME and Notes.)