TopBand: Mirror earth

[Roger Graves]

> Date:          Sun, 6 Jul 1997 16:53:59 -0800 (AKDT)
> To:            topband@contesting.com
> From:          Dan Robbins <kl7y@Alaska.NET>
> Subject:       TopBand: Mirror earth

> Some seem to have the impression that a signal bouncing off the earth can
> only do so with a tremendous loss.  According to the Radio Propagation
> Handbook, this isn't so.  Remember we're talking Top Band here and with
> common radiation angles, say 10 to 50 degrees.  Under those conditions a
> seawater reflection has 0.1 dB or less loss.  In the absence of ionospheric
> absorption, it is expected that over mostly water paths the signal levels at
> amateur power levels on the low bands will exceed S9 on paths well in excess
> of 10,000 miles.  The lossy parameter in this equation is not Mirror Earth.
> Land reflections due impose a penalty anywhere from a couple to a few dB per
> hop.  Fortunately, we live on watery planet.
> 
There is also a 3 db per reflection "polarization loss". Thus a 6 hop 
path has a 36 db loss from polarization losses even if there are no 
other losses. This also applies for ducting type reflections.  
The major differences are that ducting paths can be at much lower 
angles, so there are fewer bounces, plus there are no transitions 
through the d layer, which entails substantial loss for each 
crossing (2 crossings for each "normal" hop) - one can not 
realistically assume an "absence of ionospheric absorption" on the 
low bands.  The computer model I have written clearly shows these 
effects, and also shows that paths exist for "ducting" modes when there
are no "normal" paths possible.
73, Rod VE7FPT


Roger Graves
Victoria, BC
CANADA

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