TopBand: Look Ma, no math!

[Robert Brown]

Friends in Radio Land -

I thought you might like to know more about my latest "ducting theory"; 
it is based on my experience teaching at the Big U in California.
 
When I taught there, I tried to illustrate my lectures.  In the case of
simple harmonic motion, I liked to talk about the problem of "weighing
the baby".  That could be done quasi-statically by lowering the baby
onto a scale and seeing what the dial scale showed.  Simple enough!
 
The dynamical approach was to hold the baby just above the scale, then 
suddenly take one's hands away so the baby dropped onto the scale.  
Then, knowing the spring constant (pounds force/inch compression) and 
measuring the period of vertical oscillations of the baby, one could 
calculate the mass of the baby using results from the theory of simple 
harmonic motion.  That's not recommended but you get the idea; OK?
 
Now for the "Baby's Crib theory of Top Band propagation".  There, 
the baby (signal) climbs into his/her crib (electron density valley 
above the E-region) but is trapped there when the railing (foE value) 
is raised (by an acoustic/gravity wave changing the local electron 
density) or the bottom of the crib is lowered (by a F-region tilt 
that changes the radiation angle in the duct).  But 3D ray-traces 
from the PropLab Program show that the bottom is lowered naturally  
when the earth's magnetic field affects the radiation angle by 
refracting the wave sideways (off great-circle) into weaker ionization 
in the F-region.  That's the news!
 
The interesting thing is that ionosphere really "puts the squeeze" on 
the baby as "the crib" (electron density valley) becomes narrower and 
shallower as the dawn terminator is approached.  PropLab Pro shows 
the baby (signal) struggling to get out but it stays trapped in the
crib (showing smaller vertical excursions of the ducted signal as it 
goes toward the terminator).
 
How does the kid (signal) get out?  Sunrise (larger F-region tilting 
and fluctuations) shakes the bottom of the crib until the kid flies 
out.  Poor kid!
 
So that's the "Baby's Crib Theory of Top Band Ducting".  No math, at 
least that's visible to the casual observer.
 
73,
 
Bob, NM7M
73,
 
Bob, NM7M


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