"Denny" wrote
When Terman did his work what were considered to be
"the broadcast frequencies"?

540 kHz to 1,600 kHz.

Another issue in Termans work is his decision to characterize
the F layer as a reflecting mirror...

Sorry, that's a misunderstanding. He states using the E layer in his Fig 55
and in his text.

... I can suggest that too low of a launch angle of the main wave
is deleterious for HF DX in that such a shallow angle of incidence
against the lower boundary of the F layer allows only reflection and
does not allow the wave front to enter the ducting region higher up,
which limits you to less than 1000 miles first hop.

On page 749 of this chapter, Terman writes, "If it assumed that 3-1/2
degrees is the minimum practical angle above the horizon at which rays can
depart from the transmitting antenna, the maximum skip distance
theoretically possible is about 1,700 km for E-layer transmission and 3,000
to 3,500 km for F2-layer transmission. When communication is to be carried
on over longer distances the transmission path must include two or more
hops..."

But if the vertical monopole has an unobstructed path for rays between the
optical horizon and +3-1/2 degrees elevation, then those ranges for a single
hop would increase.

And the reason that 3-1/2 degrees might be a practical limit is not related
to the belief that a ground-mounted vertical monopole has no radiation in
that sector. In fact its relative field in that sector for all practical
purposes is the same as it is in the horizontal plane, where it equals 100%.

The link below leads to another of Terman's graphs, this time showing the
measured fields vs distance out to about 2600 miles with MW freqs between
640 kHz and 1190 kHz . About these curves Terman writes," At great
distances from the transmitter the skywave intensity drops off more rapidly
than inversely with distance (see Fig 56) presumably because the reflection
coefficient becomes less when the angle of incidence of the skywave
approaches glancing, and also because at at considerable distances the
skywave may have made two or three round trips between the ionosphere and
the earth before reaching the receiving point."

http://i62.photobucket.com/albums/h8...ermanFig56.jpg

RF




Whereas a steeper launch angle results
in the wave front penetrating the bottom of the F layer, being
refracted to a shallower angle once inside the ducting region, being
ducted long distances with far less absorption than it would for
multiple hops, and then again refracting and exiting the duct at a
steeper angle than one would expect for such a distance... This to my
mind is a common mechanism for those amazingly strong DX openings we
see...

denny / k8do