When Terman did his work what were considered to be "the broadcast
frequencies"? Even on 80 meters we are well above that range from that
era... There is a major change in polarization response as we go from
160 to 80 meters, and again to 40 meters, etc.

As an active antenna experimenter on 80 meters who keeps records of
which antenna made which contacts, and with the ability to switch
between relatively high horizontal antennas, and vertical antennas of
both a quarter wave and a half wave in height (80 meters), I can tell
you with certainty that less than 5% of my 80 meter DX contest contacts
in 04 and 05 were made on the vertical antennas ... It is the rare DX
station that is clearly better on the verticals compared to the high
dipoles - and when they are stronger on the low angle verticals it is
usually right as the band opens or closes... Based upon my antenna
testing in previous years, for this year's CQWW I am replacing all the
vertical arrays with horizontal arrays... At that point the only
vertical arrays I will have left is on 160...

My gut feeling, based on real results over multiple years, is that the
incoming HF signals rarely peak below 10 degrees - probably less than
5% of the time...

Another issue in Termans work is his decision to characterize the the F
layer as a reflecting mirror... We now know that the ionosphere
refracts and ducts signals as often as it reflects them... 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.. 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