Topband: Topband Geomagnetic Absorption

[KN4LF, T. F. Giella]

Tom Asked:
"I wonder if anyone can offer opinions on the advantages that
vertically polarized antennas have over horizontals on 160M related to
geomagnetic Earth absorption?"

Thomas KN4LF Says:
Antenna polarization plays a large role in the success of a long haul DX
contact. As a medium frequency RF signal traverses our planets magnetic
lines of force in a perpendicular manner on high and mid latitude paths say
between W3 land and SM, higher angle horizontally polarized signals  are
more readily absorbed then lower angle vertically polarized signals. On
other propagation paths on the globe opposite results can be found.

The majority of the time medium frequency RF signals in excess of
approximately 3100 miles propagate via the E valley-F layer propagation
mechanism or via the Chordal Hop propagation mechanism. High solar flux
values can aid in long haul medium frequency propagation, as high solar flux
values ensure a strong F layer half of the E Valley-F layer duct mechanism.
Typically the majority of transmit antenna's radiation must be focused under
30 degrees (18-22 deg. being optimum) to enter the E Valley-F layer duct and
(((((lower angle vertical polarization is best suited for this))))).

A note, high solar activity in the form of increased ionization created by
ultraviolet and x-ray radiation, can fill in the E Valley-F layer ducting
region with medium frequency absorptive ionization and interfere with the E
Valley-F layer ducting mechanism. In a sense the E/F layer duct is shut down
and the medium frequency RF signal can only propagate between the E layer
and land/ocean surface, at a higher angle and with more signal loss. This
closing of the duct can be reciprocal on each end of the propagation path or
one way only. (((((When closing of the duct occurs the advantage of a low
angle vertical radiator is lost, with a higher takeoff angle horizontal
dipole making the contact still possible, albeit maybe weaker.)))))

73,
T. F. Giella, KN4LF
Plant City, FL

KN4LF 160 Meter Amateur Radio Resources And More:
http://www.kn4lf.com

KN4LF 7 Day Medium Wave Radio Propagation Outlook:
http://www.kn4lf.com/kn4lf5.htm

Florida Meteorological & Climatic Institute:
http://64.176.157.9/sub/fmci.htm




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