Topband: More on 160 dipoles

[Robert Brown]

Friends in Radio Land -
 
Power coupling is greatest when the E-field from an antenna is
parallel to the geomagnetic field and the least when the two are
perpendicular to each other.  For the parallel case, the E-field
drives ionospheric electrons in the direction of the magnetic
field.  But there is no magnetic force on the moving electron
in that case and the RF field is fully effective in causing the
electron to re-radiate RF energy.  For the perpendicular case, the
full magnetic field strength is effective in resisting the
electron being driven by the RF field and the coupling is least.
 
A favorable example of power coupling would be propagation in all
directions from a vertical antenna at one of the magnetic poles
where the magnetic field is vertical.  Least favorable would be
propagation in all directions from horizontal antennas at one of
the magnetic poles.  Another favorable case would be propagation
in the geomagnetic E-W direction from a horizontal antenna
oriented in the N-S magnetic direction at the geomagnetic dip
equator, where the field is horizontal; an unfavorable case would
be propagation in the geomagnetic E-W direction from a vertical
antenna at the geomagnetic dip equator.
 
Numerically, power coupling is calculated by finding the degree
to which the radiation field at a given radiation angle has an
E-field component parallel to the magnetic field.  It involves
the elliptically polarized waves propagated in the field and the
geometry of the waves relative to the antenna.  In short, the
numerical factor by which power coupling affects the signals
entering or leaving the lower ionosphere is largely geometrical in
origin.
 
Of course, there is an infinity of antenna situations and a
similar degree of multiplicity when it comes to magnetic field
inclinations and radiation angles.  By way of illustration, the
vertical variation of signal intensity from a simple vertical
antenna over a perfect ground plane and power coupling was given
earlier for propagation of signals to the north and south of
Omaha, NE, where the field points 4.7 degrees east, is at 20.4
degrees with the zenith and directed down into the ground,
 
With the field inclined equatorward from the vertical direction,
the discussion above leads one to expect the most favorable power
coupling for signals propagated to the north and at a radiation
angle that puts the signal's vertical E-field parallel to the
inclined magnetic field.  Considering power coupling to be like an
insertion loss between the lower ionosphere and the antenna on
the ground, the results of calculations for the vertical antenna
are given in logarmthic notation:
 
  Rad. Angle       Coupling         Coupling
    (deg)         (to North)       (to South)
 
      3             -0.2             -1.2
      6             -0.1             -1.3
      9             -0.1             -1.4
     12             -0.1             -1.5
     15             -0.1             -1.6
     18             -0.1             -1.8
     21             -0.1             -1.9
     24             -0.1             -2.0
     27             -0.1             -2.1
     30             -0.2             -2.2
     33             -0.4             -2.3
     36             -0.5             -2.4
     39             -0.6             -2.5
     42             -0.8             -2.6
     45             -0.9             -2.7
 
It is seen that the insertion loss from power coupling is least
to the north at about 18-20 degrees elevation, close to the
inclination of the magnetic field.  To the south, the insertion
loss increases as the field to the south is tilted away from the
wavefront in the direction of propagation.
 
Replacing the vertical antenna with a horizontal dipole results in
an unfavorable propagation situation, at least for those northern
directions where the horizontal E-field in the wavefront from the
antenna is perpendicular to the magnetic field lines.  That is
shown in the table below for power coupling losses for different
orientations of a dipole:
 
   Rad. Angle       N-Loss        E-W Loss         S-Loss
     (degs)          (dB)           (dB)            (dB)
 
       3            -14.2           -8.2            -6.0
       6            -15.3           -8.0            -5.8
       9            -17.2           -8.0            -5.5
      12            -20.4           -7.4            -5.0
      15            -27.0           -7.8            -5.0
      18            -25.0           -6.6            -4.8
      21            -24.0           -6.2            -4.5
      24            -18.2           -5.8            -4.3
      27            -15.1           -5.4            -4.1
      30            -12.8           -5.1            -3.9
      33            -11.1           -4.8            -3.8
      36             -9.7           -4.5            -3.6
      39             -8.7           -4.3            -3.5
      42             -7.8           -4.1            -3.4
      45            - 7.1           -3.8            -3.3
 
Here, it is seen the greatest insertion loss for a horizontal
dipole is to the north and in the angular range which is most
favorable for vertical antennas.
 
In considering these features of power coupling, it should be
borne in mind that the results only apply to the first and last
transits of the lower ionosphere.  There are other aspects of
power coupling along a path but those involve changes in wave
polarization on ground reflection, a very involved process.
 
In any event, the power coupling values given here may be added to
other features of 160 meter antenna systems to evaluate their
potential performance, as suggested recently in a posting by W4ZV.
 
73,
 
Bob, NM7M