Topband: Ground conductivity, permittivity measurement

[Richard Fry]

>Conclusion:  The less ground conductivity the higher is the antenna 
>elevation radiation angle. This is a negative impact for DX!

Cris, Tom, Paul et al

This belief is common when looking at the far-field elevation pattern of a 
vertical monopole in MoM results, or in antenna textbooks.  That pattern is 
what remains of the radiated field at an infinite distance from the 
monopole, over an infinite, flat ground plane.

--> But in reality all vertical monopoles of 5/8-wavelength and less radiate 
their maximum relative field (E/Emax) in the horizontal plane.<--

A NEC near-field evaluation can show the field produced in and near the 
horizontal plane closer to the radiator, for earth of defined 
conductivity/permittivity, and it will not be zero as is shown in a NEC 
far-field plot.

The NEC study at the link below illustrates this.  The groundwave field 
(0-deg elevation) is plotted out to 8 km.  Note the good correlation between 
the NEC GW field and the GW field measured by a broadcast consulting 
engineer using an accurately-calibrated field intensity meter.

On that same chart is plotted the field existing from that radiator at an 
elevation of 100 meters above the earth.  Note that it is lower near the 
radiator than the GW field, because the relative field radiated by a 
1/4-wave monopole at higher elevation angles is less than in the horizontal 
plane.  In fact at the zenith it will be zero.  At ~8km downrange it has 
reached the value of the GW field, and further downrange it will exceed the 
GW field.

>From the NEC chart it can be seen that the field at low vertical angles 
(less than 3 degrees) is at least as great as it is at zero degrees.  There 
is no physical reason for that low-angle radiation NOT to continue on to the 
ionosphere to produce a skywave signal, given the right conditions.

http://i62.photobucket.com/albums/h85/rfry-100/Measured_vs_NEC2D_Fields2.jpg

Supporting this below is a clip from the Radio Engineers' Handbook (Terman, 
1943) showing this same reality, in that the angle at which radiation leaves 
that antenna for greatest 1-hop skywave range is less than 3 degrees.

http://i62.photobucket.com/albums/h85/rfry-100/TermanFig55.jpg

Probably the description "takeoff angle" commonly applied to vertical 
monopoles is a rather misleading specification.

R. Fry