Topband: Ground conductivity, permittivity measurement

[Paul Christensen]

To Rich's point, I created a buried field in 4NEC2 using the radial geometry 
wizard and a vertical wire radiator 40m tall, all wires being of #12.  The 
radial field consists of 64, 0.5 wavelength radials, buried 0.1m deep in 
moderate soil conductivity of 3 mS/m.  As a bit of a sanity check, the base 
impedance computes to 40.2-j18.9.  Rich can keep me straight on my 
assumptions, but I believe they're valid.

http://72.52.250.47/images/160m.jpg

Using NEC version 4.2, surface wave field strength is calculated at 1km as a 
function of varying Z value from 0-500m with 1500W from the source at 1.8 
MHz.  Notice how the field strength at zero elevation does not produce a 
null as we typically see in far-field calculations over average soil.  So, 
without knowing the surface wave component, it would appear the traditional 
far-field plots only tell half the story about low-angle radiation from 
base-fed monopoles.

My NEC input file is available to anyone who wants to see it.  Before 
asking, please ensure your modeling program runs under NEC 4 and not NEC 2.

Paul, W9AC


----- Original Message ----- 
From: "Richard Fry" <rfry at adams.net>
To: <topband at contesting.com>
Sent: Tuesday, October 02, 2012 2:58 PM
Subject: Re: Topband: Ground conductivity, permittivity measurement


> >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
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK