Topband: Skywaves from Monopole Surface Waves

[Michael Tope]

Hi Dick,

I get that at any point in the far field there is RF current in the 
ground due to the space wave from the transmitter reflecting obliquely 
off ground. I was, however, under the impression that at very low angles 
the field from this reflection cancelled direct radiation for earth of 
finite conductivity. I think you can see this in the Distance=20 mile 
E-field plot you provided for the 1.83 MHz radiator. In that plot there 
is an inflection in the E-field versus height very close to the horizon. 
If you ignore that inflection and extrapolate a smooth curve from the 
E-field at higher elevation angles down to zero elevation angle you can 
see that the field would go to zero. I believe that this extrapolated 
curve is the component of the E-field attributable to the space wave and 
that E-field in the small segment of the original curve below the 
inflection point is the E-field attributable to the surface wave which 
arrived at that point as a "guided-wave" hugging the surface of the ground.

73, Mike W4EF

On 10/4/2012 2:54 AM, Richard Fry wrote:
> Guy Olinger wrote:
>> But to prove it we can't use standing-man-with-meter. He may just be 
>> sensing the current in the ground just below his feet that will never 
>> be airborne. We need sitting-man-with-meter-in-helicopter to go up 
>> there and prove that what you get from the ground up to twenty 
>> thousand feet out 20 miles is a blend, and not a notch.
>
> The r-f current flowing in real earth at a given distance beyond a few 
> wavelengths from a vertical monopole did not travel through the earth 
> from the monopole site to get there.  It is the result of the EM wave 
> radiated by the monopole through space, at the earth boundary of that 
> wave.
>
> Here is Terman's description: "The electric vector of the wave 
> produced at the surface of the earth by a vertical antenna possesses a 
> slight forward tilt.  As a result the electric vector has, in addition 
> to the normal vertical component a slight horizontal component 
> parallel to the surface of the earth, and lying in the direction of 
> propagation. ... The component of the electric vector lying parallel 
> to the earth's surface that is present because of wave tilt represents 
> a voltage applied across earth's surface. This  voltage causes a 
> current to penetrate down into the earth."
>
> So those r-f currrents in the earth don't ORIGINATE radiation into 
> space, rather they are a consequence of it.
>
> As for helicopter measurements, below is a clip from a website used 
> mostly by broadcast engineers, containing part of a post I made and a 
> response to it from an active, full-time broadcast engineering 
> consultant.  The consultant's comment should be enlightening.  Not 
> only does he recognize the validity of my NEC analysis, he reports his 
> experience with helicopter measurements confirmiing what is shown in 
> that NEC analysis.
>
> CLIP:
>
> The link below was prepared using values generated by 4nec2, but 
> plotted in
> other software. It shows that even NEC2 can be used to accurately 
> calculate
> low-angle fields radiated by a monopole over real earth -- and that 
> the NEC
> fields are very close to the accurately measured fields for those 
> conditions
> shown in the plot at the bottom of that page.
>
> The NEC plot also shows that very significant space wave (skywave) is
> generated by radiation from a monopole from elevation angles down to 
> about
> one degree, where a NEC far-field plot shows very little relative field.
>
> http://i62.photobucket.com/albums/h85/rfry-100/Measured_vs_NEC2D_Fields2.jpg 
>
>
> R. Fry
> ______________
>
> Re: Monopole Radiation Patterns
>
> Post by XXXXX » Thu May 17, 2012 7:16 pm
> Interesting. As a side note I learned early on the hard way that when 
> taking FI readings in a helicopter to do the vertical calibration at 
> least a couple KM away from the antenna, otherwise all you're 
> measuring is the vertical plane radiation pattern and you end up 
> somewhere on the left end of the green line on the graph and screw 
> everything else up.
> _______________________________________________
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>
>