Takeoff Angle of Vertical Monopole

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R. Fry
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Takeoff Angle of Vertical Monopole

Post by R. Fry » Wed Sep 04, 2013 6:16 am

Below is a post I made on a "ham" board that might be interesting here -- although I don't think that many readers of this board believe the way a lot of hams without a broadcast background do about the subject.
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That a vertical monopole 5/8 wavelength and less in height, using a less than perfect ground plane has a certain takeoff angle above the horizon where radiated field is maximum is a very common (almost universal) belief of ham radio operators, based on their use of MoM software such as NEC.

The graphic below shows a different conclusion when considering the surface wave in such NEC evaluations, for the parameters shown.

The NEC far-field pattern for 0.1 km shows a maximum field intensity of 590 mV/m at an elevation angle of 23 degrees (the assumed "takeoff angle"). It also shows that the field at an elevation angle of 5 degrees is 348 mV/m.

The NEC surface-wave pattern for 0.1 km shows that the maximum field lies in the horizontal plane rather than at 23 degrees, and is about 890 mV/m rather than 590 mV/m.

The surface wave analysis also shows that the field radiated toward 5 degrees elevation is about 850 mV/m, rather than the 348 mV/m shown by the far-field analysis. Of course, the ratios of these fields are even greater for elevation angles below 5 degrees, and infinite in the horizontal plane.

But it is the radiation from these low elevation angles that can provide the greatest single-hop range for skywave service, even though its existence might be disregarded by some.

Discussion/comments welcome.

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Dave Loudin
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Location: King George, VA

Re: Takeoff Angle of Vertical Monopole

Post by Dave Loudin » Wed Sep 04, 2013 10:37 pm

We struggled with this at VOA when trying to model a nominal whip antenna located at a random height for reception. Obviously, the result was a statistic, not an actual measured pattern, but the it was consistent with what you are showing. We needed some rigor in order to properly use the propagation model IONCAP, as it gives statistics for each propagation mode possible in the circuit. Bias of low-angle response (positive or negative) could tip the scales to favor the wrong propagation mode (usually 1-hop vice 2-hop), and that could lead to selecting a frequency with as low as 10% reliability instead of the desired 90%.
What's that do....?

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