Monopole Low-Angle Radiation 1

For consideration / comment...

Radiation launched by vertical monopoles of 5/8-wavelength and less in height always is maximum in the horizontal plane -- regardless of earth conductivity and the number/length of buried radials they use.

This point is illustrated in the NEC4 analysis shown/linked below, comparing low-angle radiation for sea water, average, and poor earth conductivity. The shapes of these patterns (their relative fields) are essentially the same, even though the lower conductivities produce lower fields.

The fields present at these elevation angles below ~ 26 degrees can continue on to reach the ionosphere, and given the right conditions return to the earth as skywaves. Radiation from the lowest angles provides the greatest single-hop skywave range.

The belief that monopoles need a near-perfect ground plane to radiate high relative fields at low elevation angles is a common interpretation when considering only the far-field patterns shown in NEC and in antenna textbooks as the shape of the fields actually leaving the monopole. But they are not -- they are only what remains of those fields at an infinite distance over an infinite, flat ground plane.

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

 

[VEBill]

On my phone, the linked image is presented at very low resolution, and it's difficult to read the text. Does the header at the top indicate that your analysis is of a monopole over a ground plane that includes 120 radials?

 

[fmradio]

Yes. All three plots were made using 120 x 1/4-wave buried radials, and the same Z-matched power (100 watts) at the feedpoint terminals.

Using a web browser on a PC or Mac to open the URL in my original post will show the graphic with better resolution.

 

[VEBill]

"The belief that monopoles need a near-perfect ground plane..."

What point are you trying to make? 120 radials is pretty near-perfect.

 

[fmradio]

The point that fields produced/launched over a real earth path from monopoles of up to 5/8-wave height are not really zero in the horizontal plane, and not much more than zero at low angles above it -- as shown in NEC far-field elevation patterns.

If they were, then AM broadcast band stations would have ~ zero groundwave coverage areas, and no possibility of daytime listeners.

 

[VEBill]

Perhaps I missed something in my understanding to date.

I thought that the oft-repeated point being made was that one should install a good ground plane (as you have included). If you omit the ground radials, then you would expect to find a hole in the coverage at very low angles (depending on the ground conditions). By including the 120 ground radials, the monopole (with its reflection in the near-perfect ground plane) becomes equivalent to a dipole.

Elsewhere, the 120 count of radials was demonstrated to be sufficient.

Another real world option is to raise the monopole's feed point above the ground (on a mast), then the number of radials can be decreased (e.g. 4). Drooping the ground radials is another real world trick.

Last time that I was paying any attention to this sort of topic (2001 ?), there was an active discussion about relative field strengths from the monopole over a perfect infinite ground plane as compares to the equivalent dipole. There was a reported 2:1 discrepancy in the calculated field strengths. I thought perhaps thus might be due to closing off half the Universe space with the infinite ground plane. It seemed to be a foolish discrepancy report this late in the game. I'm not sure what happened after that.

It's been my understanding that with a very good ground plane (120 radials), one would expect coverage right down to zero degrees elevation. Perhaps I missed that there was a NEC caused concern. I've not used NEC2 very much, and NEC4 not at all.

 

[fmradio]

VE1BLL wrote: "It's been my understanding that with a very good ground plane (120 radials), one would expect coverage right down to zero degrees elevation."
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Yes, that is true. But it is also true for monopoles using short/sparse buried radials, or just a few ground rods.

The link below compares the low-angle fields produced by a 1/4-wave monopole using good and poor ground systems, when driven with 100 watts on 1.85 MHz. This 10 km path length has an earth conductivity of 5 mS/m, d.c. 13.

Note in both cases that these relative fields are maximum in the horizontal plane, even though earth conductivity is far less than perfect.

Of course the system with a good set of buried radials produces higher fields at all elevation angles than the one with a poor set.

This performance is not shown in a NEC far-field pattern. A more complete NEC analysis is required.

http://s15.postimg.org/plohwmwtn/Compare_Good_Poor_Radial_Systems.jpg

 

[VEBill]

"...even though earth conductivity is far less than perfect."

Because of my line of work (aircraft comms), I never fall into the trap of consciously or unconsciously assuming that ground or earth is somehow necessary for efficient radio communications (or, for that matter, effective lightning protection).

A monopole installed on a good (local) ground plane (e.g. most VHF to L-band aircraft antennas) radiate just fine at zero degrees elevation, even when the nearest earth (of whatever conductivity) might be 35,000 feet below.

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The 30 ohms case in your latest link; I suspect that's just a virtual series R in the feed point (within the simulation). In a sense, probably an uninteresting over-simplification (by the purveyors of the software). Conservation of energy indicates that the RF power has to go somewhere. It's either field strength somewhere, or heat.

Few radials don't necessarily have to be lossy; elevated ground plane antennas (with just 3 or 4 radials) could easily be 95%+ efficient.

 

[fmradio]

VE1BLL wrote: "The 30 ohms case in your latest link; I suspect that's just a virtual series R in the feed point (within the simulation)."
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NEC4 calculates the r-f loss resistance in whatever set of buried wires is defined for a monopole antenna in the model. That loss depends on their length, number, geometric arrangement, the earth conductivity defined, and the frequency. NEC4 does not simulate that loss by inserting a virtual series resistance at the feedpoint.

The link below compares the low-angle radiation performance of a 1/4-wave monopole 10 km from its base for a perfect ground and a 5 mS/m dc 13 ground. In both cases the model used a single, 3-meter vertical ground rod buried directly below the base of the monopole. The only change made to the model for this comparison was to change the conductivity of the ground plane in use.

Note that the ~ 10 mV/m field at zero degrees elevation at 10 km when using a perfect ground plane is as predicted by theory when 100 watts is radiated by a 1/4-wave monopole.

Once again the results show that low-angle fields launched by monopoles over real earth paths are maximum in the horizontal plane.

Understand about airborne monopoles at VHF and above using the fuselage of the aircraft as a counterpoise. Also agree that 4 horizontal, 1/4-wave wires at 90-deg intervals elevated a few meters above the earth are the equivalent of 120 x 1/4-wave wires buried in the earth.

 

[VEBill]

Just to confirm: What's the Y axis (0.0 to 50 m) on this latest plot? Is it height above ground in m at the 10km point?

 

[fmradio]

Yes, the Y axis is the height above ground in meters, 10 km downrange (for a flat ground plane).

 

[VEBill]

So that's a very very tiny slice of the overall elevation pattern (50m high at 10km range). If you've not already done so, you should zoom out to examine the larger picture that encompasses the entire elevation range (0-90°).

 

[fmradio]

That last NEC4 graphic was posted to show that fields from a monopole are not zero in the horizontal plane even at 10 km downrange over a real earth path, as some expect by looking only at a NEC far-field analysis.

For a NEC4 analysis and discussion showing elevation angles and skywave generation potential to about +27 degrees please refer to my opening post in this thread. Those relative fields also are maximum in the horizontal plane.

Radiation toward the ionosphere is a function of the fields/angles existing just beyond the near-field radius of the monopole antenna system, rather than from some downrange location.

 

[VEBill]

Yes, the first graph is at 0.1km range and thus shows a good range of elevation angles.

"...zero in the horizontal plane..., as some expect..."

I've not met anyone that actually believed what is so obviously untrue, based on the real world fact that radio communications do actually work (as you pointed out).

None of my reference books show zero in the horizontal plane for such cases. The editors know that isn't true.

In a sense, you're preaching to the converted with this limited audience. That's why I had difficulty initially understanding your point.

As you must know, NEC4 is (still?) less widely available than NEC2 engines. The limitations of NEC2 w.r.t. imperfect ground planes is well known; I suppose that's why NEC4 was written. I had assumed that it was well known that NEC2 users need to be cautious when applying NEC2 to cases with complex ground conditions.

The same caution should be applied to NEC4 results; e.g. the real world is not flat, real world ground conditions are vastly more complex than can be entered into NEC4, etc.

Over the years we've done endless antenna (installed) modelling, using various tools - including GTD/UTD due to the relative scale and application. Our approach is very cautious; we never implicitly trust the results, we cross check, we dither the input assumptions to reveal inexplicable discontinuities, we may even integrate over the sphere to confirm things add up to unity.

 

[fmradio]

VE1BLL wrote (in part): "The limitations of NEC2 w.r.t. imperfect ground planes is well known; ... I had assumed that it was well known that NEC2 users need to be cautious when applying NEC2 to cases with complex ground conditions. The same caution should be applied to NEC4 results; e.g. the real world is not flat, real world ground conditions are vastly more complex than can be entered into NEC4" etc.
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Thanks for your replies.

The groundwave fields calculated even by a suitable NEC2 model of a real-world, AM broadcast monopole system using buried radials closely can duplicate the fields accurately measured for that real-world system by a professional broadcast engineering firm using a calibrated FI meter (link below).

My NEC2D calculations are shown at the top of that graphic, and are based on the physical conditions generating the real-world, measured results shown at the bottom of that graphic.

Details about those real-world measurements/conditions are available on further request.

Note that the path length in the link below, and the path lengths in my NEC4 graphics posted earlier in this thread are short enough so that they do not depart significantly from a flat surface.

 

[fmradio]

On 25 Aug 2013 VE1BLL posted:
"...zero in the horizontal plane..., as some expect..."

I've not met anyone that actually believed what is so obviously untrue, based on the real world fact that radio communications do actually work (as you pointed out).
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This is a very common (almost universal) belief of ham radio operators, based on their use of MoM software such as NEC to evaluate the elevation patterns of monopoles using less than perfect ground planes. This leads to a further belief that the maximum gain/field of these systems is centered at some "takeoff angle" above the horizon.

The link below shows a different conclusion when including 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^o (the assumed "takeoff angle"). It also shows that the field at an elevation angle of 5^o is 348 mV/m.

The surface-wave pattern for 0.1 km shows that the maximum field lies in the horizontal plane rather than at 23^o, and is about 900 mV/m rather than 590 mV/m.

The surface wave analysis also shows that the field radiated toward 5^o 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^o, and infinite in the horizontal plane.

Hopefully this illustrates the basis for my earlier comments in this thread.