Using NEC (M-o-M) Software to Calculate the Radiated Fields of a Vertical Monopole

Amateur radio operators tend to use the NEC-calculated far-field analysis almost exclusively. However the far field as defined in antenna engineering textbooks begins at a distance of about 2L²/λ meters [L = Radiator Length(Height), in meters; λ = wavelength, in meters].

Note that at the distance of 1 kilometer specified for the e-fields of the NEC calculations shown below, both the far-field and the surface-wave values lie in far-field of that radiator.

Using only the NEC far-field pattern to analyze the performance of a vertical monopole antenna system driven against a lossy ground plane can lead to a belief that it has zero radiation in the horizontal plane, and not much more than zero at very low elevation angles.

However that is not always the case, when NEC analyzes the surface wave at ranges within distances of 30 miles or even more from the antenna site (depending on radiated power, frequency, ground plane conductivity etc).

 

What is a "surface wave"? Why should I be interested in it?

 

First below is a clip defining the surface and space waves taken from Radio Engineers' Handbook by F. E Terman.

Because the fields radiated by vertical monopoles toward very low (non-zero) elevation angles in reality are space waves. Space waves decay at an inverse distance (1/r) rate, whereas surface waves decay at a rate greater than 1/r, because of the effects of the lossy ground plane (Earth).

But as noted above by Terman, the space wave quickly dominates the radiated fields of a vertical monopole for non-zero elevation angles.

Below is a NEC graphic showing that the field radiated by a vertical monopole along a 5° departure angle decays at a 1/r rate. Therefore that field has the properties of a space wave. Given the needed conditions it can propagate to the ionosphere while decaying at the 1/r rate, and reflect from the ionosphere at a 1/r rate to return to Earth as a useful skywave.

That is why amateurs might want to take an interest in evaluating the "surface wave" fields calculated by NEC, and other M-o-M software.

 

Quite recently I built a vertical dipole for 20 m band. I located it on the balcony railing 4 m a.g.l.

I compared its performance with the performance of my old multiband commercial GP standing some 16 meters away on a 5 m high mast. The active part of the GP on 20 m band is 6.1 m long. There are 7 counterpoises (1.5 m long) at the bottom and a capacitance hat made of 4 spokes 1.26 m long that terminates the GP for the 20 m band. The active part of this GP is longer than a quarter of a WL and I guess people call such antennas 3/8 lambda GPs.

I modeled the vertical dipole and the GP and got very similar gain plots. So, I did not expect big difference in signal strength tests. And indeed, both reception tests as well as transmission tests (for which I used the websdr receievers located in USA, Russia and Europe) showed that both antennas are almost identical in gain.

However, when I arranged a test with a fellow ham living 7.4 km away from my QTH, he reported 1.5 S unit difference. The vertical dipole was 9 dB stronger than the GP! My friend's GP was located 85 meters higher than my both antennas, so the elevation angle at which my antennas saw his was only 0.65 degree. Evidently, for such low angles, the surface wave from the vertical dipole was much stronger than that from the GP. I really can not explain such difference in any other way but by the impact of the surface wave.

 

But yet, there was no difference over paths of a few hundred to thousands of km...

So, I ask again, why should hams care about a surface wave on HF bands? If you want to talk to someone a few km away, use VHF.

 

My posts in this thread are not pitched at using HF to talk to someone a few km away, but at understanding and utilizing the skywave (DX) signals that result from the low-angle radiation of vertical monopoles.

 

Well, perhaps a reason to care for a surface wave on HF could be a likelyhood of drawing a wrong conclusion when testing antennas.
Imagine I had a vertical dipole for 10 m band and a GP for that band. Because propagation is hopeless on this band nowadays, I would only test the two antennas against a local station. My conclusion would be: "something is terribly wrong with the GP" or "my vertical dipole has an extraordinary gain".
It is good to remember that the surface wave can significantly distort antenna comparison.

 

Agreed. However, the question that I am not getting an answer to is: "does an HF antenna's surface wave say anything about how useful that antenna will be for ionospheric DX communication?"

I see nothing in Richard's postings that begin to answer that question. Looking at Terman's definitions and figure 1, it is the space wave that matters to hams, and it is the space wave that MOM calculates and displays as a function of elevation, based on the properties of the reflecting earth beyond the ends of the radials.

Furthermore, the space wave at low angles is reduced if the earth beyond the ends of the radials is less than ideal, which Richard seems reluctant to admit.

 

If that appears to be true to anyone, it can only be traced to a lack of understanding of the information I posted in this thread.

Below is a clip from one of the NEC graphics I posted here earlier, but this time with large annotations added to aid in a correct understanding of what the graphic shows.

It does NOT show that the surface wave "morphs" into a space wave. A vertical monopole radiates (launches) such fields at different elevation angles.

At some horizontal distance away from that radiator, the radiation toward 5 degrees elevation will start to decay at a fixed 1/r rate as the wavefront sufficiently clears Earth's surface. That 1/r radiation will reach the ionosphere, and under the right conditions be reflected back to the Earth as a skywave.

The reduction of a 1/r space wave field with distance beyond the lengths of the buried radials of a vertical monopole is always (and only) a function of the change in path length along that elevation angle. It has nothing to do with Earth conductivity and its propagation loss for the surface wave from that radiator.

 

true, but only for antennas that can put energy into the surface wave modes of the air-earth system. Typical broadcasting and ham radio antennas can’t do that (more precisely, they can’t do that to an actually measurable, either in the real world or in the simulated world, extent).