Elevation Patterns of Ground Mounted Vertical Monopoles

[fmradio]

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Possibly of interest to some readers of this Forum, here is part of a post I wrote for an antenna newsgroup for amateur radio operators ...

Ground-mounted verticals up to 5/8-wave high used by ham operators have the same elevation pattern shapes as those used by broadcast stations. The peak radiation launched from all of them occurs in the horizontal plane, and reduces slowly and smoothly for lower elevation angles above the horizontal plane. It could well be that the DX you do work results from radiation at a much lower elevation angle than believed possible when looking at the usual NEC calculations and plots for that vertical antenna.

The first link below leads to a scan of a graphic from Section 10 of Terman's Radio Engineers' Handbook (1943). It shows the "takeoff angles" needed to serve various distances from a ground-mounted, vertical monopole radiator via its skywave, and the resulting skywave fields there for the conditions stated. The reflection coefficients apply to the E-layer.

Terman's work shows that the elevation pattern of such a radiator over lossy earth does not approach zero field near the horizontal plane -- as is a common interpretation when looking at their NEC evaluations.

Terman's text (p. 743) also states that the reduction in skywave field after a peak at ~130 miles results from the ERP at the elevation angles serving those ranges not compensating for the greater losses of those longer paths.

But the skywave fields at 1000+ miles with takeoff angles of 1 degree and less are far from approaching zero (no matter what we think NEC is telling us).

http://i62.photobucket.com/albums/h85/rfry-100/TermanFig55.jpg

Discussion invited...

RF

http://rfry.org

 

[VEBill]

On Figure 55 as referenced, what is the meaning of the horizontal axis for the 'Angle' curve - is that angle vs. distance?

 

[fmradio]

Yes, it is.

 

[VEBill]

Why is radiation angle a function of distance?

(Doesn't seem to make sense on its face.)

 

[fmradio]

It is _distance_ that is a function of radiation angle (for a single hop of skywave).

 

[VEBill]

Isn't it more usual to plot the independent variable (angle in this case) on the X-axis?

 

[fmradio]

This was Terman's method to show the data in a manner he wanted.

 

[VEBill]

Getting back to the primary point: I suspect that there isn't as much of a gap between Terman and NEC as there might sometimes appear. If there are differences in the output, it almost certainly comes down to different assumptions on the input. I suspect that Terman would have happily embraced NEC if he'd been born just a bit later.

 

[fmradio]

"Getting back to the primary point: I suspect that there isn't as much of a gap between Terman and NEC as there might sometimes appear."
_________

Agree. NEC will give ~ the same output result as developed by Terman and other early authors as long as its operation is understood, and it is used correctly for the model circumstances.

But a common belief when looking at the NEC far-field elevation pattern of a ground-mounted vertical monopole up to 5/8-wave high over real ground is that it radiates zero field in the horizontal plane. But this is untrue. If it was, then MW broadcast stations would have no daytime groundwave coverage, yet that field is the source of most of their income.

The error isn't the fault of NEC, but the operator.

 

[VEBill]

Aren't most MW (AM broadcast) verticals typically installed over a field of quarterwave radials? I've read that any more than Qty 120 is just a waste...

 

[fmradio]

Yes, they are. A benchmark, 1937 IRE paper by Brown, Lewis & Epstein of RCA Labs first investigated the effectiveness of such buried ground systems using radials of various numbers and lengths. They found that a vertical monopole using about 120 radials each 0.41-lambda in length radiates an h-plane field at one mile that was within a few percent of the theoretical maximum for the applied power and antenna configuration, no matter what the frequency and earth conductivity at the antenna site.

More and longer radials weren't worth the investment for the small improvement in radiated fields that they could provide.

BL&E's findings were the basis for the FCC rules applying to AM broadcast station antenna requirements to the present day.

 

[VEBill]

Then the typical MW (AM Broadcast) antenna is installed over 'real GOOD ground', as opposed to 'real (lossy) ground'. Thus we can't really use them as an example to support any conclusions about the accuracy of NEC over 'real ground'.

I think I read somewhere that one of the advantages of NEC4 over NEC2 is the accuracy of the ground modeling. This seems to confirm that the ground modeling in NEC2 isn't 'The Last Word' on what must be a very complicated subject.

Then again, I'm not sure that a guy with a slide rule in the 1930s could have done much better either, no matter how smart he was.

 

[fmradio]

"Then again, I'm not sure that a guy with a slide rule in the 1930s could have done much better either, no matter how smart he was."
__________

BL&E's investigations were not a slide rule study, they used real hardware in the real world. And THAT is why their conclusions have proven valid in practice, ever since (some NEC conclusions notwithstanding).

 

[VEBill]

I meant Terman.

(Sorry, I wasn't specific.)