Richard Fry wrote:
"Roy Lewallen" wrote
With EZNEC, you have to use the near field analysis to include
the ground wave; direct ground wave analysis isn't included in
EZNEC because (etc).
_________

After the comments of Richard Clark and you, Roy, I attempted to use
EZNEC to determine the ground wave (see link below).

The near-field analysis of EZNEC for radiation in the horizontal plane
at a point 1 km from a 1/4-wave monopole having two ohms in series
with a Mininec r-f ground, while radiating 1 kW over an earth
conductivity of 8 mS/m is shown as 72 mV/m.

The same setup when analyzed using the FCC's radiation efficiency for
this monopole height, and their propagation charts for these
conditions shows about 295 mV/m as the result, which value is
supported by the measured performance of real-world AM broadcast
stations, and is also a value in a range that could be expected from
the BL&E data.

Hopefully you or Richard Clark can tell me the reason(s) for this
difference, which could easily be my own setup of the NEC model.

Roy, would you mind posting the ground wave value EZNEC Pro reports
for these conditions?

http://i62.photobucket.com/albums/h8...FldExample.gif

RF

My model has 120 0.5 wavelength radials buried 1.2 feet deep (the
unusual depth due to rescaling another model). Ground conductivity 8
mS/m, dielectric constant 13. The antenna is 0.25 wavelength high. The
whole structure is made from #12 wire to eliminate any problems due to
dissimilar diameters. Field strength is Ez at 1000 meters with 1000
watts applied.

Using the NEC-4D calculating engine, EZNEC Pro/4 shows (NF = near field
analysis, GW = far field analysis with ground wave):

Z = 40.08 + j27.91
GW = 297.7 mV/m
NF = 297.7 mV/m

Same, but with 0.25 wavelength radials:

Z = 39.56 + j26.55
GW = 292.7 mV/m
NF = 292.7 mV/m

Note that the feedpoint R and field strength don't exactly correlate if
you make the assumption that the resistance difference is due to loss.
This would be due to a slightly different current distribution on the
radiator due to interaction with the different ground fields. Other
experiments have shown that the impedance will also vary some with
radial burial depth.

Following are the results using the NEC-2D engine with a 0.25 wavelength
vertical and 120 0.5 wavelength radials one foot above the ground, all
other conditions otherwise the same. This analysis can be run with
EZNEC+, but only the NF results will be available:

Z = 66.83 + j1.894
GW = 230.0
NF = 229.7

As above, but 0.25 wavelength radials:

Z = 32.42 + j18.87
GW = 311.4
NF = 311.4

Elevated radials, even when elevated only this amount, show distinct
resonance effects, and making them longer than about 0.25 wavelength
often results in reduced efficiency which I think is due to movement of
the radial current maxima away from the center. The above results
illustrate these phenomena. While slightly elevated radials can be used
to approximate buried ones, as you can see the substitution isn't perfect.

The same 0.25 wavelength vertical over perfectly conducting (or MININEC)
ground showed a Z of 37.95 + j21.49 ohms. However, the resistances of
the various examples above aren't just this resistance plus loss
resistance, since the current distribution isn't quite the same when
radials are present.

The results you got weren't valid due to use of MININEC ground with near
field analysis, as I explained in another posting. As you can see, you
can get reasonably good results using EZNEC+ and near field analysis,
although the vast majority of people this intensely interested in the
mechanisms of AM broadcasting aren't hobbyists but rather professional
engineers who are using EZNEC Pro/4.

Roy Lewallen, W7EL