Low-angle Elevation Gain of a 1/4-wave Vertical Monopole
 

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

Low-angle Elevation Gain of a 1/4-wave Vertical Monopole
 

On Nov 29, 1:10*pm, Roy Lewallen wrote:

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.
________

Thanks very much for your numbers and comments, Roy.

I would never have thought to try to use EZNEC near-field analysis to
compute the groundwave if I hadn't read the suggestion to do so in
this thread. That was my first, and will be my last attempt at that.

When I need to calculate the MW ground wave for a particular distance,
monopole height, frequency and ground conductivity I use the FCC
method of first determining the inverse distance field of the radiator
at 1 km for 1 kW of radiated power, and then using that value in a
program I have with the FCC's MW propagation curves in digitized form.

My point when starting this thread was to show that the elevation
pattern radiation actually launched by vertical monopoles on any
frequency does not have a zero/very low relative amplitude at/near the
horizontal plane, which from what I read on these NGs seems to be a
popular belief.

RF

Low-angle Elevation Gain of a 1/4-wave Vertical Monopole
 

Richard Fry wrote:
On Nov 29, 1:10 pm, Roy Lewallen wrote:

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.
________

Thanks very much for your numbers and comments, Roy.

I would never have thought to try to use EZNEC near-field analysis to
compute the groundwave if I hadn't read the suggestion to do so in
this thread. That was my first, and will be my last attempt at that.

I've suggested it to you on at least one of the several occasions you've
brought this subject up, in the thread "Rhombics" on Oct. 1, 2006.
I've also mentioned it at least 10 other times on this newsgroup going
back as far as 1998. Reg used to entertain himself by periodically
complaining about EZNEC's lack of ground wave analysis, and most of
those postings mentioning the near field technique were in response to
his postings. I see you've taken on that aspect of Reg's former source
of entertainment. You and Reg were just about the only hobbyists who
have this intense interest in EZNEC and ground wave analysis, and now
that Reg is gone it's pretty much down to you. Of course you could
directly get the results you want from NEC-2, which is free and readily
available. I assume the reason you don't simply do that is that it
wouldn't be as amusing.

When I need to calculate the MW ground wave for a particular distance,
monopole height, frequency and ground conductivity I use the FCC
method of first determining the inverse distance field of the radiator
at 1 km for 1 kW of radiated power, and then using that value in a
program I have with the FCC's MW propagation curves in digitized form.

Since you can use this method to get results you believe to be correct,
why do you need EZNEC? If you want another program to give you the same
answers, why not use NEC-2? NEC uses the same method as the one used to
generate the FCC's curves. But I believe the FCC curves account for
Earth curvature while NEC doesn't, so I'm told they begin deviating at
somewhere around a couple of hundred miles.

My point when starting this thread was to show that the elevation
pattern radiation actually launched by vertical monopoles on any
frequency does not have a zero/very low relative amplitude at/near the
horizontal plane, which from what I read on these NGs seems to be a
popular belief.

I don't believe I've ever read that. But if anyone does believe it, a
much larger number believe just about the opposite -- that the signal
strength from a vertical is maximum at zero elevation angle at great
distances from the antenna. This of course comes from the ubiquitous
plots of the pattern of a vertical over perfect ground.

Guess that's enough for now. Maybe you can go a little longer before
bringing it up again the next time? In the meantime, I suggest you
either update your v. 4.0 EZNEC demo program or replace it with v. 5.0.
The demo programs are still free.

Roy Lewallen, W7EL

Low-angle Elevation Gain of a 1/4-wave Vertical Monopole
 

On Nov 29, 4:51*pm, Roy Lewallen wrote:

Since you can use this method to get results you believe
to be correct, why do you need EZNEC?

Roy:

Don't _you_ believe that the results I posted using the FCC method I
described to be "correct?"

Regardless, and to answer your question -- I don't really need
EZNEC. But it can be interesting to see how various analytic methods
compare.

Quite a few years ago and after due investigation/consideration, I
paid about $300 for the NEC-2 products of one of EZNEC's competitors,
because I preferred its graphical output choices and print quality,
its higher segment limit compared to EZNEC+, and the customization it
allowed in its printed output legends. This capability included the
synthesis and import into the NEC model of any one of many dozens of 2-
D and 3-D structures, to their specific mechanical specifications
defined by the NEC user.

The main reason I use EZNEC occasionally is to investigate the claims
of others who use EZNEC.

Sorry to be blunt , Roy, but then you asked.

RF wrote:
My point when starting this thread was to show that the elevation
pattern radiation actually launched by vertical monopoles on any
frequency does not have a zero/very low relative amplitude at/near the
horizontal plane, which from what I read on these NGs seems to be a
popular belief.

Roy Lewallen responded:
I don't believe I've ever read that. But if anyone does believe it, a
much larger number believe just about the opposite -- that the signal
strength from a vertical is maximum at zero elevation angle at great
distances from the antenna.

?? My reading of these NGs shows that many/most amateur radio
operators ignore/discount the fact that the peak radiation launched by
a vertical monopole of 5/8 lambda or less in height ALWAYS occurs in
the horizontal plane., regardless of the operating frequency, or the r-
f ground in use.

The radiation/reception characteristics at low elevation angles of
such an antenna can be useful in establishing contacts with the most
distant possible single-hop DX sites, can they not?

RF

Low-angle Elevation Gain of a 1/4-wave Vertical Monopole
 

Richard Fry wrote:
. . .
?? My reading of these NGs shows that many/most amateur radio
operators ignore/discount the fact that the peak radiation launched by
a vertical monopole of 5/8 lambda or less in height ALWAYS occurs in
the horizontal plane., regardless of the operating frequency, or the r-
f ground in use.

I don't think most amateurs care about the locally launched radiation,
except when dealing with local RFI. That low angle radiation decays to
essentially nothing within a few miles at HF and even less at VHF and
above. So it's of no use for communicating beyond a few miles.

The radiation/reception characteristics at low elevation angles of
such an antenna can be useful in establishing contacts with the most
distant possible single-hop DX sites, can they not?

RF

They can not. I see you're still a bit confused about what happens to
that ground wave signal. Beyond a few miles at HF, that low angle
radiation decays to essentially zero. The pattern of the field beyond
that distance resembles the one reported by EZNEC and other programs
giving distant far field data. And they correctly show that unless the
ground has very high conductivity at the reflection point, there will be
very little field remaining at very low angles beyond that ground wave
decay distance.

The performance of an antenna when communicating with a distant station
is precisely what EZNEC is attempting to show you. If you want to know
how it will do at various elevation angles for DX, or even at distances
of a few hundred miles, look at those plots. At HF, ground wave analysis
will only tell you how well the antenna will do when talking with
someone across town. Which is why there's very little interest in ground
wave analysis among amateurs, HF broadcasters, or just about anyone
except AM broadcasters.

Roy Lewallen, W7EL

Low-angle Elevation Gain of a 1/4-wave Vertical Monopole
 

On Nov 29, 9:21*pm, Roy Lewallen wrote:
Richard Fry wrote:
The radiation/reception characteristics at low elevation angles of
such an antenna can be useful in establishing contacts with the most
distant possible single-hop DX sites, can they not?

They can not. I see you're still a bit confused about what happens to
that ground wave signal. Beyond a few miles at HF, that low angle
radiation decays to essentially zero. The pattern of the field beyond
that distance resembles the one reported by EZNEC and other programs
giving distant far field data. And they correctly show that unless the
ground has very high conductivity at the reflection point, there will be
very little field remaining at very low angles beyond that ground wave
decay distance.
_______

I'm not considering that the ground wave signal _provides_ any of that
low-angle DX coverage. It is the direct radiation existing in the
radiation pattern of the monopole at low elevation angles that can do
so. No ground reflection is necessary to create that field - it is
launched by the monopole itself.

Below is a link to a clip from Terman's Radio Engineers Handbook, 1st
edition, showing that the greatest single-hop range for skywave
signals occurs from the radiation of the monopole at elevation angles
of less than ten degrees. But looking at a NEC far-field analysis
this would seem impossible, due to the greatly reduced fields in this
sector that NEC shows for a vertical monopole over real earth.

This clip was done for MW frequencies, but the concept would apply
equally at HF, would it not?

http://i62.photobucket.com/albums/h8...ermanFig55.jpg

RF

Low-angle Elevation Gain of a 1/4-wave Vertical Monopole
 

On Sun, 30 Nov 2008 04:20:33 -0800 (PST), Richard Fry
wrote:

showing that the greatest single-hop range for skywave
signals occurs from the radiation of the monopole at elevation angles
of less than ten degrees.

Which is uniformly poorer by 12 dB than that launched at 40°.

73's
Richard Clark, KB7QHC

Low-angle Elevation Gain of a 1/4-wave Vertical Monopole
 

On Sun, 30 Nov 2008 04:20:33 -0800 (PST), Richard Fry
wrote:

But looking at a NEC far-field analysis
this would seem impossible, due to the greatly reduced fields in this
sector that NEC shows for a vertical monopole over real earth.

NEC is not a propagation modeler.

However, resourcing the top engineers of the AM field for their
observations of sky-wave and ground-wave field strengths (a typical
service application) where they combine destructively (the "fading
wall"); at a distance of 70 miles, for 50% of the time, both signals
are equal (with propagation variations of phase accounting for
fading).

The graph you supply suggests that this 70 mile distance is obtained
by a launch angle (for the sky-wave) of 60 degrees. The NEC far-field
analysis for the BL&E antenna of 70 foot tall radiator in a field of
113 135 foot radials over average ground has a response of -2.32dB @
60° and -1.61dB @ 1° which shows a pretty close accord with field
reports from Laport (Fig 2.7).

73's
Richard Clark, KB7QHC

Low-angle Elevation Gain of a 1/4-wave Vertical Monopole
 

Richard Fry wrote:

_______

I'm not considering that the ground wave signal _provides_ any of that
low-angle DX coverage. It is the direct radiation existing in the
radiation pattern of the monopole at low elevation angles that can do
so. No ground reflection is necessary to create that field - it is
launched by the monopole itself.

Below is a link to a clip from Terman's Radio Engineers Handbook, 1st
edition, showing that the greatest single-hop range for skywave
signals occurs from the radiation of the monopole at elevation angles
of less than ten degrees. But looking at a NEC far-field analysis
this would seem impossible, due to the greatly reduced fields in this
sector that NEC shows for a vertical monopole over real earth.

This clip was done for MW frequencies, but the concept would apply
equally at HF, would it not?

http://i62.photobucket.com/albums/h8...ermanFig55.jpg

RF


I for one, think you correct. What is "launched" at the antenna obeys
physics laws at 100 ft., 1000 ft., 10,000 ft., 100,000 ft., 1,000,000
ft. ... the signal does not ever suffer magical, mystical, supernatural
manipulations--EVER!

Regards,
JS

Low-angle Elevation Gain of a 1/4-wave Vertical Monopole
 

On Nov 30, 5:57*pm, Richard Clark wrote:
On Sun, 30 Nov 2008 04:20:33 -0800 (PST), Richard Fry wrote:
showing that the greatest single-hop range for skywave
signals occurs from the radiation of the monopole at elevation angles
of less than ten degrees.

Which is uniformly poorer by 12 dB than that launched at 40°.
_________

Radiation from the monopole from zero to 10 degree elevation is not
"poorer by 12 dB" than that launched at 40 degrees. It is greater.

The _reception_ of such radiation is a different matter, as the total,
skywave path length, and therefore the propagation losses are
different for those elevation sectors. This accounts for the lower
value of received field at the greater distances, as shown in Terman's
Fig 55.

RF