*The idea that coverage is maximum for the 5/8th wave
radiator is common but in practice, (maybe we are saying the
same thing) a straight 1/2 wave may have a smaller fading
ring because it does not have the high-angle lobe wich
appears on the 5/8th wave pattern.

My comments also were addressing the belief of the OP that the peak
gain of a 5/8-wave vertical was very little different than for a 1/4-
wave, because of a high-angle lobe developed by a 5/8-wave.

It is true that such a high-angle lobe develops to some extent for all
vertical monopoles between 1/2-wave and 5/8-wave in electrical
height. This can be seen in the plots linked below (FCC method).

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

But regardless, the 5/8-wave has the greatest peak gain of the five
monopoles shown.

In situations where the combination of frequency, earth conductivity
etc (my 6 points above) limits the useful groundwave coverage radius
closer to the transmitter site than is served by the high-angle lobe
from a 5/8-wave radiator, then the 5/8-wave would produce the greatest
fade-free groundwave coverage area day and night (other things equal).

However this isn't the case for most "Class A" (50 kW, non-directional
day/night) AM stations. The most common radiator height used by them
is about 195 degrees, which provides a little more groundwave range
than a 1/2-wave, and about the greatest distance/smallest width for
the fade zone. WJR, in fact, uses a 195 degree vertical.

I certainly agree that patterns calculated for "ideal" ground
are not matched by practical ground systems except, perhaps,
sea-water grounds.

The FCC approach for AM broadcast stations is to use the pattern/gain
of the radiator over a perfect ground as a basis for the groundwave
field intensity at a given distance over real ground, as determined by
the FCC's MW propagation curves -- which curves are based on real-
world, measured performance.

With the advent of NEC and NEC-2, some have been misled by the
elevation pattern shown for a vertical radiator at an infinite
distance over real ground as being that of the radiation launched by
that vertical radiator. But it is not, it is only the amount of that
original radiation that survives at an infinite distance, for those
ground conditions (and for flat earth, at that).

This has led to the concept of a "take-off angle" from a ground-
mounted vertical where peak radiation occurs, and that little to no
radiation occurs from the monopole in and near the horizontal plane.
But that isn't the case -- the relative field over real ground at low
elevation angles close to the vertical radiator can be very high, and
will continue onward to produce a long-range skywave. Even radiation
at an elevation angle of one degree will reach the ionosphere, due to
earth curvature.

The theoretical elevation patterns shown in my plots don't exist very
far from the antenna, but they are a closer to reality over real
ground than those shown by NEC and NEC-2 for an infinite distance over
the same ground, and assumed also to exist close to the radiator.

The results obtained using NEC-4 to calculate the groundwave field
intensity within the useful daytime coverage areas of AM broadcast
stations give much better correlation to the measured fields, and to
the methodology of using the theoretical pattern with the FCC's MW
propagation curves.

RF

"Richard Fry" wrote in message
...
The idea that coverage is maximum for the 5/8th wave
radiator is common but in practice, (maybe we are saying
the
same thing) a straight 1/2 wave may have a smaller fading
ring because it does not have the high-angle lobe wich
appears on the 5/8th wave pattern.

My comments also were addressing the belief of the OP that
the peak
gain of a 5/8-wave vertical was very little different than
for a 1/4-
wave, because of a high-angle lobe developed by a 5/8-wave.

It is true that such a high-angle lobe develops to some
extent for all
vertical monopoles between 1/2-wave and 5/8-wave in
electrical
height. This can be seen in the plots linked below (FCC
method).

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

But regardless, the 5/8-wave has the greatest peak gain of
the five
monopoles shown.

Snipping here...........

I was not disagreeing with what you say above, perhaps
I did not express it well.


--

--
Richard Knoppow
Los Angeles
WB6KBL

Richard Fry wrote:

The FCC approach for AM broadcast stations is to use the pattern/gain
of the radiator over a perfect ground as a basis for the groundwave
field intensity at a given distance over real ground, as determined by
the FCC's MW propagation curves -- which curves are based on real-
world, measured performance.

With the advent of NEC and NEC-2, some have been misled by the
elevation pattern shown for a vertical radiator at an infinite
distance over real ground as being that of the radiation launched by
that vertical radiator. But it is not, it is only the amount of that
original radiation that survives at an infinite distance, for those
ground conditions (and for flat earth, at that).

That is correct, except only the people who misunderstand surface wave
propagation are being misled.

This has led to the concept of a "take-off angle" from a ground-
mounted vertical where peak radiation occurs, and that little to no
radiation occurs from the monopole in and near the horizontal plane.
But that isn't the case -- the relative field over real ground at low
elevation angles close to the vertical radiator can be very high, and
will continue onward to produce a long-range skywave. Even radiation
at an elevation angle of one degree will reach the ionosphere, due to
earth curvature.

Radiation at one degree will indeed reach the ionosphere. But not the
radiation propagating as a surface wave, as I'll show.

The field launched at very low angles contacts the ground and in doing
so, induces a current into it. This extracts power from the wave as it
propagates. The result is that the surface wave field is attenuated
quite rapidly with distance. At AM broadcast frequencies, it propagates
far enough to be useful for local broadcasting, but it doesn't reach the
ionosphere. If it did, fading (due to surface and sky wave interference
at distant points) would be a much more serious problem for broadcasters
than it is. As you go higher in frequency, the attenuation becomes
greater, so the surface wave propagates even less distance before
dropping below the noise level. No measurable fraction of it ever goes
anywhere near far enough to reach the ionosphere.

The theoretical elevation patterns shown in my plots don't exist very
far from the antenna, but they are a closer to reality over real
ground than those shown by NEC and NEC-2 for an infinite distance over
the same ground, and assumed also to exist close to the radiator.

NEC (NEC-2 and NEC-4) does a very good job of showing surface wave
("ground wave") field strength. It uses the same calculation method as
used by the FCC and, I understand that NEC modeling results are now
being accepted by the FCC in lieu of measurement for AM broadcast proof
of performance.

The results obtained using NEC-4 to calculate the groundwave field
intensity within the useful daytime coverage areas of AM broadcast
stations give much better correlation to the measured fields, and to
the methodology of using the theoretical pattern with the FCC's MW
propagation curves.

NEC-2 and NEC-4 give virtually identical results for both ground and sky
wave propagated fields. The results below were done using EZNEC Pro with
the double precision NEC-4 calculating engine. Results using the double
precision NEC-2 engine were different by less than 0.1 dB.

Here are some numerical values to illustrate what happens to the ground
wave field(*). Two antennas are analyzed, one at 1 MHz and the other at
7 MHz. Each is 1/4 wavelength high with effectively a zero loss ground
system. The 1 MHz antenna is 7 inches in diameter, the 7 MHz antenna one
inch diameter. (Diameter in this range makes no significant difference.)
Ground is "average" -- 5 mS/m conductivity, dielectric constant of 13.
Data are for elevation angles of zero, one, and two degrees, to
distances of 50 miles. In that distance range, the difference between
flat and curved Earth is negligible. The reported field strengths are in
dBi for easy comparison; to convert to mV/m, use

mV/m = 1000 / Distance(m) * Sqrt(30 * Power(w)) * 10 ^ (dBi / 20)

dBi is a comparison to the field from an isotropic antenna with the same
input power, measured at the same distance. So if the attenuation of all
parts of the field is the same with distance (in other words, if the
pattern is the same shape at all distances), the dBi field strength will
be the same at all distances.

First, the results. Field intensities are shown at various distances
from the antenna. These are the entire field, including surface wave.
View with a fixed-width font.

Field strengths, dBi, are under each elevation angle:

-- Elev angle --
Freq(MHz) Dist(mi) 0 deg 1 deg 2 deg
1 inf -inf -11.8 -6.8
1 1 4.0 3.6 3.4
1 10 -2.8 -5.0 -4.6
1 50 -18.7 -12.6 -7.0
7 inf -inf -18.1 -12.7
7 1 -15.8 -15.4 -12.2
7 10 -36.7 -18.3 -12.8
7 50 -50.8 -18.1 -12.7

The two lines with "inf" distance are sky wave only, that is, the field
is evaluated at a great enough distance that the ground wave has decayed
to effectively zero. This is what you'll see with EZNEC's Far Field
analysis, or with NEC if you don't include the ground wave. The
difference between these and the entries at other distances represents
the contribution of the ground wave at those distances.

As Richard has observed, the field strength is zero at zero elevation
angle and long distances. At the AM broadcast frequency of 1 MHz,
there's still the useful field strength of -18.7 dBi at the surface at
50 miles (even though the field strength is 20 dB greater -- not shown
-- at an elevation angle of 20 degrees). Notice, though, how the zero
degree field relative field strength continues getting smaller with
distance. Fortunately for the broadcasters, the surface wave component
doesn't detach itself from the Earth and head for the ionosphere as the
Earth curves away, but follows the curvature of the Earth. This allows
broadcasting beyond the horizon without ionospheric skip, and prevents
fading from the ground wave alone. It doesn't reach the ionosphere as
Richard has claimed.

The ground wave contribution is noticeable at one and two degrees also.
But look at what happens with distance (at 1 MHz). At 50 miles, the
ground wave contribution at those angles is hardly noticeable, as you
can see by comparing the dBi field strength at that distance with the
dBi field strength at an infinite distance. Interestingly, the dBi field
strength is shown to be very slightly lower at 50 miles than at an
infinite distance. This might be due to some of the power in the surface
wave at 50 miles being transferred to the sky wave at greater distances,
or it might be due to small calculation errors. But the differences are
slight, indicating that no significant ground wave energy remains at 50
miles at elevation angles of 1 and 2 degrees. There's no noticeable
ground wave energy at higher angles at or beyond 50 miles, and very
little at closer distances.

An analysis of the 7 MHz antenna shows the same thing, except that the
ground wave decays faster as expected. At that frequency, the one and
two degree field strengths have reached their sky wave-only values at 10
miles -- indicating essentially full decay of the ground wave -- , and
the zero degree field strength is less than -50 dBi at 50 miles.

The _ARRL Antenna Book_ propagation chapter devotes only one paragraph
to surface wave propagation(*). It summarizes that "The surface wave is
of little value in amateur communication, except possibly at 1.8 MHz."
Analysis which concludes that the surface wave plays a role in
ionospheric skip is erroneous and leads to conclusions which are simply
and demonstrably not true. Ground wave field strength is of interest at
HF only for communicating with stations within a few miles, and at MF
with stations within a few tens of miles.

(*) Technically, the surface wave is only one kind or component of a
ground wave, as explained in the _ARRL Antenna Book_. However, NEC uses
"ground wave" to mean the wave in contact with the ground, which some
others call the surface wave. This discussion is about the wave in
contact with the ground, for which I've used "ground wave" and "surface
wave" pretty much interchangeably. There's a good discussion of surface
wave propagation in Terman's _Radio Engineering_ as well as other
references.

Roy Lewallen, W7EL

Roy Lewallen's post of Dec 29, 9:45 pm generally supports the points I
have been writing about.

But two observations are due:

Fortunately for the broadcasters, the surface wave component
doesn't detach itself from the Earth and head for the ionosphere
as the Earth curves away, but follows the curvature of the Earth.
This allows broadcasting beyond the horizon without ionospheric
skip, and prevents fading from the ground wave alone. It doesn't
reach the ionosphere as Richard has claimed.

The last sentence above is incorrect in that I made no such claim. My
post stated only that radiation from elevation angles as small as one
degree will reach the ionosophere. See the paste below.

"But that isn't the case -- the relative field over real ground at low
elevation angles close to the vertical radiator can be very high, and
will continue onward to produce a long-range skywave. Even radiation
at an elevation angle of one degree will reach the ionosphere, due to
earth curvature."

A low elevation angle does not include zero degrees (the horizontal
plane).

The reported field strengths are in dBi for easy comparison; ...

The term "dBi" is not a unit of field strength. Field strength is a
voltage existing between two points in space typically one meter
apart, and is expressed in terms of that voltage with respect to that
distance, as in volts/meter (V/m).

Field strengths can be compared using decibels, but such comparisons
are referenced to the field strength shown in standard form. For
example, a field strength of 1,000 µV/m may be expressed as 60 dBµV/m.

RF

Richard Fry wrote:
Roy Lewallen's post of Dec 29, 9:45 pm generally supports the points I
have been writing about.

That surprises me. Apparently I didn't understand your points, which
seem to emphasize the importance of considering the field very close to
the antenna in evaluating an antenna's performance for long-distance
skip communication. The data I posted show clearly that this isn't so,
because that strong field at low angles is attenuated to virtually zero
well before it can reach the ionosphere. The low elevation angle field
close to the antenna is of interest only if the other station is close
to the antenna.

But two observations are due:

Fortunately for the broadcasters, the surface wave component
doesn't detach itself from the Earth and head for the ionosphere
as the Earth curves away, but follows the curvature of the Earth.
This allows broadcasting beyond the horizon without ionospheric
skip, and prevents fading from the ground wave alone. It doesn't
reach the ionosphere as Richard has claimed.

The last sentence above is incorrect in that I made no such claim. My
post stated only that radiation from elevation angles as small as one
degree will reach the ionosophere. See the paste below.

Sorry, I interpreted your postings to state that the surface wave was an
important factor to consider in determining the strength of the field
from a vertical for working skip communication. If that's not what you
meant, then exactly what is the point you were trying to make regarding
the importance of considering the surface wave for amateur communication?

"But that isn't the case -- the relative field over real ground at low
elevation angles close to the vertical radiator can be very high, and
will continue onward to produce a long-range skywave. Even radiation
at an elevation angle of one degree will reach the ionosphere, due to
earth curvature."

The data I gave shows this to be incorrect. While the field at low
angles close to the radiator are very high, they don't "continue outward
to produce a long-range skywave". The very low angle field, as I've
shown, decays rapidly with distance and is virtually gone well short of
the distance needed to reach the ionosphere. From the data, at one
degree elevation angle, the surface wave has decayed to nearly zero
within 50 miles of the antenna (the difference between sky wave +
surface wave and sky wave only is 0.8 dB), and at 7 MHz, within 10
miles. This means that the surface wave makes no contribution to the one
degree elevation angle wave reaching the ionosphere. So there's no point
in calculating or considering the surface wave if your interest is in
ionospheric, or anything other than ground wave, communication.

A low elevation angle does not include zero degrees (the horizontal
plane).

Ok.

The reported field strengths are in dBi for easy comparison; ...

The term "dBi" is not a unit of field strength. Field strength is a
voltage existing between two points in space typically one meter
apart, and is expressed in terms of that voltage with respect to that
distance, as in volts/meter (V/m).

dBi is a direct expression of field strength, but normalized for power
and distance. At any particular distance and power level, for any field
strength in V/m there is only one corresponding value of dBi, and
vice-versa. I gave the conversion equation in my posting.

Field strengths can be compared using decibels, but such comparisons
are referenced to the field strength shown in standard form. For
example, a field strength of 1,000 µV/m may be expressed as 60 dBµV/m.

dBi is more than just dB. It's field strength (in dB) relative to a
known standard. That enables direct calculation of field strength in V/m
for any power level and distance, given the dBi value.

But that's really beside the point. Anyone with a calculator and the
posted equation can quickly convert the table I gave into V/m for
whatever power level you'd like. The conclusions are the same.

Roy Lewallen, W7EL

On Dec 31 2008, 4:10*pm, Roy Lewallen wrote:
Richard Fry wrote:
Roy Lewallen's post of Dec 29, 9:45 pm generally supports the points I
have been writing about.

That surprises me. Apparently I didn't understand your points, which
seem to emphasize the importance of considering the field very close to
the antenna in evaluating an antenna's performance for long-distance
skip communication. The data I posted show clearly that this isn't so,
because that strong field at low angles is attenuated to virtually zero
well before it can reach the ionosphere. The low elevation angle field
close to the antenna is of interest only if the other station is close
to the antenna.

If this belief were true then the long-distance coverage possible for
some MW broadcast stations would have to be made using more than a
single reflection from the ionosphere. Yet the texts of Terman
( http://i62.photobucket.com/albums/h8...ermanFig55.jpg ) and
Laport ( http://i62.photobucket.com/albums/h8...aportFig23.jpg
) show that such coverage is possible from single-hop skywave radiated
at elevation angles of just a few degrees.

And as this is true for MW broadcast monopoles, it is equally true for
the HF monopoles used by amateurs.

Sorry, I interpreted your postings to state that the surface wave was an
important factor to consider in determining the strength of the field
from a vertical for working skip communication. If that's not what you
meant, then exactly what is the point you were trying to make regarding
the importance of considering the surface wave for amateur communication?

Again, I do not, and never have considered the surface wave to be
important in skywave communications. The reason I referred to it was
to show that if it exists with substantial relative field close to the
radiator, then so does substantial radiation exist there at low
elevation angles, and which can serve the most distance ranges using a
single reflection from the ionosphere.

RF

On Jan 1, 9:28*am, Richard Fry wrote:


Again, I do not, and never have considered the surface wave to be
important in skywave communications. *The reason I referred to it was
to show that if it exists with substantial relative field close to the
radiator, then so does substantial radiation exist there at low
elevation angles, and which can serve the most distance ranges using a
single reflection from the ionosphere.

RF

I haven't really given this much thought, but seems to me the
low angle radiation that does reach the ionosphere and would
be useful for very long ranges would be considered the lower angles
of the space wave, and would be separate from the ground or surface
wave, whichever you would want to call it.. I tend to use "ground
wave",
but I've always considered it separate from the "space wave" as
I call it..
As a difference between the two, the ground wave would follow the
curvature of the earth, but the lowest angles of the space wave would
not. They would continue at the original angle, which naturally would
lead them to the ionosphere eventually. At low angles too if
measured from the transmitter location.
Anyway, seems to me almost all radiation that strikes the
ionosphere at low angles would be from the space wave, not the
ground wave.
I dunno if this makes any sense or is totally correct.. MPG will
vary..

On Jan 1, 11:21*am, wrote:
I haven't really given this much thought, but seems to me the
low angle radiation that does reach the ionosphere and would
be useful for very long ranges would be considered the lower angles
of the space wave, and would be separate from the ground or surface
wave...

I totally agree. That is the point I have been trying to make:
radiation from low elevation angles is not attenuated virtually to
zero before it reaches the ionosphere.

RF

wrote:
On Jan 1, 9:28 am, Richard Fry wrote:

Again, I do not, and never have considered the surface wave to be
important in skywave communications. The reason I referred to it was
to show that if it exists with substantial relative field close to the
radiator, then so does substantial radiation exist there at low
elevation angles, and which can serve the most distance ranges using a
single reflection from the ionosphere.

RF

I haven't really given this much thought, but seems to me the
low angle radiation that does reach the ionosphere and would
be useful for very long ranges would be considered the lower angles
of the space wave, and would be separate from the ground or surface
wave, whichever you would want to call it.. I tend to use "ground
wave",
but I've always considered it separate from the "space wave" as
I call it..
As a difference between the two, the ground wave would follow the
curvature of the earth, but the lowest angles of the space wave would
not. They would continue at the original angle, which naturally would
lead them to the ionosphere eventually. At low angles too if
measured from the transmitter location.
Anyway, seems to me almost all radiation that strikes the
ionosphere at low angles would be from the space wave, not the
ground wave.
I dunno if this makes any sense or is totally correct.. MPG will
vary..

Yes, that's correct. And NEC (and EZNEC) correctly show this low angle
sky wave radiation in their "far field" (sky wave only) analysis. The
surface wave isn't of much interest to anyone except AM broadcasters,
except perhaps some amateurs interested in local (a few tens of miles)
ground wave communication on 160 meters.

Roy Lewallen, W7EL

On Dec 31 2008, 4:10*pm, Roy Lewallen wrote:
dBi is more than just dB. It's field strength (in dB) relative to a
known standard.

Roy, certainly you must recognize that the term dBi is not a direct
unit, or measure of field intensity (field strength). The direct,
primary unit of measure of field intensity in accurate, common use is
volts/meter (V/m).

The term dBi is only a measure of the relative gain of a particular
radiator in particular directions stated in decibels with respect to
an isotropic radiator.

Other things equal, the field intensity at a given location is
determined by the gain of a transmit antenna system along paths
serving that location ALONG WITH the matched, r-f power applied to the
feedpoint of that antenna.

The gain of a transmit antenna system in dBi includes nothing about
the absolute amount of power it radiates in various directions.
Therefore by itself, antenna gain(s) in dBi cannot define the absolute
field intensity in standard units/sub-units of V/m that such an
antenna can produce at a given location.

Ergo "dBi" is not a measure of absolute field intensity (field
strength).

RF