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

Richard Fry wrote:
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

I'm completely confused about the point you were trying to make. You
called NEC results "misleading" when showing only "far field" (sky wave,
without surface wave) results, and implied that the surface wave must be
considered when determining skip performance. Are you now agreeing that
it correctly shows the amount of radiation at low angles which is
capable of reaching the ionosphere? If so, what's misleading about it?
Or are you saying that the field strength capable of reaching the
ionosphere at low angles is greater than NEC "far field" (sky wave)
analysis reports? And if so, how much greater and why?

Roy Lewallen, W7EL

On Jan 1, 3:46*pm, Roy Lewallen wrote:

I'm completely confused about the point you were trying to make.
You called NEC results "misleading" when showing only "far field"
(sky wave, without surface wave) results, and implied that the
surface wave must be considered when determining skip per-
formance.

No, I did not write that the surface wave must be considered when
determining skywave performance. Your understanding of what I posted
is incorrect. Please re-read what I posted previously, and quote us
any of my text that you believe supports your present conclusion about
this.

Do you reject the data in the Terman and Laport plots I linked to
showing that the most distant, single-hop skywave coverage over a
real, curved earth originates from space wave radiation at very low
elevation angles (less than 5 degrees above the horizontal plane at
the transmit antenna site)?

Are you now agreeing that it (far-field NEC) correctly shows the
amount of radiation at low angles which is capable of reaching
the ionosphere?

Absolutely not, and I am rather surprised that, apparently,
you believe and support this concept.

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