With multi-hops through the ionosphere there will be reflections from
the ground between adjacent hops. If there are N hops there will be
N-1 ground reflections.

At each reflection a signal loss is incurred. But the ground is a
variable quantity.

What rule of thumb relating to reflection loss is used by radio
professionals when calculating path loss?

At the back of my mind I have a figure of 5 dB per reflection. Is this
in the right ball park?

Thank you.
----
Reg.

"Reg Edwards" wrote
With multi-hops through the ionosphere there will be reflections
from
the ground between adjacent hops. If there are N hops there will be
N-1 ground reflections.

At each reflection a signal loss is incurred. But the ground is a
variable quantity.

What rule of thumb relating to reflection loss is used by radio
professionals when calculating path loss?

At the back of my mind I have a figure of 5 dB per reflection. Is
this
in the right ball park?

Thank you.
----
Reg.

=========================================
It has just occurred to me that the loss due to ground reflections may
be related to the angle at which waves strike the earth. It is the
same angle as the transmit elevation angle and can be quite small.
----
Reg.

Reg Edwards wrote:
=========================================
It has just occurred to me that the loss due to ground reflections may
be related to the angle at which waves strike the earth. It is the
same angle as the transmit elevation angle and can be quite small.
----
Reg.

The amplitude and phase of a field after ground reflection depends on
the polarization, and is quite different for horizontal and vertical. It
of course also depends on ground conductivity and permittivity,
frequency, and angle. The equations are simple, and can be found in
Kraus and other references. Those equations are used by NEC for
determination of the far field pattern.

Roy Lewallen, W7EL

The amplitude and phase of a field after ground reflection depends
on
the polarization, and is quite different for horizontal and
vertical. It
of course also depends on ground conductivity and permittivity,
frequency, and angle. The equations are simple, and can be found in
Kraus and other references. Those equations are used by NEC for
determination of the far field pattern.

Roy Lewallen, W7EL

===========================================

Yes! But what's the ball park, rule-of-thumb value in dB ?
----
Reg

Reg Edwards wrote:
The amplitude and phase of a field after ground reflection depends
on
the polarization, and is quite different for horizontal and
vertical. It
of course also depends on ground conductivity and permittivity,
frequency, and angle. The equations are simple, and can be found in
Kraus and other references. Those equations are used by NEC for
determination of the far field pattern.

Roy Lewallen, W7EL

===========================================

Yes! But what's the ball park, rule-of-thumb value in dB ?
----
Reg

I dunno. What's the ball park, rule-of-thumb value of a resistor in ohms?

Roy Lewallen, W7EL

Uh, uh,
Wait until Richard Clark sees this and descends on you!!! :-)

Reg,
you can't just look at HF waves propagation through simple "reflective"
glasses. There is considerable amount of propagating going on by refraction,
ducting, polarization gets all tumbled around and ground conditions vary so
much that even those orderly, same current in a loading coil believers get
drowned. There are some propagation prediction programs that will do some
predicting +- 59%, but that's about it. Otherwise as W7EL says, if you are
looking at the antenna pattern forming properties within few wavelengths you
need to consider polarization and ground conditions within the zone, but
once you get beyond "first hop" you are on the mercy of propagation Gods.
You can look at my oooold article
http://members.aol.com/ve3bmv/bmvpropagation.htm
which at the time was judged "ridiculous" and "nothing new" at the same
time. Now a days, especially low band crowds are coming around and admitting
that there is perhaps more refracting/ducting than reflecting going on and
trying to figure out when, why, how.
W8JI fought another losing battle claiming that there is no high angle
propagation on extreme DX signals on 160/80, or skewed path. Now he is
"guru" on the subject.
Soooo, depends.... what you are after? For "regular" conditions you can
apply some ballparks, but for extreme DXing and weak signal comms, there is
whole different world outside of formulas.

Yuri K3BU, VE3BMV



"Reg Edwards" wrote in message
...
The amplitude and phase of a field after ground reflection depends
on
the polarization, and is quite different for horizontal and
vertical. It
of course also depends on ground conductivity and permittivity,
frequency, and angle. The equations are simple, and can be found in
Kraus and other references. Those equations are used by NEC for
determination of the far field pattern.

Roy Lewallen, W7EL

===========================================

Yes! But what's the ball park, rule-of-thumb value in dB ?
----
Reg

Roy wrote -

I dunno. What's the ball park, rule-of-thumb value of a resistor in
ohms?

=========================================

Lord Kelvin was correct.

"Reg Edwards" wrote in message
...
With multi-hops through the ionosphere there will be reflections from
the ground between adjacent hops. If there are N hops there will be
N-1 ground reflections.

At each reflection a signal loss is incurred. But the ground is a
variable quantity.

What rule of thumb relating to reflection loss is used by radio
professionals when calculating path loss?

At the back of my mind I have a figure of 5 dB per reflection. Is this
in the right ball park?

Thank you.
----
Reg.

Reflection loss can vary from zero to infinity; depending on the material
and angle of incidence.
For example; take the oblique incidence of a horizontally polarized EM wave
on an air/rocky
ground interface of:: conductivity 2 mS/m, and relative permittivity 15.
The reflection loss
at zero degrees is 4.4 dB, increasing to 21.4 dB at the pseudo Brewster
angle of 76 degrees, and
0 dB at 90 degrees. Ref. Advanced Engineering Electromagnetics, C. A.
Balanis, pp 206 - 214.
I have copies of the Mathcad calculations if anybody is interested.

Frank

Reg, G4FGQ wrote:
"At the back of my mind I have a figure of 5 dB per reflection. Is this
the right ball park?"

It could be right for some reflections.

E.A. Laport was Chief Engineer of RCA International when he wrote "Radio
Antenna Engineering". On page 236 Fig. 3.17 shows the effect of ground
conductivity on maximum field strength from a horizontal dipole antenna
versus its height in vavelengths.

Optimum height would be about 0,50 wavelength to most concentrate energy
at a certain vertical angle, 30-degrees according to the RAF Signal
Manual quoted by Laport. 30-degrees might hop 1000 kilometers.

At the antenna earth reflection point, frequencies between 2 and 16 MHz
are reduced to 95% of their prereflection field strengths by ordinary
soil from a dipole at 1/2-wavelength height. A reduction to 70.7% of
prereflection strength would represent a 3 dB power loss. So no harm
done yet by the reflection from an antenna over good soil. Lower antenna
height and poorer soil would attenuate more.

The angle at which rhe signal strikes the earth in subsequent
reflections should be the same as the first reflection from the antenna.
Conductivity and dielectric constant at subsequent earth reflection
points are what they are.

Shortwave broadcasters use vertically stacked horizontal elements to
concentrate the vertical beam to avoid multipath interference. They also
prefer targets reached on the first reflection from the ionosphere..

Best regards, Richard Harrison, KB5WZI

On Thu, 27 Apr 2006 08:22:17 -0400, "Yuri Blanarovich"
wrote:

Uh, uh,
Wait until Richard Clark sees this and descends on you!!! :-)

Hi Yuri,

You mean he might stand a chance of obtaining data instead of griping:

W8JI fought another losing battle

Reggie,

Rule of hand with 5 thumbs....

Land Loss
deg 10 20 30 40
band
80 3 4+ 5+ 6
40 3+ 5+ 6 7
30 3+ 6 7 8
20 3+ 6+ 7+ 8+
10 3 6+ 7+ 8+

Sea Loss
deg 10 20 30 40
band
80 .2 .14 .12 .1
40 .3 .18 .15 .13
30 .4 .2 .17 .15
20 .45 .27 .22 .18
10 .52 .38 .28 .27

Washington D.C. - London
Autumn Ionospheric 1.35MHz Loss
UT dB
00 0
02 0
04 0
06 0
08 0
10 2
12 1.5
14 1
16 1.5
18 1
20 .5
22 0

Earth/Ionospheric Waveguide Loss/10000kM
band
80 20+
40 8
30 3
20 2
10 2

73's
Richard Clark, KB7QHC